WAA:  An acronym for Warm Air Advection.  See Warm Air Advection.

Wall Cloud: It is formed in a supercell thunderstorm.  A localized, persistent, often abrupt lowering from a rain-free base. Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm.  Even though this cloud is lowering, it remains attached to the rain free cloud base of the thunderstorm. It is usually located south or southwest of the visible precipitation area, and marks the strongest updraft in the thunderstorm. The wall cloud develops as the strong updraft draws in surface moisture from several miles away. Eventually, this updraft will pull air from the rain cooled area of the thunderstorm. Since the rain cooled air is very humid, it will quickly condense in the updraft at a lower altitude than the rain free cloud base. When seen from within several miles, many wall clouds exhibit rapid upward motion and cyclonic rotation. However, not all wall clouds rotate. Rotating wall clouds usually develop before strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour. Wall clouds should be monitored visually for signs of persistent, sustained rotation and/or rapid vertical motion. See supercell. "Wall cloud" also is used occasionally in tropical meteorology to describe the inner cloud wall surrounding the eye of a tropical cyclone, but the proper term for this feature is eyewall.

Wave SteepnessThe ratio of wave heights to wave length.

Warm Air Advection:  Transport of warm air into an area by horizontal winds.  Low-level warm advection sometimes is referred to (erroneously) as overrunning. Although the two terms are not properly interchangeable, both imply the presence of lifting in low levels.

Warm Core Low: A low pressure area which is warmer at its center than at its periphery. Tropical cyclones exhibit this temperature pattern. Unlike cold core lows, these lows produce much of their cloud cover and precipitation during the nighttime.

Warm Front: A front that moves in such a way that warm air replaces cold air.

Warning:  A product issued by NWS local offices indicating that a particular weather hazard is either imminent or has been reported. A warning indicates the need to take action to protect life and property. The type of hazard is reflected in the type of warning (e.g., tornado warning, blizzard warning). See short-fuse warning.

WatchAn NWS product indicating that a particular hazard is possible, i.e., that conditions are more favorable than usual for its occurrence. A watch is a recommendation for planning, preparation, and increased awareness (i.e., to be alert for changing weather, listen for further information, and think about what to do if the danger materializes).

Watch Box (or Box):  Slang for a severe thunderstorm or tornado watch.

Watch Cancellation (SEL) This product will be issued to let the public know when either a Tornado Watch or Severe Thunderstorm Watch has been canceled early.  It is issued by the Storm Prediction Center (SPC) in Norman, Oklahoma.  In the text of the statement it will specify the severe weather watch number and the area which the watch covered.

Watch Redefining Statement (SLS):  This product tells the public which counties/parishes are included in the watch.  This is done not only by writing them all out, but by using the county FIPS codes in the Header of the product.  It is issued by the local National Weather Service Forecast Office (NWFO).

Watch Status Reports (WWA):  This product lets the NWFO know of the status of the current severe weather watch (Tornado or Severe Thunderstorm).  During the severe weather watch, the Storm Prediction Center (SPC) will issue these reports periodically.  These reports will describe, in plain language, the current evaluation of the severe weather situation and whether the watch will expire or be reissued.  A status report is not issued if a cancellation or replacement has been issued at least 1 hour prior to the expiration time of the original watch.

Water Equivalent: The liquid content of solid precipitation that has accumulated on the ground (snow depth). The accumulation may consist of snow, ice formed by freezing precipitation, freezing liquid precipitation, or ice formed by the refreezing of melted snow.

Water Pollution:  The alteration of the constituents of a body of water by man to such a degree that the water loses its value as a natural resource.

WatershedThe total area drained by a river and its tributaries. Sometimes called a basin.
Water Supply Outlook:  A seasonal volume forecast, generally for a period centered around the time of spring snowmelt (e.g., April-July). The outlooks are in units of acre-feet and represent the expected volume of water to pass by a given point during a snowmelt season. The outlook categories include Most Probable, Reasonable Maximum, and Reasonable Minimum.
Water Supply Outlook (ESS) Product:  A public product issued by a Forecast Office which contains narrative and numeric information on current and extended water supply conditions.

Water Table:  The level below the earth's surface at which the ground becomes saturated with water. The water table is set where hydrostatic pressure equals atmospheric pressure.
Water Vapor (WV) Satellite Imagery: This satellite imagery uses the 6.7 micrometer (um) channel to detect moisture between 700 and 200 mb; therefore, it is good for determining mid and upper level moisture in the atmosphere. Abundant water vapor appears white in this imagery. Meanwhile, dry air appears black in this satellite imagery. This satellite imagery can be used both day and night.

Water Vapor Plume: This appear in the water vapor satellite imagery. It is a plume-like object that extends from the Intertropical Convergence Zone (ITCZ) northward or southward into the higher latitudes. It is usually located over a 850 to 700 mb theta-e ridge axis. As a result, it is a favored location for the development of a Mesoscale Convective Complex (MCC). Researchers have found it to be a favored region for very heavy rain. It is thought that the ice crystals located in this plume help thunderstorms to become highly efficient rainfall producers. In North America, this is sometimes called the "Mexican Connection".

Water Year:  The time Period form October 1 through September 30.

Watercourse:  Any surface flow such as a river, stream, tributary.

Watershed: Land area from which water drains toward a common watercourse in a natural basin.  They range in size from a few acres to large areas of the country.

Waterspout: A violently rotating column of air, usually a pendant to a cumulus or cumulonimbus cloud, over a body of water with its circulation reaching the water. In the summer and spring, these phenomena are usually "tornadoes over water" that have been generated by thunderstorms. In the fall months, these most often begin as "cold air funnels", being generated by a cold air mass passing over much warmer waters. Such waterspouts are generally much less intense than tornadoes and usually dissipate upon approaching shore. Waterspouts are most common over tropical or subtropical waters.
The exact definition of waterspout is debatable. In most cases the term is reserved for small vortices over water that are not associated with storm-scale rotation (i.e., they are the water-based equivalent of landspouts). But there is sufficient justification for calling virtually any rotating column of air a waterspout if it is in contact with a water surface.

Watt:  The unit of power in the MKS system of units; energy per unit time, one Joule/second.

Wave: An identifiable, periodic disturbance or motion in a medium that shows displacement. The most commonly referred medium is water, followed by the atmosphere. The forecasted heights of waves on the Great Lakes or in the oceans are those heights expected at the end of the fetch for that body of water.

Waveguide:  A hollow conductor, usually rectangular or round in cross-section, used to carry radar waves between various components of a radar.

Wavelength:  The distance a wave will travel in the time required to generate one cycle. The distance between two consecutive wave peaks (or other reference points) in space. Weather radar wavelengths typically range from 1 mm to 50 cm.

Wave SpectrumThe distribution of wave energy with respect to wave frequency or period.  Wave spectra assist in differentiating between wind waves and swells.

Weather Forecast Office (WFO): This National Weather Service office is responsible for issuing advisories, warnings, statements, and short term forecasts for its county warning area.

Wedge (or Wedge Tornado):  Slang for a large tornado with a condensation funnel that is at least as wide (horizontally) at the ground as it is tall (vertically) from the ground to cloud base. The term "wedge" often is used somewhat loosely to describe any large tornado. However, not every large tornado is a wedge. A true wedge tornado, with a funnel at least as wide at the ground as it is tall, is very rare.
Wedges often appear with violent tornadoes (F4 or F5 on the Fujita Scale), but many documented wedges have been rated lower. And some violent tornadoes may not appear as wedges (e.g., Xenia, OH on 3 April 1974, which was rated F5 but appeared only as a series of suction vortices without a central condensation funnel). Whether or not a tornado achieves "wedge" status depends on several factors other than intensity - in particular, the height of the environmental cloud base and the availability of moisture below cloud base. Therefore, spotters should not estimate wind speeds or F-scale ratings based on visual appearance alone. However, it generally is safe to assume that most (if not all) wedges have the potential to produce strong (F2/F3) or violent (F4/F5) damage.

Weekly Weather and Crop Bulletin:  This bulletin is jointly prepared by the U.S. Department of Commerce, National Oceanic and Atmospheric Administration, and the U.S. Department of Agriculture. The bulletin is published weekly on Tuesday.

Weighing-Type Precipitation Gage:  A rain gage that weighs the rain or snow which falls into a bucket set on a platform of a spring or lever balance.  The increasing weight of its contents plus the bucket are recorded on a chart. The record thus shows the accumulation of precipitation.

Weir:  a) A low dam built across a stream to raise the upstream water level (fixed-crest weir when uncontrolled); b) A structure built across a stream or channel for the purpose of measuring flow (measuring or gaging weir).

WER - Weak Echo Region 1) Radar term for a region of relatively weak (reflectivity at low levels on the inflow side of a thunderstorm echo, topped by stronger reflectivity in the form of an echo overhang directly above it (see Fig. 2). The WER is a sign of a strong updraft on the inflow side of a storm, within which precipitation is held aloft. When the area of low reflectivity extends upward into, and is surrounded by, the higher reflectivity aloft, it becomes a BWER. 2) An WSR-88D radar product which displays reflectivity for up to 8 elevation angles for a radar operator selected location as a set presentation of a storm.  The plains in this product are presented in an ascending order, lowest plain is lowest elevation angle selected.  It is used to depict storm tilt and to identify Weak Echo Regions (WER) and Bounded Weak Echo Regions (BWER) in thunderstorms.

West African Disturbance Line (WADL): It is a line of convection about 300 miles long, similar to a squall line. It forms over west Africa north of the equator and south of 15 degrees North latitude. It moves faster than an Easterly Wave between 20 and 40 mph. They move off the African coast every 4 to 5 days mainly in the summer. Some reach the American tropics and a few develop into tropical cyclones.

Wet-Bulb Temperature: The lowest temperature that can be obtained by evaporating water into air.

Wet-Bulb Zero (WBZ): It is the height where the wet-bulb temperature goes below 0 o F. It is important because WBZ heights between 7000 ft and 10,500 ft (above ground level) correlate well with large hail at the surface when storms develop in an airmass primed for strong convection. Higher values infer mid and upper level stability and also indicate a large melting area for falling hail. Lower WBZ heights indicate that the low level atmosphere is often too cool and stable to support large hail.

Wet Floodproofing:  An approach to floodproofing which usually is a last resort. Flood waters are intentionally allowed into the building to minimize water pressure on the structure. Wet Floodproofing can include moving a few valuable items to a higher place or completely rebuilding the floodable area. Wet floodproofing has an advantage over other approaches: not matter how little is done, flood damage will be reduced. Thousands of dollars in damage can be avoided just by moving furniture and appliances out of the flood-prone area.

Wet Microburst:  A microburst accompanied by heavy precipitation at the surface. A rain foot may be a visible sign of a wet microburst. See dry microburst.

Wetland An area that is regularly wet or flooded and has a water table that stands at or above the land surface for at least part of the year.

WFOA National Weather Service Weather Forecast Office.

WhirlwindA small, rotating column of air; may be visible as a dust devil.

Wideband Communications:  The high speed (1.54 megabits per second of data) communication lines between the Radar Data Acquisition (RDA) and the Radar Product Generator (RPG) on the WSR-88D radar.  There are 4 ways that this data is transmitted from the RDA to the RPG:

    1) Wire:  It is used for distances between 0 and 400 feet.

    2) Fiber Optics:  It is used between 400 feet and 11 miles.

    3) Microwave Line-Of-Site (MLOS):  It is used between from 3,280 feet to 25 miles.  The problem with this type of  link is that it will experience loss of data from thunderstorms.  Only 4 sites
        in the entire country have this due to the possibility of the loss of data.

    4) T1 A contract service provided by a telecommunications company.  This will be used in place of MLOS.

Widely ScatteredA National Weather Service convective precipitation descriptor for a 20 percent chance of measurable precipitation (0.01 inch).  See Precipitation Probability (PoP).

WildfireAny free burning uncontainable wildland fire not prescribed for the area which consumes the natural fuels and spreads in response to its environment.

Wildfire Prevention Advisory:  This product is issued through the National Weather Service whenever either the Department of Natural Resources (DNR) or United State Forest Service (USFS) requests one due to a high fire danger.  In the state of Michigan, this product will be issued by NWFO Gaylord and NWFO Marquette.

Wildlands:  Any nonurbanized land not under extensive agricultural cultivation, e.g., forests, grasslands, rangelands.

Willy-WillyA tropical cyclone of hurricane strength near Australia.

Wind: The horizontal motion of the air past a given point.  Winds begin with differences in air pressures. Pressure that's higher at one place than another sets up a force pushing from the high toward the low pressure. The greater the difference in pressures, the stronger the force. The distance between the area of high pressure and the area of low pressure also determines how fast the moving air is accelerated. Meteorologists refer to the force that starts the wind flowing as the "pressure gradient force." High and low pressure are relative. There's no set number that divides high and low pressure.  Wind is used to describe the prevailing direction from which the wind is blowing with the speed given usually in miles per hour or knots.  The following table gives descriptions of winds used in National Weather Service forecasts.

Sustained Wind Speed

Descriptive Term

0-5 mph

Light or light and variable wind

5-15 mph or 10-20 mph


15-25 mph

Breezy, Brisk, or Blustery

20-30 mph


30-40 mph

Very Windy

40 mph or greater

Strong, dangerous, or damaging

Wind Chill: It is based on the rate of heat loss from exposed skin caused by the combined effect of wind and cold. As the wind increase, heat is carried away from the body at an accelerated rate, driving down the body temperature. Animals are also affected by wind chill.  The term wind chill was coined by Paul Siple. He was a pioneer in determining the relationship between heat loss to wind and temperature. During the 2nd Byrd Expedition to Antarctica in 1939-40, Siple exposed water-filled plastic cylinders to various temperatures and wind speeds. He then recorded the time that it took for the water to freeze in the cylinders.  With this data and the assistance of his colleague Charles Passel, he developed a formula for calculating wind chill.

The wind chill equation was updated in 2001 by an international group of governmental and academic organizations. The new equation makes use of advances in science, technology, and computer modeling to provide a more accurate, understandable, and useful formula for calculating the dangers from winter winds and freezing temperatures. In addition, clinical trials have been conducted and the results of those trials have been used to verify and improve the accuracy of the new formula. The National Weather Service began using the new wind chill equation in November, 2001. The new equation is:

Wind Chill (°F) = 35.74 + 0.6215T - 35.75(V0.16) + 0.4275T(V0.16)

where: T = air temperature is in degrees Fahrenheit (°F) and
V = wind speed in miles per hour (MPH)


Wind Chill Index

Temperature (°F)
40 35 30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30 -35 -40 -45
  5 36 31 25 19 13 7 1 -5 -11 -16 -22 -28 -34 -40 -46 -52 -57 -63
  10 34 27 21 15 9 3 -4 -10 -16 -22 -28 -35 -41 -47 -53 -59 -66 -72
W 15 32 25 19 13 6 0 -7 -13 -19 -26 -32 -39 -45 -51 -58 -64 -71 -77
i 20 30 24 17 11 4 -2 -9 -15 -22 -29 -35 -42 -48 -55 -61 -68 -74 -81
n 25 29 23 16 9 3 -4 -11 -17 -24 -31 -37 -44 -51 -58 -64 -71 -78 -84
d 30 28 22 15 8 1 -5 -12 -19 -26 -33 -39 -46 -53 -60 -67 -73 -80 -87
  35 28 21 14 7 0 -7 -14 -21 -27 -34 -41 -48 -55 -62 -69 -76 -82 -89
(m 40 27 20 13 6 -1 -8 -15 -22 -29 -36 -43 -50 -57 -64 -71 -78 -84 -91
p 45 26 19 12 5 -2 -9 -16 -23 -30 -37 -44 -51 -58 -65 -72 -79 -86 -93
h) 50 26 19 12 4 -3 -10 -17 -24 -31 -38 -45 -52 -60 -67 -74 -81 -88 -95
  55 25 18 11 4 -3 -11 -18 -25 -32 -39 -46 -54 -61 -68 -75 -82 -89 -97
  60 25 17 10 3 -4 -11 -19 -26 -33 -40 -48 -55 -62 -69 -76 -84 -91 -98
  Frostbite times: 30 minutes 10 minutes 5 minutes

Wind Chill Advisory: The National Weather Service issues this product when the wind chill could be life threatening if action is not taken. The criteria for this warning varies from state to state. In Michigan, the criteria is a 10 mph or greater wind and a wind chill forecasted between minus 20 degrees F and minus 29 degrees F.

Wind Chill Warning: The National Weather Service issues this product when the wind chill is life threatening. The criteria for this warning varies from state to state. In Michigan, the criteria is a 10 mph or greater wind and a wind chill forecasted of being less than 30 degrees F below zero.

Wind Couplet: It is an area on the radar display where you have two maximum wind speeds which are blowing in opposite directions.

Wind Direction: The true direction from which the wind is moving at a given location. It is normally measured in tens of degrees from 0 to 360.

Wind Gust: They are rapid fluctuations in the wind speed with a variation of 10 knots or more between peaks and lulls. The speed of the gust will be the maximum instantaneous wind speed.

Wind Rose: A diagram that shows the percent of time that the wind blows from different directions at a given location over a given time.

Wind Shear: The rate of change of wind speed and/or direction over a given distance. Also, see shear.

Wind Shift: A change in wind direction of 45 degrees or more in less than 15 minutes with sustained wind speeds of 10 knots or more throughout the wind shift.

Wind Shift Line:  A long, but narrow axis across which the winds change direction (usually veer).

Wind Sock: A tapered fabric shaped like a cone that indicates wind direction by pointing away from the wind. It is also called a "wind cone".

Wind Speed: The rate at which air is moving horizontally past a given point. It may be a 2-minute average speed (reported as wind speed) or an instantaneous speed (reported as a peak wind speed, wind gust, or squall). The following table is a way of estimating wind speed:

Beaufort Wind Speed Estimations on Land and Water
Knots* Visual Appearance Knots* Visual Appearance
<1 Calm: smoke rises vertically; sea like a mirror 22-27 Strong Breeze: Large branches in motion; whistling heard in overhead wires; umbrellas used with difficulty; large waves begin to form; the white foam crest are more extensive everywhere (probably some spray).
1-3 Light Air: Direction of wind shown by smoke drift not by wind vanes. Ripples resembling scales are formed on water, but without foam crests. 28-33 Near Gale: Whole trees in motion; inconveniences felt against wind; sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind.
4-6 Light Breeze: Wind felt on face; leaves rustle; wind vanes moved by wind; small wavelets form on water, still short, but more pronounced; crests have glassy appearance. 34-40 Gale: Breaks twigs off trees; impedes progress; moderately high waves of greater length; edges of crests begin to break into spindrift; foam on water surface is blown in well-marked streaks along wind.
7-10 Gentle Breeze: Leaves and small twigs in constant motion; wind extends light flag; large wavelets on water; waves crests begin to break; water has a foam of glassy appearance; scattered whitecaps appear on lakes. 41-47 Strong Gale: Slight structural damage occurs; high waves; dense streaks of foam on water surface along wind; crest of waves begin to roll over; spray may affect visibility.
11-16 Moderate Breeze: Raises dust, loose paper; small branches moved; small waves, becoming longer; fairly frequent white caps appear on lakes. 48-55 Storm: Trees uprooted; considerable damage occurs; very high waves with long overhanging crests; foam on water, in great patches, is blown in dense white streaks along wind; sea takes on a white appearance; tumbling of sea becomes heavy and shocklike; visibility affected over water.
17-21 Fresh Breeze: Small trees in leaf begin to sway; crested wavelets from on inland waters; moderate waves, taking a more pronounced long form; many whitecaps appear on lakes. 56-63 Violent Storm: Widespread damage; exceptionally high waves (small and medium-sized ships might be for a time lost to view behind waves); the sea is completely covered with long white patches of foam lying along the direction of the wind; everywhere the edges of the wave crests are blown into froth; visibility affected.
*Note: MPH equals approximately 1.15 x Knots

Wind WavesLocal, short period waves generated from the action of wind on the water surface (as opposed to swell).  Commonly referred to as waves.  In a National Weather Service Coastal Marine Forecast or Offshore Forecast, wind waves are used when swells are described in the forecast.

Winds and Temperatures AloftThis NWS aviation product contains winds aloft which are computer prepared and contain forecast wind direction and speed as well at forecast temperatures. Forecast winds and temperatures aloft are prepared for:

  6,000 feet
  9,000 feet
  12,000 feet
  18,000 feet
  24,000 feet
  30,000 feet
  34,000 feet
  39,000 feet

All heights are above Mean Sea Level. Forecast winds are also prepared for 3,000 feet. Wind directions are true directions.

Windy20 to 30 mph winds

Winter Storm WarningThis product is issued by the National Weather Service when a winter storm is producing very heavy snow.  The criteria for this warning can vary from place to place.  For example, in Lower Michigan, a Winter Storm Warning is issued when winter storm is producing snow greater than or equal to 6 inches in 12 hours; or greater than or equal 8 inches in 24 hours.  In cases of mixed precipitation or blowing, these snowfall amounts may be less.  Meanwhile, in Upper Michigan, a Winter Storm Warning is issued when a winter storm is producing snow greater than or equal to 8 inches in 12 hours; or greater than or equal 10 inches in 24 hours.  Like in Lower Michigan, these snowfall amounts may be less in situations where you have mixed precipitation and/or blowing.

Winter Storm WatchThis product is issued by the National Weather Service when there is a potential of very heavy snow.  The criteria for this watch can vary from place to place.  For example, in Lower Michigan, a Winter Storm Watch is issued for the potential of a winter storm producing snow greater than or equal to 6 inches in 12 hours; or greater than or equal 8 inches in 24 hours.  In cases of mixed precipitation or blowing, these snowfall amounts may be less.  Meanwhile, in Upper Michigan, a Winter Storm Watch is issued for the potential of a winter storm producing snow greater than or equal to 8 inches in 12 hours; or greater than or equal 10 inches in 24 hours.  Like in Lower Michigan, these snowfall amounts may be less in situations where you have mixed precipitation and/or blowing.

Winter Weather Advisory:  This product is issued by the National Weather Service when a low pressure system produces a combination of winter weather (snow, freezing rain, sleet, etc.) that present a hazard, but does not meet warning criteria.  In this case, snowfall does not have to reach Snow Advisory Criteria.  The Snow Advisory criteria can vary 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.

Wire Weight Gage:  A river gage comprised of a weight which is lowered to the water level. The weight is attached to a cable; and as the weight is lowered, a counter indicates the length of cable released. The stage is determined from the length of cable required to reach the water level.

Wrapping Gust Front:  A gust front which wraps around a mesocyclone, cutting off the inflow of warm moist air to the mesocyclone circulation and resulting in an occluded mesocyclone.

WSR-57:  A NWS Weather Surveillance Radar designed in 1957.  It used to be part of weather radar network.  It was replaced by WSR-88D units.

WSR-74:  A NWS Weather Surveillance Radar designed in 1974.  It used to be part of weather radar network.  It was replaced by WSR-88D units.

WSR-88D:  Weather Surveillance Radar - 1988 Doppler; NEXRAD unit.

WSR-88D System:  The summation of all hardware, software, facilities, communications, logistics, staffing, training, operations, and procedures specifically associated with the collection, processing, analysis, dissemination and application of data from the WSR-88D unit.

WSR-88D unit:  Weather Service Radar, commissioned in 1988. Includes the associated displays, product generators, archiving facilities and communications services.

WW:  Severe Thunderstorm Watch or Tornado Watch  

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