A severe weather outbreak is likely across the Deep South on November 7, 2011, as an area of low pressure develops and produces numerous thunderstorms capable of spawning tornadoes. The biggest areas of concern are from Clinton, Oklahoma, southwestward into Wichita Falls, Texas, where the greatest tornado threat exists later this afternoon and into the evening hours. A slight chance of severe thunderstorms is outlined by the Storm Prediction Center (SPC), stretching from central Texas and Oklahoma and across southern Kansas. Cities included in the slight risk areas are Stockton and Wichita Falls, Texas, Norman and Oklahoma City, Oklahoma, and areas just south of Wichita, Kansas.
The SPC includes a 10 percent probability of seeing tornadoes across southwestern Oklahoma and the north central portions of Texas. This map (below) shows the probability of seeing a tornado within 25 miles of a point. If an area is hatched, it shows a 10 percent or greater probability of strong tornadoes of EF2- EF5 strength within 25 miles of a point. If you are living in these areas, be weather ready in case watches and warnings are issued later this afternoon.
SPC outlined the same areas with the highest tornado threat with a 30 percent hatched area for hail. The image below shows the probability of seeing one inch diameter hail or larger. The hatched area indicates a 10 percent or greater probability of seeing two inch diameter hail or larger within 25 miles of a point.
A wind threat will increase later this evening as supercell thunderstorms will likely form along the front. As evening becomes night, these individual storms will likely join together as an intense line of storms producing winds as strong as 70 miles per hour. In the image below, the SPC indicates a large area of a 15 percent probability of seeing 60 mph winds or greater within 25 miles of a point.
A trough will be pushing south into the Deep South today, and will be accompanied with strong, southerly low-level winds, which will allow the atmosphere to destabilize and allow more moisture into the area. When we look at parameters that influence severe weather, we always look at the stability in the atmosphere. If the atmosphere is stable, then an air parcel is cooler than its environment and does not rise. When this occurs, we typically see sunny weather. If the atmosphere is unstable and a forcing mechanism is involved, an air parcel becomes warmer than its surrounding environment and continues to rise.
We look at Convective Available Potential Energy (CAPE) and Lifted Indexes to provide ideas about how unstable the atmosphere will become. The picture below shows the CAPE across the central portions of the United States. CAPE numbers become larger around the same areas that are expected to see the greatest chance for tornadoes. Values of over 1500 J/kg are expected in these regions, which is high enough to support severe weather. All of the images below can be found at WeatherCaster.
The image below shows us the significant tornado parameter (STP) across the region today. Values greater than one show a decent chance for tornadoes to develop. Numbers over three, or colors in orange, red and purple, show a high chance for tornado development. The STP map uses a lot of factors to determine areas for tornado development. It looks at temperature profiles, wind shear, helicity (spin in the atmosphere) levels, instability and much more.
Severe weather is not that uncommon for this time of the year. In fact, November is considered the secondary severe weather season across much of the South. The transition from summer to winter and winter to summer (also known as fall and spring) is what triggers stronger storms during these times of year. It is simply nature’s way of transitioning into the winter months.
It is not uncommon to see tornado outbreaks as far east as Georgia and even the Carolinas during late November and early December. Outbreaks are typically less severe in the fall months versus springtime, but there have been notable autumn outbreaks. For instance, there was a large tornado outbreak on Veteran’s Day from November 9-11, 2002. There’s a small possibility that Oklahoma could experience an aftershock while this severe weather event occurs. How crazy would that be?
Bottom line: People across central Texas, a majority of Oklahoma, and southern parts of Kansas should be ready for severe thunderstorms this afternoon as daytime heating, higher dew points, and increasing instability combine with a strengthening area of low pressure. Individual storms will likely form ahead of the cold front, and may trigger a few tornadic supercell thunderstorms. By later the evening, individual storms will likely merge into a squall line of storms that will focus more on a wind threat as it pushes to the east. The severe weather today will shift east into eastern Texas/Oklahoma, Arkansas, Louisiana, and southern portions of Missouri tomorrow as a quasi-linear convective system, or a QLCS. QLCS is simply the technical term used to describes a squall line of organized thunderstorms capable of producing winds over 60 mph. A QLCS can produce small tornadoes, and they typically cover a larger area of land. All areas included in the slight risk for severe thunderstorms should be weather-ready this afternoon and evening.
Matt Daniel is weekend Meteorologist for 13WMAZ (CBS) in Macon, Georgia, and founder of the blog Athens GA Weather. He's a self-described "big weather and music geek" and has produced weather content for CNN, MSN Weather and EarthSky. He has a passion for helping to keep people safe when severe weather strikes and says if you don't have a NOAA Weather Radio ... you should get one.