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T

TAF (Terminal Aerodrome Forecast):  This NWS aviation product is a concise statement of the expected meteorological conditions at an airport during a specified period (usually 24 hours). Each country is allowed to make modifications or exceptions to the code for use in each particular country. TAFs use the same weather code found in METAR weather reports.

Tail Cloud:  A horizontal, tail-shaped cloud (not a funnel cloud) at low levels extending from the precipitation cascade region of a supercell toward the wall cloud (i.e., it usually is observed extending from the wall cloud toward the north or northeast). The base of the tail cloud is about the same as that of the wall cloud. Cloud motion in the tail cloud is away from the precipitation and toward the wall cloud, with rapid upward motion often observed near the junction of the tail and wall clouds. Compare with beaver tail, which is a form of inflow band that normally attaches to the storm's main updraft (not to the wall cloud) and has a base at about the same level as the updraft base (not the wall cloud). See supercell.

Tail-End Charlie:  Slang for the thunderstorm at the southernmost end of a squall line or other line or band of thunderstorms. Since low-level southerly inflow of warm, moist air into this storm is relatively unimpeded, such a storm often has a higher probability of strengthening to severe levels than the other storms in the line.

Tailwater Height:  Height of water immediately downstream of the dam. (Various datums may be used.)
 
Target: Precipitation or other phenomena which produces echoes on a radar display.

TCUAn acronym for Towering Cumulus.  See Towering Cumulus.

T. D.An acronym for Tropical Depression.  See Tropical Depression.

TDWR:  An acronym for Terminal Doppler Weather Radar.

Technical Support Branch (TSB):  One of three branches of the Tropical Prediction Center (TPC).  TSB provides support for TPC computer and communications systems, including the McIDAS satellite data processing systems, the N-AWIPS workstations, and the WSR-88D computer systems. TSB also maintains a small applied research unit which develops tools for hurricane and tropical weather analysis and prediction. TSB also has a storm surge group which provides information for developing evacuation procedures for coastal areas, and an oceanography unit that produces ocean current and sea surface temperature analyses.

Temperature: A measure of the warmth of the ambient air measured by a suitable instrument such as a thermometer.

Thalweg The line of maximum depth in a stream. The thalweg is the part that has the maximum velocity and causes cutbanks and channel migration.

Theodolite: An instrument used in surveying to measure horizontal and vertical angles with a small telescope that can move in the horizontal and vertical planes. It used to track the movements of either a ceiling balloon or a radiosonde.

Thermal: A relatively small-scale, rising air current produced when the Earth's surface is heated. Thermals are a common source of low level turbulence for aircraft.

Thermal Highs: Areas of high pressure that are shallow in vertical extent and are produced primarily by cold surface temperatures.

Thermal Lows: Areas of low pressure that are shallow in vertical extent and are produced primarily by warm surface temperatures.

Thermal Wind: It is a theoretical wind that blows parallel to the thickness lines, for the layer considered, analogous to how the geostrophic wind blows parallel to the height contours. The closer the thickness isopleths, the stronger the thermal wind. Cold air is always located to the left of the thermal wind (as you face downstream) and the warm air is located on the right. Since thickness contours are tighter on the cold side of thermal wind, your lower thickness values will be found on the left side of the thermal wind. The speed and direction of the thermal wind are determined by vector geometry where the geostrophic wind at the upper level is subtracted from the geostrophic wind at the lower level.

Thermistor: An electrical resistance device used in the measurement of temperature.

Thermodynamic Chart (or Thermodynamic Diagram):  A chart containing contours of pressure, temperature, moisture, and potential temperature, all drawn relative to each other such that basic thermodynamic laws are satisfied. Such a chart typically is used to plot atmospheric soundings, and to estimate potential changes in temperature, moisture, etc. if air were displaced vertically from a given level. A thermodynamic chart thus is a useful tool in diagnosing atmospheric instability.

Thermodynamics:  In general, the relationships between heat and other properties (such as temperature, pressure, density, etc.) In forecast discussions, thermodynamics usually refers to the distribution of temperature and moisture (both vertical and horizontal) as related to the diagnosis of atmospheric instability.

Thermograph: An instrument that measures and records air temperature.

Thermometer: An instrument for measuring temperature.

Theta-e (or Equivalent Potential Temperature):  The temperature a parcel of air would have if a) it was lifted until it became saturated, b) all water vapor was condensed out, and c) it was returned adiabatically (i.e., without transfer of heat or mass) to a pressure of 1000 millibars. Theta-e, which typically is expressed in degrees Kelvin, is directly related to the amount of heat present in an air parcel. Thus, it is useful in diagnosing atmospheric instability.

Theta-e Index (TEN): It represents the greatest decrease in equivalent potential temperature measured in a layer beginning at or below 700 mb. Consequently, this stability index evaluates the potential for elevated convection, and adds insight where surface-based indices fail. It also provides useful information for diagnosing the potential for short-fused flooding events, especially when warm, moist, unstable air south of a warm or stationary front is forced isentropically over the frontal boundary by significant low level winds (greater than 20-30 knots). A TEN equal to or exceeding 5-10 o C indicates a potential for elevated convection, especially when isentropic lift is probable.

Theta-e Ridge:  An axis of relatively high values of theta-e. Severe weather and excessive rainfall often occur near or just upstream from a theta-e ridge.

Three-Hour Rainfall Rate (THP):  This WSR-88D Radar product displays precipitation total (in inches) of the current and past two clock hours as a graphical image. It displays hourly precipitation total (in inches) as a graphical image (polar format with resolution 1.1 nm by 1 degree).  It is updated once an hour.  It is used to:  1) Assess rainfall intensities and amounts over a longer viewing interval; and 2) Possibly adjust flash flood guidance values since the product corresponds to the timing of Flash Flood Guidance values.

Threshold Runoff:  The runoff in inches from a rain of specified duration that causes a small stream to slightly exceed bankfull.  When available, flood stage is used instead of slightly over bankfull.

Thin Line Echo: A narrow, elongated, non-precipitating echo. It is usually associated with thunderstorm outflows, fronts, or other density discontinuities. It is also known as a Fine Line.

Thunder: The sound emitted by the rapidly expanding gases along the channel of a lightning discharge. Thunder is seldom heard farther than about 15 miles from the lightning discharge, with 25 miles an approximate upper limit and 10 miles a typical value.

Thunderstorm (TS): A local storm produced by cumulonimbus clouds. It is always accompanied by lightning and thunder. It is estimated that nearly 2,000 thunderstorms occur simultaneously around the Earth at any given instant. There are 3 types of thunderstorms. They are the following: 1) Single Cell Thunderstorms, 2) Multicell Thunderstorms, and 3) Severe Supercell Thunderstorm.

Tidal Anomaly: Actual water level minus the tide table predictions.

Tidal Cycle: The periodic changes in the intensity of tides caused primarily by the varying relations between the earth, moon, and sun.

Tidal Piling: Abnormally high water levels caused by the accumulation of successive incoming tides that do not completely drain due to opposing strong winds and/or waves.

TidesThey are the periodic (occurring at regular intervals) variations in the surface water level of the oceans, bays, gulfs, and inlets.  Tides are the result of the gravitiational attraction of the sun and the moon on the earth. The attraction of the moon is far greater than the attraction of the sun due to the close proximity of the earth and the moon. The sun is 360 times further from the earth than the moon. Therefore, the moon plays a larger role than the sun in producing tides. Every 27.3 days, the earth and the moon revolve around a common point. This means that the oceans and other water bodies which are affected by the earth-moon system experience a new tidal cycle every 27.3 days.  Because of the physical processes which occur to produce the tidal system, there are two high tides and two low tides each day. Because of the angle of the moon with respect to the earth, the two high tides each day do not have to be of equal height.  The same holds true for the two low tides each day. Tides also differ in height on a daily basis. The daily differences between tidal heights is due to the changing distance between the earth and the moon.  Scientists use measurements of the height of the water level to examine tides and the various phenomena which influence tides, such as hurricanes and winter storms.

Tilt: It describes a storm in which a line connecting the centroid of a mid level storm component to the centroid of the lowest storm component is to the right or the rear of the direction of motion.

Tilt Sequence Radar term indicating that the radar antenna is scanning through a series of antenna elevations in order to obtain a volume scan.

Tilted Storm or Tilted Updraft:  A thunderstorm or cloud tower which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for severe storm development.

Time-Height Display:  An intensity-modulated display which has height as the vertical coordinate and time as the horizontal coordinate; usually used for vertically-pointing antennas only.

Time Lag:  The time necessary for a fuel particle to lose approximately 63 percent of the difference between its initial moisture content and its equilibrium moisture content.

Tipping-Bucket Rain Gage A precipitation gage where collected water is funneled into a two compartment bucket; 0.01, 0.1 mm, or some other designed quantity of rain will fill one compartment and overbalance the bucket so that it tips, emptying into a reservoir and moving the second compartment into place beneath the funnel. As the bucket is tipped, it actuates an electric circuit.

Toe Drain (or Outfall):  A drain which carries seepage away from the dam and can allow seepage quantities to be measured.

Toe of Dam (Upstream and Downstream):  The junction of the face of a dam with the ground surface.

TonitrophobiaThe fear of thunder.  See Astraphobia, Astrapophobia, Brontophobia, Ceraunophobia, and Keraunophobia.

Topography:  The shape of the land.
 
Tornado (+FC):  A violently rotating column of air in contact with the ground and extending from the base of a thunderstorm. A condensation funnel does not need to reach to the ground for a tornado to be present; a debris cloud beneath a thunderstorm is all that is needed to confirm the presence of a tornado, even in the total absence of a condensation funnel.It nearly always starts as a funnel cloud and may be accompanied by a loud roaring noise. Tornadoes are classified by the amount of damage that they cause. See Fujita Scale.

Tornado Family:  A series of tornadoes produced by a single supercell, resulting in damage path segments along the same general line.

Tornado Warning (TOR) This is issued when a tornado is indicated by the WSR-88D radar or sighted by spotters; therefore, people in the affected area should seek safe shelter immediately.  They can be issued without a Tornado Watch being already in effect.  They are usually issued for a duration of around 30 minutes.

A Tornado Warning is issued by your local National Weather Service office (NWFO).  It will include where the tornado was located and what towns will be in its path.  If the tornado will affect the nearshore or coastal waters, it will be issued as the combined product--Tornado Warning and Special Marine Warning.  If the thunderstorm which is causing the tornado is also producing 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 tornado 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.

Tornado Watch (SEL):  This is issued by the National Weather Service when conditions are favorable for the development of tornadoes in and close to the watch area.  Their size can vary depending on the weather situation.  They are usually issued for a duration of 4 to 8 hours.  They normally are issued well in advance of the actual occurrence of severe weather.  During the watch, people should review tornado safety rules and be prepared to move a place of safety if threatening weather approaches.

A Tornado Watch is issued by the Storm Prediction Center (SPC) in Norman, Oklahoma.  Prior to the issuance of a Tornado Watch, SPC will usually contact the affected local National Weather Forecast Office (NWFO) and they will discuss what their current thinking is on the weather situation.  Afterwards, SPC will issue a preliminary Tornado Watch and then the affected NWFO will then adjust the watch (adding or eliminating counties/parishes) and then issue it to the public.  After adjusting the watch, the NWFO will let the public know which counties are included 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.

Total Gross Reservoir Capacity:  The total amount of storage capacity available in a reservoir for all purposes from the streambed to the normal water or normal water or normal pool surface level. It does not include surcharge, but does include dead storage.

Total Totals (TT): This index estimates the potential for severe convection. It combines the effects of vertical temperature lapse rate (Vertical Totals or VT) and low level moisture (Cross Totals or CT) in a given environment. The following formulas are used to create the Total Totals index:

      Vertical Totals (VT) = 850 mb temperature - 500 mb temperature

      Cross Totals (CT) = 850 mb dew point - 500 mb temperature

When the two are combined, you have the following formula:

      Total Totals (TT) = Vertical Totals (VT) + Cross Totals (CT)

The following table shows what these relationships typically mean east of the Rockies:
 

What Different Total Totals Values Mean East of the Rockies?

Cross Totals (CT)

Vertical Totals (VT)

Total Totals (TT)

Forecast

18-19

26 or more

44

Isolated or few thunderstorms

20-21

26 or more

46

Scattered thunderstorms

22-23

26 or more

48

Scattered thunderstorms, isolated severe

24-25

26 or more

50

Scattered thunderstorms, few severe, isolated tornadoes

26-29

26 or more

52

Scattered to numerous thunderstorms, few to scattered severe, few tornadoes

30

26 or more

56

Numerous thunderstorms, scattered severe, scattered tornadoes

 

High lapse rates or a source of low level moisture will yield large values of TT. However, high lapse rates can produce large TT, with little supporting low level moisture. The sounding must be examined carefully to ascertain the validity of the TT for a given environment.  Also as with any index, you must carefully examine your environment.

Tower:   Short for towering cumulus.   A cloud element showing appreciable upward vertical development.

Towering Cumulus (TCU): It signifies a relatively deep layer of unstable air. The bases are flat and usually appear darker than the bases of fair weather cumulus. They show considerable vertical development and have billowing "cauliflower" tops. Showers can result from these clouds. Same as cumulus congestus.

TPCAn acronym for the Tropical Prediction Center.  See Tropical Prediction Center (TPC).

Trace:  1) A rainfall amount less than 0.01 of an inch. 2) A hydrograph or similar plot for an extended-range time horizon showing one of many scenarios generated through an ensemble forecast process.

Trace of Icing:  Ice becomes perceptible on an aircraft.  The rate of ice accumulation is slightly greater than the rate of sublimation.  It is not hazardous even though de-icing/anti-icing equipment is not utilized, unless encountered for an extended period of time--over one hour.  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 convetion has been to designate icing intensity in terms of its operational effect upon reciprocating engine, straight wing transport aircraft as used by commuter operators.

TrackThe path that a storm or weather system follows.
 
Trade Winds: The winds that occupy most of the tropics and blow from subtropical highs to the equatorial low.

Transmitter:  The radar equipment used for generating and amplifying a radio frequency (RF) carrier signal, modulating the carrier signal with intelligence, and feeding the modulated carrier to an antenna for radiation into space as electromagnetic waves. Weather radar transmitters are usually magnetrons or klystrons.

Transpiration: Water discharged into the atmosphere from plant surfaces.

Transport Wind:  The average wind over a specified period of time within a mixed layer near the surface of the earth.

Transverse Bands:  Bands of clouds oriented perpendicular to the flow in which they are embedded. They often are seen best on satellite photographs. When observed at high levels (i.e., in cirrus formations), they may indicate severe or extreme turbulence. Transverse bands observed at low levels (called transverse rolls or T rolls) often indicate the presence of a temperature inversion (or cap) as well as directional shear in the low- to mid-level winds. These conditions often favor the development of strong to severe thunderstorms.

Transverse Rolls:  Elongated low-level clouds, arranged in parallel bands and aligned parallel to the low-level winds but perpendicular to the mid-level flow. Transverse rolls are one type of transverse band, and often indicate an environment favorable for the subsequent development of supercells. Since they are aligned parallel to the low-level inflow, they may point toward the region most likely for later storm development.

Trash Rack:  A screen located at an intake to prevent debris from entering.

Travel Time: The time required for a flood wave to travel from one location to a subsequent location downstream.

T Rolls:  Slang term for transverse rolls.

Triple DopplerSince any wind has three components (say, in the x, y and z directions), and a single radar measures in only one direction (radial), a single radar cannot give the 3D winds everywhere it samples. However, if three different radars view a storm from three different locations, the 3 measured radial velocities can be transformed into the actual 3D wind field.

Triple Point:  The intersection point between two boundaries (dry line, outflow boundary, cold front, etc.), often a focus for thunderstorm development. Triple point also may refer to a point on the gust front of a supercell, where the warm moist inflow, the rain-cooled outflow from the forward flank downdraft, and the rear flank downdraft all intersect; this point is a favored location for tornado development (or redevelopment).

Tropical Advisories:  Official information issued by the Tropical Prediction Centers describing all tropical cyclone watches and warnings in effect along with details concerning tropical cyclone locations, intensity and movement, and precautions that should be taken.  Advisories are issued to describe (a) tropical cyclones prior to issuance of watches and warnings and (b) subtropica cyclones.

    1) Scheduled Public Tropical Cyclone Advisory (TCP)This is one of the most prominent products issued by the hurricane centers.  It summarizes all watches or warnings in effect.  It includes the location of the storm center in latitude and longitude and in distance from a well known point.  The current movement is given to 16 points of the compass.  Also, included in this advisory are the maximum sustained wind, pressure, storm tide or storm surge and the radius of both hurricane force and tropical storm force winds (just tropical storm force winds for a tropical storm) and an intensity forcast.  If it is going to affect land, inland effects will also be highlighted in this advisory.  They are issued for all tropical storms and hurricanes in the Atlantic (which includes the Caribbean Sea and the Gulf of Mexico) and central Pacific.  In the eastern Pacific, they are issued for all tropical cyclones that are expected to affect land within 48 hours.

    The initial advisory is issued when there are data (satellite, ship, aircraft, etc.) that confirm a tropical cyclone has developed.  The title of the advisory will depend upon the strength of the tropical cyclone.  A tropical depression advisory will be issued for a tropical cyclone with 1-minute sustained winds up to 38 mph (33 knots).  A tropical storm advisory will be used for tropical cyclones with 1-minute sustained surface winds 39 to 73 mph (34 to 63 knots).  A hurricane/typhoon advisory is used for tropical cyclones with winds 74 mph (64 knots) or greater.

    These products are numbered consecutively, starting with each new named storm.  Once numbering begins, it is continued as long as advisories are being issued, even if the storm decreases to tropical depression strength.  They are issued at 0500, 1100, 1700, and 2300 Eastern Daylight Time (EDT) with vaid position times corresponding to the advisory time.

    They will cease to be issued when the tropical cyclone becomes extratopical (the storm no longer has tropical characteristics), drops below depression stage, or move inland over large land areas, such as the United States, Mexico, Canada, or Central America.

    2) Intermediate Public Tropical Cyclone Advisory (TCP):  They are issued to ensure a more continuous flow of information to the public whenever a tropical cyclone affects a coast or is forecast to affect a coast.  Intermediate advisories will not be used to issue tropical cyclone watches or warnings, but it may be used to clear all or portion of a watch or warning area.  The content will be similar to that of the scheduled advisory.  However, since the content may be less formal and less complete.  In the Atlantic, they will be issued either every 2 hours or every 3 hours.  The 2 hour issuances will occur at:  0100, 0300, 0700, 0900, 1300, 1500, 1900, and 2100 Eastern Daylight Time (EDT).  The 3 hour issuances will occur at 0200, 0800, 1400, 2000 Eastern Daylight Time (EDT).  When these advisories are being issued, the Tropical Cyclone Center will tell when the next advisory will be issued at the end of the advisory.

    3) Special Public Tropical Cyclone Advisory:  These will be issued whenever one or more of the following criteria are met:
     

       1) When conditions require a hurricane/typhoon or tropical storm watch or warning to be issued.

       2) A significant change has occurred, requiring the issuance of a revised forecast package.
       

    The content of special advisories should generally be similar to that of th scheduled advisory.  The meteorological condition that requires the issuance of the special advisory is normally highlighted.  However, since special advisories are designed to update earlier scheduled advisories, their format and content will be less rigid.

    4) Scheduled Tropical Cyclone Forecasts/Advisory (TCM):  This is one of the most prominent products issued by the hurricane centers.  They are prepared for all tropical cyclones within a Tropical Cyclone Center's area of responsibility.  They provide invaluable wind field information to emergency managers, local decision makers, and other users who must make preparations and take responseactions for inland wind effects of tropical cyclones.  This product along with the public advisories should be used for decision making purposes.  They will be issued at the same time as the Scheduled Public Tropical Cyclone Advisory.  Like public advisories, these are sequentially numbered beginning with the first one issued.  All advisories will contain 12-, 24-, 36-, 48- and 72-hour forecast positions.  A standard statement indicating the uncertainty associated with the 48- and 72-hour forecast positions will precede these forecasts.  They will cease to be issued when the tropical cyclone becomes either extratopical (the storm no longer has tropical characteristics) or drop below depression stage.

    5) Special Tropical Cyclone Forecast/Advisory (TCM):  This product is issued as needed for conditions that either change abruptly or significantly prior to the next regularly scheduled Tropical Cyclone Forecast/Advisory.  They will contain generally the same information as the Tropical Cyclone Forecast/Advisory; however, their format is less stringent.

    6) Subtropical Cyclone Public Advisory (SPA):  These advisories are issued when a subtropical storm is affecting or is forecasted to affect the U.S., our territories, or our installations.  Each subtropical storm is assigned a storm number to keep track of the sequence of occurrences such storms in that area.  The contents of the advisory are the same as advisories on depressions; however, subtropical storms are referred to as plain "storms".  Thus, they are entitled as storm advisories.  They are issued at the same scheduled times as hurricane and tropical storm advisories.  They are updated as needed for significant changes such as a change in course, a threat of tornadoes being added, or a major change in intensity.  For each storm, the advisories are numbered consecutively for the life of the storm.

    7) Intermediate Subtropical Cyclone Public Advisory (SPA):  These advisories function in the same manner as public intermediate advisories.  Whenever a subtropical cyclone affects or is forecasted to affect a coast, they will be issued on a 2 to 3 hourly interval between scheduled advisories.  For clarity, whenever the National Hurricane Center is issuing intermediates, a statement will be included at the end of the scheduled public subtropical advisory telling the user that an intermediate advisory will be issued.

    8) Special Subtropical Cyclone Public Advisory (SPA):  These advisories are issued to update the previously scheduled advisory whenever a significant change in the cyclone has occurred.  The change or conditions that require the issuance of the special advisory normally are highlighted.

    9) Scheduled Subtropical Forecast/Advisory (SMA):  These advisories are issued when a subtropical storm is affecting or is forecasted to affect the U.S., our territories, or our installations.  Each subtropical storm is assigned a storm number to keep track of the sequence of occurrences such storms in that area.  The contents of the advisory are the same as advisories on depressions; however, subtropical storms are referred to as plain "storms".  Thus, they are entitled as storm advisories.  They are issued at the same scheduled times as hurricane and tropical storm advisories.

    10) Special Subtropical Cyclone Forecast/Advisory (SMA)These advisories will be issued to update any abrupt or significant change that may have occurred with the subtropical cyclone.  The format is the same as the scheduled advisory that is being replaced.

Tropical Analysis and Forecast Branch (TAFB):  One of three branches of the Tropical Prediction Center (TPC).  It provides year-round products involving marine forecasting, aviation forecasts and warnings (SIGMETs), and surface analyses. The unit also provides satellite interpretation and satellite rainfall estimates for the international community. In addition, TAFB provides support to NHC through manpower and tropical cyclone intensity estimates from the Dvorak technique.

Tropical Cyclone: It is a warm-core low pressure system which is non-frontal. It originates over tropical and subtropical waters and a has a organized cyclonic (counter-clockwise) surface wind circulation.

Tropical Cyclone Plan of the Day:  A coordinated mission plan that tasks operational weather reconnaissance requirements during the next 1100 to 1100 UTC day or as required, describes reconnaissance flights committed to satisfy both operational and research requirements, and identifies possible reconnaissance requirements for the succeeding 24-hour period.

Tropical Cyclone Position Estimate (TCE) The National Hurricane Center issues a position estimate between scheduled advisories whenever the storm center is within 200 nautical miles of U.S. land-based weather radar and if sufficient and regular radar reports are available to the hurricane center.  As far as is possible, the position estimate is issued hourly near the beginning of the hour.  The location of the eye or storm center is given in map coordinates and distance and direction from a well-known point.

Tropical Cyclone Update (TCU):  This brief statement is issued by the National Hurricane Center in lieu of or preceding special advisories to inform of significant changes in a tropical cyclone or the posting or cancellation of watches and warnings.

Tropical Depression: Cyclones that have maximum sustained winds of surface wind speed (using the U.S. 1-minute average) is 33 kt (38 mph or 62 kph) or less. They are either located in the tropics or subtropics. They characteristically have one or more closed isobars. They usually intensify slowly and may dissipate before reaching Tropical Storm intensity.

Tropical Disturbance:  A discrete tropical weather system of apparently organized convection--generally 100 to 300 nautical miles in diameter---originating in the tropics or subtropics, having a nonfrontal migratory character, and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field.In successive stages of intensification, it may be subsequently classified as a tropical wave, tropical depression, tropical storm, or hurricane.

Tropical Prediction Center (TPC) One of NOAA's 9 National Centers for Environmental Prediction (NCEP).  The mission of the Tropical Prediction Center (TPC) is to save lives and protect property by issuing watches, warnings, forecasts, and analyses of hazardous weather conditions in the tropics. TPC products are generated for use in both the domestic and international communities. To fulfill its mission, the TPC is comprised of the following branches:  The National Hurricane Center, Tropical Analysis and Forecast Branch (TAFB), and Technical Support Branch (TSB).

Tropical Storm: It is a warm-core tropical cyclone that has maximum sustained surface wind speed (using the U.S. 1-minute average) ranges from 34 kt (39 mph or 63 kph) to 63 kt (73 mph or 118 kph).  

Tropical Storm Summaries (SCC):  These are written by the Hydrometeorological Prediction Center* (HPC) after subtropical and names tropical cyclones have moved inland and advisories have been discontinued.  These advisories will be terminated when the threat of flash flooding has ended or when the remnants of these storms can no longer be distinguished from other synoptic features capable of producing flash floods.  Storm summaries will not be issued for storms that enter the coast of Mexico and do not pose an immediate flash flood threat to the coterminous United States.  They will be initiated when and if flash flood watches are posted in the United States because of an approaching system.  Storm summaries will continue to be numbered in sequence with tropical cyclone advisories and will reference the former storm's name in the text.  Summaries will be issued at 0100, 0700, 1300, and 1900 Eastern Daylight Time (EDT).  The only exception will be the first one in the series may be issued at a nonscheduled time.

*Note:  HPC was formerly called the National Meteorological Center (NMC) and it is located in Washington DC.

Tropical Storm Warning A warning for tropical storm conditions including sustained winds within the range of 34 to 63 kt (39 to 73 mph or 63 to 118 kph) that are expected in a specified coastal area within 24 hours or less.

Tropical Storm Watch:  An announcement that a tropical storm poses or tropical storm conditions pose a threat to coastal areas generally within 36 hours. A tropical storm watch should normally not be issued if the system is forecast to attain hurricane strength.

Tropical Wave: A trough or cyclonic curvature maximum in the trade wind easterlies and it is not classified as a tropical cyclone. The wave may reach maximum amplitude in the lower middle troposphere.

Tropical Weather Discussion (TWD):  These messages are issued 4 times daily by the Tropical Analysis and Forecast Branch (TAFB) to describe significant synoptic weather features in the tropics.  One message will cover the Gulf of Mexico, the Caribbean, and the Atlantic between the equator and 32 degrees North and east of 140 degrees West.  Plain language is used in these discussions.

Tropical Weather Outlook (TWO):  This outlook normally covers the tropical and subtropical waters, discussing the weather conditions, emphasizing any disturbed and suspicious areas which may become favorable for tropical cyclone development within the next day to two.  In the Atlantic, the outlook is transmitted daily at 0530, 1130, 1730, and 2230 Eastern local time.  In the eastern Pacific, it is transmitted daily at 0100, 0700, 1300, and 1900 Eastern local time.  For the Central Pacific, transmission times are 1000 and 2200 UTC.  Existing tropical and subtropical cyclones are mentioned, as are depressions not threatening land.  Given for each system are its location, size, intensity, and movement.  For the first 24 hours of a depression or tropical cyclone, the outlook includes a statement identifying the AFOS and World Meteorological Organization (WMO) headers for the advisory on it.

Tropical Weather Summary (TWS):  The National Hurricane Center issues a monthly summary of tropical weather is included at the end of the month or as soon as feasible thereafter, to describe briefly the past activity or lack thereof and the reasons why.

Tropics: Areas of the Earth within 20o North and South of the Equator.

Tropopause: The upper boundary of the troposphere, usually characterized by an abrupt change in lapse rate from positive (decreasing temperature with height) to neutral or negative (temperature constant or increasing with height).  It is also the boundary between the troposphere and the stratosphere.

Troposphere The layer of the atmosphere from the earth's surface up to the tropopause, characterized by decreasing temperature with height (except, perhaps, in thin layers - see inversion, cap), vertical wind motion, appreciable water vapor content, and sensible weather (clouds, rain, etc.).

Tropopause Jets: Jet streams found near the tropopause. Examples of these types of jets are the subtropical and polar fronts.

Trough:  An elongated area of relatively low atmospheric pressure, usually not associated with a closed circulation, and thus used to distinguish from a
closed low. The opposite of ridge.

T.S.An abbreviation for Tropical Storm.  See Tropical Storm.

Tsunami: An ocean wave produced by a sub-marine earthquake, landslide, or volcanic eruption. These waves may reach enormous dimensions and have sufficient energy to travel across entire oceans.

Turbidity:  The thickness or opaqueness of water caused by the suspension of matter. The turbidity of rivers and lakes increases after a rainfall.

Turkey Tower:  Slang for a narrow, individual cloud tower that develops and falls apart rapidly. The sudden development of turkey towers from small cumulus clouds may signify the breaking of a cap.

Turning Point A temporary point whose elevation is determined by additions and subtractions of backsights and foresights respectively.

TVS (Tornado Vortex Signature): An image of a tornado on the Doppler radar screen that shows up as a small region of rapidly changing wind speeds inside a mesocyclone. The following velocity criteria is normally required for recognition: velocity difference between maximum inbound and outbound (shear) is greater than or equal to 90 knots at less than 30 nmi and is greater than or equal to 70 knots between 30 and 55 nmi. It shows up as a red upside down triangle on the Storm Relative Velocity Display. Existence of a TVS strongly increases the probability of tornado occurrence, but does not guarantee it; therefore, the feature triggering it must be examined closely by the radar operator. A TVS is not a visually observable feature.

TWEB (Transcribed Weather Broadcasts) Route ForecastThis NWS aviation product is similar to the Area Forecast (FA) except information is contained in a route format. Forecast sky cover (height and amount of cloud bases), cloud tops, visibility (including vertical visibility), weather, and obstructions to vision are described for a corridor 25 miles either side of the route. Cloud bases and tops are always Mean Sea Level (MSL) unless noted. Ceilings are always above ground level.

The Synopsis is a brief statement of frontal and pressure systems affecting the route during the forecast valid period.

The TWEB route forecasts are prepared by National Weather Service Forecast Offices (WFOs) for more than 300 selected short-leg and cross-country routes over the contiguous U.S. WFOs prepare synopses for the routes in their areas.

The TWEB route forecasts and synopses are issued by the WFOs three times per day. Route forecasts are valid for 15 hours.  This schedule provides 24-hour coverage with most frequent updating during the hours of greatest aviation activity.

Twilight:  The intervals of incomplete darkness following sunset and preceding sunrise.  The time at which evening twilight ends or morning twilight begins is determined by arbritrary convention, and several kinds of twilight have been defined and used; most commonly civil, nautical, and astronomical twilight.

    1) Civil Twilight:  The period of time before sunrise and after sunset when the sun is not more than 6 degrees below the horizon.

    2) Nautical TwilightThe period of time before sunrise and after sunset when the sun is not more than 12 degrees below the horizon.

    3) Astronomical TwilightThe period of time before sunrise and sunset when the sun is not more than 18 degrees below the horizon.

TwisterIn the United States, a colloquial terms for a tornado.

TyphoonA tropical cyclone of hurricane strength in the Eastern Hemisphere.  


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