Definitions of Severe Weather Alerts

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The NWS divides severe weather alerts into a few types of hazardous weather/hydrologic events:

• Severe local storms - Short-fused, small-scale hazardous weather or hydrologic events produced by thunderstorms, including large hail, damaging winds, tornadoes, and flash floods.
• Winter storms - Weather hazards associated with freezing or frozen precipitation (freezing rain, sleet, snow) or combined effects of winter precipitation and strong winds.
• Fire weather - Weather conditions leading to an increased risk of wildfires.
• Flooding - Hazardous hydrologic events resulting in temporary inundation of land areas not normally covered by water, often caused by excessive rainfall.
• Coastal/lakeshore hazards - Hydrological hazards that may affect property, marine or leisure activities in areas near ocean and lake waters including high surf and coastal or lakeshore flooding, as well as rip currents.
• Marine hazards - Hazardous events that may affect marine travel, fishing and shipping interests along large bodies of water, including hazardous seas and freezing spray.
• Other hazards - Weather hazards not directly associated with any of the above including extreme heat or cold, dense fog, high winds, and river or 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 five to eight hours.
  
• Tornado Warning - Strong rotation in a thunderstorm is indicated by Doppler weather radar or a tornado is sighted by Skywarn spotters or other persons, such as local law enforcement. These warnings are currently issued on a polygonal basis.
  
• Tornado Emergency - Sent as a "severe weather statement" or a complete re-issuance of the tornado warning, this is an unofficial, high-end tornado warning issued when a violent tornado is expected to impact a heavily populated area.
• Severe Thunderstorm Watch (yellow box or blue box) - Conditions are favorable for the development of severe thunderstorms in and close to the watch area. A severe thunderstorm contains large damaging hail of 1 inch (25 mm) diameter or larger, and/or damaging winds greater than 58 mph (95 km/h or 50 knots) or greater. Isolated tornadoes are also possible but not expected to be the dominant severe weather event.
• Severe Thunderstorm Warning - A severe thunderstorm is indicated by Doppler weather radar or sighted by Skywarn spotters or other persons, such as local law enforcement. A severe thunderstorm contains large damaging hail of 1 inch (2.5 cm) in diameter or larger, and/or damaging winds of 58 mph (93 km/h) or greater.
• Significant Weather Advisory - A strong thunderstorm is indicated by Doppler weather radar, containing small hail below 1 inch (2.5 cm) diameter, and/or strong winds of 39-57 miles per hour (63-92 km/h). These advisories are usually issued on a county by county basis.
• Flash Flood Watch (green box) - 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 24 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 six 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 two to six hours, although particularly during tropical cyclones a warning may last for a longer period of time, and occasionally last shorter than two hours.

Winter Storms

• Blizzard Warning - Sustained winds or frequent gusts of 35 miles per hour (56 km/h) or greater, considerable falling, and/or blowing snow reducing visibility frequently to 14 mile (0.40 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 miles per hour (56 km/h) winds will create sub-zero (below ?18 C (0 F)) wind chills.
• Blizzard Watch - Sustained winds or frequent gusts of 35 miles per hour (56 km/h) or greater, considerable falling, and/or blowing snow reducing visibility frequently to 14 mile (0.40 km) or less for a period of three hours or more are possible generally within the next 48 hours.
• 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. The National Weather Service has deprecated the Heavy Snow Warning and Sleet Warning products in favor of issuing a Winter Storm Warning for Heavy Snow or a Winter Storm Warning for Heavy Sleet, respectively.
• Winter Storm Watch - Hazardous winter weather conditions including significant accumulations of snow and/or freezing rain and/or sleet are possible generally within 48 hours. These watches are issued by the National Weather Service Forecast Office.
• 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 (which replaced the previously used "travelers advisory"), is used for a combination of two or more of the following events; snow, freezing rain or freezing drizzle, sleet, and blowing snow.

The following event-specific warnings are issued for a single weather hazard:

Freezing Rain/Ice

• Ice Storm Warning - Heavy ice accumulations are imminent and the criteria for amounts vary over different county warning areas. Accumulations range from 14 to 12 inch (6.4 to 12.7 mm) or more of freezing rain. In Canada, these are known as Freezing Rain Warnings.
• Freezing Rain Advisory - Light to moderate ice accumulations are imminent. A trace to 1/4 inch (1-6 mm) of expected freezing rain is needed in any county warning area to prompt a freezing rain advisory.
• Freezing Drizzle Advisory - Drizzle causing light accumulations of ice is imminent or occurring. A trace to 1/4 inch (1-6 mm) of expected freezing drizzle is needed in any county warning area to prompt a freezing rain advisory.
• Freezing Fog Advisory - Widespread dense fog reducing visibility to less than 14 mile (400 m) that occurs in a sub-zero environment, leaving a thin glazing of ice.

Lake-effect Snow

• Lake-Effect Snow Warning - Very 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.
• Lake-Effect Snow Advisory - Heavy lake-effect snowfall amounts of generally 4 inches (10 cm) in 12 hours or less or 6 inches (15 cm) in 24 hours or less are imminent or highly likely. Lake-effect snow squalls can significantly reduce visibilities with little notice.
• Lake-Effect Snow Watch - Very 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 possible generally within 48 hours. Lake-effect snow squalls can significantly reduce visibilities with little notice.

Wind Chill

• Wind Chill Warning - Extreme wind chills that are life-threatening are imminent or occurring; the criteria varies significantly over different county warning areas.
• Wind Chill Advisory - Dangerous wind chills making it feel very cold are imminent or occurring; the criteria varies significantly over different county warning areas.
• Wind Chill Watch - Extreme wind chills that are life-threatening are possible; the criteria varies significantly over different county warning areas.

Deprecated

• Heavy Snow Warning - Heavy snowfall amounts are imminent; the criteria for amounts vary significantly over different county warning areas.
• Sleet Warning - Heavy sleet accumulations of 2 inches (5.1 cm) or more in 12 hours or less are imminent. Usually issued as a winter storm warning for heavy sleet.
• Snow Advisory - Moderate snowfall amounts are imminent; the criteria for amounts vary significantly over different county warning areas.
• Blowing Snow Advisory - Sustained winds or frequent gusts of 25 to 35 miles per hour (40 to 56 km/h) accompanied by falling and blowing snow, occasionally reducing visibility to 14 mile (0.40 km) or less.
• Extreme Cold Watch - Dangerously low temperatures are possible for a prolonged period of time. Frostbite and hypothermia are likely if exposed to these temperatures.
• Extreme Cold Warning - Dangerously low temperatures are expected for a prolonged period of time. Frostbite and hypothermia are likely if exposed to these temperatures.

Fire Weather

• Fire Warning - A fire is currently burning in the area and evacuation is recommended.
• Red Flag Warning - A warning issued when conditions are favorable for the rapid spread of wildfires.
• Fire Weather Watch - Conditions are expected to become favorable for the rapid spread of wildfires.

Flooding

• River Flood Warning - Flooding of streams or rivers is occurring, imminent, or highly likely. These warnings are issued on a county by county basis by the local Weather Forecast Office and are generally in effect for a couple of days or longer.
• Areal 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 usually issued for flooding that occurs more than six hours after the excessive rainfall, or when flooding is imminent/occurring but is not rapid enough to prompt a flash flood warning. These warnings are issued on a polygonal basis by the local Weather Forecast Office and are generally in effect for six to twelve hours.
• River Flood Advisory - Streams or rivers reaching action stage is occurring, imminent, or highly likely. These advisories are issued on a county by county basis by the local Weather Forecast Office and are generally in effect for a couple of days or longer.
• Areal Flood Advisory - Minor general or areal flooding of streets, low-lying areas, urban storm drains, creeks, and small streams is occurring, imminent, or highly likely. These advisories are issued on a polygonal basis by the local Weather Forecast Office and are generally in effect for three to six hours.

Coastal/Lakeshore Hazard

• Coastal Flood Warning - Coastal flooding is occurring or is imminent in the next 12 hours, which poses a serious threat to life and/or property.
• Coastal Flood Watch - Coastal flooding is possible in the next 24 hours, which would pose a serious threat to life and/or property.
• Coastal Flood Advisory - Minor coastal flooding is occurring or is imminent in the next 12 hours, which poses a threat to life and/or property.

Temperature

• Excessive Heat Warning - Extreme Heat Index (HI) values forecast to meet or exceed locally defined warning criteria for at least two days. Specific criteria varies among local Weather Forecast Offices, due to climate variability and the effect of excessive heat on the local population. Typical HI values are maximum daytime temperatures above 105 to 110 F (41 to 43 C) and minimum nighttime temperatures above 75 F (24 C).
  
• Excessive Heat Watch - Conditions are favorable for an excessive heat event to meet or exceed local Excessive Heat Warning criteria in the next 24 to 72 hours.
• Extreme Cold Warning - Forecast shelter temperature of ?50 F (?46 C) or colder and air temperature remains below ?40 F (?40 C) up to the 700 mb (21 inHg) level for three or more consecutive days. Only issued by Weather Forecast Offices in Alaska. Elsewhere, an Extreme Cold Warning can be issued by local Weather Forecast Offices as an experimental product using locally appropriate thresholds, usually with little or no wind.
• Extreme Cold Watch - Conditions are favorable for an extreme cold event to meet or exceed local Extreme Cold Warning criteria. Only issued by Weather Forecast Offices in Alaska.
• Freeze Warning - Widespread sheltered temperatures are forecast to be at or below 32 F (0 C) in the next 12 to 24 hours during the locally defined growing season. A freeze may occur with or without frost.
• Freeze Watch - Conditions are favorable for a freeze event to meet or exceed Freeze Warning criteria in the next 12 to 48 hours during the locally defined growing season.
• Frost Advisory - Minimum sheltered temperatures are forecast to be 33 to 36 F (1 to 2 C) during the locally defined growing season on nights with good radiational cooling conditions (e.g., light winds and clear skies). Widespread frost can be expected
• Hard Freeze Warning - Widespread temperatures at or below 28 F (?2 C) during the growing season. A hard freeze may occur with or without frost.
• Heat Advisory - High Heat Index (HI) values are forecast to meet or exceed locally defined warning criteria for one or two days. Specific criteria vary over different county warning areas, due to climate variability and the effect of excessive heat on the local population. Typical HI values are maximum daytime temperatures above 100 to 105 F (38 to 41 C) and minimum nighttime temperatures above 75 F (24 C).

Other Hazards

• Dense Smoke Advisory - Widespread or localized fog reducing visibilities to 14 mi (0.4 km) or less.
  
• Special Weather Statement - An advisory issued when a hazard is approaching advisory level. In some areas, it is also used in lieu of a significant weather advisory.
  
• Civil Emergency Message - A message to warn the public of an in-progress or imminent significant threat(s) to public safety and/or property.

Hazardous weather Risks

The various weather conditions described above have different levels of risk. The National Weather Service 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. In order of increasing risk, these statements 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 seven 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 (such as emergency management agencies, Skywarn spotters and media outlets).

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 may cause significant inconvenience, and if caution is not exercised could lead to situations that may threaten life or property. The National Weather Service may activate weather spotters in areas affected by advisories to help them better track and analyze the event.

Emergency - An Emergency is issued when an event that by itself cannot pose a threat to life or property, but may indirectly cause other events to happen that may pose a threat to life or property. An example of this would be a power outage, which although not directly posing a hazard, may threaten public safety and critical services. The only existing exceptions to this are the tornado emergency and flash flood emergency, which are issued to get the attention of the public to a major tornado or flash flood.

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. The National Weather Service may activate weather spotters in areas affected 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. The National Weather Service may activate weather spotters in areas affected by warnings to help them better track and analyze the event.

Statement - A statement is either issued as a follow-up message to a warning, watch, or emergency, that may update, extend, or cancel the message it is following up or a notification of significant weather for which no type of advisory, watch, or warning exists.