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S

S-Band Radar: These were in use as network radars in the National Weather Service prior to the installation of the WSR 88-D radars. They were 10-centimeter wavelength radars.

Sacramento Soil Moisture Accounting Model (SACSMA): A model which simulates the movement and occurrence of water in and on top of the ground.

SafetyNET:  A satellite based part of the Global Maritime Distress and Safety System (GMDSS) for automatically disseminating safety information, including weather warnings and forecasts, to mariners almost anywhere on the world's oceans.

Sample and Hold:  The process of sampling (measuring) the signal strength at a particular point in space (i.e., at a range gate).

Sandstorm (SS): Particles of sand carried aloft by strong wind. The sand particles are mostly confined to the lowest ten feet, and rarely rise more than fifty feet above the ground.

Saffir-Simpson Hurricane Intensity Scale: This scale was developed in an effort to estimate the possible damage a hurricane's sustained winds and storm surge could do to a coastal area. The scale of numbers are based on actual conditions at some time during the life of the storm. As the hurricane intensifies or weakens, the scale number is reassessed accordingly. The following table shows the scale broken down by central pressure, winds, and storm surge:
 

Saffir-Simpson Hurricane Intensity Scale

Scale  Number (Category)

Central Pressure

Winds (MPH)

Storm Surge (Feet)

  Damage

Millibars

Inches

1

greater than 980

greater than 28.94

74 - 95

4 - 5

Minimal

2

965 - 979

28.50 - 28.91 

96 - 110

6 - 8

Moderate

3

945 - 964

27.91 - 28.47

111 - 130

9 - 12

Extensive

4

920 - 944

27.17 -27.88

131 - 155

13 - 18

Extreme

5

less than 920

less than 27.17

greater than 155

greater than 18

Catastrophic

 
 

Damage Associated with the Saffir-Simpson Hurricane Intensity Scale
Category Damage Description
1 Damage mainly to trees, shrubbery, and unanchored mobile homes. There is no substantial damage to other structures. Some damage occurs to poorly constructed signs. Low lying coastal roads are inundated. There is some minor damage to piers. Some small craft in exposed anchorages are torn from their moorings.
2 There is considerable damage to shrubbery and to tree foliage, with some trees blown down. Major damage occurs to exposed mobile homes. There is extensive damage to poorly constructed signs and some damage to roofing materials of buildings, windows, and doors. No major destruction occurs to buildings. Coastal roads and low-lying escape routes inland are cutoff by rising water about 2 to 4 hours before the arrival of the hurricane center. There is considerable damage done to piers, and marinas are flooded. Small craft in unprotected anchorages are torn from their moorings. Evacuation of some shoreline residences and of low-lying areas is required.
3 Foliage removed from trees and large trees blown down. Nearly all poorly constructed signs are blown down. There is some damage to roofing materials of buildings, windows, and doors. Some structural damage occurs to small buildings. Mobile homes are destroyed. Serious flooding occurs at the coast and many smaller structures near the coast are damaged by battering waves and floating debris. Low-lying escape routes inland are cut by rising water about 3 to 5 hours before the hurricane center arrives. Flat terrain 5 feet or less above sea-level is flooded up to 8 or more miles inland. Evacuations of low-lying residences within several blocks of the shoreline may be required.
4 Shrubs, trees, and all signs are blown down. There is extensive damage to roofing materials, windows, and doors, with complete failure of roofs on many small residences. Mobile homes are demolished. Flat terrain which 10 feet or less above sea-level is flooded inland as far as 6 miles. The flooding and the battering by waves and floating debris cause major damage to the lower floors of structures near the shore. Low-lying escape routes inland are cut off by rising water about 3 to 5 hours before the arrival of the hurricane center. There is major erosion of beaches. Massive evacuation of all residences within 500 yards of the shore may be required, as well as of single-story residences on low ground within 2 miles of shore.
5 Trees, shrubs, and all signs are blown down. There is considerable damage to roofs of buildings, with very severe and extensive damage to windows and doors. Indeed, complete failure of roofs occur on many residences and industrial buildings. There is extensive shattering of glass in windows and doors. Some buildings are destroyed completely. Small buildings are overturned or blown away, and mobile homes are demolished. There is major damage to lower floors of all structures which are less than 15 feet above sea-level within 1,500 feet of the shore. Low-lying escape routes inland are cut off by rising water about 3 to 5 hours before the arrival of the hurricane center. Massive evacuations of residential areas on low ground within 5 to 10 miles of the shore may be required.

 

Santa Ana Wind: A strong, hot, dry foehn-like wind that blows from the north, northeast, or east into southern California.

Satellite Hydrology Program:  A NOHRSC program that uses satellite data to generate areal extent of snow cover data over large areas of the western United States.

SAWRS (Supplementary Aviation Weather Reporting Station)The SAWRS program addresses the concerns of users who depend on weather observations for air operations.  If the cooperator is collocated with a commissioned automated system, they ensure continuity during outage periods of the automated system. The requirement for a SAWRS arises from the FAA validated need for observations to satisfy FAR 121 or 135 operations or for the safe conduct of other aircraft.  The classification of a SAWRS operation is directly related to the automated observing platforms.  There are three distinct SAWRS classifications:

    1) SAWRS:  Indicates manual weather observations are the primary source of reporting the weather at the airport.

    2) Backup SAWRS:  Indicates automated observations taken by a commissioned version 3 of an
    Automated Weather Observing System (AWOS), better known as an AWOS III, is the primary source of reporting
    the airport's weather. AWOS III systems may be FAA owned units or privately non-federal units owned by airports
    or local governments.

    3) SAWRS-II:  Indicates automated observations taken by a commissioned Automated Surface Observing System
    (ASOS), is the primary source of reporting the airport's weather. An ASOS unit is federally owned by the Federal
    Aviation Administration (FAA) or the National Weather Service (NWS).

SBCAPECAPE calculated using a Surface based parcel.  See Convective Available Potential Energy (CAPE).

SBLI:  Lifted Index (LI) calculated using surfaced based parcel.  See Lifted Index.

Scattered (SCT)1) An official sky cover classification for aviation weather observations, descriptive of a sky cover of 3/8 to 4/8.  This is applied only when obscuring phenomenon aloft are present--that is, not when obscuring phenomenon are surface-based, such as fog. 2) A National Weather Service convective precipitation descriptor for a 30, 40, and 50 percent chance of measurable precipitation (0.01 inch).  See Precipitation Probability (PoP).

Scattering: The change in direction, frequency, or polarization of electromagnetic waves. See also Back scatter.

Scotophobia:  The fear of the darkness.  See Achluophobia.

SCS:  The Soil Conservation Service.

Scud (or Fractus):  Small, ragged, low cloud fragments that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gust fronts. Such clouds generally are associated with cool moist air, such as thunderstorm outflow.

SDM:  an acronym for Station Duty Manual

Sea Breeze:  A current of air flowing inland, associated with warmer surface temperatures inland than at sea. Often shows up as a long, thin radar feature as insects get caught up in the frontal region. Temperature and moisture gradients across the front may also contribute to its reflectivity.

Seas:  This term is used in National Weather Service Marine Forecasts to describe the combination or interaction of wind waves and swells (combined seas) in which the spearate components are not distinguished.  This includes the case when swells are negligible or are not considered in describing sea state.

Sea Level Pressure: The pressure value obtained by the theoretical reduction or increase of barometric pressure to sea-level.

Second-Day Feet (SDF): The volume of water represented by a flow of one cubic foot per second for 24 hours; equal to 86,400 cubic feet. This is used extensively as a unit of runoff volume.

Sector Visibility: The visibility in a specific direction that represents at least a 45o arc of a horizontal circle.

Sectorized Hybrid Scan:  A single reflectivity scan composed of data from the lowest four elevation scans. Close to the radar, higher tilts are used to reduce clutter. At further ranges, either the maximum values from the lowest two scans are used or the second scan values are used alone.

Sediment Storage Capacity The volume of a reservoir planned for the deposition of sediment.

Seepage:  The interstitial movement of water that may take place through a dam, its foundation, or abutments.

Seiche: A standing wave oscillation in any enclosed lake which continues after the forcing mechanism has ceased. In the Great Lakes, this forcing mechanism may be either strong winds along the axis of a lake, a pressure jump, or down draft winds associated with fast moving squall lines over the lake. In either case, water is piled up at one end. The water sloshes from one end of the lake to the other causing fluctuations of perhaps several feet before damping out.

SELS - SEvere Local Storms Unit:  The former name of the Operations Branch of the Storm Prediction Center (SPC) in Norman, OK (formerly in Kansas City, MO).

Serial Derecho: It consists of an extensive squall line which is oriented such that the angle between the mean wind direction and the squall line axis is small. A series of LEWPs and bow echoes move along the line. The downburst activity is associated with the LEWPs and bows. A Serial Derecho tends to be more frequent toward the north end of the line during the late winter and spring months. It occurs less frequently than its cousin the "progressive derecho".

It is associated with a linear type mesoscale convective system that moves along and in advance of a cold front or dry line. These boundaries are often associated with a strong, migratory surface low pressure system and strong short wave trough at 500 mb (strong dynamic forcing). Lifted Indices are typically -6 or lower and the advection of dry air in the mid-troposphere (3-7 km above ground) by relatively strong winds leads to high convective instability and increased downdraft potential. The bow echoes move along the line in the direction of the mean flow, often southwest to northeast. These storms move at speeds exceeding 35 knots. Squall line movement is often less than 30 knots.

Service Hydrologist:  The designated expert of the hydrology program at a WFO.

Set:  The direction towards which a current is headed.  For example, a current moving from west to east is said to be set to east.

Set-up:  The process whereby strong winds blowing down the length of a lake cause water to "pile up" at the downwind end, raising water levels there and lowering them at the upwind end of the lake.

Severe IcingThe rate of ice accumulation on an aircraft is such that de-icing/anti-icing equipment fails to reduce or control the hazard.  Immediate diversion is necessary.  This standard of reporting this type of icing was based on a recommendation set forth by the subcommittee for Aviation Meterorological Services in the Office of the Federal Coordinator for Meteorology in November 1968.  The convection has been to designate icing intensity in terms of its operational effect upon reciprocating engine, straight wing transport aircraft as used by commuter operators.

Severe StormA storm with a tornado, surface hail >= 3/4 inch, or wind gusts >= 50 knots, or all three.

Severe Supercell Thunderstorm: It is potentially the most dangerous of the convective storm types. Storms possessing this structure have been observed to generate the vast majority of long-lived strong and violent (F2-F5) tornadoes, as well as downburst damage and large hail. It is defined as a thunderstorm consisting of one quasi-steady to rotating updraft which may exist for several hours. Supercells usually move to the right of the mean wind. These are called "Right Movers" and they are favored with veering winds. Occasionally, these thunderstorms will move to the left of the mean wind. These thunderstorms are called "Left Movers". These supercells typically don't last as long as their "Right Mover" cousins and they usually only produce large hail (greater than 3/4 inch in diameter) and severe wind gusts in the excess of 58 miles an hour. Left Movers are favored when you have backing winds.

Radar will observe essentially one long-lived cell, but small perturbations to the cell structure may be evident. The stronger the updraft, the better the chance that the supercell will produce severe (hail greater than 3/4 inch in diameter, wind gusts greater than 58 miles an hour, and possibly a tornado) weather.

Severe supercell development is most likely in an environment possessing great buoyancy (CAPE) and large vertical wind shear. A Bulk Richardson Number of between 15 and 35 favor supercell development. Typically, the hodograph will look like a horse shoe. This is due to the wind speed increasing rapidly with height and the wind direction either veering or backing rapidly with height.

Severe Thunderstorm: A thunderstorm that produces either of the following: winds of 58 miles an hour or greater (these speeds can result in structural or tree damage), hail 3/4 of an inch in diameter or larger, or a tornado. Lightning frequency is not a warning criteria for issuing a severe thunderstorm warning. Severe thunderstorms can result in the loss of life and property. They can also produce a tornado with little or no advanced warning. A table of hail sizes can be found in this glossary, under the definition of hail. The following table shows an estimate of winds that meet Severe Thunderstorm criteria:
 

Estimated Winds that Meet Severe Thunderstorm Criteria

Wind Speed Damage
58-72 mph Damage to chimneys and TV antennae; uproots shallow rooted trees and blows down limbs or branches.
73-112 mph Peels surface off roofs; windows broken; mobile homes moved or overturned; moving automobiles pushed off roads
113-157 mph Roofs torn off houses; weak buildings and mobile homes destroyed; large trees snapped or uprooted
158+ mph Severe damage; vehicles lifted off ground

 

See approaching (severe).

Severe Thunderstorm Warning (SVR):  This is issued when either a severe thunderstorm is indicated by the WSR-88D radar or a spotter reports a thunderstorm producing hail 3/4 inch or larger in diameter and/or winds equal or exceed 58 miles an hour; therefore, people in the affected area should seek safe shelter immediately.  Severe thunderstorms can produce tornadoes with little or no advance warning.  Lightning frequency is not a criteria for issuing a severe thunderstorm warning.  They are usually issued for a duration of one hour.  They can be issued without a Severe Thunderstorm Watch being already in effect.

Like a Tornado Warning, the Severe Thunderstorm Warning is issued by your National Weather Service Forecast Office (NWFO).  Severe Thunderstorm Warnings will include where the storm was located, what towns will be affected by the severe thunderstorm, and the primary threat associated with the severe thunderstorm warning.  If the severe thunderstorm will affect the nearshore or coastal waters, it will be issued as the combined product--Severe Thunderstorm Warning and Special Marine Warning.    If the severe thunderstorm is also causing torrential rains, this warning may also be combined with a Flash Flood Warning.   If there is an ampersand (&) symbol at the bottom of the warning, it indicates that the warning was issued as a result of a severe weather report.

After it has been issued, the affected NWFO will followed it up periodically with Severe Weather Statements.  These statements will contain updated information on the severe thunderstorm and  they will also let the public know when warning is no longer in effect.

Severe weather reports will either be reported in one the following 3 products:  1) another severe weather warning (Tornado or Severe Thunderstorm), 2) in a Severe Weather Statement, or 3) in a Local Storm Report.

Severe Thunderstorm Watch (WWA):  This is issued by the National Weather Service when conditions are favorable for the development of severe thunderstorms in and close to the watch area.  A severe thunderstorm by definition is a thunderstorm that produces 3/4 inch hail or larger in diameter and/or winds equal or exceed 58 miles an hour.  The size of the watch can vary depending on the weather situation.  They are usually issued for a duration of 4 to 8 hours.  They are normally issued well in advance of the actual occurrence of severe weather.  During the watch, people should review severe thunderstorm safety rules and be prepared to move a place of safety if threatening weather approaches.

A Severe Thunderstorm Watch is issued by the Storm Prediction Center in Norman, Oklahoma.  Prior to the issuance of a Severe Thunderstorm Watch, SPC will usually contact the affected local National Weather Service Forecast Office (NWFO) and they will discuss what their current thinking is on the weather situation.  Afterwards, SPC will issue a preliminary Severe Thunderstorm Watch and then the affected NWFO will then adjust the watch (adding or eliminating counties/parishes) and then issue it to the public by way of a Watch Redefining Statement.  During the watch, the NWFO will keep the public informed on what is happening in the watch area and also let the public know when the watch has expired or been cancelled.

Severe Weather Analysis (SWA):  This WSR-88D radar product provides 3 base products (reflectivity (SWR), radial velocity (SWV), and spectrum width (SWW)) at the highest resolution available along with radial shear (SWS).  These products are mapped into a 27 nm by 27 nm region centered on a point which the operator can specify anywhere within a 124 nm radius of the radar.  It is most effective when employed as an alert paired product with the product centered on alert at height that caused the alert.  It is used to examine 3 base products simultaneously in a 4 quadrant display; and analyze reflectivity and velocity products at various heights to gain a comprehensive vertical analysis of the thunderstorm.

Severe Weather Probability (SWP):  This WSR-88D radar product algorithm displays numerical values proportional to the probability that a storm will produce severe weather within 30 minutes.  Values determined using a statistical regression equation which analyzes output from the VIL algorithm.  It is used to quickly identify the most significant thunderstorms.

Severe Weather Potential Statement (SPS or HWO):   This statement is issued designed to alert the public and state/local agencies to the potential for severe weather up to 24 hours in advance.  It is issued the local National Weather Service office.

Severe Weather Statement (SWS):  A National Weather Service product which provides follow up information on severe weather conditions (severe thunderstorm or tornadoes) which have occurred or are currently occurring.

Servo Loop:  In radar meteorology, a generic description of hardware needed to remotely control the motion of the antenna dish.

Sferic: A transient electric or magnetic field generated by any feature of lightning discharge (entire flash).

Shear:  Variation in wind speed (speed shear) and/or direction (directional shear) over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.

Sheet Flow:  Flow that occurs overland in places where there are no defined channels, the flood water spreads out over a large area at a uniform depth. This also referred to as overland flow.

SHEF (Standard Hydrologic Exchange Format):  A documented set of rules for coding data for operational day-to-day use in a form for both visual and computer recognition.
 
SHEFPARS:  A software decoder for SHEF Data.
 
Shelf Cloud:  A low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms). Unlike the roll cloud, the shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn. It is accompanied by gusty, straight-line winds and is followed by precipitation.

ShoalingThe process whereby waves coming into shallow waters are slowed by bottom friction and become closer together and steeper.

Shore Ice:  An ice sheet in the form of a long border attached to the bank or shore.  See border ice.

Short-Fuse Warning:  A warning issued by the NWS for a local weather hazard of relatively short duration. Short-fuse warnings include tornado warnings, severe thunderstorm warnings, and flash flood warnings. Tornado and severe thunderstorm warnings typically are issued for periods of an hour or less, flash flood warnings typically for three hours or less.

Short Term Forecast This National Weather Service narrative summary describes the weather in the local area and includes a short-range forecast (usually not more than 6 hours).  This product will be updated more frequently when it is used during active weather.  This product is also sometimes referred to as a "NOWcast".

Shortwave (or Shortwave Trough):  A disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave.

Showalter Index (SWI): It is a stability index used to determine thunderstorm potential. The SWI is calculated by lifting an air parcel adiabatically from 850 mb to 500 mb. The algebraic difference between the air parcel and the environmental temperature at 500 mb represents the SWI. It is especially useful when you have a shallow cool airmass below 850 mb concealing greater convective potential aloft. However, the SWI will underestimate the convective potential for cool layers extending above 850 mb. It also does not take in account diurnal heating or moisture below 850 mb. As a result, one must be very careful when using this index. The following table shows what the SWI value means:
 

Showalter Index (SWI) vs. Thunderstorm Consideration

Showalter Index (SWI)

Thunderstorm Consideration

3 to 1

Thunderstorm possible - strong trigger needed

0 to -3

Unstable - thunderstorms probable

-4 to -6

Very unstable - good heavy thunderstorm potential

less than -6

Extremely unstable - good strong thunderstorm potential

 

Shower (SH): It implies short duration, intermittent, and scattered precipitation (rain, snow, ice pellet) of a more unstable, convective nature.

Shortwave: A small wave that moves around long waves in the same direction as the air flow in the middle and upper troposphere. Shortwaves are also called shortwave troughs.

Sidelobe: A secondary energy maximum located outside the main radar beam. Typically, it contains a small percentage of energy compared to the main lobe, but it may produce erroneous echoes.

Significant Wave Height: The average height (trough to crest distance) of the one-third highest waves.  An experienced observer will most frequently report heights equivalent to the average of the highest one-third of all waves observed.

Single Cell Thunderstorm: This type of thunderstorm develops in weak vertical wind shear environments. On a hodograph, this would appear as a closely grouped set of random dots around the center of the graph. They are characterized by a single updraft core and a single downdraft that descends into the same area as the updraft. The downdraft and its outflow boundary then cut off the thunderstorm inflow. This causes the updraft and the thunderstorm to dissipate. Single cell thunderstorms are short-lived. They only last about 1/2 hour to an hour. These thunderstorms will occasionally become severe (3/4 inch hail, wind gusts in the excess of 58 miles an hour, or a tornado), but only briefly. In this case, they are called: "Pulse Severe Thunderstorms".

Site-Specific:  Term used in conjunction with "forecast" or "warning" to convey the fact that a hydrologic (stream) forecast is produced for an individual stream gage location as opposed to a general area (e.g., a city, zone, or county) as is commonly done in many types of weather forecasts.

Site Specific Hydrologic Prediction System (SSHP):  The WFO hydrologic forecast model for small rivers and streams that uses RFC soil moisture state variables, stage and precipitation data. Routing capabilities may be added to future builds.
 
Skew-T Hodograph Analysis and Research Program (SHARP): This computer program is used by forecasters to interpret soundings. From this program, they are able to calculate the stability of the atmosphere, which way a thunderstorm will move once it forms, and what type of thunderstorm it will be (single cell, multicell, or supercell).

Signal-to-Noise Ratio (SNR):  A ratio that measures the comprehensibility of data, usually expressed as the signal power divided by the noise power, usually expressed in decibels (dB).

Sky ConditionUsed in a forecast to describes the predominant/average sky condition based upon octants (eighths) of the sky covered by opaque (not transparent) clouds.
 

Sky Condition 

Cloud Coverage

Clear or Sunny

0/8

Mostly Clear, or Mostly Sunny

1/8 to 2/8

Partly Cloudy, or Partly Sunny 

3/8 to 4/8

Mostly Cloudy, or Considerable Cloudiness 

5/8 to 7/8

Cloudy

8/8

Fair (used mostly for nighttime periods)  Less than 4/10 opaque clouds, no precipitation, no extremes of visibility, temperature or winds.  Describes generally pleasant weather conditions.

 
Sleet (PL): Describes solid grains of ice formed by the freezing of raindrops or the refreezing of largely melted snowflakes. These grains usually bounce upon impact with the ground or pavement. Heavy sleet is a relatively rare event defined as an accumulation of ice pellets covering the ground to a depth of 1/2 inch or more. See Ice Pellets.

Slight ChanceA National Weather Service precipitation descriptor for a 20 percent chance of measurable precipitation (0.01 inch).  When the precipitation is convective in nature, the term widely scattered is used.  See Precipitation Probability (PoP).

Slight Risk of Severe Thunderstorms:  Severe thunderstorms are expected to affect between 2 and 5 percent of the area. A slight risk generally implies that severe weather events are expected to be isolated. See high risk, moderate risk, convective outlook.

SMA:  The Soil Moisture Accounting Model.

Small Craft Advisory:  This is issued by the National Weather Service to alert small boats to sustained (more than 2 hours) hazardous weather or sea conditions.  These conditions may be either present or forecasted.  The threshold conditions for it are usually sustained winds of 18 knots (21 mph) (less than 18 knots in some dangerous waters) to 33 knots (38 mph) inclusive or hazardous wave conditions (such as 4 feet or greater).  In the Great Lakes, this advisory relates to conditions within 5 nautical miles of shore.  As a result, these will be only issued in the Nearshore Forecast.  Along the coastal regions of the East Coast, Gulf of Mexico, and West Coast, this advisory relates to conditions out to as much as 100 nautical miles of shore (coastal waters).  As a result, these will be only issued in the Coastal Marine Forecast.  Mariners learning of this advisory are urged to determine immediately the reason by turning their radios to the latest marine broadcast.  Decisions as to the degree of the hazard will be left to the boater, based on experience and size and type of boat.  There is no legal definition for a "small craft".

Small Stream Flooding:  Flooding of small creeks, streams, or runs.

Smog: Originally smog meant a mixture of smoke and fog. Now, it means air that has restricted visibility due to pollution or pollution formed in the presence of sunlight--photochemical smog.

Smoke (FU): A suspension in the air of small particles produced by combustion. A transition to haze may occur when smoke particles have traveled a great distance (25 to 100 miles or more) and when the larger particles have settled out and the remaining particles have become widely scattered through the atmosphere.

Smoke ManagementConducting a prescribed fire or slash burn with firing techniques and meteorological conditions that keep the smoke's impact on the environment with acceptable limits.

SMPDBK:  The Simplified Dam Break (DAMBRK) Model

Snow (SN): Precipitation of snow crystals, mostly branched in the form of six-pointed stars. It usually falls steadily for several hours or more. Qualifiers, such as occasional or intermittent, are used when a steady, prolonged (for several hours or more) fall is not expected. Like drizzle, its intensity is based on visibility. The following table shows snow intensity versus visibility:
 

Snow Intensity versus Visibility

Snow Intensity

Visibility

Light

greater than statue mile

Moderate

1/4 to statue mile

Heavy

less 1/4 statue mile

 

The amount of snow that falls is highly dependent upon temperature.  For example, at 10 degrees F, one inch of precipitation will produce 30 inches of snow.  At 20 degrees F, one inch of precipitation will produce 20 inches of snow.  At 30 degrees F, one inch of precipitation produces 10 inches of snow.  At freezing, one inch precipitation will produce approximately 6 inches of snow.

Snow Accumulation and Ablation Model:  A model which simulates snow pack accumulation, heat exchange at the air-snow interface, areal extent of snow cover, heat storage within the snow pack, liquid water retention, and transmission and heat exchange at the ground-snow interface.
 
Snow Advisory:  This product is issued by the National Weather Service when a low pressure system produces snow that may cause significant inconveniences, but do not meet warning criteria and if caution is not exercised could lead to life threatening situations.  The advisory criteria varies from area to area.  In Michigan, the criteria for its issuance is a snow event that is forecasted to produce snow (average of forecast range) greater than 3 inches, but less than warning criteria (6 inches in Lower Michigan and 8 inches in Upper Michigan) in 12 hours.  If the forecaster feels that it is warranted, he or she can issued it for amounts less than the minimum criteria.  For example, it may be issued for the first snow of the season or when snow has not fallen in long while.

Snow and Blowing Snow Advisory:  This product is issued by the National Weather Service during situations that cause significant inconveniences, but do not meet warning criteria and if caution is not exercised could lead to life threatening situations.  The warning criteria in this definition varies from area to area.  For example, in Lower Michigan, it refers to a snow of greater than or equal to 6 inches in a 12 hour period; or greater than or equal to 8 inches in a 24 hour period.  Meanwhile, in Upper Michigan, it refers to a snow of greater than or equal to 8 inches in a 12 hour period; or greater than or equal to 10 inches in a 24 hour period.

Snow Core:  A sample of either freshly fallen snow, or the combined old and new snow on the ground. This is obtained by pushing a cylinder down through the snow layer and extracting it.

Snow Density:  The mass of snow per unit volume which is equal to the water content of the snow divided by its depth.

Snow Depth The combined total depth of both the old and new snow on the ground.

Snow Flurries: They are intermittent light snowfalls of short duration (generally light snow showers) with no measurable accumulation.

Snow Grains (SG): Precipitation of very small, white, and opaque grains of ice. They can be distinguished from ice pellets, because ice pellets bounce and snow grains do not bounce at all.

Snow Pack: The combined layers of snow and ice on the ground at any one time. It is also called snowcover.

Snow Pillow:  An instrument used to measure snow water equivalents. Snow pillows typically have flat stainless steel surface areas. The pillow below this flat surface is filled with antifreeze solution and the pressure in the pillow is related to the water-equivalent depth of the snow on the platform. One great advantage of snow pillows over a snow survey is the frequency of observations, which can be as high as twice per day.

Snow Pellets (GS): Precipitation of white, opaque grains of ice. The grains are round or sometimes conical. Diameters range from about 0.08 to 0.2 inch (2 to 5 mm). This is also referred to as Small Hail.

Snow Shower (SHSN): It is a moderate snowfall of short duration. Some accumulation is possible.

Snow Squalls (SQSN): They are intense, but limited duration, periods of moderate to heavy snowfall. They are accompanied by strong, gusty surface winds, and possibly lightning (generally moderate to heavy snow showers). Snow accumulations may be significant.

Snow Stake:  A 1-3/4 inch square, semi-permanent stake, marked in inch increments to measure snow depth.
 
Snow Stick:  A portable rod used to measure snow depth.
 
SNOw TELemetry (SNOTEL):  An automated network of snowpack data collection sites. The Natural Resources Conservation Service (NRCS), formerly the Soil Conservation Service (SCS), has operated the Federal-State-Private Cooperative Snow Survey Program in the western United States since 1935. A standard SNOTEL site consists of a snow pillow, a storage type precipitation gage, air temperature sensor and a small shelter for housing electronics.

Snow Water Equivalent:  The water content obtained from melting accumulated snow.

Snowboard A flat, solid, white material, such as painted plywood, approximately two feet square, which is laid on the ground, or snow surface by weather observers to obtain more accurate measurements of snowfall and water content.

Snowmelt Flooding:  Flooding caused primarily by the melting of snow.

Snowpack:  The total snow and ice on the ground, including both the new snow and the previous snow and ice which has not melted.

Soil Conservation Service:  The former name of a branch of the United States Department of Agriculture, renamed the Natural Resources Conservation Service (NRCS). NRCS has responsibilities in soil and water conservation, and flood prevention.

Soil Moisture: Water contained in the upper part of the soil mantle. This moisture evaporates from the soil and is the used and transpired by vegetation.

Sounding: A plot of the vertical profile of temperature and dew point (and often winds) above a fixed location. Soundings are used extensively in severe weather forecasting, e.g., to determine instability, locate temperature inversions, measure the strength of the cap, obtain the convective temperature, etc.

Southern Oscillation (SO): A "see-saw" in surface pressure in the tropical Pacific characterized by simultaneously opposite sea level pressure anomalies at Tahiti, in the eastern tropical Pacific and Darwin, on the northwest coast of Australia. The SO was discovered by Sir Gilbert Walker in the early 1920's. Walker was among the first meteorologists to use the statistical techniques to analyze and predict meteorological phenomena. Later, the three-dimensional east-west circulation related to the SO was discovered and named the "Walker Circulation". The SO oscillates with a period of 2-5 years. During one phase, when the sea level pressure is low at Tahiti and High at Darwin, the El Nino occurs. The cold phase of the SO, called "La Nina" by some, is characterized by high pressure in the eastern equatorial Pacific, low in the west, and by anomalously cold sea surface temperature (SST) in the central and eastern Pacific. This is called El Nino Southern Oscillation or ENSO. For additional information see El Nino.

Space Environment Center (SEC)This center provides real-time monitoring and forecasting of solar and geophysical events, conducts research in solar-terrestrial physics, and develops techniques for forecasting solar and geophysical disturbances. SEC's parent organization is the National Oceanic and Atmospheric Administration (NOAA). SEC is one of NOAA's 12 Environmental Research Laboratories (ERL) and one of NOAA's 9 National Centers for Environmental Prediction (NCEP). SEC's Space Weather Operations is jointly operated by NOAA and the U.S. Air Force and is the national and world warning center for disturbances that can affect people and equipment working in the space environment.

SPC - Storm Prediction Center:  A national forecast center in Norman, Oklahoma, which is part of NCEP. The SPC is responsible for providing short-term forecast guidance for severe convection, excessive rainfall (flash flooding) and severe winter weather over the contiguous United States.

SPC Mesoscale Discussions (SWO):  A mesoscale discussion issued by the Storm Prediction Center (SPC) in Norman, Oklahoma as a routine/daily, but nonscheduled, short-term (0-6 hours) product to communicate the current judgment of the SPC to the user community.  It also provides guidance on other short-term mesoscale phenomenon that may be of significance (for example heavy snow potential, the formation of dense fog, etc.).

Spearhead Echo: A radar echo associated with a Down burst with a pointed appendage extending toward the direction of the echo motion. The appendage moves much faster than the parent echo, which is drawn into the appendage. During it's mature stage, the appendage turns into a major echo and the parent echo loses its identity.

Special Avalanche Warning:  Issued by the National Weather Service when avalanches are imminent or occurring in the mountains.  It is usually issued for a 24 hour period.

Special Fire Weather:  Meteorological services uniquely required by user agencies which cannot be provided at an NWS office during normal working hours.  Examples are on-site support, weather observer training, and participation in user agency training activities.

Special Marine Warning (SMW):  This is issued by the National Weather Service for hazardous weather conditions (thunderstorms over water, thunderstorms that will move over water, cold air funnels over water, or waterspouts) usually of short duration (2 hours or less) and producing sustained winds or frequent gusts of 34 knots or more that is not covered by existing marine warnings.  These are tone alerted on NOAA Weather Radio.  Boaters will also be able to get this information by tuning into Coast Guard and commercial radio stations that transmit marine weather information.

Special Tropical Disturbance Statement (DSA):  This statement issued by the National Hurricane Center furnishes information on strong and formative non-depression systems.  This statement focuses on the major threat(s) of the disturbance, such as the potential for torrential rainfall on an island or inland area.  The statement is coordinated with the appropriate forecast office(s).

Special Weather Statement (SPS) This is used by the National Weather Service to provide additional information about expected or ongoing significant weather changes not covered in other statements.  This would include non-severe convective, winter weather, and non-precipitation events.

Specific Humidity:  In a system of moist air, the ratio of the mass of water vapor to the total mass of the system.

Specific Yield:  The ratio of the water which will drain freely from the material to the total volume of the aquifer formation. This value will always be less than the porosity.

Spectral Density:  A radar term for the distribution of power by frequency.

Spectrum Width (SW):  This WSR-88D radar product depicts a full 360 degree sweep of spectrum width data indicating a measure of velocity dispersion within the radar sample volume.  It is available for every elevation angle sampled, it provides a measure of the variability of the mean radial velocity estimates due to wind shear, turbulence, and/or the quality of the velocity samples.  It is used to estimate turbulence associated with boundaries, thunderstorms, and mesocyclones; check the reliability of the velocity estimates; and locate boundaries (cold front, outflow, lake breeze, etc.).

Spectrum Width Cross Section (SCS):  This WSR-88D radar product displays a vertical cross section of spectrum width on a grid with heights up to 70,000 feet on the vertical axis and distance up to 124 nm on the horizontal axis.  Two end points to create cross section are radar operator selected along a radial or from one AZRAN to another AZRAN within 124 nm of the radar that are less than 124 nm apart.  It is used to:  1) Verify features on the Reflectivity Cross Section (RCS) and Velocity Cross Section (VCS) and to evaluate the quality of the velocity data and 2) Estimate vertical extent of turbulence (aviation use).

Speed Shear:  The component of wind shear which is due to a change in wind speed with height, e.g., southwesterly winds of 20 mph at 10,000 feet increasing to 50 mph at 20,000 feet. Speed shear is an important factor in severe weather development, especially in the middle and upper levels of the atmosphere.

SPENESAn acronym for NESDIS Satellite Precipitation Estimates.

Sphere Calibration Reflectivity calibration of a radar by pointing the dish at a metal sphere of (theoretically) known reflectivity. The sphere is often tethered to a balloon.  Here's an image of a sphere calibration at sea. The sphere is visible just above the horizon.

Spillway:  A structure over or through which excess or flood flows are discharged. If the flow is controlled by gates, it is a controlled spillway, if the elevation of the spillway crest is the only control, it is an uncontrolled spillway.  Some various types of spillways include:

    Auxiliary or Emergency Spillway:  A secondary spillway designed to operate only during exceptionally large flood flows. Allows inflows from large storms to be released from the reservoir before the water level raises high enough to overtop the dam.
     
    Fuse Plug Spillway: An auxiliary or emergency spillway comprising a low embankment or a natural saddle designed to be overtopped and eroded away during flood flows.
     
    Primary (or Principal) Spillway:  The spillway which would be used first during normal inflow and flood flows.
     
    Shaft or Morning Glory Spillway:  A vertical or inclined shaft into which flood water spills and then is conducted through, under, or around a dam by means of a conduit or tunnel. If the upper part of the shaft is splayed out and terminates in a circular horizontal weir, it is termed a "bellmouth" or "morning glory" spillway.
     
    Side Channel Spillway:  A spillway whose crest is roughly parallel to the channel immediately downstream of the spillway.
     
    Siphon Spillway:  A spillway with one or more siphons built at crest level. This type of spillway is sometimes used for providing automatic surface-level regulation within narrow limits or when considerable discharge capacity is necessary within a short period of time.

Spillway Crest:  The elevation of the highest point of a spillway.

Spin-up:  Slang for a small-scale vortex initiation, such as what may be seen when a gustnado, landspout, or suction vortex forms.

Splitting Storm:  A thunderstorm which splits into two storms which follow diverging paths (a left mover and a right mover). The left mover typically moves faster than the original storm, the right mover, slower. Of the two, the left mover is most likely to weaken and dissipate (but on rare occasions can become a very severe anticyclonic-rotating storm), while the right mover is the one most likely to reach supercell status.

Spot ForecastsThese are NWS site-specific fire weather forecasts.  They are issued upon request of User Agencies for wildfires, prescribed burns, or special projects.

Spray (PY): An ensemble of water droplets torn by the wind from the surface of the of an extensive body of water, generally from crests of waves, and carried a short distance into the air.

Spring:  An issue of water from the earth; a natural fountain; a source of a reservoir of water.

Spring Tide: A tide higher than normal which occurs around the time of the new and full moon.

Squall (SQ): A strong wind characterized by a sudden onset in which the wind speed increases at least 16 knots and is sustained more than 22 knots or more for at least one minute.

Squall Line: A line or narrow band of active thunderstorms. The line may extend across several hundred miles. It forms along and ahead of an advancing cold front.

SRHAn acronym for Storm-relative Helicity.

Stability Index: The overall stability or instability of a sounding is sometimes conveniently expressed in the form of a single numerical value. Used alone, it can be quite misleading, and at times, is apt to be worthless. The greatest value of an index lies in alerting the forecaster to those soundings which should be examined more closely.

StableAn atmospheric state with warm air above cold air which inhibits the vertical movement of air.

Staccato Lightning:  A Cloud to Ground (CG) lightning discharge which appears as a single very bright, short-duration stroke, often with considerable branching.

Staff Gage: A vertical staff graduated in appropriate units which is placed so that a portion of the gage is in the water at all times. Observers read the river stage off the staff gage.

Stage: The level of the water surface above a given datum at a given location along a river or stream.

Stage I Precipitation Processing:  The first level of precipitation processing, occurring within the WSR-88D computer and performance for each volume scan of the radar. Base reflectivity data are converted to a precipitation estimate for each grid in the radar umbrella using a complex algorithm that includes quality control procedures, a Z/R relationship, and a bias adjustment using data from a ground-based precipitation gage network. Several graphical and digital products are produced for Weather Forecast Offices (WFO) operations and subsequent processing.

Stage II Precipitation Processing:  The second level of precipitation processing, occurring within the WFO Advanced Weather Interactive Processing System (AWIPS) and performed on an hourly basis. Stage I precipitation estimates are further refined using data from additional precipitation gages and other sources such as rain/no rain determinations from satellite imagery. Stage II may also be executed at RFCs for backup purposes.

Stage III Precipitation Processing:  The third level of precipitation processing, performed interactively at RFCs. Stage II precipitation estimates from multiple radars are mosaicked into an RFC area-wide product for use in river basin hydrologic modeling operations. RFC forecasters can review the mosaicked product, interactively edit areas of bad data, and substitute gage-only fields into portions of the mosaicked radar based product.

Stage IV Precipitation Processing The fourth level of precipitation processing, performed automatically and/or interactively at NCEP. Stage III precipitation estimates from RFCs are mosaicked into a Nation-wide product for use in various real-time forecast activities and forecast verification operations.

Stair Stepping:  The process of continually updating river forecasts for the purpose of incorporating the effects rain that has fallen since the previous forecast was prepared. The goal of using QPF is to minimize "stair-stepping."

Standard Deviation:  The positive square root of the signal variance. The velocity standard deviation is often called spectrum width.

State Forecast Discussion (SFD):  This National Weather Service product is intended to provide a well-reasoned discussion of the meteorological thinking which went into the preparation of the Zone Forecast Product.  The forecaster will try to focus on the most particular challenges of the forecast.  The text will be written in plain language or in proper contractions.  At the end of the discussion, there will be a list of all advisories, non-convective watches, and non-convective warnings.  The term non-convective refers to weather that is not caused by thunderstorms.  Intermediate State Forecast Discussion will be issued when either significant forecast updates are being made or if interesting weather is expected to occur.  Most states are going away from this product and more toward the Area Forecast Discussion (AFD).

State Forecast Product (SFP):  This National Weather Service product is intended to give a good general picture of what weather may be expected in the state during the next 5 days.  The first 2 days of the forecast is much more specific than the last 3 days.  In comparison with the Zone Forecast Product, this product will be much more general.

State Maximum/Minimum Temperature and Precipitation Table (STP):  This tabular product is issued by the National Weather Service once in the morning and evening.  The morning product will contain the current weather conditions, yesterday's daytime high temperature (in Fahrenheit), the 12-hour low temperature (in Fahrenheit) ending at a specified time, and 24-hour precipitation (in inches) ending at a specified time from available reporting stations within the state or NWFO forecast area.  The evening product will contain the same information; however, the daytime high temperature will be today's high temperature instead of yesterday's high temperature.  In the winter time, this product will contain the snow depth in inches if it is available.  In the state of Michigan, this product is prepared by NWFO Grand Rapids.  They issue this product twice daily--usually somewhere between 7:00 and 8:00 AM/PM EST.

State Weather Roundup (SWR) This is a National Weather Service tabular product which provides routine hourly observations within the state through the National Weather Wire Service (NWWS).  It gives the current weather condition in one word (cloudy, rain, snow, fog, etc.), the temperature and dew point in Fahrenheit, the relative humidity, wind speed and direction, and finally additional information (wind chill, heat index, a secondary weather condition).  These reports are broken up regionally.  When the complementary satellite product is not available, reports from unaugmented ASOS stations will report "fair" in the sky/weather column when there are few or no clouds (i.e., scattered or less) below 12,000 feet with no significant weather and/or obstructions to visibility.  In the state of Michigan, this product is done automatically at NWFO Detroit/Pontiac about 10 minutes past the hour.

Stationary Front: A front that barely moves with winds blowing in almost parallel, but in opposite directions on each side of the front. Occasionally, these fronts can cause widespread flooding, because showers and thunderstorms moving along them will continue to move across the same area. This weather situation is called "Train Echoing".

Steam Fog: It forms as cold air moves over warm water. Water evaporates from the warm water surface and immediately condenses in the cold air above. Heat from the water warms the lower levels of the air creating a shallow layer of instability. It rises like smoke from the warm surface. The low level convection can become quite turbulent. Steam fog is most common in Arctic regions where it is called "Arctic Sea Smoke", but it can and does occur occasionally at all latitudes.

Steering Winds (or Steering Currents):  A prevailing synoptic scale flow which governs the movement of smaller features embedded within it.

Stepped Leader: A leader which initiates the very first stroke and establishes the channel for all subsequent streamers of a lightning discharge.

Stilling Basin:  A basin constructed to dissipate the energy of fast-flowing water (e.g., from a spillway or bottom outlet), and to protect the streambed from erosion.

Stoplogs:  Large logs, timbers or steel beams placed on top of each other with their ends held in guides on each side of a channel or conduit providing a temporary closure versus a permanent bulkhead gate.

Storage:  1) Water artificially impounded in surface or underground reservoirs for future use. 2) Water naturally detained in a drainage basin, such as ground water, channel storage, and depression storage.

Storage Equation:  The equation for the conservation of mass.

Storm:  Any disturbed state of the atmosphere, especially affecting the Earth's surface, and strongly implying destructive and otherwise unpleasant weather.  Storms range in scale from tornadoes and thunderstorms through tropical cyclones to widespread extratropical cyclones.
 
Storm Data (SD)This National Climatic Data Center (NCDC) monthly publication documents a chronological listing, by states, of occurrences of storms and unusual weather phenomena. Reports contain information on storm paths, deaths, injuries, and property damage. An "Outstanding storms of the month" section highlights severe weather events with photographs, illustrations, and narratives.  The December issue includes annual tornado, lightning, flash flood, and tropical cyclone summaries.

Storm Hydrograph: A hydrograph representing the flow or discharge of water past a point on a river.

Storm Motion: The speed and direction at which a thunderstorm travels.

Storm Relative:  Measured relative to a moving thunderstorm, usually referring to winds, wind shear, or helicity.

Storm Relative Mean Radial Velocity Map (SRM):  This WSR-88D radar product depicts a full 360 degree sweep of radial velocity data with the average motion of all identified storms subtracted out.  It is available for every elevation angle sampled.  It is used to aid in displaying shear and rotation in storms and storm top divergence that might otherwise be obscured by the storm's motion, investigate the 3-D velocity structure of a storm, and help with determining rotational features in fast and uniform moving storms.

Storm Relative Mean Radial Velocity Region (SRR):  This WSR-88D radar product depicts a 27 nm by 27 nm region of  storm relative mean radial velocity centered on a point which the operator can specify anywhere within a 124 nm  radius of the radar.  The storm motion subtracted defaults to the motion of the storm closest to the product center, or can be input by the operator.  It is used to examine the 3-dimensional storm relative flow of a specific thunderstorm (radar operator centers product on a specific thunderstorm; aid in displaying shear and rotation in thunderstorms and storm top divergence that might otherwise be obscured by storm motion; and gain higher resolution velocity product

Storm Scale:  Referring to weather systems with sizes on the order of individual thunderstorms. See synoptic scale and mesoscale.

Storm Surge: A rise above the normal water level along a shore caused by strong onshore winds and/or reduced atmospheric pressure. The surge height is the difference of the observed water level minus the predicted tide. Most hurricane deaths are caused by the storm surge. It can be 50 or more miles wide and sweeps across the coastline around where the hurricane makes landfall. The maximum rises in sea-level move from under the storm to the right of the storm's track, reaching a maximum amplitude of 10 to 30 feet at the coast. The storm surge may even double or more in height when the hurricane's track causes it to funnel water into a bay. The storm surge increases substantially as it approaches the land because the normal water depth decreases rapidly as it approaches the beaches. The moving water contains the same amount of energy; thus, resulting in an increase of storm surge. Typically, the stronger the hurricane, the greater the storm surge.

Storm Tide: The actual sea level resulting from astronomical tide combined with the storm surge. This term is used interchangeably with "hurricane tide".

Storm Total Precipitation (STP) This WSR-88D radar product displays the total precipitation (in inches) as a graphical image.  It displays hourly precipitation total (in inches) as a graphical image.  Currently , this product is done in a polar format with resolution 1.1 nm by 1 degree.  It will reset after one hour of no precipitation.  It is used to monitor total precipitation accumulation; observe short term trends of precipitation tracks with time lapse of this product; and estimate total basin runoff and ground saturation.

Storm Tracking Information (STI):  This WSR-88D radar product displays the previous, current, and projected locations of storm centroids (forecast and past positions are limited to one hour or less).  Forecast tracks are based upon linear extrapolation of past storm centroid positions, and they are intended for application to individual thunderstorms not lines or clusters.  It is used to provide storm movement:  low track variance and/or 2 or more plotted past positions signify reliable thunderstorm movement.

Stormwater Discharge:  Precipitation that does not infiltrate into the ground or evaporate due to impervious land surfaces, but instead flows onto adjacent land or water areas and is routed into drain/sewer systems.

Storm Warning:  A warning of 1-minute sustained surface winds of 48 kt (55 mph or 88 kph) or greater, either predicted or occurring, not directly associated with tropical cyclones.

Straight-Line Hodograph: The name pretty well describes what it looks like on the hodograph. What causes this shape is a steady increase of winds with height (vertical wind shear). This shape of hodograph favors multicell thunderstorms.

Straight Line Winds:  Generally, any wind that is not associated with rotation, used mainly to differentiate them from tornadic winds.

Stratiform: Descriptive of clouds of extensive horizontal development, as contrasted to the more narrow and vertically developed cumuliform type. Stratiform clouds cover large areas but show relatively little vertical development. Stratiform precipitation, in general, is relatively continuous and uniform in intensity (i.e., steady rain versus rain showers).

Stratiform Rain: Horizontally widespread rain, uniform in character, typically associated with macroscale fronts and pressure systems.

Stratiform Rings and Bands: These occur between the active convective bands of a hurricane outside of the eye wall. Inner stratiform bands often exhibit the bright band aloft, a VIP Level 2, and in the lower layers typically show a VIP Level 1.

Stratiform Snow: Same as for stratiform rain except precipitation is in the form of snow.

Stratocumulus (Sc): It has globular masses or rolls unlike the flat, sometimes definite, base of stratus. This cloud often forms from stratus as the stratus is breaking up or from spreading out of cumulus clouds. They usually consist of mainly water vapor and are located between the ground and 6,500 feet. Stratocumulus often reveals the depth of the moist air at low levels, while the speed of the cloud elements can reveal the strength of the low-level jet.

Stratus (St): It is a low, uniform sheet-like cloud. Stratus may appear in the form of ragged patches, but otherwise does not exhibit individual cloud elements as do cumulus and stratocumulus clouds. It usually is located between the ground and 6,500 feet. It usually consist of mainly water vapor. Fog is a stratus cloud with its base located at the ground.

Stream Gage A site along a stream where the stage (water level) is read either by eye or measured with recording equipment.

Stream Segment:  Refers to the surface waters of an approved planning area exhibiting common hydrological, natural, physical, biological, or chemical processes. Segments will normally exhibit common reactions to external stresses such as discharge or pollutants.
 
Streamer: A channel of very high ion density which propagates through the air by the continual establishment of an electron avalanche ahead of its tip.

Streamflow:  Water flowing in the stream channel. It is often used interchangeably with discharge.

Striations:  Grooves or channels in cloud formations, arranged parallel to the flow of air and therefore depicting the airflow relative to the parent cloud. Striations often reveal the presence of rotation, as in the barber pole or "corkscrew" effect often observed with the rotating updraft of an Low Precipitation (LP) storm.

Subrefraction: The bending of the radar beam in the vertical which is less than under standard refractive conditions. This causes the beam to be higher than indicated, and lead to the underestimation of cloud heights.

Subsidence: 1) The slow sinking of air usually associated with high pressure areas. It is usually over a broad area. 2) Sinking down of part of the earth's crust due to underground excavation, such as the removal of groundwater

Subsidence Inversion: It is produced by adiabatic heating of air as it sinks and is associated with anticyclones (high pressure) and/or stable air masses. These inversions form between sinking heated air and air below and they are characterized by temperature increase with height through the inversion, while above the inversion, the temperature cools almost dry adiabatically. The dew point temperature, relative humidity, and mixing ratio values all decrease with height through the inversion.

Subsurface Storm Flow:  The lateral motion of water through the upper layers until it enters a stream channel. This usually takes longer to reach stream channels than runoff. This also called interflow.

Substation:  A location where observations are taken or other services are furnished by people not located at NWS offices who do not need to be certified to take observations.

Sub-Synoptic Low:  Essentially the same as mesolow.

Subtle Heavy Rainfall Signature ("SHARS"): This heavy rain signature is often difficult to detect on satellite. These warm top thunderstorms are often embedded in a synoptic-scale cyclonic circulation. Normally, they occur when the 500 mb cyclonic circulation is quasi-stationary or moves slowly to the east or northeast (about 2 degrees per 12 hours). The average surface temperature is 68oF with northeasterly winds. The average precipitable water (P) value is equal to or greater than 1.34 inches and the winds veer with height, but they are relatively light. The heavy rain often occurs north and east of the vorticity maximum across the lower portion of the comma head about 2 to 3 degrees north or northeast of the 850 mb low.

Subtropical Cyclone: A low pressure system that develops over subtropical waters that initially has a non-tropical circulation, but in which some elements of tropical cyclone cloud structure are present. Subtropical cyclones can evolve into tropical cyclones. Subtropical cyclones are generally of two types:

      1) Cold Low Type: This type has a circulation extending from the surface to the upper troposphere, with the maximum sustained low-level winds typically extending to a radius of 100 miles or more from the center.

      2) Mesoscale (Sub-Synoptic Scale) Cyclone Type: This type develops in or near a dying frontal zone with horizontal wind shear. This low is compact and develops a tight pressure gradient with the maximum sustained low-level winds, which can reach hurricane intensity, typically located less than 30 miles from the center. The whole storm circulation may initially be no more than 100 miles in diameter. These lows are typically short- lived and spend their lives usually over water. They may be cold core or warm core. This strange hybrid was once referred to as a "neutercane" after being discovered by satellite imagery.

Subtropical Depression:  A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 33 kt (38 mph or 62 kph) or less.

Subtropical Storm:  A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 34 kt (39 mph or 63 kph) or more.

Subtropical Jet: This jet stream is usually found between 20o and 30o latitude at altitudes between 12 and 14 km.

Suction Vortex (sometimes Suction Spot):  A small but very intense vortex within a tornado circulation. Several suction vortices typically are present in a multiple-vortex tornado. Much of the extreme damage associated with violent tornadoes (F4 and F5 on the Fujita scale) is attributed to suction vortices.

Sun Pointing:  Alignment of the radar antenna by locating the position of the sun in the sky, which has an exactly known position given the radar's location and the present time. This may be necessary to verify that when we think we're pointing "north", we actually are! The sun's signal is usually several dB above the background noise, and this technique is also sometimes used to examine the receiver sensitivity.

SupercellA thunderstorm with a persistent rotating updraft. Supercells are rare, but are responsible for a remarkably high percentage of severe weather events - especially tornadoes, extremely large hail and damaging straight-line winds. They frequently travel to the right of the main environmental winds (i.e., they are right movers). Radar characteristics often (but not always) include a hook or pendant, bounded weak echo region (BWER), V-notch, mesocyclone, and sometimes a TVS. Visual characteristics often include a rain-free base (with or without a wall cloud), tail cloud, flanking line, overshooting top, and back-sheared anvil, all of which normally are observed in or near the right rear or southwest part of the storm. Storms exhibiting these characteristics often are called classic supercells; however HP (High Precipitation) storms and LP (Low Precipitation) storms also are supercell varieties. 

Supercooled Liquid WaterIn the atmosphere, liquid water can survive at temperatures colder than 0 degrees Celsius; many vigorous storms contain large amounts of supercooled liquid water at cold temperatures. Important in the formation of graupel and hail.

Superrefraction: Bending of the radar beam in the vertical which is greater than sub-standard refractive conditions. This causes the beam to be lower than indicated, and often results in extensive ground clutter as well as an overestimation of cloud top heights.

Surcharge Capacity:  The volume of a reservoir between the maximum water surface elevation for which the dam is designed and the crest of an uncontrolled spillway, or the normal full-pool elevation of the reservoir with the crest gates in the normal closed position.
 
Surface Based Convection:  Convection occurring within a surface-based layer, i.e., a layer in which the lowest portion is based at or very near the earth's surface. Compare with elevated convection.
 
Surface Impoundment:  An indented area in the land's surface, such as a pit, pond, or lagoon.
 
Surface Runoff The part of runoff, caused by precipitation and/or snowmelt, that moves over the soil surface to the nearest stream channel.  Rain that falls on the stream channel is often lumped with this quantity.
 
Surface Water:  Water that flows in streams and rivers and in natural lakes, in wetlands, and in reservoirs constructed by humans.
 
Sustained Wind: Wind speed determined by averaging observed values over a 2-minute period.

Severe WEAther Threat Index (SWEAT Index):   A stability index developed by the Air Force which incorporates instability, wind shear, and wind speeds.  The index combines the effects of low-level moisture (850 mb dew point), convective instability (Total Totals (TT) Index), jet maxima (850 mb and 500 mb wind speed), and warm air advection (veering directional shear between 850 mb and 500 mb). It was designed to discriminate between ordinary and severe convection by incorporating thermodynamic information (850 mb dew point and Total Totals Index) and kinematic information (low and mid level flow characteristics showing strong wind fields and veering directional shear). SWEAT Index - Severe Weather ThrEAT index as follows:

    SWEAT = (12 Td 850 ) + (20 [TT-49]) +( 2 f 850) + f 500 + (125 [s+0.2]) where

    Td 850 is the dew point temperature at 850 mb,
    TT is the total-totals index,
    f 850 is the 850-mb wind speed (in knots),
    f 500 is the 500-mb wind speed (in knots), and 
    s is the sine of the angle between the wind directions at 500 mb and 850 mb (thus representing the directional shear in this layer).

SWEAT values of about 250-300 or more indicate a greater potential for severe weather, but as with all stability indices, there are no magic numbers. The SWEAT index has the advantage (and disadvantage) of using only mandatory-level data (i.e., 500 mb and 850 mb), but has fallen into relative disuse with the advent of more detailed sounding analysis programs.

SunnyWhen there are no opaque (not transparent) clouds.  Same as Clear.

Surge:  A rise in water level caused by strong wind or fluctuations in the atmospheric pressure.  This term is usually applied to a sharp set-up effect at the downwind end of the lake, but thunderstorms can cause brief local storm surges in bays and harbours.  See Set-up.

Sustained Overdraft:  Long-term withdrawal from the aquifer of more water than is being recharged.

SWE:  Snow Water Equivalent
 
Swell: Wind-generated waves that have travelled out of their generating area.  Swells characteristically exhibit smoother, more regular and uniform crests and a longer period than wind waves.

SWODY1 (sometimes pronounced swoe-dee):  The Day-1 Convective Outlook, sometimes called the "AC" is a guidance product issued by the Operational Guidance Branch (OGB) unit of the Storm Prediction Center (SPC) in Norman, Oklahoma. The Day 1 outlook outlines areas in the continental United States where severe thunderstorms may develop during the next 6 to 30 hours. The Convective Outlook is issued 5 times daily: at 06Z (the initial day-1 outlook, valid 12Z that day until 12Z the following day), 11Z (the "two-part outlook"), 15Z (the "morning update," valid until 12Z the next day), 19Z (the "afternoon update, " valid until 12Z the next day), and the 02Z (the "evening update," valid until 12Z the following day).

SWODY2 (sometimes pronounced swoe-dee):  The Day 2 Convective Outlook is very similar to the Day 1 Outlook. It is issued only twice a day, at 08Z and 18Z, and covers the period from 12Z the following day to 12Z the day after that. For example, if today is Monday then the Day 2 Outlook will cover the period 12Z Tuesday to 12Z Wednesday. The outlook issued at 08Z now qualifies the degree of risk like the Day 1 has (i.e. SLGT, MDT, and HIGH risk areas). The Day 2 Outlook has also includes a general thunderstorm outline.
 
Symmetric Double Eye: A concentrated ring of convection that develops outside the eye wall in symmetric, mature hurricanes. The ring then propagates inward and leads to a double-eye. Eventually, the inner eye wall dissipates while the outer intensifies and moves inward.

Synchronous Detection: Radar processing that retains the received signal amplitude and phase but that removes the intermediate frequency carrier.

Synoptic Scale (or Large Scale): The typical weather map scale that shows features such as high and low pressure areas and fronts over a distance spanning a continent. Also called cyclonic scale. Compare with mesoscale and storm-scale.

Synoptic Track:  Weather reconnaissance mission flown to provide vital meteorological information in data sparse ocean areas as a supplement to existing surface, radar, and satellite data. Synoptic flights better define the upper atmosphere and aid in the prediction of tropical cyclone development and movement.

Syzygy: The instance (new moon or full moon) when the earth, sun, and moon are all in a straight line.  


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