Hazardous Weather Definitions

The NWS divides hazardous weather conditions 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 five to eight hours.

Tornado Warning - Strong rotation in a thunderstorm is indicated by Doppler radar or a tornado is sighted by skywarn spotters. These warnings are currently issued on a county by county basis, but will become more localized in 2007 with the development of the polygon warning system.

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. A tornado is possible. These watches are issued for large areas by the Storm Prediction Center in Norman, Oklahoma, and are usually valid for five to eight hours.

Severe Thunderstorm Warning - A severe thunderstorm is indicated by Doppler 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 county by county basis, but will become more localized in 2007 with the development of the polygon warning system.

(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 Storm Watch - Hazardous winter weather conditions including significant accumulations of snow and/or freezing rain and/or sleet are possible generally within 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 winds will create sub-zero 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 in 12 hours or less or 8 inches 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 or more of freezing rain. In Canada, these are known as Freezing Rain Warnings.

• Sleet Warning - Heavy sleet accumulations of 2 inches or more in 12 hours or less are imminent.

Winter Weather Advisory - Hazardous winter weather conditions are occurring, imminent or likely. Conditions will cause a significant inconvenience and if caution is not exercised, and 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, blowing snow.

The following event-specific advisories are issued for these single weather hazards:

• Snow Advisory - Moderate snowfall amounts are imminent and the criteria for amounts varies significantly over different county warning areas. They can range from as little as one inch in Florida to up to half a foot out West.

• Freezing Rain Advisory - A trace to 1/4 inch of freezing rain 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.

Other hazards

Wind Chill Warning - Extreme wind chills making it feel very cold, criteria varies significantly over different county warning areas.

Wind Chill Advisory - Dangerous wind chills making it feel cold, criteria varies significantly over different county warning areas.

Excessive Heat Warning - Extreme heat index making it feel very hot, typically above 110°F for 3 hours or more during the day and at or above 80°F 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 for 3 hours or more during the day and at or above 75°F 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.

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 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.