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 (1:00 am central time, CST or CDT) and 1730z (around noon).|
|Day 3||This is the forecast for the subsequent 24 hours. Day 3 forecasts are issued daily by 2:30 a.m. central time (0830z on standard time and 0730z on daylight time).|
|Days 4-8||A severe weather area depicted in the Day 4-8 period (issued at 10z (4:00 am central time, CST or CDT) 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).|
In convective outlook graphics, the green shading depicts a 10% or higher probability of thunderstorms during the valid period.
A yellow shaded area indicates a slight (SLGT) risk of severe thunderstorms during the forecast period. Depending on the size of the area, approximately 5-25 reports of one inch or larger hail, and/or 5-25 wind events, and/or 1-5 tornadoes would be possible.
The red shaded area indicates a moderate (MDT) risk of severe thunderstorms are expected. The moderate risk indicates a potential for a greater concentration of severe thunderstorms than the slight risk, and in most situations, greater intensity of the severe weather.
The fuschia shaded area indicates a high (HIGH) risk of severe thunderstorms are expected. A high risk area suggests a major severe weather outbreak is expected, with a high concentration of severe weather reports and an enhanced likelihood of extreme severe (i.e., violent tornadoes or very damaging convective wind events occurring across a large area).
With a HIGH RISK event, the potential exists for 20 or more tornadoes, some possibly EF2 or stronger, or an extreme derecho potentially causing widespread wind damage and higher end wind gusts (80+ mph) that may result in structural damage.
Finally, a "SEE TEXT" label will be used for areas where a 5% probability of severe is forecast, but the coverage or intensity is not expected to be sufficient for a slight risk.
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.