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 Department of Commerce (DoC). Until October 1995, the SPC was known as the National Severe Storms Forecast Center (NSSFC) and was located near Kansas City, Missouri.
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[edit] Overview
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 forecasted goes from more general to more specific, also with heightening alertness.
[edit] Convective outlooks
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 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.
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. Most days during the severe weather season will have at least a SLGT risk somewhere in the US, with MDT risks being issued a few times a month on average from spring to fall and HIGH risks are quite rare, only issued a few times a year on average.
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 MDT risks are uncommon (only several times a year on average), 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. While a MDT can be issued for Day 3, there are no known occurrences of such.
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, 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 of any point in the delineated region (H = Hatched area; 2% and 10% are only used for tornadoes):
| Tornado | Wind | Hail | Day 2/3 | |
| 2% | SEE TEXT | |||
| 5% | SLGT | SEE TEXT | SEE TEXT | SEE TEXT¹ |
| 10% | SLGT | |||
| 15% | MDT | SLGT | SLGT | SLGT |
| 30% | HIGH | SLGT | SLGT | SLGT |
| 45% | HIGH | MDT | SLGT | SLGT |
| 45% (H) | HIGH | MDT | MDT | MDT |
| 60% | HIGH | MDT | MDT | MDT² |
| 60% (H) | HIGH | HIGH | MDT | HIGH² |
¹Can be a SLGT if the risk is only for tornadoes (usually occurs as a result of a tropical system).
²Not issued for Day 3.
[edit] Mesoscale discussions
Mesoscale 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.
[edit] Weather watches
Watches (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 an extremely high threat, a watch with special wording of "particularly dangerous situation" (PDS) is subjectively issued. It is occasionally issued with tornado watches, normally for more organized or destructive tornado events. A PDS Severe Thunderstorm Watch is very rare and usually issued for 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."
