Storm Prediction Center
Encyclopedia
The Storm Prediction Center (SPC), located in Norman
, Oklahoma
, is tasked with forecasting the risk of severe thunderstorms and tornadoes in the contiguous United States. The agency issues convective outlooks, mesoscale discussions, and watches as a part of this process. Convective outlooks are issued for Day 1, Day 2, Day 3, and Day 4–8, and detail the risk of severe thunderstorms and tornado
es 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 used a probabilistic scale, determining the probability for a severe weather event in percent. Mesoscale discussions are issued to give information on a region that is becoming a severe weather threat and states whether a watch is likely and details thereof, 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.
The agency is also responsible for forecasting fire weather (conditions favorable for wildfires) in the contiguous US, and issues Day 1, 2, and 3–8 fire weather outlooks. These outlooks detail areas with critical or extremely critical fire conditions.
The Storm Prediction Center 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 Oceanic and Atmospheric Administration
(NOAA) of the United States
Department of Commerce
(DoC).
The Storm Prediction Center was previously known as the National Severe Storms Forecast Center and was located in Kansas City
, Missouri
. In October 1995, the National Severe Storms Forecast Center relocated to Norman and was renamed the Storm Prediction Center. From the time of the move until 2006, it was co-located with the National Severe Storms Laboratory
at University of Oklahoma Westheimer Airport
. In 2006, they moved into the National Weather Center
.
as SELS (Severe Local Storms Unit), a special unit of forecasters in the Weather Bureau. In 1954, the unit moved to Kansas City. SELS began issuing convective outlooks in 1955, and began issuing radar summaries every three hours in 1960; with the increased duties of radar summaries this unit became the National Severe Storms Forecast Center (NSSFC) in 1966.
In 1968 the National Severe Storms Forecast Center began issuing status reports on watches, and in 1971 the agency made their first computerized data transmission. On April 2, 1982 the first particularly dangerous situation
watch was issued. Two new products were introduced in 1986: the Day 2 Convective Outlook and the Mesoscale Discussion.
The National Severe Storms Forecast Center remained located in Kansas City, Missouri until October 1995, when it moved to Norman, Oklahoma and was renamed the Storm Prediction Center. In 1998, the Center began issuing the National Fire Weather Outlook. The Day 3 Convective Outlook was first issued on an experimental basis in 2000, and was made an official product in 2001. From 1995 to 2006 the Storm Prediction Center was housed at University of Oklahoma Westheimer Airport
, in the same building as the National Severe Storms Laboratory, after which it moved to the National Weather Center
.
The Storm Prediction Center continues operations out of the National Weather Center building as of 2011.
caused by severe thunderstorms, specifically those producing tornadoes, hail 1 inch (2.5 cm) or larger, and winds 58 mph (93 km/h) or greater. The agency also forecasts hazardous winter
and fire weather. It does so primarily by issuing convective outlooks, severe thunderstorm watch
es, tornado watch
es, and mesoscale discussions.
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, where they are expected to occur, and in what time frame. If warranted, forecasts will also increase in severity through this three-stage process.
The Storm Prediction Center employs a total of 43 personnel, including five lead forecasters, ten mesoscale/outlook forecasters, and seven assistant mesoscale forecasters.
The Storm Prediction Center 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 Oceanic and Atmospheric Administration
(NOAA) of the United States
Department of Commerce
(DoC).
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.
The Storm Prediction Center issues categorical and probability forecasts describing the general threat of severe convective storms over the contiguous United States for the next 6–192 hours (Day 1–Day 8). They are labeled and issued by day and are issued up to five times a day.
The categorical risks are general thunderstorms (green shaded area/previously brown line before April 2011), "SEE TEXT" (black, textual label on map indicating potential for isolated severe storms or near-severe storms), "SLGT" (yellow shaded area/previously green line indicating slight risk of severe weather), "MDT" (red shaded area/previously red line indicating moderate risk of severe weather), and "HIGH" (pink shaded area/previously fuchsia line indicating 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" (F2/EF2
or stronger tornado, 2 inches (5 cm) or larger hail, or 75 mph (120 km/h) winds or greater) level.
In April of 2011, the SPC began issuing new graphics for categorical and probabilistic outlooks. The new format includes shading of risk areas and population, county, and interstate overlays. The colors were changed as mentioned above as well. The new shaded maps also include changes to the probability color shades on each outlook.
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.
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 (including some strong tornadoes), more widespread or severe wind damage and very large/destructive hail could occur. Major events, such as large tornado outbreaks, are sometimes also possible on moderate risk days, but with greater uncertainty. Moderate risk days are not uncommon and typically occur several times a month, especially during peak season. A slight risk area typically surrounds a moderate risk area, where the threat is lower.
A high risk day indicates a considerable likelihood of 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, including widespread strong or violent tornadoes and/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 hail as well). Many of the most prolific severe weather days were high risk days. Such days are quite rare; a high risk is typically issued only a few times each year (see List of SPC 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.
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. SLGT is slight risk, meaning that well organized severe thunderstorms are expected, but low in number or coverage. MDT moderate risk, indicating that greater concentration and magnitude of severe weather is expected than would be expected in a slight risk. HIGH a high risk of severe weather, and indicates that a major severe weather outbreak is expected, with a high concentration of severe weather and enhanced risk of extremely severe weather. On high risk days the potential exists for 20 or more tornadoes (with some EF2
or stronger possible) or a extreme derecho
(with winds of 80 miles per hour (128.7 km/h) or greater possible). SEE TEXT means that there is a threat for severe weather, but the threat is not high enough to warrant a slight risk.
The Day 1 Convective Outlook, issued five times per day at 0600Z (valid 1200Z that day until 1200Z the following day), 1300Z and 1630Z (the "morning updates," valid until 1200Z the next day), 2000Z (the "afternoon update," valid until 1200Z the next 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 probabilities specifically for tornadoes, hail, or wind. It is the most descriptive and highest accuracy outlook.
Day 2 outlooks, issued twice daily at 0600Z and 1730Z, refer to tomorrow's weather (1200Z–1200Z of the next calendar day; for example a day 2 outlook issued on April 12, 2100 would be valid from 1200Z April 13, 2100 through 1200Z 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 once (for April 7, 2006).
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. Higher probability forecasts are less and less likely as the forecast period increases due to lessening forecast ability farther in advance. No attempt is made to forecast general thunderstorms and a high risk is never issued that far out. Day 3 moderate risks are also quite rare; it has been used only ten times since the product became operational (most recently for April 27, 2011).
Day 4–8 outlooks are the longest-term official SPC Forecast Product, and often change significantly from day to day. They were an experimental product until March 22, 2007 when they became an official product. Areas are delineated in this forecast that have least a 30% chance of severe weather in the day 4–8 period (equivalent to a mid-range slight risk).
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.
es organized severe thunderstorms are expected but conditions are not thought to be especially favorable for tornadoes, whereas for tornado watch
es conditions are thought favorable for severe thunderstorms producing tornadoes. In situations where a forecaster expects a significant threat of extremely severe and life-threatening weather, a watch with special wording of "particularly dangerous situation
" (PDS) is subjectively issued. It is occasionally issued with tornado watches, normally for the potential of major tornado outbreaks. A PDS severe thunderstorm watch is very rare and usually issued for the potential of major derecho events.
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. Warnings are issued by local National Weather Service offices, not the Storm Prediction Center, which is a national guidance center.
Watches are canceled by the local National Weather Service office.
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 Extreme Conditions of Wind and Relative Humidity", and a "Critical Fire Weather Area for Dry Thunderstorms". The outlook type depends on forecast weather conditions, severity of the predicted threat, and local climatology of a forecast region. "See Text" is a label on the map for pointing out areas where fire potential is great enough to pose a limited threat, but not enough to warrant a critical area, similar to "See Text" areas in convective outlooks. Critical Fire Weather Areas for Wind and Relative Humidity are typically issued when strong winds (>20 mph) and low Relative Humidity
are expected to occur where dried fuels exist, similar to a slight or moderate risk of severe weather. Critical Fire Weather Areas for Dry Thunderstorms are typically issued when widespread or numerous thunderstorms producing little wetting rain (<0.10 in) 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 RH are expected to occur with very dry fuels. Extremely Critical areas are rarely issued, similar to the very low frequency of high risk areas in convective outlooks.
Norman, Oklahoma
Norman is a city in Cleveland County, Oklahoma, United States, and is located south of downtown Oklahoma City. It is part of the Oklahoma City metropolitan area. As of the 2010 census, Norman was to have 110,925 full-time residents, making it the third-largest city in Oklahoma and the...
, Oklahoma
Oklahoma
Oklahoma is a state located in the South Central region of the United States of America. With an estimated 3,751,351 residents as of the 2010 census and a land area of 68,667 square miles , Oklahoma is the 28th most populous and 20th-largest state...
, is tasked with forecasting the risk of severe thunderstorms and tornadoes in the contiguous United States. The agency issues convective outlooks, mesoscale discussions, and watches as a part of this process. Convective outlooks are issued for Day 1, Day 2, Day 3, and Day 4–8, and detail the risk of severe thunderstorms and tornado
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
es 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 used a probabilistic scale, determining the probability for a severe weather event in percent. Mesoscale discussions are issued to give information on a region that is becoming a severe weather threat and states whether a watch is likely and details thereof, 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.
The agency is also responsible for forecasting fire weather (conditions favorable for wildfires) in the contiguous US, and issues Day 1, 2, and 3–8 fire weather outlooks. These outlooks detail areas with critical or extremely critical fire conditions.
The Storm Prediction Center is part of the National Centers for Environmental Prediction
National Centers for Environmental Prediction
The United States National Centers for Environmental Prediction delivers national and global weather, water, climate and space weather guidance, forecasts, warnings and analyses to its Partners and External User Communities...
(NCEP), operating under the control of the National Weather Service
National Weather Service
The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...
(NWS), which in turn is part of the National Oceanic and Atmospheric Administration
National Oceanic and Atmospheric Administration
The National Oceanic and Atmospheric Administration , pronounced , like "noah", is a scientific agency within the United States Department of Commerce focused on the conditions of the oceans and the atmosphere...
(NOAA) of the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
Department of Commerce
United States Department of Commerce
The United States Department of Commerce is the Cabinet department of the United States government concerned with promoting economic growth. It was originally created as the United States Department of Commerce and Labor on February 14, 1903...
(DoC).
The Storm Prediction Center was previously known as the National Severe Storms Forecast Center and was located in Kansas City
Kansas City, Missouri
Kansas City, Missouri is the largest city in the U.S. state of Missouri and is the anchor city of the Kansas City Metropolitan Area, the second largest metropolitan area in Missouri. It encompasses in parts of Jackson, Clay, Cass, and Platte counties...
, Missouri
Missouri
Missouri is a US state located in the Midwestern United States, bordered by Iowa, Illinois, Kentucky, Tennessee, Arkansas, Oklahoma, Kansas and Nebraska. With a 2010 population of 5,988,927, Missouri is the 18th most populous state in the nation and the fifth most populous in the Midwest. It...
. In October 1995, the National Severe Storms Forecast Center relocated to Norman and was renamed the Storm Prediction Center. From the time of the move until 2006, it was co-located with the National Severe Storms Laboratory
National Severe Storms Laboratory
The National Severe Storms Laboratory is a National Oceanic and Atmospheric Administration weather research laboratory located at the National Weather Center in Norman, Oklahoma....
at University of Oklahoma Westheimer Airport
University of Oklahoma Westheimer Airport
University of Oklahoma Westheimer Airport , also known as Max Westheimer Airport, is a public general aviation airport located three miles northwest of the city of Norman in Cleveland County, Oklahoma, USA. The airport is run by the University of Oklahoma...
. In 2006, they moved into the National Weather Center
National Weather Center
The National Weather Center , located on the campus of the University of Oklahoma, is a confederation of federal, state, and academic organizations that work together in partnership to improve understanding of events occurring in Earth's atmosphere over a wide range of time and space scales...
.
History
The Storm Prediction Center began in 1952 in Washington, D.C.Washington, D.C.
Washington, D.C., formally the District of Columbia and commonly referred to as Washington, "the District", or simply D.C., is the capital of the United States. On July 16, 1790, the United States Congress approved the creation of a permanent national capital as permitted by the U.S. Constitution....
as SELS (Severe Local Storms Unit), a special unit of forecasters in the Weather Bureau. In 1954, the unit moved to Kansas City. SELS began issuing convective outlooks in 1955, and began issuing radar summaries every three hours in 1960; with the increased duties of radar summaries this unit became the National Severe Storms Forecast Center (NSSFC) in 1966.
In 1968 the National Severe Storms Forecast Center began issuing status reports on watches, and in 1971 the agency made their first computerized data transmission. On April 2, 1982 the first particularly dangerous situation
Particularly dangerous situation
A particularly dangerous situation is a type of enhanced wording first used by the National Weather Service's Storm Prediction Center in Norman, Oklahoma on certain severe weather watches...
watch was issued. Two new products were introduced in 1986: the Day 2 Convective Outlook and the Mesoscale Discussion.
The National Severe Storms Forecast Center remained located in Kansas City, Missouri until October 1995, when it moved to Norman, Oklahoma and was renamed the Storm Prediction Center. In 1998, the Center began issuing the National Fire Weather Outlook. The Day 3 Convective Outlook was first issued on an experimental basis in 2000, and was made an official product in 2001. From 1995 to 2006 the Storm Prediction Center was housed at University of Oklahoma Westheimer Airport
University of Oklahoma Westheimer Airport
University of Oklahoma Westheimer Airport , also known as Max Westheimer Airport, is a public general aviation airport located three miles northwest of the city of Norman in Cleveland County, Oklahoma, USA. The airport is run by the University of Oklahoma...
, in the same building as the National Severe Storms Laboratory, after which it moved to the National Weather Center
National Weather Center
The National Weather Center , located on the campus of the University of Oklahoma, is a confederation of federal, state, and academic organizations that work together in partnership to improve understanding of events occurring in Earth's atmosphere over a wide range of time and space scales...
.
The Storm Prediction Center continues operations out of the National Weather Center building as of 2011.
Overview
The Storm Prediction Center is responsible for forecasting the risk of severe weatherSevere weather
Severe weather phenomena are weather conditions that are hazardous to human life and property.- Examples Include :Severe weather can occur under a variety of situations, but three characteristics are generally needed: a temperature or moisture boundary, moisture, and , instability in the...
caused by severe thunderstorms, specifically those producing tornadoes, hail 1 inch (2.5 cm) or larger, and winds 58 mph (93 km/h) or greater. The agency also forecasts hazardous winter
Winter storm
A winter storm is an event in which the dominant varieties of precipitation are formed that only occur at low temperatures, such as snow or sleet, or a rainstorm where ground temperatures are low enough to allow ice to form...
and fire weather. It does so primarily by issuing convective outlooks, severe thunderstorm watch
Severe thunderstorm watch
[File:Annual_severe_thunderstorm_watch_frequency_in_the_United_States.svg|thumb|right|300px|Map of average annual severe thunderstorm watches in the United States between 1999 and 2008.Legend:...
es, tornado watch
Tornado watch
A tornado watch is issued when conditions are right for a tornado to form. Since any thunderstorm capable of producing a tornado is defined as severe, a tornado watch is also automatically a severe thunderstorm watch...
es, and mesoscale discussions.
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, where they are expected to occur, and in what time frame. If warranted, forecasts will also increase in severity through this three-stage process.
The Storm Prediction Center employs a total of 43 personnel, including five lead forecasters, ten mesoscale/outlook forecasters, and seven assistant mesoscale forecasters.
The Storm Prediction Center is part of the National Centers for Environmental Prediction
National Centers for Environmental Prediction
The United States National Centers for Environmental Prediction delivers national and global weather, water, climate and space weather guidance, forecasts, warnings and analyses to its Partners and External User Communities...
(NCEP), operating under the control of the National Weather Service
National Weather Service
The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...
(NWS), which in turn is part of the National Oceanic and Atmospheric Administration
National Oceanic and Atmospheric Administration
The National Oceanic and Atmospheric Administration , pronounced , like "noah", is a scientific agency within the United States Department of Commerce focused on the conditions of the oceans and the atmosphere...
(NOAA) of the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
Department of Commerce
United States Department of Commerce
The United States Department of Commerce is the Cabinet department of the United States government concerned with promoting economic growth. It was originally created as the United States Department of Commerce and Labor on February 14, 1903...
(DoC).
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.
Convective outlooks
The categorical risks are general thunderstorms (green shaded area/previously brown line before April 2011), "SEE TEXT" (black, textual label on map indicating potential for isolated severe storms or near-severe storms), "SLGT" (yellow shaded area/previously green line indicating slight risk of severe weather), "MDT" (red shaded area/previously red line indicating moderate risk of severe weather), and "HIGH" (pink shaded area/previously fuchsia line indicating 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" (F2/EF2
Enhanced Fujita Scale
The Enhanced Fujita Scale rates the strength of tornadoes in the United States based on the damage they cause.Implemented in place of the Fujita scale introduced in 1971 by Ted Fujita, it began operational use on February 1, 2007. The scale has the same basic design as the original Fujita scale:...
or stronger tornado, 2 inches (5 cm) or larger hail, or 75 mph (120 km/h) winds or greater) level.
In April of 2011, the SPC began issuing new graphics for categorical and probabilistic outlooks. The new format includes shading of risk areas and population, county, and interstate overlays. The colors were changed as mentioned above as well. The new shaded maps also include changes to the probability color shades on each outlook.
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.
Categories
A slight risk day typically will mean the threat exists for scattered severe weather, including scattered wind damage or severe hail and possibly some isolated tornadoes. During the peak severe weather season, most days will have a slight risk somewhere in the US. Isolated significant severe events are possible in some circumstances, but are generally not widespread.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 (including some strong tornadoes), more widespread or severe wind damage and very large/destructive hail could occur. Major events, such as large tornado outbreaks, are sometimes also possible on moderate risk days, but with greater uncertainty. Moderate risk days are not uncommon and typically occur several times a month, especially during peak season. A slight risk area typically surrounds a moderate risk area, where the threat is lower.
A high risk day indicates a considerable likelihood of a major tornado outbreak or (much less often) an extreme derecho
Derecho
A derecho is a widespread and long-lived, violent convectively induced straight-line windstorm that is associated with a fast-moving band of severe thunderstorms in the form of a squall line usually taking the form of a bow echo...
event. On these days, the potential exists for extremely severe and life-threatening weather, including widespread strong or violent tornadoes and/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 hail as well). Many of the most prolific severe weather days were high risk days. Such days are quite rare; a high risk is typically issued only a few times each year (see List of SPC 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.
Issuance and usage
Convective outlooks are issued in Zulu time (also known as UTC).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. SLGT is slight risk, meaning that well organized severe thunderstorms are expected, but low in number or coverage. MDT moderate risk, indicating that greater concentration and magnitude of severe weather is expected than would be expected in a slight risk. HIGH a high risk of severe weather, and indicates that a major severe weather outbreak is expected, with a high concentration of severe weather and enhanced risk of extremely severe weather. On high risk days the potential exists for 20 or more tornadoes (with some EF2
Enhanced Fujita Scale
The Enhanced Fujita Scale rates the strength of tornadoes in the United States based on the damage they cause.Implemented in place of the Fujita scale introduced in 1971 by Ted Fujita, it began operational use on February 1, 2007. The scale has the same basic design as the original Fujita scale:...
or stronger possible) or a extreme derecho
Derecho
A derecho is a widespread and long-lived, violent convectively induced straight-line windstorm that is associated with a fast-moving band of severe thunderstorms in the form of a squall line usually taking the form of a bow echo...
(with winds of 80 miles per hour (128.7 km/h) or greater possible). SEE TEXT means that there is a threat for severe weather, but the threat is not high enough to warrant a slight risk.
The Day 1 Convective Outlook, issued five times per day at 0600Z (valid 1200Z that day until 1200Z the following day), 1300Z and 1630Z (the "morning updates," valid until 1200Z the next day), 2000Z (the "afternoon update," valid until 1200Z the next 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 probabilities specifically for tornadoes, hail, or wind. It is the most descriptive and highest accuracy outlook.
Day 2 outlooks, issued twice daily at 0600Z and 1730Z, refer to tomorrow's weather (1200Z–1200Z of the next calendar day; for example a day 2 outlook issued on April 12, 2100 would be valid from 1200Z April 13, 2100 through 1200Z 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 once (for April 7, 2006).
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. Higher probability forecasts are less and less likely as the forecast period increases due to lessening forecast ability farther in advance. No attempt is made to forecast general thunderstorms and a high risk is never issued that far out. Day 3 moderate risks are also quite rare; it has been used only ten times since the product became operational (most recently for April 27, 2011).
Day 4–8 outlooks are the longest-term official SPC Forecast Product, and often change significantly from day to day. They were an experimental product until March 22, 2007 when they became an official product. Areas are delineated in this forecast that have least a 30% chance of severe weather in the day 4–8 period (equivalent to a mid-range slight risk).
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.
Mesoscale discussions
Mesoscale discussions (MCDs) generally precede a tornado watch or severe thunderstorm watch, by 1–3 hours when possible. 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. MCDs contain meteorological information on what is happening and what is expected to happen in the next few hours, and forecast reasoning in regard to weather watches. Mesoscale discussions are often issued to update information on watches already issued, and sometimes when one is to be canceled. Mesoscale discussions are also issued for winter weather and heavy rainfall events.Example
Weather watches
Watches issued by the SPC are generally less than 20000–50000 sq mi (51,799.8–129,499.4 km2) in area and are normally preceded by a mesoscale discussion. Watches are intended to be issued preceding arrival of severe weather by 1–6 hours. They indicate that conditions are favorable for severe thunderstorms or tornadoes. In the case of severe thunderstorm watchSevere thunderstorm watch
[File:Annual_severe_thunderstorm_watch_frequency_in_the_United_States.svg|thumb|right|300px|Map of average annual severe thunderstorm watches in the United States between 1999 and 2008.Legend:...
es organized severe thunderstorms are expected but conditions are not thought to be especially favorable for tornadoes, whereas for tornado watch
Tornado watch
A tornado watch is issued when conditions are right for a tornado to form. Since any thunderstorm capable of producing a tornado is defined as severe, a tornado watch is also automatically a severe thunderstorm watch...
es conditions are thought favorable for severe thunderstorms producing tornadoes. In situations where a forecaster expects a significant threat of extremely severe and life-threatening weather, a watch with special wording of "particularly dangerous situation
Particularly dangerous situation
A particularly dangerous situation is a type of enhanced wording first used by the National Weather Service's Storm Prediction Center in Norman, Oklahoma on certain severe weather watches...
" (PDS) is subjectively issued. It is occasionally issued with tornado watches, normally for the potential of major tornado outbreaks. A PDS severe thunderstorm watch is very rare and usually issued for the potential of major derecho events.
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. Warnings are issued by local National Weather Service offices, not the Storm Prediction Center, which is a national guidance center.
Watches are canceled by the local National Weather Service office.
Example
Fire weather products
The Storm Prediction Center also is responsible for issuing fire weather outlooks 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. 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 for days 3–8.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 Extreme Conditions of Wind and Relative Humidity", and a "Critical Fire Weather Area for Dry Thunderstorms". The outlook type depends on forecast weather conditions, severity of the predicted threat, and local climatology of a forecast region. "See Text" is a label on the map for pointing out areas where fire potential is great enough to pose a limited threat, but not enough to warrant a critical area, similar to "See Text" areas in convective outlooks. Critical Fire Weather Areas for Wind and Relative Humidity are typically issued when strong winds (>20 mph) and low Relative Humidity
Relative humidity
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...
are expected to occur where dried fuels exist, similar to a slight or moderate risk of severe weather. Critical Fire Weather Areas for Dry Thunderstorms are typically issued when widespread or numerous thunderstorms producing little wetting rain (<0.10 in) 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 RH are expected to occur with very dry fuels. Extremely Critical areas are rarely issued, similar to the very low frequency of high risk areas in convective outlooks.