The weather office in charge of charged with monitoring and forecasting the potential for severe weather over the 48 continental United States is the Storm Prediction Center (SPC) located in Norman, OK. The information provided by SPC will give you critical information concerning the threat of severe weather in your locale.
Convective Outlooks consist of a narrative and a graphic depicting severe thunderstorm threats across the continental United States. The outlook narratives are written in technical language, intended for sophisticated weather users, and provide the meteorological reasoning for the risk areas.
This product also provides explicit information regarding the timing, the greatest severe weather threat and the expected severity of the event. The graphics that accompany the narratives provide vital information to help plan your day.
Convective Outlooks are divided into four periods.
|Day 1||This is the risk of severe weather today through early morning of the following day. Day 1 forecasts are issued five times daily; 06z (around midnight), 13z (around sunrise), 1630z (mid-morning), 20z (mid-afternoon), and 01z (early evening). This is the forecast you will see on SPC's frontpage. (What is Z-time?)|
|Day 2||Day 2 continues from the ending of Day 1 (tomorrow morning) for the next 24 hours. These are issued twice daily; 07z (around midnight) and 1730z (around noon).|
|Day 3||This is the forecast for the subsequent 24 hours. Day 3 forecasts are issued daily by 0830z on standard time and 0730z on daylight time (after midnight).|
|Days 4-8||A severe weather area depicted in the Day 4-8 period (issued at 10z (early morning) indicates a 30% or higher probability for severe thunderstorms (e.g. a 30% chance that a severe thunderstorm will occur within 25 miles of any point).|
General ThunderstormsThe light green shading depicts a 10% or higher probability of non-severe or near severe thunderstorms during the valid period.
Be aware that a thunderstorm producing ¾" hail and wind gusts at 55 mph wind is officially a NON-severe storm but can produce damage. So, just because you may be in an area of "general thunderstorms", you need to keep alert for the possibility of rapidly changing weather conditions.
Severe Category 1 - MarginalThe dark green shading area indicates a marginal (MRGL) risk of severe thunderstorms during the forecast period. This means a...
2% probability or greater tornado probability OR 5% probability for severe hail (≥1") OR severe wind. (≥58 mph/93 km/h).
Severe Category 2 - SlightThe yellow shaded area indicates a slight (SLGT) risk of severe thunderstorms during the forecast period. This means a...
5% probability or greater tornado probability OR 15% probability for severe hail or severe wind probability WITH OR WITHOUT 10% or greater probability of hail 2" or greater in diameter OR wind gusts 75 mph or greater (120 km/h or greater).
Severe Category 3 - EnhancedThe orange shaded area indicates an enhanced (ENH) risk of severe thunderstorms during the forecast period. This means a...
10% probability for any tornado WITH OR WITHOUT 10% or greater probability of an EF2+ tornado OR 15% probability for any tornado OR 30% severe hail or severe wind probability WITH OR WITHOUT 10% or greater probability of hail 2" or greater in diameter, or wind gusts 75 mph or greater OR 45% probability of severe hail or wind.
Severe Category 4 - ModerateThe red shaded area indicates a moderate (MDT) risk of severe thunderstorms are expected. This means a...
15% tornado probability AND 10% or greater probability of an EF2+ tornado OR 30% probability for any tornado OR 45% severe wind probability AND 10% or greater probability of a wind gusts 75 mph or greater OR 45% severe hail probability AND 10% or greater probability of hail 2 inches or greater in diameter OR 60% severe wind probability OR 60% severe hail probability WITH OR WITHOUT 10% or greater probability of hail 2 inches or greater in diameter.
Severe Category 5 - HighThe fuschia shaded area indicates a high (HIGH) risk of severe thunderstorms are expected. This means a...
30% tornado probability AND 10% or greater probability of an EF2+ tornado OR 45% or greater probability for any tornado WITH OR WITHOUT 10% or greater probability of an EF2+ tornado OR 60% severe wind probability AND a 10% or greater probability of a wind gust 75 mph or greater.
These are the official definitions. The reason for the "AND's", "OR's" and "WITH OR WITHOUT's" is to help cover the many different states that occur in the atmosphere. There will be times when the number of severe weather events will be high but the overall intensities will not necessarily be extreme.
This (below) is a simplified version of the official definitions.
There will be other times when the atmosphere will be extremely unstable and will produce extreme damaging weather but the number of events will be fairly small. Or any combination of the above. Still Confused?!? Just know that the greater the threat (from Slight to High), the greater the risk for severe weather which could be either in number of events or intensity or both.
The Public Severe Weather Outlooks (PWO) are issued when a potentially significant or widespread tornado outbreak is expected. This plain-language forecast is typically issued 12-24 hours prior to the event and is used to alert NWS field offices and other weather customers concerned with public safety of a rare, dangerous situation. The PWO is reserved for for all high risks and for moderate risks with a strong risk for tornadoes and/or widespread damaging winds. The SPC issues about 30 PWOs each year.
The MCD basically describes what is currently happening, what is expected in the next few hours, the meteorological reasoning for the forecast, and when/where SPC plans to issue the watch (if dealing with severe thunderstorm potential). Severe thunderstorm MCDs provide you with extra lead time on the severe weather development.
When conditions become favorable for severe thunderstorms and tornadoes to develop, SPC usually issues a severe thunderstorm or tornado watch. Tornadoes can occur in either type of watch, but tornado watches are issued when conditions are especially favorable for tornadoes. Severe thunderstorm watches are blue with tornado watches in red.
Watches are large areas, 20,000 to 40,000 square miles, and are issued by county. They are numbered sequentially (the count is reset at the beginning of each year). A typical watch has a duration of about four to six hours but it may be canceled, replaced, or re-issued as required. A watch is not a warning, and should not be interpreted as a guarantee that there will be severe weather!
When a watch is issued, stay alert for changing weather conditions and possible warnings. Any warnings will be issued by your local NWS Weather Forecast Office.
Usually this decision is based on a number of atmospheric clues and parameters, so the decision to issue a PDS watch is subjective. There are no hard threshold or criteria. PDS watches are most often issued in association with high risk convective outlooks.