Last Thursday (2/24) marked the first day of the year that significant severe weather occurred in the lower 48 states. A total of 289 reports were made from the extreme weather, 24 of which were tornado related.
Severe weather is once again going to sweep through the South and Midwest regions of the United States today and tonight. I just wanted to touch on the reason why:
At the surface, there is a very evident sharp temperature gradient (change in temperature) caused by an area of low pressure in New Mexico. What this sharp temperature gradient at the surface does is affect the winds aloft, divergence. Strong divergence aloft is a good indication of a low at or near the surface. Take a look at the current temperatures around the nation:
This difference in temperatures can spark precipitation, if conditions are just right. This means that there must be enough moisture in the lower levels and upper air support (lift). In order to determine whether it will be a severe weather day, one must look at ALL levels of the atmosphere. We’ll start at 850mb:
From the 850mb map above, you can see that there is a closed low over 4-corners of the U.S. The flow around this low is advecting warm air into the Southern Plains and Midwest areas. This is important because warmer air holds in more moisture; more moisture means a better chance of precipitation. Now where is this moisture coming from? If we go back to the surface and look at winds, we can see that the flow from the Gulf of Mexico is feeding right into the Southern Plains and the Midwest:
This moisture flow from the Gulf will increase dewpoint temperatures as well. Dewpoint temperatures have to be decently high for severe weather to occur, on most occasions. Now that we know that there is significant warming and moisture in the lower levels, there needs to be lifting to elevate the storms. For that we need to look at the mid-levels of the atmosphere:
AT 500 mb:
First thing you should see is a big circle, or a closed low, over southern California and western Arizona. To the east of the low, more warm air is being advected into the southwestern, south-central, and mid-western states. Since this image was valid at 12Z (6:00 AM CST) today, it’s just a bit outdated. What’s going to happen is the low will move east and eject into the south-central U.S. as a trough. This trough will bring the lifting needed for thunderstorm activity:
From figure 5 above, you can see that the highest winds (jet) will be over central and eastern Oklahoma around midnight, central time. The greater the jet, the more unstable the atmosphere is, therefore greater lift, most of the time.
AT 300 mb:
The map above is to show the areas of divergence associated with the upper lever trough. Again, divergence means rising motion at or near the surface, precipitation. Black lines are streamlines, or the flow aloft. Notice the yellow areas are on the southeast side of the trough. This will be advected into the Southern Plains and Midwest by this afternoon and evening.
All of the ingredients for a severe weather day/night are there. The only problem is the alignment of everything I discussed. We’ve been looking at layers of the atmosphere (Surface, 850 mb, 500mb, etc), but the timing and alignment of all the severe weather criteria have to be correct. If everything isn’t aligned correctly, the greatest potential for severe weather won’t occur. I’m not saying severe weather won’t occur, but I am saying it most likely won’t be a significant severe weather outbreak with multiple strong tornadoes. This is my belief:
Anywhere from 5-15 tornadoes may form from this system. I do not anticipate any tornadoes being stronger than EF-2. Finally, I also expect large hail to fall with some of these storms, as well as 60+ mph wind speeds and gusts.