Storm Prediction Center

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Storm Prediction Center
The logo of the Storm Prediction Center.
Agency overview
FormedOctober 1995 (1995-10)
Preceding agencies
  • National Severe Storms Forecast Center (1966–1995)
  • SELS (1953–1966)
Jurisdiction Federal government of the United States
Headquarters Norman, Oklahoma
Agency executive
  • Russell Schneider, Director
Parent agency National Centers for Environmental Prediction

The Storm Prediction Center (SPC) is a government agency that is part of the National Centers for Environmental Prediction (NCEP), operating under the control of the National Weather Service (NWS), [1] which in turn is part of the National Oceanic and Atmospheric Administration (NOAA) of the United States Department of Commerce (DoC). [2]

A government or state agency, sometimes an appointed commission, is a permanent or semi-permanent organization in the machinery of government that is responsible for the oversight and administration of specific functions, such as an intelligence agency. There is a notable variety of agency types. Although usage differs, a government agency is normally distinct both from a department or ministry, and other types of public body established by government. The functions of an agency are normally executive in character, since different types of organizations are most often constituted in an advisory role—this distinction is often blurred in practice however.

The United States National Centers for Environmental Prediction (NCEP) delivers national and global weather, water, climate and space weather guidance, forecasts, warnings and analyses to its Partners and External User Communities. These products and services are based on a service-science legacy and respond to user needs to protect life and property, enhance that nation's economy and support the nation's growing need for environmental information. The centers form part of the National Weather Service.

National Weather Service United States weather agency

The National Weather Service (NWS) is an agency of the United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weather-related products to organizations and the public for the purposes of protection, safety, and general information. It is a part of the National Oceanic and Atmospheric Administration (NOAA) branch of the Department of Commerce, and is headquartered in Silver Spring, Maryland, within the Washington metropolitan area. The agency was known as the United States Weather Bureau from 1890 until it adopted its current name in 1970.


Headquartered at the National Weather Center in Norman, Oklahoma, the Storm Prediction Center is tasked with forecasting the risk of severe thunderstorms and tornadoes in the contiguous United States. It issues convective outlooks, mesoscale discussions, and watches as a part of this process. Convective outlooks are issued for the following eight days (issued separately for Day 1, Day 2, Day 3, and Days 4–8), and detail the risk of severe thunderstorms and tornadoes during the given forecast period, although tornado, hail and wind details are only available for Day 1. Days 2 and 3, as well as 4–8 use a probabilistic scale, determining the probability for a severe weather event in percentage categories.

National Weather Center

The National Weather Center (NWC), on the campus of the University of Oklahoma, is a confederation of federal, state, and academic organizations that work together to better understand events that take place in Earth's atmosphere over a wide range of time and space scales. The NWC partners give equal attention to applying that understanding to the development of improved observation, analysis, assimilation, display, and prediction systems. The National Weather Center also has expertise in local and regional climate, numerical modeling, hydrology, and weather radar. Members of the NWC work with a wide range of federal, state, and local government agencies to help reduce loss of life and property to hazardous weather, ensure wise use of water resources, and enhance agricultural production. They also work with private sector partners to develop new applications of weather and regional climate information that provide competitive advantage in the marketplace.

Norman, Oklahoma City in Oklahoma, United States

Norman is a city in the U.S. state of Oklahoma located 20 miles (32 km) south of downtown Oklahoma City. As the county seat of Cleveland County and a part of the Oklahoma City metropolitan area, its population was 110,925 at the 2010 census. Norman's estimated population of 122,843 in 2017 makes it the third-largest city in Oklahoma.

Oklahoma State of the United States of America

Oklahoma is a state in the South Central region of the United States, bordered by Kansas on the north, Missouri on the northeast, Arkansas on the east, Texas on the south, New Mexico on the west, and Colorado on the northwest. It is the 20th-most extensive and the 28th-most populous of the fifty United States. The state's name is derived from the Choctaw words okla and humma, meaning "red people". It is also known informally by its nickname, "The Sooner State", in reference to the non-Native settlers who staked their claims on land before the official opening date of lands in the western Oklahoma Territory or before the Indian Appropriations Act of 1889, which dramatically increased European-American settlement in the eastern Indian Territory. Oklahoma Territory and Indian Territory were merged into the State of Oklahoma when it became the 46th state to enter the union on November 16, 1907. Its residents are known as Oklahomans, and its capital and largest city is Oklahoma City.

Mesoscale discussions are issued to provide information on certain individual regions where severe weather is becoming a threat and states whether a watch is likely and details thereof, particularly concerning conditions conducive for the development of severe thunderstorms in the short term, as well as situations of isolated severe weather when watches are not necessary. Watches are issued when forecasters are confident that severe weather will occur, and usually precede the onset of severe weather by one hour, although this sometimes varies depending on certain atmospheric conditions that may inhibit or accelerate convective development.

The agency is also responsible for forecasting fire weather (indicating conditions that are favorable for wildfires) in the contiguous U.S., issuing fire weather outlooks for Days 1, 2, and 3–8, which detail areas with various levels of risk for fire conditions. (such as fire levels and fire alerts. )


The Storm Prediction Center began in 1952 as SELS (Severe Local Storms Unit), the U.S. Weather Bureau in Washington, D.C. In 1954, the unit moved its forecast operations to Kansas City, Missouri. SELS began issuing convective outlooks for predicted thunderstorm activity in 1955, and began issuing radar summaries in three-hour intervals in 1960; [3] with the increased duties of compiling and disseminating radar summaries, this unit became the National Severe Storms Forecast Center (NSSFC) in 1966, [4] remaining headquartered in Kansas City.

Washington, D.C. Capital of the United States

Washington, D.C., formally the District of Columbia and commonly referred to as Washington or D.C., is the capital of the United States. Founded after the American Revolution as the seat of government of the newly independent country, Washington was named after George Washington, the first President of the United States and a Founding Father. As the seat of the United States federal government and several international organizations, Washington is an important world political capital. The city is also one of the most visited cities in the world, with more than 20 million tourists annually.

Kansas City, Missouri City in western Missouri

Kansas City is the largest city in the U.S. state of Missouri. According to the U.S. Census Bureau, the city had an estimated population of 491,918 in 2018, making it the 38th most-populous city in the United States. It is the central city of the Kansas City metropolitan area, which straddles the Kansas–Missouri state line. Kansas City was founded in the 1830s as a Missouri River port at its confluence with the Kansas River coming in from the west. On June 1, 1850 the town of Kansas was incorporated; shortly after came the establishment of the Kansas Territory. Confusion between the two ensued and the name Kansas City was assigned to distinguish them soon after.

Missouri State of the United States of America

Missouri is a state in the Midwestern United States. With over six million residents, it is the 18th-most populous state of the Union. The largest urban areas are St. Louis, Kansas City, Springfield, and Columbia; the capital is Jefferson City. The state is the 21st-most extensive in area. In the South are the Ozarks, a forested highland, providing timber, minerals, and recreation. The Missouri River, after which the state is named, flows through the center of the state into the Mississippi River, which makes up Missouri's eastern border.

In 1968, the National Severe Storms Forecast Center began issuing status reports on weather watches; the agency then made its first computerized data transmission in 1971. [3] On April 2, 1982, the agency issued the first "Particularly Dangerous Situation" watch, which indicates the imminent threat of a major severe weather event over the watches' timespan. [3] In 1986, the NSSFC introduced two new forecast products: the Day 2 Convective Outlook (which include probabilistic forecasts for outlined areas of thunderstorm risk for the following day) and the Mesoscale Discussion (a short-term forecast outlining specific areas under threat for severe thunderstorm development). [3]

A Particularly Dangerous Situation (PDS) is a type of enhanced wording first used by the Storm Prediction Center (SPC), a national guidance center of the United States National Weather Service, for tornado watches and eventually expanded to use for other severe weather watches and warnings by local NWS forecast offices. It is issued at the discretion of the forecaster composing the watch or warning and implies that there is an enhanced risk of very severe and life-threatening weather, usually a major tornado outbreak or a long-lived, extreme derecho event, but possibly another weather hazard such as an exceptional flash flood or fire.

In October 1995, the National Severe Storms Forecast Center relocated its operations to Norman, Oklahoma, and was rechristened the Storm Prediction Center. At that time, the guidance center was housed at Max Westheimer Airport (now the University of Oklahoma Westheimer Airport), co-located in the same building as the National Severe Storms Laboratory and the local National Weather Service Weather Forecast Office (the latter of which, in addition to disseminating forecasts, oversees the issuance of weather warnings and advisories for the western two-thirds of Oklahoma and western portions of North Texas, and issues outline and status updates for SPC-issued severe thunderstorm and tornado watches that include areas served by the Norman office). [1] In 1998, the Center began issuing the National Fire Weather Outlook to provide forecasts for areas potentially susceptible to the development and spread of wildfires based on certain meteorological factors. [3] The Day 3 Convective Outlook (which is similar in format to the Day 2 forecast) was first issued on an experimental basis in 2000, and was made an official product in 2001. [3]

University of Oklahoma Westheimer Airport Airport in Oklahoma, United States of America

University of Oklahoma Westheimer Airport is a public use airport in Norman, in Cleveland County, Oklahoma. It is owned by the University of Oklahoma. The National Plan of Integrated Airport Systems for 2011–2015 categorized it as a reliever airport.

The National Severe Storms Laboratory (NSSL) is a National Oceanic and Atmospheric Administration (NOAA) weather research laboratory under the Office of Oceanic and Atmospheric Research. It is one of seven NOAA Research Laboratories (RLs).

National Weather Service Norman, Oklahoma

National Weather Service - Norman, Oklahoma is a Weather Forecast Office (WFO) of the National Weather Service based in Norman, Oklahoma, which is responsible for forecasts and the dissemination of weather warnings and advisories for central and most of western Oklahoma, and western portions of north Texas. It is located in the National Weather Center on the University of Oklahoma campus, where it acts as one of the NOAA Weather Partners, a group of close-together weather-related agencies of the National Oceanic and Atmospheric Administration. NWS Norman is currently overseen by David Andra, who serves as the Meteorologist In Charge of the office.

In 2006, the Storm Prediction Center, National Severe Storms Laboratory and National Weather Service Norman Forecast Office moved their respective operations into the newly constructed National Weather Center, near Westheimer Airport. [3] [5] Since the agency's relocation to Norman, the 557th Weather Wing at Offutt Air Force Base would assume control of issuing the Storm Prediction Center's severe weather products in the event that the SPC is no longer able to issue them in the event of an outage (such as a computer system failure or building-wide power disruption) or emergency (such as an approaching strong tornadic circulation or tornado on the ground) affecting the Norman campus; on April 1, 2009, the SPC reassigned responsibilities for issuing the center's products in such situations to the 15th Operational Weather Squadron based out of Scott Air Force Base. [6]


The Storm Prediction Center is responsible for forecasting the risk of severe weather caused by severe thunderstorms, specifically those producing tornadoes, hail of one inch (2.5 cm) in diameter or larger, and/or winds of 58 miles per hour (93 km/h) [50 knots] or greater. The agency also forecasts hazardous winter and fire weather conditions. It does so primarily by issuing convective outlooks, severe thunderstorm watches, tornado watches and mesoscale discussions. [7]

There is a three-stage process in which the area, time period, and details of a severe weather forecast are refined from a broad-scale forecast of potential hazards to a more specific and detailed forecast of what hazards are expected, and where and in what time frame they are expected to occur. If warranted, forecasts will also increase in severity through this three-stage process. [7]

The Storm Prediction Center employs a total of 43 personnel, including five lead forecasters, ten mesoscale/outlook forecasters, and seven assistant mesoscale forecasters. [8] Many SPC forecasters and support staff are heavily involved in scientific research into severe and hazardous weather. This involves conducting applied research and writing technical papers, developing training materials, giving seminars and other presentations locally and nationwide, attending scientific conferences, and participating in weather experiments. [9]

Convective outlooks

2019-05-20 1630 UTC Day 1 convective outlook.gif
2019-05-20 1630 UTC Day 1 tornado outlook.gif
Day 1 Convective Outlook and probabilistic maps issued by the Storm Prediction Center on May 20, 2019. The top map indicates the risk of general severe weather (including large hail, damaging winds, and tornadoes), while the bottom map specifically shows the risk percentage of a tornado forming within 25 miles (40 km) of any point within the enclosed area. The hatched area on the bottom map indicates a 10% or greater risk of an EF2 or stronger tornado forming within 25 miles (40 km) of a point.

The Storm Prediction Center issues convective outlooks (AC), consisting of categorical and probabilistic forecasts describing the general threat of severe convective storms over the contiguous United States for the next six to 192 hours (Day 1 through Day 8). These outlooks are labeled and issued by day, and are issued up to five times per day. [10]

The categorical risks are TSTM (for Thunder Storm: light green shaded area – rendered as a brown line prior to April 2011 – indicating a risk for general thunderstorms), "MRGL" (for Marginal: darker green shaded area, indicating a very low but present risk of severe weather); "SLGT" (for Slight: yellow shaded area – previously rendered as a green line – indicating a slight risk of severe weather); "ENH" (for Enhanced: orange shaded area, which replaced the upper end of the SLGT category on October 22, 2014); "MDT" (for Moderate: red shaded area – previously rendered as a red line – indicating a moderate risk of severe weather); and "HIGH" (pink shaded area – previously a rendered as a fuchsia line – indicating a high risk of severe weather). Significant severe areas (referred to as "hatched areas" because of their representation on outlook maps) refer to a threat of increased storm intensity that is of "significant severe" levels (F2/EF2 or stronger tornado, 2 inches (5.1 cm) or larger hail, or 75 miles per hour (121 km/h) winds or greater). [11]

In April 2011, the SPC introduced a new graphical format for its categorical and probability outlooks, which included the shading of risk areas (with the colors corresponding to each category, as mentioned above, being changed as well) and population, county/parish/borough and interstate overlays. The new shaded maps also incorporated a revised color palette for the shaded probability categories in each outlook.

In 2013, the SPC incorporated a small table under the Convective Outlook's risk category map that indicates the total coverage area by square miles, the total estimated population affected and major cities included within a severe weather risk area. [12]

Public severe weather outlooks (PWO) are issued when a significant or widespread outbreak is expected, especially for tornadoes. From November to March, it can also be issued for any threat of significant tornadoes in the nighttime hours, noting the lower awareness and greater danger of tornadoes at that time of year. [13]


A marginal risk day indicates storms of only limited organization, longevity, coverage and/or intensity, typically isolated severe or near-severe storms with limited wind damage, large hail and perhaps a tornado. Wind gusts of at least 60 miles per hour (97 km/h) and hailstones of around 1 inch (2.5 cm) in diameter are common storm threats within a marginal risk; depending on the sufficient wind shear, a tornado – usually of weak (EF0 to EF1) intensity and short duration – may be possible. This category replaced the "SEE TEXT" category on October 22, 2014. [11]

A slight risk day typically will indicate that the threat exists for scattered severe weather, including scattered wind damage (produced by straight-line sustained winds and/or gusts of 60 to 70 mph), scattered severe hail (varying in size from .25 inches (0.64 cm) to 1.75 inches (4.4 cm)) and/or isolated tornadoes (often of shorter duration and varying weak to moderate intensity, depending on the available wind shear and other sufficient atmospheric parameters). During the peak severe weather season, most days will have a slight risk somewhere in the United States. Isolated significant severe events are possible in some circumstances, but are generally not widespread. [10]

An enhanced risk day indicates that there is a greater threat for severe weather than that which would be indicated by a slight risk, but conditions are not adequate for the development of widespread significant severe weather to necessitate a moderate category. Severe storms are expected to be more concentrated and of varying intensities. These days are quite frequent in the peak severe weather season and occur occasionally at other times of year. This risk category replaced the upper end of "slight" on October 22, 2014, although a few situations that previously warranted a moderate risk were reclassified as enhanced (i.e. 45% wind or 15% tornado with no significant area). [11]

A moderate risk day indicates that more widespread and/or more dangerous severe weather is possible (sometimes with major hurricanes), with significant severe weather often more likely. Numerous tornadoes (some of which may be strong and potentially long-track), more widespread or severe wind damage and/or very large/destructive hail (up to or exceeding 2 inches (5.1 cm) in diameter) could occur. Major events, such as large tornado outbreaks or widespread straight-line wind events, are sometimes also possible on moderate risk days, but with greater uncertainty. Moderate risk days are not terribly uncommon, and typically occur several times a month during the peak of the severe weather season, and occasionally at other times of the year. Slight and enhanced risk areas typically surround areas under a moderate risk, where the threat is lower. [10]

A high risk day indicates a considerable likelihood of significant to extreme severe weather, generally a major tornado outbreak or (much less often) an extreme derecho event. On these days, the potential exists for extremely severe and life-threatening weather. This includes widespread strong or violent tornadoes which may be on the ground for a half-hour or longer, or very destructive straight-line winds. Hail cannot verify or produce a high risk on its own, although such a day usually involves a threat for widespread very large and damaging hail as well. Many of the most prolific severe weather days were high risk days. Such days are rare; a high risk is typically issued (at the most) only a few times each year (see List of Storm Prediction Center high risk days). High risk areas are usually surrounded by a larger moderate risk area, where uncertainty is greater or the threat is somewhat lower. [10]

The Storm Prediction Center began asking for public comment on proposed categorical additions to the Day 1-3 Convective Outlooks on April 21, 2014, for a two-month period. [14] The Storm Prediction Center broadened this system beginning on October 22, 2014 by adding two new risk categories to the three used originally. The new categories that were added are a "marginal risk" (replacing the "SEE TEXT" contours, see below) and an "enhanced risk". The latter is used to delineate areas where severe weather will occur that would fall under the previous probability criteria of an upper-end slight risk, but do not warrant the issuance of a moderate risk. In order from least to greatest threat, these categories are ranked as: marginal, slight, enhanced, moderate, and high. [15] [16] [17]

Issuance and usage

Note: SIGNIFICANT SEVERE area needed where denoted by bold italic type – otherwise default to next lower category.

Day 1 probability to categorical outlook conversion [11] [18]
Outlook probabilityTORNWINDHAIL
No thunderstormsNo Thunder
< 2%No Severenot usednot used
2%MRGLnot usednot used
< 5%not usedNo SevereNo Severe
10%ENHnot usednot used
Day 2 and Day 3 probability to categorical outlook conversion [11]
* Not used on Day 3 Convective Outlooks
Outlook probabilityCombined TORN, WIND, and HAIL
No thunderstormsNo Thunder
< 5%No Severe
Day 4–8 probability to categorical outlook conversion [11]
Outlook probabilityCombined TORN, WIND, and HAIL
< 15%No Area
15%Severe (15%)
30%Severe (30%)

Convective outlooks are issued by the Storm Prediction Center in Zulu time (also known as Universal Coordinated Time or UTC). [19]

The categories at right refer to the risk levels for the specific severe weather event occurring within 25 miles (40 km) of any point in the delineated region, as described in the previous section. The Day 1 Convective Outlook, issued five times per day at 0600Z (valid from 1200Z of the current day until 1200Z the following day), 1300Z and 1630Z (the "morning updates," valid until 1200Z the following day), 2000Z (the "afternoon update," valid until 1200Z the following day), and the 0100Z (the "evening update," valid until 1200Z the following day), provides a textual forecast, map of categories and probabilities, and chart of probabilities. The Day 1 is currently the only outlook to issue specific probabilities for tornadoes, hail or wind. It is the most descriptive and highest accuracy outlook, and typically has the highest probability levels. [10]

Day 2 outlooks, issued twice daily at 0600Z and 1730Z, refer to predicted risks of convective weather for the following day (1200Z to 1200Z of the next calendar day; for example, a Day 2 outlook issued on April 12, 2100 would be valid from 1200Z on April 13, 2100 through 1200Z on April 14, 2100) and include only a categorical outline, textual description, and a probability graph for severe convective storms generally. Day 2 moderate risks are fairly uncommon, and a Day 2 high risk has only been issued twice (for April 7, 2006 and for April 14, 2012). [10]

Day 3 outlooks refer to the day after tomorrow, and include the same products (categorical outline, text description, and probability graph) as the Day 2 outlook. As of June 2012, the SPC forecasts general thunderstorm risk areas. [20] Higher probability forecasts are less and less likely as the forecast period increases due to lessening forecast ability farther in advance. Day 3 moderate risks are quite rare; these have been issued only eighteen times since the product became operational (most recently for May 20, 2019). [10] [21] Day 3 high risks are never issued and the operational standards do not allow for such. This is most likely because it would require both a very high degree of certainty (60%) for an event which was still at least 48 hours away and a reasonable level of confidence that said severe thunderstorm outbreak would include significant severe weather (EF2+ tornadoes, hurricane-force winds, and/or egg-sized hail).

Day 4–8 outlooks are the longest-term official SPC Forecast Product, and often change significantly from day to day. This extended forecast for severe weather was an experimental product until March 22, 2007, when the Storm Prediction Center incorporated it as an official product. Areas are delineated in this forecast that have least a 15% or 30% chance of severe weather in the Day 4–8 period (equivalent to a slight risk and an enhanced risk, respectively); as forecaster confidence is not fully resolute on how severe weather will evolve more than three days out, the Day 4–8 outlook only outlines the areas in which severe thunderstorms are forecast to occur during the period at the 15% and 30% likelihood, and does not utilize other categorical risk areas or outline where general (non-severe) thunderstorm activity will occur. [10]

Local forecast offices of the National Weather Service, radio and television stations, and emergency planners often use the forecasts to gauge the potential severe weather threats to their areas. [10] Even after the marginal and enhanced risk categories were added in October 2014, some television stations have continued to use the original three-category system to outline forecasted severe weather risks (though stations that do this may utilize in-house severe weather outlooks that vary to some degree from the SPC convective outlooks), while certain others that have switched to the current system have chosen not to outline marginal risk areas.

Generally, the convective outlook boundaries or lines – general thunderstorms (light green), marginal (dark green), slight (yellow), enhanced (orange), moderate (red) and high (purple) – will be continued as an arrow or line not filled with color if the risk area enters another country (Canada or Mexico) or across waters beyond the United States coastline. This indicates that the risk for severe weather is also valid in that general area of the other side of the border or oceanic boundary.

Mesoscale discussions

SPC mesoscale discussions (MDs) once covered convection (mesoscale convective discussions [MCDs]) and precipitation (mesoscale precipitation discussions [MPDs]); MPDs are now issued by the Weather Prediction Center (WPC). MCDs generally precede the issuance of a tornado or severe thunderstorm watch, by one to three hours when possible. [22] Mesoscale discussions are designed to give local forecasters an update on a region where a severe weather threat is emerging and an indication of whether a watch is likely and details thereof, as well as situations of isolated severe weather when watches are not necessary. [22] MCDs contain meteorological information on what is happening and what is expected to occur in the next few hours, and forecast reasoning in regard to weather watches. [22] Mesoscale discussions are often issued to update information on watches already in effect, and sometimes when one is to be canceled. Mesoscale discussions are occasionally used as advance notice of a categorical upgrade of a scheduled convective outlook. [22]


Graphic associated with the example mesoscale discussion SPC Mesoscale discussion 2017-04-26.gif
Graphic associated with the example mesoscale discussion
     Mesoscale Discussion 0562    NWS Storm Prediction Center Norman OK    0105 PM CDT Wed Apr 26 2017     Areas affected...central AR into northwester{{Clarification}} LA and northeast TX     Concerning...Tornado Watch 162...     Valid 261805Z - 261930Z     The severe weather threat for Tornado Watch 162 continues.     SUMMARY...Isolated severe hail/gusts are the main severe risks in    the short term (next 1-3 hours).  The tornado threat may increase    later this afternoon dependent on quasi-discrete supercells    developing ahead of the cold front.     DISCUSSION...Radar mosaic shows a relatively weak squall line over    central AR with the mean flow largely parallel to the gust front of    the squall line.  Severe weather is not expected in the short term    associated with this part of the convective line.  Farther south,    strengthening updrafts on or immediately behind the surface wind    shift have led to the development of strong/severe storms developing    from northeast TX into far southwestern AR.  Isolated large    hail/severe gusts are the primary hazards with this activity over    the next 1-3 hours.       A very stout cap was noted in the 12z SHV raob and the 18z SHV raob    exhibited much reduced convective inhibition compared to this    morning in the 850-650 mb layer.  Although strengthening and veering    flow with height in the lowest 1 km was noted --resulting in some    enlargement to the hodograph-- considerable backing of flow in the    750-500 mb layer would tend to be suboptimal for tornadic low-level    mesocyclones.  Nonetheless, gradual moistening in the low levels is    expected.  It appears the risk for strong tornadoes may be less than    previously thought---although some risk remains.     ..Smith.. 04/26/2017  

Weather watches

Watches (WWs) issued by the SPC are generally less than 20,000–50,000 square miles (52,000–129,000 km2) in area and are normally preceded by a mesoscale discussion. [23] Watches are intended to be issued preceding the arrival of severe weather by one to six hours. [23] They indicate that conditions are favorable for thunderstorms capable of producing various modes of severe weather, including large hail, damaging straight-line winds and/or tornadoes. In the case of severe thunderstorm watches organized severe thunderstorms are expected but conditions are not thought to be especially favorable for tornadoes (although they can occur in such areas where one is in effect, and some severe thunderstorm watch statements issued by the SPC may note a threat of isolated tornadic activity if conditions are of modest favorability for storm rotation capable of inducing them), whereas for tornado watches conditions are thought to be favorable for severe thunderstorms to produce tornadoes. [23]

In situations where a forecaster expects a significant threat of extremely severe and life-threatening weather, a watch with special enhanced wording, "Particularly Dangerous Situation" (PDS), is subjectively issued. [24] It is occasionally issued with tornado watches, normally for the potential of major tornado outbreaks, especially those with a significant threat of multiple tornadoes capable of producing F4/EF4 and F5/EF5 damage and/or staying on the ground for long-duration – sometimes uninterrupted – paths. [24] A PDS severe thunderstorm watch is very rare and is typically reserved for derecho events impacting densely populated areas. [24]

Watches are not "warnings", where there is an immediate severe weather threat to life and property. Although severe thunderstorm and tornado warnings are ideally the next step after watches, watches cover a threat of organized severe thunderstorms over a larger area and may not always precede a warning; watch "busts" do sometimes occur should thunderstorm activity not occur at all or that which does develop never reaches the originally forecast level of severity. Warnings are issued by local National Weather Service offices, not the Storm Prediction Center, which is a national guidance center. [23]

The process of issuing a convective watch begins with a conference call from SPC to local NWS offices. If after collaboration a watch is deemed necessary, the Storm Prediction Center will issue a watch approximation product which is followed by the local NWS office issuing a specific county-based watch product. The latter product is responsible for triggering public alert messages via television, radio stations and NOAA Weather Radio. The watch approximation product outlines specific regions covered by the watch (including the approximate outlined area in statute miles) and its time of expiration (based on the local time zone(s) of the areas under the watch), associated potential threats, a meteorological synopsis of atmospheric conditions favorable for severe thunderstorm development, forecasted aviation conditions, and a pre-determined message informing the public of the meaning behind the watch and to be vigilant of any warnings or weather statements that may be issued by their local National Weather Service office. [23]

Watch outline products provide a visual map depiction of the issued watch; the SPC typically delineates watches within this product in the form of "boxes," which technically are represented as either squares, rectangles (horizontal or vertical) or parallelograms depending on the area it covers. Jurisdictions outlined by the county-based watch product as being included in the watch area may differ from the actual watch box; as such, certain counties, parishes or boroughs not covered by the fringes of the watch box may actually be included in the watch and vice versa. Watches can be expanded, contracted (by removing jurisdictions where SPC and NWS forecasters no longer consider there to be a viable threat of severe weather, in which case, the watch box may take on a trapezoidal representation in map-based watch products) or canceled before their set time of expiration by local NWS offices. [23]


Graphic associated with the example watch. Ww2.gif
Graphic associated with the example watch.
URGENT - IMMEDIATE BROADCAST REQUESTED    Tornado Watch Number 162    NWS Storm Prediction Center Norman OK    1005 AM CDT Wed Apr 26 2017     The NWS Storm Prediction Center has issued a     * Tornado Watch for portions of       Much of Arkansas      Northwest Louisiana      Northeast Texas     * Effective this Wednesday morning and afternoon from 1005 AM      until 500 PM CDT.     * Primary threats include...      A few tornadoes likely with a couple intense tornadoes possible      Widespread large hail likely with isolated very large hail        events to 2 inches in diameter possible      Widespread damaging wind gusts to 70 mph likely     SUMMARY...Severe storms will move across the watch area through this    afternoon, posing a risk of locally damaging winds, large hail, and    a few tornadoes.  An isolated strong tornado or two is also    possible.     The tornado watch area is approximately along and 90 statute miles    east and west of a line from 25 miles north of Walnut Ridge AR to 35    miles southeast of Longview TX. For a complete depiction of the    watch see the associated watch outline update (WOUS64 KWNS WOU2).     PRECAUTIONARY/PREPAREDNESS ACTIONS...     REMEMBER...A Tornado Watch means conditions are favorable for    tornadoes and 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.     &&  

Source: [25]

Fire weather products

Day 3–8 probability to categorical fire weather outlook conversion [26]
Outlook probabilityCRITICALDRY TSTM
< 10%not usedNo Area
10%not usedMARGINAL
< 40%No Areanot used
70%CRITICALnot used
An example of a Day 1 fire outlook, issued in the midst of the October 2007 California wildfires. Day 1 fire outlook October 21, 2007.png
An example of a Day 1 fire outlook, issued in the midst of the October 2007 California wildfires.

The Storm Prediction Center also is responsible for issuing fire weather outlooks (FWD) for the continental United States. These outlooks are a guidance product for local, state and federal government agencies, including local National Weather Service offices, in forecasting the potential for wildfires. [27] The outlooks issued are for Day 1, Day 2, and Days 3–8. The Day 1 product is issued at 4:00 a.m. Central Time and is updated at 1700Z, and is valid from 1200Z to 1200Z the following day. The Day 2 outlook is issued at 1000Z and is updated at 2000Z for the forecast period of 1200Z to 1200Z the following day. The Day 3–8 outlook is issued at 2200Z, and is valid from 1200Z two days after the current calendar date to 1200Z seven days after the current calendar date. [27]

There are four types of Fire Weather Outlook areas: "See Text", a "Critical Fire Weather Area for Wind and Relative Humidity", an "Extremely Critical Fire Weather Area for Wind and Relative Humidity", and a "Critical Fire Weather Area for Dry Thunderstorms". [28] The outlook type depends on the forecast weather conditions, severity of the predicted threat, and local climatology of a forecast region. [27] "See Text" is a map label used for outlining areas where fire potential is great enough to pose a limited threat, but not enough to warrant a critical area, similar to areas using the same notation title that were formerly outlined in convective outlooks. Critical Fire Weather Areas for Wind and Relative Humidity are typically issued when strong winds ( > 20 mph (32 km/h); 15 mph (24 km/h) for Florida) and low relative humidity (usually < 20%) are expected to occur where dried fuels exist, similar to a slight, enhanced, or moderate risk of severe weather. Critical Fire Weather Areas for Dry Thunderstorms are typically issued when widespread or numerous thunderstorms producing rainfall of little accumulation to provide sufficient ground wetting ( < 0.10 inches (2.5 mm)) are expected to occur where dried fuels exist. Extremely Critical Fire Weather Areas for Wind and Relative Humidity are issued when very strong winds and very low humidity are expected to occur with very dry fuels. Extremely Critical areas are issued relatively rarely, similar to the very low frequency of high risk areas in convective outlooks (see List of Storm Prediction Center extremely critical days). [28]

See also

Related Research Articles

Tornado watch A watch issued when conditions are favorable for tornadoes.

A tornado watch is issued when weather conditions are favorable for the development of severe thunderstorms that are capable of producing tornadoes. A tornado watch therefore implies that it is also a severe thunderstorm watch. A tornado watch must not be confused with a tornado warning. In most cases, the potential exists for large hail and/or damaging winds in addition to tornadoes.

Severe thunderstorm watch A watch that is issued when conditions are favorable for severe thunderstorms.

A Severe Thunderstorm Watch is issued when weather conditions are favorable for the development of severe thunderstorms that are not capable of producing tornados. If thunderstorms are expected to be of sufficient strength such that there is a significant risk that they may produce tornadoes, then a Tornado Watch is issued. A Severe Thunderstorm Watch can also be upgraded to a Tornado Watch if conditions originally forecasted for limited to no tornadic development change to allow possible tornado formation. A watch must not be confused with a Severe Thunderstorm Warning.

2000 Fort Worth tornado

During the evening hours of March 28, 2000, a powerful F3 tornado struck downtown Fort Worth, Texas, United States and adjacent areas, causing significant damage to numerous buildings and skyscrapers as well as two deaths. The tornado was part of a larger severe weather outbreak that caused widespread storms across Texas and Oklahoma in late-March, spurred primarily by the moist and unstable atmospheric environment over the South Central United States as a result of an eastward-moving upper-level low and shortwave trough. The tornado outbreak was well forecast by both computer forecast models and the National Weather Service, though the eventual focal point for the severe weather—North Texas—only came into focus on March 28 as the conditions favorable for tornadic development quickly took hold.

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.

The May 2009 derecho series was an unusually strong sequence of derecho events and tornadoes beginning on May 2, 2009 and continuing through May 8, which primarily affected the Southern United States. At least seven people were killed by the storms. An associated tornado outbreak also resulted in nearly 100 tornadoes, some strong, with most strong tornadoes, most damage, and all of the deaths on May 8. In total, nine people were killed, dozens were injured and at least $70 million in damage occurred, $58 million on May 8.

2010 New Years Eve tornado outbreak

The 2010 New Year's Eve tornado outbreak was a three-day-long tornado outbreak that impacted the central and lower Mississippi Valley from December 30, 2010 to January 1, 2011. Associated with a low pressure system and a strong cold front, 37 tornadoes tracked across five states over the length of the severe event, killing nine and injuring several others. Activity was centered in the states of Missouri and later Mississippi on December 31. Seven tornadoes were rated EF3 on the Enhanced Fujita Scale; these were the strongest during the outbreak. Non-tornadic winds were recorded to have reached as high as 80 mph (130 km/h) at eight locations on December 31, while hail as large as 2.75 in (7.0 cm) was documented north-northeast of Mansfield, Missouri. Overall, damage from the outbreak totaled US$123.3 million, most of which was related to tornadoes.

Tornado outbreak of April 9–11, 2011

The tornado outbreak of April 9–11, 2011 was one of several tornado outbreaks in the U.S. to take place during the record month of April 2011. Producing 49 tornadoes across the Midwest and Southeast, widespread damage took place; however, no fatalities resulted from the event due to timely warnings. In Wisconsin, 16 tornadoes touched down, ranking this outbreak as the state's largest April event on record as well as one of the largest single-day events during the course of any year. The strongest of these storms was an EF4 tornado that touched down south of Pocahontas, Iowa on April 9. Between 0256 and 0257 UTC that day, five tornadoes were on the ground simultaneously in Pocahontas County, Iowa, all of which were from one supercell thunderstorm.

2011 Tuscaloosa–Birmingham tornado

The 2011 Tuscaloosa–Birmingham tornado was a large and violent EF4 multiple-vortex tornado that devastated portions of Tuscaloosa and Birmingham, Alabama, as well as smaller communities and rural areas between the two cities, during the late afternoon and early evening of Wednesday, April 27, 2011. It is one of the costliest tornadoes on record. It was one of the 360 tornadoes in the 2011 Super Outbreak, the largest tornado outbreak in United States history. The tornado reached a maximum path width of 1.5 miles (2.4 km) during its track through Tuscaloosa, and once again when it crossed Interstate 65 north of Birmingham, and attained estimated winds of 190 mph (310 km/h) shortly after passing through the city. It then went on to impact parts of Birmingham as a high-end EF4 before dissipating. This was the third tornado to strike the city of Tuscaloosa in the past decade, and the second in two weeks.

2012 Tropical Storm Debby tornado outbreak

The 2012 Tropical Storm Debby tornado outbreak was a tropical cyclone-produced severe-weather event that affected the U.S. state of Florida for nearly 3 days on June 23–26, 2012. As of 12:25 p.m. EDT on June 25, the Storm Prediction Center in Norman, Oklahoma, had received 25 tornado reports from Florida, including one fatality near Venus. Throughout the entire event, 25 tornadoes touched down across the state, making the outbreak the second largest on record in Florida, behind only that spawned by Hurricane Agnes, which produced 28 tornadoes on April 18–19, 1972. At least ten of the tornadoes—the largest 24-hour total in South Florida since Hurricane Isbell produced eight in 1964—had been confirmed in four South Florida counties by the National Weather Service forecast office in Miami.

Tornado outbreak of May 18–21, 2013 tornado outbreak in the Midwestern and Southern United States

The tornado outbreak of May 18–21, 2013 was a significant tornado outbreak that affected parts of the Midwestern United States and lower Great Plains. This event occurred just days after a deadly outbreak struck Texas and surrounding southern states on May 15. On May 16, a slow moving trough crossed the Rockies and traversed the western Great Plains. Initially, activity was limited to scattered severe storms; however, by May 18, the threat for organized severe thunderstorms and tornadoes greatly increased. A few tornadoes touched down that day in Kansas and Nebraska, including an EF4 near Rozel, Kansas. Maintaining its slow eastward movement, the system produced another round of severe weather nearby. Activity significantly increased on May 19, with tornadoes confirmed in Oklahoma, Kansas, Iowa, Missouri, and Illinois. In Oklahoma, two strong tornadoes, one rated EF4, caused significant damage in rural areas of the eastern Oklahoma City metropolitan area; two people lost their lives near Shawnee. The most dramatic events unfolded on May 20 as a large EF5 tornado devastated parts of Moore, Oklahoma, killing 24 people. Thousands of structures were destroyed, with many being completely flattened. Several other tornadoes occurred during the day in areas further eastward, though the majority were weak and caused little damage.

Tornado outbreak of November 17, 2013

The tornado outbreak of November 17, 2013, was the deadliest and costliest in the U.S. state of Illinois to occur in the month of November and fourth largest for the state overall. With more than 30 tornadoes in Indiana, it was that state's largest tornado outbreak for the month of November, and the second larget outbreak recorded in Indiana. Associated with a strong trough in the upper levels of the atmosphere, the event resulted in 73 tornadoes tracking across regions of the Midwest United States and Ohio River Valley, impacting seven states. Severe weather during the tornado outbreak caused over 100 injuries and eleven fatalities, of which eight were tornado related. Two tornadoes—both in Illinois and rated EF4 on the Enhanced Fujita Scale—were the strongest documented during the outbreak and combined for five deaths. In addition to tornadoes, the system associated with the outbreak produced sizeable hail peaking at 4.00 in (10.2 cm) in diameter in Bloomington, Illinois, as well as damaging winds estimated as strong as 100 mph (160 km/h) in three locations.

The following is a glossary of tornado terms. It includes scientific as well as selected informal terminology.

Tornado outbreak of June 16–18, 2014

The tornado outbreak of June 16–18, 2014, was a tornado outbreak concentrated in the Great Plains and the Midwestern United States. Two tornadoes also occurred in Ontario. The severe weather event most significantly affected the state of Nebraska, where twin EF4 tornadoes killed two and critically injured twenty others in and around the town of Pilger on the evening of June 16. The two Pilger tornadoes were part of a violent tornado family that produced four consecutive EF4 tornadoes and was broadcast live on television. The outbreak went on to produce multiple other strong tornadoes across the northern Great Plains states throughout the next two days.

Tornado outbreak of April 8–9, 2015

The tornado outbreak of April 8–9, 2015 was a relatively small but damaging outbreak of tornadoes that occurred in parts of the Great Plains and in the Midwestern United States. 27 tornadoes were confirmed during the two days, most of them weak, however a select few of them were powerful and damaging.

Tornado outbreak of November 30 – December 2, 2018 Weather event in the central US

The tornado outbreak of November 30 – December 2, 2018 was a late-season tornado outbreak that occurred across portions of the West South Central states and Midwestern United States. As a potent shortwave trough moved across the southern portions of the country, it was met with ample moisture return and destabilization, resulting in widespread severe thunderstorms that produced damaging winds, hail, and tornadoes. The event began late on November 30 in Oklahoma, spreading east and resulting in one fatality in Aurora, Missouri. Several tornadic supercells moved across portions of Illinois on December 1, and resulted in 29 confirmed tornadoes. This outbreak was the largest December tornado event on record in Illinois history, surpassing the December 1957 tornado outbreak sequence. The most significant tornado of the event was an EF3 that impacted Taylorville, Illinois, damaging or destroying hundreds of structures and injuring 22 people.

Tornado outbreak of April 17–19, 2019

The tornado outbreak of April 17–19, 2019 was a multi-day, widespread severe weather event stretching from the South Central United States to the East Coast of the United States. On the heels of a significant tornado outbreak just a few days prior, another potent upper-level trough progressed eastward and served as the impetus for widespread, damaging thunderstorms. The outbreak began on April 17 with several short-lived, generally weak tornadoes across Kansas, Oklahoma, and Texas. The following day, a total of 44 tornadoes were recorded across central Mississippi, tying the Hurricane Rita tornado outbreak as the largest in Mississippi state history. On April 19, the event spread eastward. North Carolina recorded 12 tornadoes, the state's sixth largest outbreak in a single day, while Virginia recorded 16 tornadoes, its third-most in a 24-hour period. Overall, 96 tornadoes were confirmed, the strongest of which was a high-end EF3 that heavily damaged homes and outbuildings north of Oak Level, Virginia. There were no fatalities recorded in association with tornadic activity, but four people were killed by fallen trees in strong straight-line winds.

Tornado outbreak sequence of May 2019 Severe weather event

The tornado outbreak sequence of May 2019 was a prolonged series of destructive tornadoes and tornado outbreaks affecting the United States over the course of nearly two weeks, producing at least 301 tornadoes, including 50 significant events (EF2+). Eighteen of these were EF3 tornadoes, spanning over multiple states, including Nebraska, Kansas, Texas, Missouri, Oklahoma, Indiana, Iowa, and Ohio, with additional tornadoes confirmed across a region extending from California to New Jersey. Two EF4 tornadoes occurred, one in Dayton, Ohio, and the other in Linwood, Kansas. Four tornadoes during this outbreak were fatal, causing a total of 8 fatalities. The deadliest of these occurred on May 22 near Golden City, Missouri, where an EF3 tornado took three lives, including an elderly couple in their eighties.


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Coordinates: 35°10′53″N97°26′25″W / 35.18139°N 97.44028°W / 35.18139; -97.44028