Annual severe thunderstorm watch frequency in the United States

A severe thunderstorm watch ( SAME code: SVA) is a statement issued by

weather forecasting Weather forecasting or weather prediction is the application of science and technology forecasting, to predict the conditions of the Earth's atmosphere, atmosphere for a given location and time. People have attempted to predict the weather info ...

agencies to advise the public that atmospheric conditions in a given region may lead to the development of

severe thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustics, acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorm ...

s 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

tornado A tornado is a violently rotating column of air that is in contact with the surface of Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, although the ...

es. 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 A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale conv ...

), or more clustered or discrete storm activity (of the single-cell, multicell and/or

supercell A supercell is a thunderstorm characterized by the presence of a mesocyclone, a deep, persistently rotating updraft. Due to this, these storms are sometimes referred to as rotating thunderstorms. Of the four classifications of thunderstorms (su ...

varieties). A severe thunderstorm watch, like a

tornado watch A tornado watch ( SAME code: TOA) 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 tornadoes within (or near) the region over a period of seve ...

, 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 In meteorology, a downburst is a strong downward and outward gushing wind system that emanates from a point source above and blows radially, that is, in straight lines in all directions from the area of impact at surface level. It originate ...

, intense

lightning Lightning is a natural phenomenon consisting of electrostatic discharges occurring through the atmosphere between two electrically charged regions. One or both regions are within the atmosphere, with the second region sometimes occurring on ...

, torrential rainfall and/or

flash flood A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, or tropical storm, or by meltwater from ice and snow. Flash f ...

ing caused by high rainfall accumulations. Depending on

storm cell A storm cell is an air mass that contains up and down vertical draft, drafts in convective loops and that moves and reacts as a single entity, functioning as the smallest unit of a storm-producing system. An organized grouping of thunder clouds wi ...

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 A blunt trauma, also known as a blunt force trauma or non-penetrating trauma, is a physical trauma due to a forceful impact without penetration of the body's surface. Blunt trauma stands in contrast with penetrating trauma, which occurs when a ...

, wind-generated

debris Debris (, ) is rubble, wreckage, ruins, litter and discarded waste, garbage/refuse/trash, scattered remains of something destroyed, or, as in geology, large rock fragments left by a melting glacier, etc. Depending on context, ''debris'' can ref ...

or intense flooding). If severe weather actually does occur, a

severe thunderstorm warning A severe thunderstorm warning ( SAME code: SVR) is a type of public warning for severe weather that is issued by weather forecasting agencies worldwide when one or more severe thunderstorms have been detected by Doppler weather radar, observed ...

tornado warning A tornado warning ( SAME code: TOR) is a public warning that is issued by weather forecasting agencies to an area in the direct path of a tornado, or a severe thunderstorm capable of producing one, and advises individuals in that area to take c ...

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

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

es, if modest

wind shear Wind shear (; also written windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical ...

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 A cloud base (or the base of the cloud) is the lowest altitude of the visible portion of a cloud. It is traditionally expressed either in metres or feet above mean sea level or above a planetary surface, or as the pressure level corresponding t ...

and may spawn tornadoes if advanced

tornadogenesis Tornadogenesis is the process by which a tornado forms. There are many types of tornadoes, varying in methods of formation. Despite ongoing scientific study and high-profile research projects such as VORTEX projects, VORTEX, tornadogenesis is a ...

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

downdraft In meteorology, an updraft (British English: ''up-draught'') is a small-scale current of rising air, often within a cloud. Overview Vertical drafts, known as updrafts or downdrafts, are localized regions of warm or cool air that move vertically ...

s, 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 A weather radio is a specialized radio receiver that is designed to receive a public broadcast service, typically from government-owned radio stations, dedicated to broadcasting weather forecasts and reports on a continual basis, with the routine w ...

, weather app alert notifications and/or

SMS Short Message Service, commonly abbreviated as SMS, is a text messaging service component of most telephone, Internet and mobile device systems. It uses standardized communication protocols that let mobile phones exchange short text messages, t ...

notifications to receive warnings and updated storm information.

United States

In the United States, severe thunderstorm watches are issued by the

Storm Prediction Center The Storm Prediction Center (SPC) is a US government agency that is part of the National Centers for Environmental Prediction (NCEP), operating under the control of the National Weather Service (NWS), which in turn is part of the National Oceani ...

(SPC), a national guidance center of the

National Weather Service The National Weather Service (NWS) is an Government agency, agency of the Federal government of the United States, United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weathe ...

(NWS) branch of the

National Oceanic and Atmospheric Administration The National Oceanic and Atmospheric Administration (NOAA ) is an American scientific and regulatory agency charged with Weather forecasting, forecasting weather, monitoring oceanic and atmospheric conditions, Hydrography, charting the seas, ...

(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 Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...

and

Hawaii Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...

are respectively handled by local NWS forecast offices in Fairbanks,

Anchorage Anchorage, officially the Municipality of Anchorage, is the most populous city in the U.S. state of Alaska. With a population of 291,247 at the 2020 census, it contains nearly 40 percent of the state's population. The Anchorage metropolita ...

and

Juneau, Alaska Juneau ( ; ), officially the City and Borough of Juneau, is the List of capitals in the United States, capital of the U.S. state of Alaska, located along the Gastineau Channel and the Southeast Alaska, Alaskan panhandle. Juneau was named the ...

, and

Honolulu, Hawaii Honolulu ( ; ) is the List of capitals in the United States, capital and most populous city of the U.S. state of Hawaii, located in the Pacific Ocean. It is the county seat of the Consolidated city-county, consolidated City and County of Honol ...

. 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 In Euclidean geometry, geometry a quadrilateral is a four-sided polygon, having four Edge (geometry), edges (sides) and four Vertex (geometry), corners (vertices). The word is derived from the Latin words ''quadri'', a variant of four, and ''l ...

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 In geometry, two geometric objects are perpendicular if they intersect at right angles, i.e. at an angle of 90 degrees or π/2 radians. The condition of perpendicularity may be represented graphically using the '' perpendicular symbol'', � ...

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 Columbia is the List of capitals in the United States, capital city of the U.S. state of South Carolina. With a population of 136,632 at the 2020 United States census, 2020 census, it is List of municipalities in South Carolina, the second-mo ...

to 15 miles south-southwest of

Montgomery, Alabama Montgomery is the List of capitals in the United States, capital city of the U.S. state of Alabama. Named for Continental Army major general Richard Montgomery, it stands beside the Alabama River on the Gulf Coastal Plain. The population was 2 ...

"). 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

and

hail Hail is a form of solid Precipitation (meteorology), precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailsto ...

from the convective activity, the intensified wording "

particularly dangerous situation A PDS tornado watch issued on December 14, 2022. In weather forecasting in the United States, "particularly dangerous situation" (PDS) is the wording used by the National Weather Service and the Storm Prediction Center to convey special urgency in ...

" (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 A bow echo is the characteristic radar return from a mesoscale convective system that is shaped like an archer's bow. These systems can produce severe straight-line winds and occasionally tornadoes, causing major damage. They can also become ...

derecho A ''derecho'' (, from , 'straight') is a widespread, long-lived, straight-line wind storm that is associated with a fast-moving group of Severe weather#Categories, severe thunderstorms known as a mesoscale convective system. Derechos cause Bea ...

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 The National Oceanic and Atmospheric Administration (NOAA ) is an American scientific and regulatory agency charged with forecasting weather, monitoring oceanic and atmospheric conditions, charting the seas, conducting deep-sea exploratio ...

satellites, and over

NOAA Weather Radio NOAA Weather Radio (NWR), also known as NOAA Weather Radio All Hazards, is an automated 24-hour network of Very high frequency, VHF Frequency modulation, FM weather radio stations in the United States which broadcast weather information direct ...

. 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 (in many cases, severe weather alert displays used by many local television stations usually reserve yellow as a highlighter for severe thunderstorm warnings, and blue as a highlighter for severe thunderstorm watches). 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 (e.g., "widespread large hail and isolated very large hail events to 3 inches in diameter likely, scattered damaging wind gusts to 70 mph likely, a tornado or two possible") 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 . (Typically, the minimum event probabilities for a severe thunderstorm watch issuance require an ~30% chance of 10 or more wind and hail reports and, for PDS watches, a 70% chance of one or more reports of >65 kt winds and/or >2-inch hail within the watch area over the valid time period.) The SPC produces two separate products listing all

counties A county () is a geographic region of a country used for administrative or other purposesL. Brookes (ed.) '' Chambers Dictionary''. Edinburgh: Chambers Harrap Publishers Ltd, 2005. in some nations. The term is derived from the Old French denoti ...

or equivalent subdivisions (

parishes A parish is a territorial entity in many Christian denominations, constituting a division within a diocese. A parish is under the pastoral care and clerical jurisdiction of a priest, often termed a parish priest, who might be assisted by one or ...

independent cities An independent city or independent town is a city or town that does not form part of another general-purpose local government entity (such as a province). Historical precursors In the Holy Roman Empire, and to a degree in its successor states ...

, and coastal marine zones) included in the broader watch area: Watch County Lists (WCL), 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 (WOU) 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 (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. 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 (such as

cold front A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface Trough (meteorology), trough of Low-pressure area, low pressure. It often forms behind an extratropica ...

s and

dryline A dry line (also called a dew point line, or Marfa front, after Marfa, Texas) is a line across a continent that separates moist air and dry air. One of the most prominent examples of such a separation occurs in central North America, especially ...

s)—and the NWS offices decide what counties to remove from the watch (the local offices will almost always follow the SPC recommendation on the status messages). 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.

Examples

This is an example of a PDS severe thunderstorm watch issued for parts of Kansas, Oklahoma, and Texas. SEL0 URGENT - IMMEDIATE BROADCAST REQUESTED Severe Thunderstorm Watch Number 260 NWS Storm Prediction Center Norman OK 235 PM CDT Sun May 19 2024 The NWS Storm Prediction Center has issued a * Severe Thunderstorm Watch for portions of Western and Central Kansas Northwestern Oklahoma Northeastern Texas Panhandle * Effective this Sunday afternoon and evening from 235 PM until 1000 PM CDT. ...THIS IS A PARTICULARLY DANGEROUS SITUATION... * Primary threats include... Widespread damaging winds expected with scattered significant gusts to 90 mph likely Widespread large hail expected with scattered very large hail events to 4 inches in diameter likely A few tornadoes possible SUMMARY...Numerous severe thunderstorms are expected to develop and track rapidly eastward across the watch area through the afternoon evening. Supercells capable of giant hail, damaging winds, and isolated tornadoes will be the main threat early. Storms will organize into multiple fast-moving bowing lines through the evening with a risk of widespread damaging winds. The severe thunderstorm watch area is approximately along and 135 statute miles east and west of a line from 55 miles southwest of Alva OK to 45 miles northwest of Russell KS. For a complete depiction of the watch see the associated watch outline update (WOUS64 KWNS WOU0). PRECAUTIONARY/PREPAREDNESS ACTIONS... REMEMBER...A Severe Thunderstorm Watch means conditions are favorable for severe thunderstorms in and close to the watch area. Persons in these areas should be on the lookout for threatening weather conditions and listen for later statements and possible warnings. Severe thunderstorms can and occasionally do produce tornadoes. && OTHER WATCH INFORMATION...CONTINUE...WW 258...WW 259... AVIATION...A few severe thunderstorms with hail surface and aloft to 4 inches. Extreme turbulence and surface wind gusts to 80 knots. A few cumulonimbi with maximum tops to 500. Mean storm motion vector 24035. ...Hart

Canada

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

Environment and Climate Change Canada Environment and Climate Change Canada (ECCC; )Environment and Climate Change Canada is the applied title under the Federal Identity Program; the legal title is Department of the Environment (). is the department of the Government of Canada res ...

(ECCC) issues severe thunderstorm watches through regional

Meteorological Service Meteorology is the scientific study of the Earth's atmosphere and short-term atmospheric phenomena (i.e. weather), with a focus on weather forecasting. It has applications in the military, aviation, energy production, transport, agriculture, ...

(MSC) offices based in

Vancouver Vancouver is a major city in Western Canada, located in the Lower Mainland region of British Columbia. As the List of cities in British Columbia, most populous city in the province, the 2021 Canadian census recorded 662,248 people in the cit ...

Edmonton Edmonton is the capital city of the Provinces and territories of Canada, Canadian province of Alberta. It is situated on the North Saskatchewan River and is the centre of the Edmonton Metropolitan Region, which is surrounded by Central Alberta ...

Winnipeg Winnipeg () is the capital and largest city of the Provinces and territories of Canada, Canadian province of Manitoba. It is centred on the confluence of the Red River of the North, Red and Assiniboine River, Assiniboine rivers. , Winnipeg h ...

Toronto Toronto ( , locally pronounced or ) is the List of the largest municipalities in Canada by population, most populous city in Canada. It is the capital city of the Provinces and territories of Canada, Canadian province of Ontario. With a p ...

Montreal Montreal is the List of towns in Quebec, largest city in the Provinces and territories of Canada, province of Quebec, the List of the largest municipalities in Canada by population, second-largest in Canada, and the List of North American cit ...

and Dartmouth. 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 The census geographic units of Canada are the census subdivisions defined and used by Canada's federal government statistics bureau Statistics Canada to conduct the country's quinquennial census. These areas exist solely for the purposes of sta ...

, 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 The Weather Network (TWN) is a Canadian English-language discretionary weather information specialty channel available in Canada, the United States and the United Kingdom. It delivers weather information on television, digital platforms (respon ...

MétéoMédia MétéoMédia is a Canadian French-language weather information specialty channel and web site owned by Pelmorex. MétéoMédia primarily serves viewers in Quebec, although some cable TV systems in Ontario and New Brunswick carry the channel as w ...

), and

Weatheradio Canada Weatheradio Canada () is a Canadian weather radio network owned and operated by Environment and Climate Change Canada's Meteorological Service of Canada division. It is one of the two weather radio systems across North America along with the NOAA ...

; 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 Alberta Emergency Alert (AEA) was a public warning system in Alberta. The system was implemented in October 2011, replacing the former Alberta Emergency Public Warning System (EPWS). Based on Common Alerting Protocol (CAP), alerts were dissemin ...

and SaskAlert) to distribute the alert to local broadcast media and cellular phones.

Philippines

In the

Philippines The Philippines, officially the Republic of the Philippines, is an Archipelagic state, archipelagic country in Southeast Asia. Located in the western Pacific Ocean, it consists of List of islands of the Philippines, 7,641 islands, with a tot ...

, the

Philippine Atmospheric, Geophysical and Astronomical Services Administration The Philippine Atmospheric, Geophysical and Astronomical Services Administration (, abbreviated as PAGASA , which means "hope" as in the Tagalog word ''pag-asa'') is the National Meteorological and Hydrological Services (NMHS) agency of the P ...

(PAGASA), through its regional services divisions, issues thunderstorm watches to alert

regions In geography, regions, otherwise referred to as areas, zones, lands or territories, are portions of the Earth's surface that are broadly divided by physical characteristics (physical geography), human impact characteristics (human geography), and ...

and/or

provinces A province is an administrative division within a country or state. The term derives from the ancient Roman , which was the major territorial and administrative unit of the Roman Empire's territorial possessions outside Italy. The term ''provi ...

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.

References

External links

Meteorological Service of Canada

Current U.S. Watches
{{SevWea nav Weather warnings and advisories