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 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 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 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.
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.
The following event-specific warnings are issued for a single weather hazard:
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.
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.
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 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.
|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.|
|Hailstone size||Measurement in||Measurement cm||Updraft Speed mph|| Updraft Speed m/s |
|half dollar||1 1/4||3.2||54||24|
|golf ball||1 3/4||4.4||64||29|
|tennis ball||2 1/2||6.4||77||34|
|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.|
|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 (37 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||12|
The Fujita scale, developed by Dr. Theodore Fujita, assigns a numerical rating from F0 to F5 to rate the damage intensity of tornadoes. 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), which makes it difficult to rate tornadoes that strike in sparsely populated areas, where few man-made structures are found.
The National Weather Service adopted a new Fujita Scale (the Enhanced Fujita Scale) and went into effect on February 1, 2007.
|F number||Wind speed||Damage|
|0||< 73 mph (<118 km/h)||Light damage. Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages sign boards.|
|1||73 to 112 (118 to 181 km/h)||Moderate damage. The lower limit is the beginning of hurricane-force wind speed; peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off roads.|
|2||113 to 157 (182 to 252 km/h)||Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light-object missiles generated; Cars lifted off ground.|
|3||158 to 206 (253 to 331 km/h)||Severe damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown.|
|4||207 to 260 (332 to 418 km/h)||Devastating damage. Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated.|
|5||261 to 318 (419 to 510 km/h)||Incredible damage. Strong frame houses lifted off foundations and carried considerable distance to disintegrate; automobile-sized missiles fly through the air in excess of 100 meters (109 yd); trees debarked; incredible phenomena will occur.|
Crest - The highest level of a flood wave as it passes a point.
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.
Funnel Cloud - A rotating column of water vapor or 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 Effect Snow - Snow that occurs to the lee of the Great Lakes when cold air moves across relatively warm waters of the lakes.
Pneumonia front - A rare, meteorological phenomenon observed on the western Lake Michigan shoreline during the warm season . These fronts are defined as lake-modified synoptic scale cold fronts that result in one-hour temperature drops of 16 degrees F or greater. This extreme change in temperatures can result in a flash severe thunderstorm and/or a microburst.
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.
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 in contact with both a thunder cloud and 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.