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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. 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. NWS may activate Weather Spotter s in areas effected by watches to help them better track and analyze the event. 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. NWS may activate weather spotters in areas effected by warnings to help them better track and analyze the event. 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. NWS may activate weather spotters in areas effected by advisories to help them better track and analyze the event. Table of weather statements as defined by the NWS HAZARDOUS WEATHER DEFINITIONS AS DEFINED BY THE NWS The hazardous weather warnings are divided into three types of hazardous weather/hydrologic events: :# 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. :# 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. :# Other hazards - Weather hazards not directly associated with thunderstorms or winter storms including extreme Heat or Cold , dense Fog , high winds, River Flood ing and lakeshore flooding. Severe local storms Tornado watch - Conditions are favorable for the development of severe thunderstorms and Tornado es 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 Spotter s. These warnings are issued on a county by county basis. Severe thunderstorm watch - Conditions are favorable for the development of severe Thunderstorm s 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 Flood ing 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 Flood ing 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 Flood ing 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 Thunderstorm s or Squall s 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:
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:
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 .'' Beaufort wind scale and other wind terms Beaufort wind scale ''Main article Beaufort Scale .'' Other wind terms ''Main article Wind .'' 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 Tornado es. 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. Saffir-Simpson hurricane category scale See Also: Saffir-Simpson Hurricane Scale 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: :The following terms of duration imply a high probability (80 to 100%) of precipitation occurrence:
Sky condition ''Main articles Sky , Cloud .'' 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. Virga - Precipitation that drys up in the atomosphere and therefore does not reach 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. A waterspout may rotate counter-clockwise or clockwise. 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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