Forecasting Extreme Spring Weather
Storm Prediction Center Issues Thunderstorm and Tornado Watches
Spring months can bring severe weather such as strong thunderstorms and damaging tornadoes. The Storm Prediction Center monitors severe weather, issuing storm watches.
In terms of weather, spring means a transition from cold weather to more warm, humid air. This transition can cause turmoil in the skies, leading to extreme weather. Severe thunderstorms and tornadoes occur more often in the spring months than during the rest of the year.
Severe Thunderstorms
In order for severe thunderstorms to form, moisture, instable air, lift and vertical wind shear are needed. These ingredients produce a variety of thunderstorms, from a small, short local storm to the largest supercell. Supercell thunderstorms are intense, rotating thunderstorms, can be long-lived, producing the largest hail and the most destructive tornadoes.
Tornadoes
The Storm Prediction Center monitors tornado data and trends. The SPC calculates the three-year average for the number of tornadoes per year to be 1159. Most tornadoes occur between the months of April to June, with three-year averages recorded as:
- April – 181 confirmed tornadoes
- May – 171 confirmed tornadoes
- June - 188 confirmed tornadoes
Storm Prediction Center
The Storm Prediction Center (SPC) in Norman, Oklahoma is a part of the National Weather Service (NWS) and one of the National Centers for Environmental Prediction (NCEP). The center was developed in order to “provide timely and accurate forecasts and watches for severe thunderstorms and tornadoes over the contiguous United States” (SPC, 2008). One of the main duties of the Storm Prediction Center is to monitor severe weather and issues watches when severe weather is possible.
Watches Issued by the Storm Prediction Center
The Storm Prediction Center issues severe weather watches for severe thunderstorms and tornadoes, described as:
- Severe Thunderstorm Watch – severe weather with hail ¾ inch in diameter or larger, damaging winds, affecting an area during a period of 3 to 8 hours
- Tornado Watch – damaging wind and hail resulting from a severe thunderstorm, but more conductive for tornado formation
The SPC issues an average of 1,000 watches each year. Watches are not always issued for every possible storm because some storms only affect a small area or are short-lived. Watches are usually issued for more significant, organized severe weather, which is considered persistent.
Difference Between Watches and Warnings
Storm and tornado watches mean that severe weather is possible, within a given amount of time. Watches usually last between four and six hours. Warnings on the other hand, mean severe weather has been detected or observed, and it is moving in a given direction.
The Storm Prediction Center has the authority to issue watches, yet it does not have the power to issue warnings. Severe thunderstorm and tornado warnings are issued by the local offices of the National Weather Service. It would be unreasonable for the SPC to issue warnings for all severe thunderstorms across the nation, because some storms can be localized to a small area or short-lived. Local NW offices are better equipped to deal with a more specific area.
Severe Weather Forecasts
The Storm Prediction Center provides a number of different forecasts and status reports for severe weather:
- Lead forecasters issue watches, bulletins and provide leadership for the SPC
- Mesoscale forecasters provide a timeframe for dangerous weather, with short-term forecasts and status reports
- Outlook forecasters provides national forecasts for thunderstorm activity on a daily basis. These meteorologists analyze weather trends, current weather and computer model forecasts in order to form national forecasts, used by local NWS offices.
