The Storm Prediction Center (SPC), located in Norman, Oklahoma, 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).
Until October 1995, the SPC was known as the National Severe Storms Forecast Center (NSSFC) and was located in Kansas City, Missouri. From 1995 to 2006 it was housed in the same building as the National Severe Storms Laboratory, after which it moved to the National Weather Center. It began in 1952 in Washington, D.C. as a special unit of forecasters in the Weather Bureau before moving to Kansas City in 1954; and with increased duties this unit became the NSSFC in 1966.
The Storm Prediction Center is tasked with forecasting the risk of convective severe weather in the contiguous United States, and 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 tornadoes during the given forecast period. 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 1 hour.
The Storm Prediction Center is responsible for forecasting (identifying, describing, and quantifying) the risk of severe weather caused by severe convective storms - specifically, those producing tornadoes, hail 3/4" (2 cm) or larger, and winds 58 mph (93 km/h) or greater - as well as winter and fire weather. It does so primarily by issuing convective outlooks (AC), severe thunderstorm watches (WS) and tornado watches (WT) ("weather watches" (WW)), and mesoscale discussions (MCD or MD).
There is a three-stage process in which the area, time period, and details of weather parameters forecast goes from more general to more specific, also with heightening alertness.
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 (GEN TSTMS) (brief textual description only - brown line), "SEE TEXT" (area on map for focusing attention only for isolated severe or near-severe weather), "SLGT" (slight risk of severe weather - green line), "MDT" (moderate risk of severe weather - red line), and "HIGH" (high risk of severe weather - fuchsia line). Critical areas (referred to as "hatched areas" because of their representation on outlook maps) refer to a threat of increased magnitude; that is of "significant severe" (F2/EF2 or stronger tornado, 2" (5 cm) or larger hail, 75 mph (120 km/h) winds or greater) either generally or specific for the phenomenon.
Public severe weather outlooks (PWO) are issued when a significant or widespread outbreak is expected, especially for tornadoes.
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. These are not uncommon and are typically issued several times each month during the 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. This is quite rare; a high risk is typically issued only a few times each year. High risk areas are usually surrounded by a larger moderate risk area, where uncertainty is greater or the threat is somewhat lower.
Note that these are merely forecasts and are not always correctly hit; tornado outbreaks are known to have happened when only a slight risk was forecast, and high risk busts are not uncommon.
The Day 1 Convective Outlook, issued five times per day at 0600Z (the initial day-1 outlook, valid 1200Z that day until 1200Z the following day), 1300Z and 1630Z (the "morning updates," valid until 12Z 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 06Z and 1730Z, refer to tomorrow's weather (12Z-12Z of the next calendar day) 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 "break outs" 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. Day 3 moderate risks are also quite rare; it has been used less than ten times (most recently for May 13, 2009).
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.
The categories refer to the following risk levels for the specific severe weather event occurring within 25 miles (40 km) of any point in the delineated region (H = Hatched area; 2% and 10% are only used for tornadoes):
|5%||SLGT||SEE TEXT||SEE TEXT||SEE TEXT¹|
Mesoscale discussions 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 that is becoming a severe weather threat 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.
MESOSCALE DISCUSSION 0685 NWS STORM PREDICTION CENTER NORMAN OK 0253 PM CDT FRI MAY 04 2007 AREAS AFFECTED...WRN KS...PARTS OF WRN OK/ERN TX PNHDL CONCERNING...SEVERE POTENTIAL...TORNADO WATCH LIKELY VALID 041953Z - 042200Z TRENDS ARE BEING CLOSELY MONITORED FOR SIGNS OF CONVECTIVE INITIATION. ALTHOUGH TIMING IS STILL A BIT UNCERTAIN...ONE OR MORE TORNADO WATCHES WILL PROBABLY BE REQUIRED LATE THIS AFTERNOON. MOISTENING SOUTHERLY LOW-LEVEL FLOW AND STRONG DAYTIME HEATING ALONG SOUTHERN HIGH PLAINS DRY LINE...INTO THE VICINITY OF WEAK SURFACE LOW OVER SOUTHWEST KANSAS...IS CONTRIBUTING TO STRONG DESTABILIZATION. RUC GUIDANCE INDICATES MIXED LAYER CAPE IS INCREASING INTO THE 3000-4000 J/KG RANGE...THOUGH MID-LEVEL SUBSIDENCE/SHORT WAVE RIDGING ALOFT IS CURRENTLY INHIBITING CONVECTIVE DEVELOPMENT. HOWEVER...MID/UPPER FORCING ASSOCIATED WITH AN IMPULSE LIFTING OUT OF AMPLIFIED WESTERN TROUGH IS BEGINNING TO SHIFT EAST OF THE CENTRAL/SOUTHERN ROCKIES. AND...LATEST RUC GUIDANCE SUGGESTS WEAK LOWER/MID TROPOSPHERIC COOLING HAS OCCURRED ACROSS WESTERN KANSAS INTO THE TEXAS PANHANDLE ASSOCIATED WITH IMPULSE ALREADY LIFTING NORTHWARD THROUGH THE NORTH CENTRAL HIGH PLAINS. ALTHOUGH UNCERTAINTY DOES EXISTS CONCERNING TIMING OF CONVECTIVE INITIATION...MUCH OF MODEL GUIDANCE SUGGESTS THAT THIS COULD OCCUR AS EARLY AS 22-23Z. THIS SEEMS MOST PROBABLE WHERE FORCING WILL BE STRONGEST NEAR SURFACE LOW...BUT INITIATION OF WIDELY SCATTERED STORMS MAY QUICKLY FOLLOW SUIT. STORM DEVELOPMENT/INTENSIFICATION WILL LIKELY BE VERY RAPID ONCE CAP BREAKS. AND...LARGE CLOCKWISE CURVED LOW-LEVEL HODOGRAPHS BENEATH 40-50 KT CYCLONIC WEST SOUTHWESTERLY 500 MB FLOW WILL BE FAVORABLE FOR TORNADOES...IN ADDITION TO THE RISK OF VERY LARGE HAIL. ISOLATED STRONG TORNADOES ARE POSSIBLE...PARTICULARLY AS LOW-LEVEL JET STRENGTHENS NEAR/SHORTLY AFTER 04/00-01Z. ..KERR.. 05/04/2007 ATTN...WFO...ICT...OUN...GID...DDC...GLD...LUB...AMA...
Watches issued by the SPC are generally less than 20,000-50,000 square miles 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 watches ("blue box"), organized severe thunderstorms are expected but conditions aren't thought to be especially favorable for tornadoes, whereas for tornado watches ("red box") 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.
URGENT - IMMEDIATE BROADCAST REQUESTED TORNADO WATCH NUMBER 232 NWS STORM PREDICTION CENTER NORMAN OK 955 AM CDT SAT MAY 5 2007 THE NWS STORM PREDICTION CENTER HAS ISSUED A TORNADO WATCH FOR PORTIONS OF PARTS OF WESTERN AND CENTRAL KANSAS PARTS OF SOUTHWEST AND CENTRAL NEBRASKA EFFECTIVE THIS SATURDAY MORNING AND EVENING FROM 955 AM UNTIL 1000 PM CDT. ...THIS IS A PARTICULARLY DANGEROUS SITUATION... DESTRUCTIVE TORNADOES...LARGE HAIL TO 4 INCHES IN DIAMETER... THUNDERSTORM WIND GUSTS TO 90 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS. THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 100 STATUTE MILES EAST AND WEST OF A LINE FROM 45 MILES NORTH NORTHWEST OF BROKEN BOW NEBRASKA TO 55 MILES SOUTHWEST OF RUSSELL KANSAS. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU2). 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. DISCUSSION...VERY POTENT TORNADIC SUPERCELL PATTERN IN PLACE ACROSS WATCH AREA AS AIR MASS IS EXTREMELY UNSTABLE WITH VERY FAVORABLE SHEAR PROFILES. WITH LITTLE INHIBITION REMAINING ALONG E OF DRY LINE...STORMS WILL RAPIDLY BECOME SEVERE BY EARLY THIS AFTERNOON WRN KS INTO SWRN NEB. TORNADIC SUPERCELLS WILL DEVELOP WITH POTENTIAL FOR LONG TRACK/VIOLENT TORNADOS. AS DRY LINE REMAINS WRN KS THRU THE AFTERNOON...ADDITIONAL DEVELOPMENT OF TORNADIC SUPERCELLS ARE LIKELY OFF THE DRY LINE THRU THE EVENING HOURS. AVIATION...TORNADOES AND 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 600. MEAN STORM MOTION VECTOR 22040. ...HALES