Severe weather terminology (United States) Travel

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This article describes severe weather terminology used by the National Weather Service (NWS) in the United States. The NWS, a government agency operating as an arm of the National Oceanic and Atmospheric Administration (NOAA) branch of the United States Department of Commerce (DoC), defines precise meanings for nearly all of its weather terms. This article describes NWS terminology and related weather scales used by the agency. Some terms may be specific to certain cities or regions.

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Definitions of severe weather alerts

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

Severe local storms

• Tornado Watch (yellow box in some select NWS documentation, red box in most other media) - 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.
• Severe Thunderstorm Watch (pink box or blue box in NWS documents, typically yellow box in other media) - 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. 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 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. These warnings are currently issued on a polygonal basis.
• 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, and are issued as special weather statements written in the style of severe thunderstorm and other short-fused warnings, rather than being an official product itself. Some areas use an entirely different format (most notably WFOs in the Northeast and Mid-Atlantic), noting where in each county the thunderstorm will affect.
• 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.
• 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 (2.5 cm) diameter or larger, or waterspouts.

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 ¹/₄ 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 of at least 0 °C (32 °F) and 35 miles per hour (56 km/h) winds will create wind chills of at least -8.5 °C (16.7 °F).
• 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 ¹/₄ 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 National Weather Service has deprecated the Snow Advisory and Blowing Snow Advisory products in favor of issuing a Winter Weather Advisory for Snow or a Winter Weather Advisory for Snow and Blowing Snow, respectively.

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

Freezing rain/ice

Lake-effect snow

Windchill

Deprecated

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.
• Urban and Small Stream Flood Advisory - Another form of the areal flood advisory, where ponding of water on 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 three hours after the excessive rainfall. These advisories are issued on a polygonal basis by the local Weather Forecast Office and are generally in effect for three to four hours (however, the NWS website does not the advisory as an area-specific polygon, instead indicating the individual counties/parishes/boroughs affected).

Coastal/lakeshore hazards

• 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.
• Storm Surge Warning (proposed) - Localized heavy flooding due to storm surge caused by a tropical cyclone is occurring or is imminent in the next 12 hours, which poses a threat to life and/or property.
• Lakeshore Flood Warning - Lakeshore flooding is occurring or is imminent in the next 12 hours, which poses a serious threat to life and/or property.
• Lakeshore Flood Watch - Lakeshore flooding is possible in the next 24 hours, which would pose a serious threat to life and/or property.
• Lakeshore Flood Advisory - Minor lakeshore flooding is occurring or is imminent in the next 12 hours, which poses a threat to life and/or property.
• Lake Wind Warning
• High Surf Warning - Destructive, pounding surf poses a danger to those in and near the water and may damage property near the shoreline.
• High Surf Advisory - Pounding surf poses a danger to those in the water.
• Rip Current Statement - Describes a risk of rip currents present in the specified area (may be issued as a beach hazards statement).
• Beach Hazards Statement - Issued for rip currents, chemical hazards, or biological hazards in lake or ocean waters.

Marine hazards

• Heavy Freezing Spray Warning - Usually issued for shipping interests when conditions are favorable for the rapid freezing of sea spray on vessels at a rate of more than 2 centimetres (0.79 in) per hour.
• Freezing Spray Advisory - Usually issued for shipping interests when conditions are probable for the freezing of sea spray on vessels.
• Hazardous Seas Warning - Issued when rough surf is expected, but strong winds are not.
• Hazardous Seas Watch - Issued when rough surf is possible, but strong winds are not expected.
• Low Water Advisory - Issued when critically low water levels present a navigation hazard.
• Marine Weather Statement - The equivalent of a special weather statement at sea, indicating potentially hazardous marine conditions.

Temperature

See also Windchill section above.

• 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).

Aviation

The following advisories are issued by the National Weather Service Aviation Weather Center (outside of Alaska) or Alaska Aviation Weather Unit. Atmospheric ash plume advisories/warnings are also issued by the United States Geological Survey (Aviation Color Codes).

• Center Weather Advisory - Advisories issued when conditions just below severe (SIGMET) criteria. CWAs are issued for thunderstorms, turbulence, icing, and ceiling and visibility limits (IFR).
• SIGMET (Significant Meteorological Information) - Advises of weather that is potentially hazardous to all aircraft and is affecting or is forecast to affect at least 3,000 square miles (8,000 km²).
  • Convective SIGMET - A convective SIGMET implies severe or greater turbulence, severe icing, and low level wind shear. Issued for severe surface weather (including surface winds greater than or equal to 50 knots (93 km/h; 58 mph), hail at the surface greater than or equal to ³/₄ inch (19 mm) in diameter, or tornadoes); embedded thunderstorms; line of thunderstorms; thunderstorms greater than or equal to VIP level 4 affecting 40% or more of an area at least 3,000 square miles (7,800 km²).
  • Non-Convective SIGMET - These SIGMETs may be issued for: severe icing; severe or extreme turbulence; dust storms and/or sand storms lowering visibilities to less than 3 miles (5 km); or volcanic ash. SIGMET advisories are issued for six hours during hurricanes and four hours for other weather-related events.
• Volcanic Ash Advisories - Advisory issued for all ash plumes detected by satellite imagery, including the location of the volcano, location/description of ash plume, forecast (at 6, 12 and 18 hours), and a graphic of the ash plume location/forecast. VAAs are issued by Volcanic Ash Advisory Centers in Anchorage (Alaska) and Washington (Contiguous United States, Caribbean, Central America, most of the North Pacific, and South America north of 10°S).

VAAs are standardized worldwide by the International Civil Aviation Organization.

Other hazards

• Air Stagnation Advisory - Atmospheric conditions stable enough to cause air pollutants to accumulate in a given area. Criteria developed in conjunction with the local or state EPA and the product issued at their request.
• Ashfall Advisory - Airborne ash plume resulting in ongoing deposition at the surface. Ashfall may originate directly from a volcanic eruption or from the re-suspension (by wind) of a significant amount of relic ash.
• Blowing Dust Advisory - Strong winds and considerable blowing sand or dust reducing visibilities.
• Dense Fog Advisory - Widespread or localized fog reducing visibilities to ¹/₄ mi (0.4 km) or less.
• Dense Smoke Advisory - Widespread or localized smoke reducing visibilities to ¹/₄ mi (0.4 km) or less.
• Dust Storm Warning - Widespread or localized blowing dust reducing visibilities to ¹/₄ mi (0.4 km) or less. Sustained winds of 25 miles per hour (40 km/h) or greater are usually required.
• 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.

Travel Weather Alerts Video

Wind and tropical cyclones

Wind alerting is classified into groups of two beaufort numbers, beginning at 6-7 for the lowest class of wind advisories. The last group includes three beaufort numbers, 14-16. The actual alerts can be categorized into three classes: maritime wind warnings, land wind warnings, and tropical cyclone warnings. Advisory-force and gale-force winds will not trigger a separate wind advisory or warning if a Blizzard warning is already in effect. However, as seen with Hurricane Sandy, if widespread high wind warnings are in effect prior to the issuance of a blizzard warning, the high wind warnings may be continued.

Wind alert terms and signals

* Tropical Storm Warning flags and lights will always be displayed the same as Storm Warning flags and lights.
+ A tropical storm with winds in this range is sometimes referred to as a "severe tropical storm".
? The Extreme Wind Warning is issued shortly before the eyewall makes landfall

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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.

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Media distribution

Hazardous weather forecasts and alerts are provided to the public using the NOAA Weather Radio All Hazards system and through news media such as television, radio and internet sources. Many local television stations have overlay graphics which will either show a map or a list of the affected areas. The most common NWS weather alerts to be broadcast over NOAA Weather Radio using SAME technology are described in the following table:

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Related weather scales as defined by the NWS

The NWS uses several scales in describing weather events or conditions. Several common scales are described below.

Hail diameter sizes

The size of individual hailstones that reach surface level is determined by speed of the updraft which create the individual ice crystals at atmospheric levels. Larger hailstones are capable of producing damage to property, and particularly with very large hailstones, resulting in serious injury or death due to blunt-force trauma induced by the impact of the hailstones. Hailstone size is typically correspondent to the size of an object for comparative purposes.

* Begins hail sizes within the severe hail criterion.
+ Begins hail sizes within the Storm Prediction Center's significant severe criterion.

Beaufort wind scale

The Beaufort scale is an empirical measure that correlates wind speed to observed conditions at sea or on land.

⁺:Beaufort levels above 12 are non-standard in the United States. Instead, the Saffir-Simpson Hurricane Scale (Category 1, Category 2, etc.) is used.

Enhanced Fujita tornado intensity scale

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, with winds of at least major hurricane force, where EF4 and EF5 are categorized as "violent" tornadoes, with winds corresponding to category 5 hurricane winds and rising to match or exceed the strongest tropical cyclones on record. 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.

Saffir-Simpson hurricane category scale

The Saffir-Simpson hurricane wind scale, assigns a numerical classification of hurricanes into five categories distinguished by the intensities of their sustained winds. The scale spans from Category 1 (winds of at least 74 miles per hour (119 km/h)) to Category 5 (exceeding 156 miles per hour (251 km/h)). Unlike the Enhanced Fujita Scale, which assigns ratings for tornadoes after damage has been incurred and thoroughly assessed, categories on the Saffir-Simpson scale are assigned to most active cyclones that reach the minimum hurricane threshold, even before landfall.

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