Storm Prediction Center
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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.
OverviewThe 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 forecasted goes from more general to more specific, also with heightening alertness. Convective outlooksImage:SPC severe outlook 04072006.png
Day 1 Convective Outlook and Probabilistic maps issued by the Storm Prediction Center during the heart of a tornado outbreak on April 7, 2006. The top map indicates the risk of general severe weather (including large hail, damaging winds, and tornadoes), while the bottom map specifically shows the percent risk 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 F2 or stronger tornado forming within 25 miles (40 km) of a point.
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 6-73 hours. They are labeled and issued by day and are issued up to five times a day (current convective outlooks). The categorical risks are general thunderstorms (GEN TSTMS) (brief textual description only), "SEE TEXT" (area on map for focusing attention only for isolated severe or near-severe weather), "SLGT" (slight risk of severe weather), "MDT" (moderate risk of severe weather), and "HIGH" (high risk of severe weather). 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. CategoriesA 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. A moderate risk day indicates that more widespread and/or more dangerous severe weather is possible, with some significant severe weather. Numerous tornadoes (including some strong tornadoes), more widespread or severe wind damage and very large hail (including destructive hailstorms) could occur. Some moderate risk days will bring a significant tornado outbreak or a major derecho event. These are not uncommon and are typically issued several times each month during the peak season. 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 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. 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 forecasted, and high risk busts are not uncommon. The outlooksThe 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 08Z 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 only been used six times (four of them in 2007, most recently for October 18, 2007). Day 4-8 outlooks primarily are only text-based, and can often completely change from day to day due to low confidence levels and high volatility. They were an experimental product until March 22, 2007 when they became an official product. Maps only show areas with at least a 30% chance of severe weather in the day 4-8 period (equivalent to a mid-range slight risk), which is fairly uncommon. 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):
¹Can be an SLGT if the risk is only for tornadoes (usually occurs as a result of a tropical system). Mesoscale discussionsMesoscale discussions (current 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. Here is an example of a typical MD: 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... Weather watchesWatches (current severe weather 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." A watch will usually be something like the following:
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