thumb|right|300px|Map of average annual severe thunderstorm watches in the United States between 2006 and 2025.<br>
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.
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. (The criteria for thunderstorms to be classified as severe varies by country; e.g., in the United States, severe thunderstorms must produce winds exceeding , and/or hailstones larger than 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.
United States
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 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 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 (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.
SPC meteorologists utilize WarnGen software integrated into the National Centers Advance Weather Interactive Processing System (N-AWIPS) and/or the SPC Product Generator (PRODGEN) 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. Local NWS offices concurrently issue Watch County Notification (WCN) 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 (if atmospheric conditions have become less conducive to form severe thunderstorms or were insufficient for severe weather development compared to earlier forecast analysis). 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. Unlike the box/subdivision structure of watches issued by the U.S. Storm Prediction Center, severe thunderstorm watches issued by the Meteorological Service are issued strictly for groups of census subdivisions, often covering a total area that is relatively smaller than severe thunderstorm watches in the U.S.). Watches are disseminated to the public through broadcast and online media outlets (including local television stations and The Weather Network/MétéoMédia), and Weatheradio Canada; depending on regional office discretion, the watch may require activation of the National Public Alerting System (Alert Ready) () and feeding provincial alerting systems (such as Alberta Emergency Alert and SaskAlert) to distribute the alert to local broadcast media and cellular phones.
Philippines
In the Philippines, the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA), through its regional services divisions, issues thunderstorm watches to alert regions and/or provinces of thunderstorm development within a period of twelve hours. Watches are disseminated to the public through terrestrial television and radio stations, and online media outlets.