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Definitions of Hazardous Weather Events


The NWS divides hazardous weather conditions 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 flooding and lakeshore flooding.

Severe Local Storms

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 five to eight hours.

Particularly dangerous situation Tornado Watch - Conditions are favorable for the development of severe thunderstorms and destructive tornadoes in and close to the watch area. These watches are occasionally issued, and usually mean that a major tornado outbreak is possible. These watches are usually issued for a larger area by the Storm Prediction Center in Norman, Oklahoma than a regular Tornado Watch, and are usually valid for a longer period of time than a regular Tornado Watch. This type of watch is usually only reserved for forecast "high-end" severe weather events.

  • Tornado Warning - Strong rotation in a thunderstorm is indicated by Doppler weather radar or a tornado is sighted by SKYWARN spotters. These warnings are currently issued on a polygonal basis.
  • Tornado Emergency - Unofficial, high end tornado warning issued when a violent tornado is expected to impact a heavily populated area. Such warnings have been issued for the 1999 F5 Moore, Oklahoma tornado, and the 2007 EF5 Greensburg, Kansas tornado.
  • Severe Thunderstorm Watch - Conditions are favorable for the development of severe thunderstorms in and close to the watch area. A severe thunderstorm contains large damaging hail of 3/4 inch (20 mm) diameter or larger, and/or damaging winds greater than 58 mph (95 km/h or 50 knots) or greater. Isolated tornadoes are possible but not expected to be the dominant severe weather event, hence these watches are very rarely issued. An expected severe wind event (derecho) is the mostly likely reason for a PDS Severe Thunderstorm Watch to be issued, with widespread winds greater than 90 mph (150 km/h or 80 knots) possible. These watches are usually issued for a larger area by the Storm Prediction Center in Norman, Oklahoma than a regular Severe Thunderstorm Watch, and are usually valid for a longer period of time than a regular Severe Thunderstorm Watch. This type of watch is usually only reserved for forecast "high-end" severe weather events.
  • Severe Thunderstorm Warning - A severe thunderstorm is indicated by Doppler weather radar or sighted by SKYWARN spotters. A severe thunderstorm contains large damaging hail of 3/4 inch (20 mm) diameter or larger, and/or damaging winds greater than 58 mph (95 km/h or 50 knots) or greater. These warnings are currently issued on a polygonal basis.
  • Flash Flood Watch - Conditions are favorable for (flash) flooding in and close to the watch area. These watches are issued by the Weather Forecast Office and are usually issued six to twenty-four hours in advance of expected flood potential. 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 Weather Advisory - Hazardous winter weather conditions are occurring, imminent or likely. Conditions will cause a significant inconvenience and if caution is not exercised, may 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 freezing drizzle, sleet, and blowing snow.
  • Winter Storm Watch - Hazardous winter weather conditions including significant accumulations of snow and/or freezing rain and/or sleet are possible generally within the next 36 hours. These watches are issued by the Weather Service Forecast Office.
  • Winter Storm Warning - Hazardous winter weather conditions that pose a threat to life and/or property are occurring, imminent, or highly 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.
  • Blizzard Watch - 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 are possible generally within the next 36 hours.
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 (56 km/h) winds will create sub-zero (below -18°C) wind chills.
  • Heavy Snow Warning - Heavy snowfall amounts are imminent and the criteria for amounts varies significantly over different county warning areas.
  • Lake Effect Snow Warning - Heavy lake-effect snowfall amounts of generally 6 inches (15 cm) in 12 hours or less or 8 inches (20 cm) in 24 hours or less are imminent or highly likely. Lake-effect snow squalls can significantly reduce visibilities with little notice.
  • Ice Storm Warning - Heavy ice accumulations are imminent and the criteria for amounts varies over different county warning areas. Accumulations range from 1/4 to 1/2 inch (6 to 12 mm) or more of freezing rain. In Canada, these are known as Freezing Rain Warnings.
  • Sleet Warning - Heavy sleet accumulations of 2 inches (5 cm) or more in 12 hours or less are imminent.
  • Wind Chill Warning - Extreme wind chills making it feel very cold, criteria varies significantly over different county warning areas.
  • Snow Advisory - Moderate snowfall amounts are imminent and the criteria for amounts varies significantly over different county warning areas.
  • Freezing Rain Advisory - A trace to 1/4 inch (1 - 6 mm) of freezing rain is expected is needed in any county warning area to prompt a freezing rain advisory.
  • Freezing Drizzle Advisory - A trace to 1/4 inch (1 - 6 mm) of freezing drizzle is expected is needed in any county warning area to prompt a freezing rain advisory.
  • 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.
  • 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.
  • Wind Chill Advisory - Dangerous wind chills making it feel cold, criteria varies significantly over different county warning areas.

Other Hazards

  • Special Weather Statement - An advisory issued when a hazard is approching advisory level.
  • Significant weather alert - A strong thunderstorm is indicated by Doppler weather radar, containing small hail below 3/4 inch (20 mm) diameter, and/or strong winds of under 58 mph (95 km/h). These advisories are issued on a county by county basis. These are issued as special weather statements, rather than an official product itself.
  • Urban and Small Stream Flood Advisory - Ponding of water of streets, low-lying areas, highways, underpasses, urban storm drains and elevation of creek and small stream levels is occurring or imminent. Urban and small stream flood advisories are issued for flooding that occurs within 3 hours after the excessive rainfall. These advisories are issued on a county by county basis by the local Weather Forecast Office and are generally in effect for 3 to 4 hours.
  • Excessive Heat Warning - Extreme heat index making it feel very hot, typically above 110°F (43°C) for 3 hours or more during the day and at or above 80°F (27°C) at night. Specific criteria varies over different county warning areas.
  • Heat Advisory - Extreme heat index making it feel hot, typically between 105°F to 110°F (40°C to 43°C) for 3 hours or more during the day and at or above 75°F (24°C) at night. Specific criteria varies over different county warning areas.
  • High Wind Warning - Sustained winds of 40 mph (65 km/h) or greater for a duration of one hour or longer or frequent gusts to 58 mph (93 km/h) or greater.
  • Extreme Wind Warning - Sustained winds of 115 mph (185 km/h) or greater during a land-falling hurricane.
  • 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).
  • Freezing Fog Advisory - Widespread dense fog reducing visibility to less than 1/4 mile (0.4 km) that occurs in a sub-zero environment, leaving a thin glazing of ice.
  • 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 Flood 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 (similar to the sloshing in a bath tub).
  • Red Flag Warning - Highly favorable conditions for wildfires, typically for areas under drought conditions with low humidity and high winds.

Hazardous Weather Risks

The various weather conditions described above have different levels of risk. The NWS uses a multi-tier system of weather statements to notify the public of threatening weather conditions. These statements are used in conjunction with specific weather phenomenea to convey different levels of risk. The below levels are color coded as they would appear on this site. In order of increasing risk, these statements are:

OUTLOOK (also can be titled SHORT TERM FORECAST)
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.

SPECIAL WEATHER STATEMENT
A special weather statement is a form of weather advisory. There is no set criteria for special weather statements. A special weather statement may be issued by the NWS for hazards that have not yet reached warning or advisory status or that do not have a specific code of their own, such as widespread funnel clouds. They are also occasionally used to clear counties from severe weather watches. A common form of special weather statement is a "Significant Weather Advisory."

WEATHER 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. A Winter Storm Watch mean that hazardous winter weather conditions including significant accumulations of snow and/or freezing rain and/or sleet are possible generally within the next 36 hours.

WEATHER 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.

The generic term, Winter Weather Advisory, is used for a combination of two or more of the following events; snow, freezing rain or freezing drizzle, sleet, and/or blowing snow. Moderate snowfall amounts are imminent and the criteria for amounts varies significantly over different county warning areas.

WEATHER 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.

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. Heavy snowfall amounts are imminent and the criteria for amounts varies significantly over different county warning areas.

Hazardous Weather Statements

Hazardous weather forecasts are provided to the public using the NOAA Weather Radio All Hazards system and through news media such as television and radio. Some of the most common NWS hazardous weather statements are described in the following table:
Tornado Watch (WT) 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 (TOR) 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. (Also automatically indicates a Severe Thunderstorm Warning)
Severe Thunderstorm Watch (WS) 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 (SVR) 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. Tornadoes can also and do develop in severe thunderstorms without the issuance of a tornado warning.
Severe Weather Statement (SVS) 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.
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 kn (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 kn (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.

Interesting Weather Facts
? MOST 1 MINUTE RAIN FALL
Unionville, Maryland on 04 July 1956 recorded a 1-minute rainfall of 1.23 inches (31.2 mm).