Severe thunderstorm watch
A severe thunderstorm watch 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 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, or more clustered or discrete storm activity.
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.
Definition
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. Generally, 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.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.
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.
United States
In the United States, severe thunderstorm watches are issued by the Storm Prediction Center, a national guidance center of the National Weather Service branch of the National Oceanic and Atmospheric Administration, 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 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 from y miles direction of city, state, to z miles another direction of another city, state. Geographic coverage of severe thunderstorm watches 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" 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 in diameter and winds greater than being generated from the thunderstorm convection, or that downstream conditions favor the development and intensification of a bow echo or derecho moving at or faster. The inclusion of PDS wording 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 tornado watch is not warranted.
SPC meteorologists utilize WarnGen software integrated into the National Centers Advance Weather Interactive Processing System and/or the SPC Product Generator to generate the watch statement, which is disseminated through various communication routes accessed by the media and various agencies, on the internet, to NOAA satellites, and over NOAA Weather Radio. The terms "blue box" and "yellow box," often used in parlance within the meteorological community, refer to the coloring assigned to severe thunderstorm watch boxes for hazard maps used by the Storm Prediction Center and the National Weather Service; coloring used by local television outlets to highlight severe thunderstorm watches sometimes vary.
The Storm Prediction Center, in conjunction with local NWS Weather Forecast Offices, issues component watch products to communicate the approximate area, primary hazards and other pertinent information about the severe thunderstorm watch to the public, NOAA meteorologists, emergency management and aviation personnel. The graphical and text Public Watch products—in addition to outlining the approximate affected area, valid time, meteorological and aviation discussions, and other pertinent information—includes language specifying the forecast severe weather threat in the primary hazards list. The Watch Probability Table describes probabilities for all modes of severe weather, including probabilities of 10 or more severe wind or hail events and one or more reports of winds exceeding or hailstones exceeding.
The SPC produces two separate products listing all counties or equivalent subdivisions included in the broader watch area: Watch County Lists, which are produced internally preceding the watch issuance for collaborative use with local NWS offices to outline counties and equivalent subdivisions being proposed for inclusion in the watch, and Watch Outline Update messages, a public list of the determined watch subdivisions published upon the initial severe thunderstorm watch issuance. Local NWS offices concurrently issue Watch County Notification messages that list subdivisions within their designated area of responsibility that the office has considered to be in the initial watch; WCN messages—which the SPC uses as the basis for their Watch Outline Update product—are updated to denote changes to the watch by local WFOs, which are provided responsibility for adding or removing counties/subdivisions from the watch, extending its time of expiration, or cancelling the watch entirely if conditions no longer support a severe weather threat. The SPC updates Watch Outline Updates at least on an hourly basis to incorporate changes made by the accordant WFOs in their Watch County Notification messages.
The SPC issues Watch Status Messages to designate areas considered to have a continuing severe weather threat, based primarily on the position of surface features —and the NWS offices decide what counties to remove from the watch. If conditions are no longer favorable for severe convective thunderstorms in the watch area, the severe thunderstorm watch may be cancelled outright; occasionally, a severe thunderstorm watch, if not merely a section of it, may be replaced by a tornado watch should conditions that were originally forecast to be conducive for non-tornadic severe thunderstorms evolve to allow an increased possibility of tornado formation, and likewise may replace a tornado watch if conditions for tornadogenesis are no longer considered favorable. If no convective development or reported severe weather occurs, this leads to a severe thunderstorm watch "bust", which can factor into determinations by the SPC and National Weather Service offices on whether to cancel the watch.
Because the Storm Prediction Center and local National Weather Service WFOs each have roles in the watch issuance process, the subdivisions listed in the Watch Outline Update and Watch County Notification products will sometimes differ from the outlined watch box area, including subdivisions located outside the outlined quadrilateral; however the local Weather Forecast Office is tasked with determining which counties should be included in or, in lieu of a new downstream watch, added to the designated watch area. The WFOs monitoring their sector of the watch area can also consult, via conference call, with the Storm Prediction Center to relay and determine locally dictated changes to the severe thunderstorm watch, regarding replacement of the watch and extensions of time and areal coverage if conditions warrant.