The NWS divides hazardous weather conditions into three types of hazardous weather/hydrologic events:
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.
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 1 inch (2.7 cm) 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. 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 weather radar or sighted by Skywarn spotters. A severe thunderstorm contains large damaging hail of 1 inch in diameter or larger, and/or damaging winds of 58 mph (93 km/h) 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.
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 1 inch (2.7 cm) diameter or larger, or waterspouts.
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 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.
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.
The following event-specific warnings are issued for a single weather hazard:
Special weather statement - An advisory issued when a hazard is approaching advisory level.
Significant weather alert - A strong thunderstorm is indicated by Doppler weather radar, containing small hail below 1 inch (2.5 cm) diameter, and/or strong winds of 30–58 miles per hour (48–93 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.
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.
Heat Advisory - Extreme heat index making it feel hot, typically between 105 and 110 °F (41 and 43 °C) for up to 3 hours during the day and at or above 80 °F (27 °C) at night for two consecutive nights. Specific criteria varies over different county warning areas.
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 for two consecutive days or above 110 °F (43 °C) at any time. Specific criteria varies over different county warning areas.
Dense Fog Advisory - Widespread dense fog reducing visibility to less than 1/4 mile (400 m).
Freezing Fog Advisory - Widespread dense fog reducing visibility to less than 1/4 mile (400 m) that occurs in a sub-zero environment, leaving a thin glazing of ice.
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 (−2 °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.
Freezing Spray Advisory - Usually issued for shipping interests when conditions are probable for the freezing of sea spray on vessels. A Heavy Freezing Spray Advisory is issued if the amount of ice is expected to accumulate more than 2 centimetres (0.79 in) per hour.
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. 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 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 (emergency management agencies, Skywarn spotters, etc.).
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. NWS may activate weather spotters in areas affected by advisories to help them better track and analyze 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 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. NWS may activate weather spotters in areas affected by warnings to help them better track and analyze the event.
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 (TOA)||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 (SVA)||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 1 inch (27 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 (FFA)||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 (FFW)||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 (TRA)||An announcement for specific coastal areas that tropical storm conditions are possible within 48 hours.|
|Tropical Storm Warning (TRW)||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 36 hours or less.|
|Hurricane Watch (HUA)||An announcement for specific coastal areas that hurricane conditions are possible within 48 hours.|
|Hurricane Warning (HUW)||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 36 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.|
The NWS uses several scales in describing weather events or conditions. Several common scales are described below.
|Hailstone size||Measurement (in)||Measurement (cm)||Updraft Speed (mph)||Updraft Speed (m/s)|
|Beaufort number||Wind speed||Conditions|
|6||25 to 31 mph (40 to 50 km/h)||Large branches in motion; whistling in telephone wires.|
|7||32 to 38 mph (51 to 62 km/h)||Whole trees in motion; inconvenience felt walking against wind.|
|8 - 9||39 to 54 mph (63 to 88 km/h)||Twigs break off trees; wind generally impedes progress.|
|10 - 11||55 to 73 mph (89 to 117 km/h)||Damage to chimneys and TV antennas; pushes over shallow-rooted trees. Severe thunderstorm criteria begins (58 mph (93 km/h)).|
|12 - 13†||74 to 112 mph (118 to 181 km/h)||Peels surfaces off roofs; windows broken; mobile homes overturned; moving cars pushed off road.|
|14 - 15†||113 to 157 mph (182 to 252 km/h)||Roofs torn off houses; cars lifted off ground.|
†:Beaufort levels above 12 are non-standard in the United States. Instead, the Saffir-Simpson Hurricane Scale (Category 1, Category 2, etc.) is used.
|Wind speed||Description||Flags||Beaufort number|
|25 to 38 mph (22 to 33 knots)||Small craft advisory||6 and 7|
|39 to 54 mph (34 to 47 knots)||Gale warning||8 and 9|
|55 to 73 mph (48 to 63 knots)||Storm warning||10 and 11|
|over 73 mph (over 64 knots)||Hurricane warning
or Hurricane Force Wind Warning if not associated with a tropical cyclone
The Enhanced Fujita Scale, an updated version of the original Fujita Scale that was developed by Ted Fujita with Allen Pearson, assigns a numerical rating from EF0 to EF5 to rate the damage intensity of tornadoes. EF0 and EF1 tornadoes are considered "weak" tornadoes, EF2 and EF3 are classified as "strong" tornadoes, where EF4 and EF5 are categorized as "violent" tornadoes. The EF scale is based on tornado damage (primarily to buildings), which makes it difficult to rate tornadoes that strike in sparsely populated areas, where few man-made structures are found. The Enhanced Fujita Scale went into effect on February 1, 2007.
|EF number||Wind speed||Damage|
|0||65–85 mph (105–137 km/h)||Light damage. Peels surface off some roofs; some damage to gutters or siding; branches broken off trees; shallow-rooted trees pushed over.|
|1||86 to 110 (138 to 178 km/h)||Moderate damage. Roofs severely stripped; mobile homes overturned or badly damaged; loss of exterior doors; windows and other glass broken.|
|2||111 to 135 (179 to 218 km/h)||Considerable damage. Roofs torn off well-constructed houses; foundations of frame homes shifted; mobile homes completely destroyed; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.|
|3||136 to 165 (219 to 266 km/h)||Severe damage. Entire stories of well-constructed houses destroyed; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown; structures with weak foundations blown away some distance.|
|4||166 to 200 (267 to 322 km/h)||Devastating damage. Well-constructed houses and whole frame houses completely leveled; cars thrown and small missiles generated.|
|5||>200 (>322 km/h)||Explosive damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 m (300 ft); steel reinforced concrete structure badly damaged; high-rise buildings have significant structural deformation; incredible phenomena will occur.|
|Category||Sustained winds||Storm surge||Central pressure||Potential damage||Example|
|No real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Also, some coastal flooding and minor pier damage.||Bess (1974)
|Some roofing material, door, and window damage. Considerable damage to vegetation, mobile homes, etc. Flooding damages piers and small craft in unprotected anchorages may break their moorings.||Carol (1954)
|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.||Alma (1966)
|More extensive curtainwall failures with some complete roof structure failure on small residences. Major erosion of beach areas. Terrain may be flooded well inland.||"Galveston" (1900)
|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.||"Labor Day" (1935)