Severe weather rarely does not happen without any warning. While we will never be able to pinpoint when and where severe weather will develop, we do know broader areas or regions where the potential for the development of severe weather. It is your responsibility to check the weather forecast, which may be often several times daily, to see if you are, or will be, under a risk of severe weather.
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 (SPC) Center 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. However, despite being technical in nature, the graphics which accompanies the narratives, you will have vital information to help plan your day.
Convective Outlooks are divided into three days.
|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; 0600z (around midnight), 1300z (around sunrise), 1630z (mid-morning), 2000z (mid-afternoon), and 0100z (early evening). This is the forecast you will see on SPC's frontpage.|
|Day 2||Day 2 continues from the ending of Day 1 (tomorrow morning) for the next 24 hours. These are issued twice daily; 1:00 a.m. (both CST/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).|
In convective outlook graphics, the brown unlabeled line depicts a 10% or higher probability of thunderstorms during the valid period. The region of these storms will be to the right of the brown line (look for the arrowhead).
A green line will be seen if there is a slight (SLGT) risk of severe thunderstorms during the forecast period. Depending on the size of the area, approximately 5-25 reports of ¾" or larger hail, and/or 5-25 wind events, and/or 1-5 tornadoes would be possible.
The red line 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 magnitude of the severe weather.
The fuschia line 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).
In a high risk, the potential exists for 20 or more tornadoes, some possibly F2 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 only the most serious weather situations where a HIGH risk is forecast for a potential tornado outbreak. The SPC issues around 5 PWO's each year.
When conditions appear favorable for severe storm development, SPC issues a Mesoscale Discussion (MCD), normally 1 to 3 hours before issuing a weather watch. SPC also puts out MCDs for mesoscale aspects of hazardous winter weather events including heavy snow, blizzards and freezing rain. MCDs are also issued on occasion for heavy rainfall or convective trends.
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.
Check for any mesoscale discussions.
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.
When a severe weather watch is issued close to your location but does not include your county, you should still remain alert. The atmosphere rarely follows straight lines, and thunderstorms do not always remain within the man-made boundaries we create around them. When SPC feels confident about the possibility of severe weather in a specific area, they try to issue a watch at least 1 hour prior the onset of severe weather.
Check for any severe weather watches.