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Severe thunderstorm watch
A severe thunderstorm watch (SAME code: SVA) is a statement issued by weather forecasting agencies to advise the public that atmospheric conditions in a given region may lead to the development of severe thunderstorms within (or near) the region over a period of several hours. The criteria for issuing a watch varies by country, and may also include torrential rainfall and tornadoes. A watch may also be issued several hours ahead of the arrival of a mature and organized complex of storms (such as a mesoscale convective system), or more clustered or discrete storm activity (of the single-cell, multicell and/or supercell varieties).
A severe thunderstorm watch, like a tornado watch, is not to be confused with a warning. A watch encourages the public to remain vigilant—to be on the watch, so to speak—for the later onset of severe weather. An area under a watch may even experience deceptively fair weather with few clouds before thunderstorms develop.
A severe thunderstorm watch indicates that atmospheric conditions observed in and close to the watch area have created a significant risk for the development and intensification of convective thunderstorms that could exceed regional severe criterion, and are normally issued in advance of the onset of severe weather. (The criteria for thunderstorms to be classified as severe varies by country; e.g., in the United States, severe thunderstorms must produce winds exceeding 58 mph [93 km/h], and/or hailstones larger than one inch [2.5 cm] in diameter, and/or one or more tornadoes.) Generally (but not always), thunderstorms that develop within the watch area may contain large hailstones, intense straight-line winds, intense lightning, torrential rainfall and/or flash flooding caused by high rainfall accumulations. Depending on storm cell intensity, severe thunderstorms can cause damage to structures or vehicles; impairment of vehicle and pedestrian travel; flooding to streets, populated neighborhoods, farmlands and other areas of poor drainage; and in extreme cases, injury or possible fatality to people and animals exposed outdoors (from either repeated hail-induced blunt trauma, wind-generated debris or intense flooding).
If severe weather actually does occur, a severe thunderstorm warning or tornado warning would then be issued. Residents and travelers in the watch area are advised to immediately undertake safety preparations ahead of the arrival of severe weather. A watch is not required for a thunderstorm- or other hydrological-based weather warning to be issued; severe thunderstorm warnings are often issued when a severe thunderstorm watch is not active (i.e., when a tornado watch is active or, less frequently, if severe convective storms are not expected to be of broad enough coverage to require a watch).
While a severe thunderstorm watch does not nominally imply the probability for tornadoes, if modest wind shear and storm-level helicity is conducive for a marginal tornado threat within the convective environment, storm cells that develop within the proximate severe thunderstorm watch area can occasionally exhibit mesocyclonic rotation at the cloud base and may spawn tornadoes if advanced tornadogenesis occurs. An existing severe thunderstorm watch, or merely a portion of it, can also be upgraded to a tornado watch, if conditions that were originally considered marginally conducive if at all for tornadic development have evolved to permit a greater risk of tornado formation; conversely, a section or the entirety of an existing tornado watch area can be replaced by a severe thunderstorm watch, if conditions are no longer considered conducive for tornadogenesis. Straight-line winds in severe thunderstorms created by strong downdrafts, however, can produce serious structural damage as severe as a lower-category tornado or hurricane over a path comparatively wider than a tornado.
When a severe thunderstorm watch is issued, people within the region of expected severe weather risk are advised to monitor conditions ahead of the developing weather situation; review thunderstorm safety precautions in the event they must seek immediate shelter; and use local broadcast media, weather radio, weather app alert notifications and/or SMS notifications to receive warnings and updated storm information.
In the United States, severe thunderstorm watches are issued by the Storm Prediction Center (SPC), a national guidance center of the National Weather Service (NWS) branch of the National Oceanic and Atmospheric Administration (NOAA), for areas of the lower 48 states where atmospheric conditions favor the development of convective thunderstorm activity reaching severe criteria. Although seldom issued in these states, responsibilities for issuing severe thunderstorm watches covering Alaska and Hawaii are respectively handled by local NWS forecast offices in Fairbanks, Anchorage and Juneau, Alaska, and Honolulu, Hawaii. Watches are typically valid for six to nine hours (extending if necessary as long as 12 hours during unusually steady-state or slow-moving severe weather events) after the time of issuance, and are intended to precede the first report of severe hail or wind by 45 minutes to one hour. SPC watch boxes—termed because the approximate watch area is represented in weather maps as a quadrilateral for aviation purposes—are usually outlined in the approximate delineation of x miles north and south, or east and west, or either side of a line (perpendicular to the center line) from y miles direction of city, state, to z miles another direction of another city, state (e.g., "50 miles either side of a line from 10 miles northeast of Columbia, South Carolina to 15 miles south-southwest of Montgomery, Alabama"). Geographic coverage of severe thunderstorm watches (which ranges from 20,000–40,000 square miles [52,000–104,000 km2] on average, encompassing portions of one or more states) vary based on the size of the land area under threat, the duration of severe weather risk, and the forward motion of the parent storm system and associated surface boundaries.
In situations where atmospheric conditions will support the production of very destructive straight-line winds and hail from the convective activity, the intensified wording "particularly dangerous situation" (PDS) can be added into the watch product. PDS severe thunderstorm watches usually suggest conditions over the approximate area present a widespread threat of destructive hail greater than two inches (5.1 cm) in diameter and winds greater than 75 mph (121 km/h) being generated from the thunderstorm convection, or that downstream conditions favor the development and intensification of a bow echo or derecho moving at 55 mph (89 km/h) or faster. The inclusion of PDS wording (more commonly used for tornado watches in areas where the environment supports the development of strong to violent tornadoes) is rare for severe thunderstorm watches since the tornado threat, in comparison to the much higher threat of extreme wind or hail, must remain low enough to where a standard (non-PDS) tornado watch is not warranted.
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Severe thunderstorm watch
A severe thunderstorm watch (SAME code: SVA) is a statement issued by weather forecasting agencies to advise the public that atmospheric conditions in a given region may lead to the development of severe thunderstorms within (or near) the region over a period of several hours. The criteria for issuing a watch varies by country, and may also include torrential rainfall and tornadoes. A watch may also be issued several hours ahead of the arrival of a mature and organized complex of storms (such as a mesoscale convective system), or more clustered or discrete storm activity (of the single-cell, multicell and/or supercell varieties).
A severe thunderstorm watch, like a tornado watch, is not to be confused with a warning. A watch encourages the public to remain vigilant—to be on the watch, so to speak—for the later onset of severe weather. An area under a watch may even experience deceptively fair weather with few clouds before thunderstorms develop.
A severe thunderstorm watch indicates that atmospheric conditions observed in and close to the watch area have created a significant risk for the development and intensification of convective thunderstorms that could exceed regional severe criterion, and are normally issued in advance of the onset of severe weather. (The criteria for thunderstorms to be classified as severe varies by country; e.g., in the United States, severe thunderstorms must produce winds exceeding 58 mph [93 km/h], and/or hailstones larger than one inch [2.5 cm] in diameter, and/or one or more tornadoes.) Generally (but not always), thunderstorms that develop within the watch area may contain large hailstones, intense straight-line winds, intense lightning, torrential rainfall and/or flash flooding caused by high rainfall accumulations. Depending on storm cell intensity, severe thunderstorms can cause damage to structures or vehicles; impairment of vehicle and pedestrian travel; flooding to streets, populated neighborhoods, farmlands and other areas of poor drainage; and in extreme cases, injury or possible fatality to people and animals exposed outdoors (from either repeated hail-induced blunt trauma, wind-generated debris or intense flooding).
If severe weather actually does occur, a severe thunderstorm warning or tornado warning would then be issued. Residents and travelers in the watch area are advised to immediately undertake safety preparations ahead of the arrival of severe weather. A watch is not required for a thunderstorm- or other hydrological-based weather warning to be issued; severe thunderstorm warnings are often issued when a severe thunderstorm watch is not active (i.e., when a tornado watch is active or, less frequently, if severe convective storms are not expected to be of broad enough coverage to require a watch).
While a severe thunderstorm watch does not nominally imply the probability for tornadoes, if modest wind shear and storm-level helicity is conducive for a marginal tornado threat within the convective environment, storm cells that develop within the proximate severe thunderstorm watch area can occasionally exhibit mesocyclonic rotation at the cloud base and may spawn tornadoes if advanced tornadogenesis occurs. An existing severe thunderstorm watch, or merely a portion of it, can also be upgraded to a tornado watch, if conditions that were originally considered marginally conducive if at all for tornadic development have evolved to permit a greater risk of tornado formation; conversely, a section or the entirety of an existing tornado watch area can be replaced by a severe thunderstorm watch, if conditions are no longer considered conducive for tornadogenesis. Straight-line winds in severe thunderstorms created by strong downdrafts, however, can produce serious structural damage as severe as a lower-category tornado or hurricane over a path comparatively wider than a tornado.
When a severe thunderstorm watch is issued, people within the region of expected severe weather risk are advised to monitor conditions ahead of the developing weather situation; review thunderstorm safety precautions in the event they must seek immediate shelter; and use local broadcast media, weather radio, weather app alert notifications and/or SMS notifications to receive warnings and updated storm information.
In the United States, severe thunderstorm watches are issued by the Storm Prediction Center (SPC), a national guidance center of the National Weather Service (NWS) branch of the National Oceanic and Atmospheric Administration (NOAA), for areas of the lower 48 states where atmospheric conditions favor the development of convective thunderstorm activity reaching severe criteria. Although seldom issued in these states, responsibilities for issuing severe thunderstorm watches covering Alaska and Hawaii are respectively handled by local NWS forecast offices in Fairbanks, Anchorage and Juneau, Alaska, and Honolulu, Hawaii. Watches are typically valid for six to nine hours (extending if necessary as long as 12 hours during unusually steady-state or slow-moving severe weather events) after the time of issuance, and are intended to precede the first report of severe hail or wind by 45 minutes to one hour. SPC watch boxes—termed because the approximate watch area is represented in weather maps as a quadrilateral for aviation purposes—are usually outlined in the approximate delineation of x miles north and south, or east and west, or either side of a line (perpendicular to the center line) from y miles direction of city, state, to z miles another direction of another city, state (e.g., "50 miles either side of a line from 10 miles northeast of Columbia, South Carolina to 15 miles south-southwest of Montgomery, Alabama"). Geographic coverage of severe thunderstorm watches (which ranges from 20,000–40,000 square miles [52,000–104,000 km2] on average, encompassing portions of one or more states) vary based on the size of the land area under threat, the duration of severe weather risk, and the forward motion of the parent storm system and associated surface boundaries.
In situations where atmospheric conditions will support the production of very destructive straight-line winds and hail from the convective activity, the intensified wording "particularly dangerous situation" (PDS) can be added into the watch product. PDS severe thunderstorm watches usually suggest conditions over the approximate area present a widespread threat of destructive hail greater than two inches (5.1 cm) in diameter and winds greater than 75 mph (121 km/h) being generated from the thunderstorm convection, or that downstream conditions favor the development and intensification of a bow echo or derecho moving at 55 mph (89 km/h) or faster. The inclusion of PDS wording (more commonly used for tornado watches in areas where the environment supports the development of strong to violent tornadoes) is rare for severe thunderstorm watches since the tornado threat, in comparison to the much higher threat of extreme wind or hail, must remain low enough to where a standard (non-PDS) tornado watch is not warranted.
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