Severe weather terminology (United States)

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Note: This article is specifically for terminology used by the U.S. National Weather Service. Some terms may be specific to certain cities or regions.


This article describes severe weather terminology used by the U.S. National Weather Service. This article focuses on terminology per se, such as tornado warning. Related weather scales and general weather terms are also addressed in this article. Additional information on specific weather phenomena can be found by following the indicated links to separate articles.


Contents

Watches, warnings and advisories as defined by the NWS

The National Weather Service has developed a multi-tier concept for forecasting all types of hazardous weather. These are:

Outlook - A hazardous weather outlook is issued daily to indicate that a hazardous weather or hydrologic event may occur in the next several days. The outlook will include information about potential severe thunderstorms , heavy rain or flooding, winter weather, extremes of heat or cold, etc., that may develop over the next 7 days with an emphasis on the first 24 hours of the forecast. It is intended to provide information to those who need considerable lead time to prepare for the event.

Watch - A watch is used when the risk of a hazardous weather or hydrologic event has increased significantly, but its occurrence, location or timing is still uncertain. It is intended to provide enough lead time so those who need to set their plans in motion can do so. A watch means that hazardous weather is possible. People should have a plan of action in case a storm threatens and they should listen for later information and possible warnings especially when planning travel or outdoor activities.

Warning - A warning is issued when a hazardous weather or hydrologic event is occurring, imminent or likely. A warning means weather conditions pose a threat to life or property. People in the path of the storm need to take protective action.

Advisory - An advisory is issued when a hazardous weather or hydrologic event is occurring, imminent or likely. Advisories are for less serious conditions than warnings, that cause significant inconvenience and if caution is not exercised, could lead to situations that may threaten life or property.


Table of weather statements as defined by the NWS

Tornado watch Conditions are favorable for the development of severe thunderstorms producing tornadoes in and close to the watch area. Watches are usually in effect for several hours, with 6 hours being the most common. (Also automatically indicates a Severe Thunderstorm Watch)
Tornado warning Tornado is indicated by radar or sighted by storm spotters. The warning will include where the tornado is and what towns will be in its path.
Severe thunderstorm watch Conditions are favorable for the development of severe thunderstorms in and close to the watch area. Watches are usually in effect for several hours, with 6 hours being the most common.
Severe thunderstorm warning Issued when a thunderstorm produces hail 3/4 inch (20 mm) or larger in diameter and/or winds which equal or exceed 58 mph (93 km/h). Severe thunderstorms can result in the loss of life and/or property. Information in this warning includes: where the storm is, what towns will be affected, and the primary threat associated with the storm.
Severe weather statement Issued when the forecaster wants to follow up a warning with important information on the progress of severe weather elements.
Flash flood watch Indicates that flash flooding is possible in and close to the watch area. Those in the affected area are urged to be ready to take quick action if a flash flood warning is issued or flooding is observed.
Flash flood warning Signifies a dangerous situation where rapid flooding of small rivers, streams, creaks, or urban areas are imminent or already occurring. Very heavy rain that falls in a short time period can lead to flash flooding, depending on local terrain, ground cover, degree of urbanization, degree of man-made changes to river banks, and initial ground or river conditions.
Urban and small stream flood advisory Alerts the public to flooding which is generally only an inconvenience and does not pose a threat to life and/or property. Issued when heavy rain will cause flooding of streets and low-lying places in urban areas, or if small rural or urban streams are expected to reach or exceed bankfull.
Flash flood statement Used as a follow-up to Flash Flood Warnings and Watches. The statement will contain the latest information on the event.
Special weather statement Issued to convey update information about severe weather watches. Also can be used to highlight ongoing or imminent weather conditions that deserve special attention but are below warning or advisory criteria.
Short term forecast (NOWCAST) A short term forecast designed to give specific, detailed forecast information for the next 1 to 6 hours on a county-by-county basis. Both routine and near-severe information are contained in these forecasts which are routinely issued several times per day, and more often during busy weather periods.
Local storm report Used to distribute severe weather reports to the media, emergency managers, and other NWS offices. It is issued as reports are received, and may also be issued as a collection of all reports received after an event is over. Delayed reports are disseminated after an event is over as well.
Weather watch redefining statement Issued for every tornado and severe thunderstorm watch that affects a state. It lists the type of watch, its corresponding number, the ending time of the watch, all counties included in the watch, and large cities and towns in the watch area.
Tropical storm watch An announcement for specific coastal areas that tropical storm conditions are possible within 36 hours.
Tropical storm warning A warning that sustained winds within the range of 34 to 63 kt (39 to 73 mph or 63 to 117 km/h) associated with a tropical cyclone are expected in a specified coastal area within 24 hours or less.
Hurricane watch An announcement for specific coastal areas that hurricane conditions are possible within 36 hours.
Hurricane warning A warning that sustained winds 64 kt (74 mph or 118 km/h) or higher associated with a hurricane are expected in a specified coastal area in 24 hours or less. A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continue, even though winds may be less than hurricane force.

Hazardous weather definitions as defined by the NWS

The hazardous weather warnings are divided into three types of hazardous weather/hydrologic events:

  1. Severe local storms - These are short-fused, small scale hazardous weather or hydrologic events produced by thunderstorms, including large hail, damaging winds, tornadoes, and flash floods.
  2. Winter storms - These are weather hazards associated with freezing or frozen precipitation (freezing rain, sleet, snow) or combined effects of winter precipitation and strong winds.
  3. Other hazards - Weather hazards not directly associated with thunderstorms or winter storms including extreme heat or cold, dense fog, high winds, river flooding and lakeshore flooding.


Severe local storms

Tornado watch - Conditions are favorable for the development of severe thunderstorms and tornadoes in and close to the watch area. These watches are issued for large areas by the Storm Prediction Center in Norman, Oklahoma, and are usually valid for four to six hours.

Tornado warning - Strong rotation in a thunderstorm is indicated by Doppler radar or a tornado is sighted by skywarn spotters. These warnings are issued on a county by county basis.

Severe thunderstorm watch - Conditions are favorable for the development of severe thunderstorms in and close to the watch area. These watches are issued for large areas by the Storm Prediction Center in Norman, Oklahoma, and are usually valid for four to six hours.

Severe thunderstorm warning - A severe thunderstorm is indicated by Doppler radar or sighted by skywarn spotters. A severe thunderstorm contains large damaging hail, 3/4 inch (20 mm) diameter or larger, and/or damaging winds of around 60 mph (95 km/h) or greater. These warnings are issued on a county by county basis.

Flood watch - Conditions are favorable for flooding in and close to the watch area. These watches are issued by the Weather Forecast Office and are usually for the first 12 to 24 hours of a forecast. In Canada, a Heavy Rainfall Warning has a similar meaning.

Flash flood warning - Flash flooding is occurring, imminent or highly likely. A flash flood is a flood that occurs within 6 hours of excessive rainfall and that poses a threat to life and/or property. Ice jams and dam failures can also cause flash floods. These warnings are issued on a county by county basis by the local Weather Forecast Office and are generally in effect for up to 6 hours.

Flood warning - General or areal flooding of streets, low-lying areas, urban storm drains, creeks and small streams is occurring, imminent, or highly likely. Flood warnings are issued for flooding that occurs more than 6 hours after the excessive rainfall. These warnings are issued on a county by county basis by the local Weather Forecast Office and are generally in effect for 6 to 12 hours.

Special marine warning - A warning to mariners of hazardous thunderstorms or squalls with wind gusts of 34 knots (39 mph or 63 km/h) or more, hail 3/4 inch (2 cm) diameter or larger, or waterspouts.

Winter storms

Winter storm watch - Conditions are favorable for hazardous winter weather conditions including heavy snow, blizzard conditions, or significant accumulations of freezing rain or sleet. These watches are issued by the Weather Service Forecast Office and are usually issued 12 to 36 hours in advance of the event.

Winter storm warning - Hazardous winter weather conditions that pose a threat to life and/or property are occurring, imminent or likely. The generic term, winter storm warning, is used for a combination of two or more of the following winter weather events; heavy snow, freezing rain, sleet and strong winds. The following event-specific warnings are issued for a single weather hazard:

  • Blizzard warning - Sustained winds or frequent gusts of 35 mph (56 km/h) or greater, considerable falling and/or blowing snow reducing visibility frequently to 1/4 mile (0.4 km) or less for a period of three hours or more. There are no temperature criteria in the definition of a blizzard but freezing temperatures and 35 mph winds will create sub-zero wind chills.
  • Heavy snow warning - Snowfall of 6 inches (15 cm) or more in 12 hours or less, or 8 inches (20 cm) or more in 24 hours or less.
  • Lake effect snow warning - Lake effect snowfall of 6 inches (15 cm) or more in 12 hours or less, or 8 inches (20 cm) or more in 24 hours or less. In Canada, the term Snowsquall Warning is has a similar meaning.
  • Ice Storm warning - Accumulations of 1/4 inch (6 mm) or more of freezing rain. In Canada, these are known as Freezing Rain Warnings.
  • Heavy sleet warning - Accumulations of 1/2 inch (12 mm) or more of sleet.

Winter Weather Advisory - Hazardous winter weather conditions are occurring, imminent or likely. Conditions will cause a significant inconvenience and if caution is not exercised, will result in a potential threat to life and/or property. The generic term, winter weather advisory, is used for a combination of two or more of the following events; snow, freezing rain or drizzle, sleet, blowing snow. The following event-specific advisories are issued for these single weather hazards:

  • Snow advisory - Snowfall of 3 to 5 inches (8 to 14 cm).
  • Lake effect snow advisory - Lake effect snowfall of 3 to 5 inches (8 to 14 cm).
  • Freezing rain advisory - Accumulations of less than 1/4 inch (6 mm) of freezing rain or freezing drizzle.
  • Sleet advisory - For accumulations of less than 1/2 inch (12 mm) of sleet.
  • Snow and blowing snow advisory - Sustained winds or frequent gusts of 25 to 35 mph (40 to 56 km/h) accompanied by falling and blowing snow, occasionally reducing visibility to 1/4 mile (0.4 km) or less.

Other hazards

Wind chill warning - Extreme wind chills making it feel like -30 °F (-34 °C) or colder. (Criteria may vary by region)

Wind chill advisory - Dangerous wind chills making it feel like -20 °F (-28 °C) to -30 °F (-34 °C). (Criteria may vary by region)

Excessive heat warning - Extreme heat index making it feel like above 110 °F (43 °C) with a minimum above 80 °F (27 °C). (Criteria may vary by region)

Heat advisory - Maximum heat index of 105 to 110 °F (40 to 43 °C) with a minimum of 75 °F (24 °C) for two or more consecutive days. Criteria may vary by region.

High wind warning - Sustained winds of 40 mph (65 km/h) or greater for a duration of one hour or longer or gusts to 58 mph (93 km/h) or greater.

Wind advisory - Sustained winds of 30 mph (48 km/h) or greater or gusts to 45 mph (72 km/h) or greater for a duration of one hour or longer.

Dense fog advisory - Widespread dense fog reducing visibility to less than 1/4 mile (0.4 km).

Flood warning (river flood)- A warning for specific communities or areas along a river where flooding is imminent or occurring. Flood warnings normally give specific crest forecasts.

Freeze warning - Widespread temperatures at or below 32 °F (0 °C) during the growing season. A freeze may occur with or without frost. A hard freeze occurs with temperatures below 28 °F (-3 °C).

Frost advisory - Widespread frost during the growing season. Frost generally occurs with fair skies and light winds.

Lakeshore warning - Lakeshore flooding that is occurring or is imminent in the next 12 hours, which poses a serious threat to life and/or property. A seiche warning is issued for rapid and large fluctuations in water level in Lake Michigan usually caused by a strong line of thunderstorms moving rapidly southeast across the lake.

Blowing Snow Advisory - Widespread blowing snow with winds 25 to 35 mph (40 to 56 km/h), occasionally reducing visibility to 1/4 mile (400 m) or less.

Related weather scales as defined by the NWS

Hail diameter sizes

Main article Hail.

1/4 in (6 mm) Pea size
1/2 in (13 mm) Small marble size
3/4 in (20 mm) (Severe Criteria) Dime/Penny/Large Marble size
7/8 in (22 mm) US 5 cent coin size
1 in (25 mm) US 25 cent coin size
1 1/4 in (32 mm) US 50 cent coin size
1 1/2 in (37 mm) Walnut or ping pong ball size
1 3/4 in (43 mm) Golf ball size
2 in (50 m) Hen Egg size
2 1/2 in (62 mm) Tennis ball size
2 3/4 in (70 mm) Baseball size
3 in (75 mm) Teacup size
4 in (100 mm) Grapefruit size
4 1/2 in (115 mm) Softball size

Beaufort wind scale and other wind terms

Beaufort wind scale

Main article Beaufort scale.

25 to 31 mph (40 to 50 km/h) Large branches in motion; whistling in telephone wires.
32 to 38 mph (51 to 62 km/h) Whole trees in motion; inconvenience felt walking against wind.
39 to 54 mph (63 to 88 km/h) Twigs break off trees; wind generally impedes progress.
55 to 73 mph (89 to 117 km/h) Damage to chimneys and TV antennas; pushes over shallow-rooted trees.
74 to 112 mph (118 to 181 km/h) Peels surfaces off roofs; windows broken; mobile homes overturned; moving cars pushed off road.
113 to 157 mph (182 to 252 km/h) Roofs torn off houses; cars lifted off ground.


Other wind terms

Main article wind.

Sustained Wind Speed Descriptive Term
0 to 5 mph (0 to 8 km/h) light or light and variable
5 to 15 mph (8 to 24 km/h) none used
10 to 20 mph (16 to 32 km/h) none used
15 to 25 mph (24 to 40 km/h) breezy (mild weather) or brisk,

blustery (cold weather)

20 to 30 mph (32 to 48 km/h) or 25 to 35 mph (40 to 56 km/h) windy
30 to 40 mph (48 to 64 km/h) or 35 to 45 mph (56 to 72 km/h) very windy
40 to 73 mph (64 to 117 km/h) high, strong, damaging or

dangerous winds

Fujita tornado intensity scale

Main article Fujita scale.

The Fujita scale, developed by Dr. Theodore Fujita, assigns a numerical rating from F0 to F5 to rate the intensity of tornadoes. F0 and F1 tornadoes are considered "weak" tornadoes, F2 and F3 are classified as "strong" tornadoes, where F4 and F5 are categorized as "violent" tornadoes. The F scale is based on tornado damage (primarily to buildings), so there is some ambiguity in the scale, but nonetheless, it provides a good baseline for classifying tornadoes according to their intensity.

F number Wind speed Damage
0 < 73 mph (<118 km/h) Light damage. Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages sign boards.
1 73 to 112 (118 to 181 km/h) Moderate damage. The lower limit is the beginning of hurricane-force wind speed; peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off roads.
2 113 to 157 (182 to 252 km/h) Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light-object missiles generated; Cars lifted off ground.
3 158 to 206 (253 to 331 km/h) Severe damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown.
4 207 to 260 (332 to 418 km/h) Devastating damage. Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated.
5 261 to 318 (419 to 510 km/h) Incredible damage. Strong frame houses lifted off foundations and carried considerable distance to disintegrate; automobile-sized missiles fly through the air in excess of 100 meters (109 yds); trees debarked; incredible phenomena will occur.
6 319 to 379 (511 to 609 km/h) Inconceivable damage. No F6 tornadoes have actually been verified, however some unofficial reports have shown that at least one tornado (a 1999 tornado in Moore, Oklahoma) was indeed briefly at this strength. It is suspected that an F6 or greater tornado would be indistinguishable from F5 because significant damage would be done by the sides before the F6 center met objects, and that only a narrow cone would exceed F5, and likely for a limited time.

Saffir-Simpson hurricane category scale

Main article Saffir-Simpson Hurricane Scale.

Category 1 Sustained winds 33–42 m/s 74–95 mph 64–82 kt 119–153 km/h
Storm surge 4–5 ft 1.2–1.5 m
Central pressure 28.94 inHg 980 mbar
Potential damage No real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Also, some coastal flooding and minor pier damage.
Example storms Hurricane AgnesHurricane DannyHurricane GastonHurricane Ophelia
Category 2 Sustained winds 43–49 m/s 96–110 mph 83–95 kt 154–177 km/h
Storm surge 6–8 ft 1.8–2.4 m
Central Pressure 28.50–28.91 inHg 965–979 mbar
Potential damage Some roofing material, door, and window damage. Considerable damage to vegetation, mobile homes, etc. Flooding damages piers and small craft in unprotected moorings may break their moorings.
Example storms Hurricane BobHurricane BonnieHurricane FrancesHurricane Juan
Category 3 Sustained winds 50–58 m/s 111–130 mph 96–113 kt 178–209 km/h
Storm surge 9–12 ft 2.7–3.7 m
Central pressure 27.91–28.47 inHg 945–964 mbar
Potential damage Some structural damage to small residences and utility buildings, with a minor amount of curtainwall failures. Mobile homes are destroyed. Flooding near the coast destroys smaller structures with larger structures damaged by floating debris. Terrain may be flooded well inland.
Example storms Great New England Hurricane of 1938Hurricane FranHurricane IsidoreHurricane Jeanne
Category 4 Sustained winds 59–69 m/s 131–155 mph 114–135 kt 210–249 km/h
Storm surge 13–18 ft 4.0–5.5 m
Central pressure 27.17–27.88 inHg 920–944 mbar
Potential damage More extensive curtainwall failures with some complete roof structure failure on small residences. Major erosion of beach areas. Terrain may be flooded well inland.
Example storms Galveston Hurricane of 1900Hurricane CharleyHurricane HugoHurricane Iris
Category 5 Sustained winds ≥70 m/s ≥156 mph ≥136 kt ≥250 km/h
Storm surge ≥19 ft ≥5.5 m
Central pressure <27.17 inHg <920 mbar
Potential damage Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. Flooding causes major damage to lower floors of all structures near the shoreline. Massive evacuation of residential areas may be required.
Example storms Hurricane AndrewHurricane CamilleHurricane GilbertLabor Day Hurricane of 1935

Precipitation probabilities

Main article precipitation.

Technically, the probability of precipitation (PoP) is defined as the likelihood of occurrence (expressed as a percent) of a measurable amount (.01 inch/0.2 mm or more) of liquid precipitation (or the water equivalent of frozen precipitation) during a specified period of time at any given point in the forecast area. Forecasts are normally issued for 12 hour time periods. Descriptive terms for uncertainty and areal coverage are used as follows:

PoP Expressions of Uncertainty Areal Coverage
0% none used none used
10% none used isolated
20% slight chance isolated
30 to 50% chance scattered
60 to 70% likely numerous
80 to 100% none used none used


The following terms of duration imply a high probability (80 to 100%) of precipitation occurrence:
  • brief
  • periods of
  • occasional
  • intermittent
  • frequent.


Sky condition

Main articles sky, cloud.

Term Predominant or Average Sky Condition
Cloudy 95 to 100% opaque cloud cover
Mostly Cloudy or Considerable Cloudiness 70 to 95% opaque cloud cover
Partly Cloudy or Partly Sunny 30 to 70% opaque cloud cover
Mostly Clear or Mostly Sunny 5 to 30% opaque cloud cover
Clear or Sunny 0 to 5% opaque cloud cover
Fair Less than 40% opaque cloud cover, no precipitation and

no extremes of temperature, visibility, or wind.

General weather terms as defined by the NWS

Main articles weather, meteorology.


Cold air funnel - A funnel cloud or (rarely) a small relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold. They are much less violent than other types of tornadoes.

Crest - The highest level of a flood wave as it passes a point.

Degree Day - Gauges the amount of heating or cooling needed for a building using 65 °F as a baseline. To compute degree days, the average temperature for a day is taken and referenced to 65 °F. An average temperature of 50 °F yields 15 heating degree days, while an average temperature of 75 °F would yield 10 cooling degree days.

Dew Point - A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming pressure and moisture content are constant).

Doppler Radar - Radar that can measure radial velocity, the instantaneous component of motion parallel to the radar beam (i.e., toward or away from the radar).

Downburst - A strong downdraft from a thunderstorm resulting in an outward burst of damaging winds on or near the ground. Downburst winds are often 50 to 100 mph (80 to 160 km/h) and in a few cases, 100 to 150 mph (160 to 240 km/h). They can do as much damage as a small tornado.

Microburst - a small downburst affecting an area less than 2 1/2 miles (4 km) in diameter with peak winds lasting generally less than five minutes.

Macroburst - a large downburst affecting an area greater than 2 1/2 miles (4 km) in diameter with peak winds generally lasting five minutes or longer.

Flood Stage - The level or stage at which a stream overflows its banks or the stage at which the overflow of a stream begins to cause damage.

Freezing Rain/Freezing Drizzle - Rain or drizzle which falls in liquid form and freezes on impact with cold surfaces to form a glaze on the ground and exposed objects.

Funnel Cloud - A rotating column of air, extending from a towering cumulus or cumulonimbus cloud, that is not in contact with the ground.

Gustnado - A gust front tornado. A small, weak, short-lived tornado that occurs along a gust front - the leading edge of a thunderstorm, caused by rain-cooled air flowing out from the storm's downdraft. A gustnado is often visible as a debris cloud or dust whirl.

Hail - Precipitation in the form of lumps of ice that form during some thunderstorms.

Heat Index - The apparent temperature that describes the combined effect of high temperatures and high levels of humidity, which reduces the body's ability to cool itself.

Lake Breeze - A local wind blowing from the cooler surface of a large lake to warmer inland areas, usually during the afternoon in spring and summer.

Lake Effect Snow - Snow that occurs to the lee of the Great Lakes when cold air moves across relatively warm waters of the lakes.

Rain/Showers - Rain is a nearly steady and uniform fall of precipitation over an area. Showers are intermittent and/or scattered convective rainfall of varying intensity.

Seiche - An oscillation of the surface of one of the Great Lakes usually caused by a squall line moving rapidly south-southeast down the lake. Rapid changes in the lake level pose a serious threat to marinas and people on piers and breakwaters. Similar to a storm surge.

Relative Humidity - The ratio of the amount of water vapor actually present in the air to the greatest amount possible at the same temperature.

Sleet - Sleet or ice pellets are solid grains of ice formed from the freezing of rain or the refreezing of melted snow, which bounce off the ground and other objects.

Squall line - A line of thunderstorms or squalls which may extend over several hundred miles.

Tornado - A violently rotating column of air, from a thunderstorm, in contact with the ground.

Waterspout - In general, a tornado occurring over water. Specifically, it refers to a small, relatively weak rotating column of air over water beneath a towering cumulus or cumulonimbus cloud.

Wind Chill - An apparent temperature that describes the combined effect of wind and low temperature on exposed skin.

See also

References

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