No one knows exactly what this tornado season will hold, but a team at Columbia University, for the first time, has published a tornado outlook for this severe weather season. The team indicates there is a 60 percent chance the season will bring an average number of tornadoes and a 30 percent chance the number will be below average. And there isMore
As forecasters had expected and warned for days, damaging thunderstorms formed Tuesday afternoon and evening in the Southern Plains. Even a few days ahead of time, the ingredients appeared to be coming together for a large severe weather outbreak there, and there was legitimate concern for numerous tornadoes developing from Kansas to Texas.
While there were more than 150 reports each of large hail and wind damage, the number of tornado reports was small — only five.
Tornadoes form in storm environments with atmospheric shear, which means that the winds move in different directions at different levels of the atmosphere. And while there was plenty of shear in the middle levels of the atmosphere Tuesday, it became apparent by late in the afternoon and evening that the shear in the lowest few thousand feet of the atmosphere was not enabling that spin to translate to the ground to form numerous tornadoes.
The thunderstorm formation began to take on a more linear pattern, more conducive to producing straight-line wind damage than tornadoes. The supercell formation that often leads to large tornadoes was also not taking taking shape.
One ingredient in creating severe storms is cold air in the middle level of the atmosphere, and that was very well forecast. Large hail was a big concern going into Tuesday evening, and the amount of hail largely met the forecast expectations. The Weather Channel’s Stu Ostro tweeted early Tuesday evening:
Very large hail was found in Lancaster, Kan., just northwest of Kansas City, where billiard-ball sized hail (about 2.25 inches in diameter) was reported. The National Weather Service also shared an image of large hail just northwest of Topeka.
Even as the storms in the Plains started to fade late into the night, the southern edge survived and marched across eastern Texas, surviving to hit Houston Wednesday morning and taking some large trees down in the metro area, still recovering from substantial flooding from recent storms.
The severe storm threat continues to shift eastward on Wednesday, with the main focus in the lower Mississippi Valley, including St. Louis, Memphis, and New Orleans. However, the concentration of damaging storms is not expected to be as high as Tuesday. The potential threats to those areas remain high winds, hail, and possible tornadoes.
The influence of climate change on local severe storms is still an active area of research, but there is some evidence that large hail could become more common as the world warms.
More water vapor trapped by a warmer atmosphere could translate to stronger updrafts within a thunderstorm, which are necessary for hail formation. But, the warmer atmosphere could also raise the freezing level, meaning that while smaller hail would be more likely to melt before reaching the ground, the larger, more robust, hailstones would survive.
Wednesday also marks the fifth anniversary for the massive 2011 tornado outbreak, which affected areas from Mississippi to Virginia. Nationwide, 199 tornadoes formed that day, with 15 of these rated EF-4 or higher, resulting in 316 fatalities.
This serves as a reminder that we are entering the peak of severe weather season, which generally lasts until late June. After that time, wind shear begins to decrease, and the middle part of the atmosphere has warmed, making a less friendly environment for storm formation.