We’ve started to hear more and more about the severe weather potential for Thursday, and since it is Wednesday, we may as well delve into things a bit further. There are several main ingredients you need for a severe thunderstorm, and we’ll be looking through Thursday for those ingredients.
Wind shear (change in direction/change in speed with height), instability (how much energy is there for storms?), moisture (how much fuel for the storms), forcing (cold front, wave of energy, etc.). Those are your main ingredients. They are all important, and without any one of them you’re not going to see a robust region of severe weather.
Let’s look toward Thursday now…
In the morning we watch a budding storm in the Central Plains become more of a mature storm system. Blizzard conditions to the North, with higher rainfall tallies and big temperature changes to the South.
Here is what the surface temperature map looks like Thursday morning:
Notice the bulge of yellows and oranges over eastern Oklahoma and southern Missouri Thursday around 6am CST. See the little circle over the KS/OK state line? That is your center of low pressure. That deepens and moves east/northeast very rapidly over the course of the day.
As it deepens, the cold air intensifies as well.
Here is what it looks like 18 hours later:
Notice that the big “L” indicating the center of low pressure, is located in northern Michigan at that point. Colder air (blues and greens) has already made its way into the middle of Tennessee and Kentucky. That is close to midnight/1am Thursday night/Friday morning.
So, we found our forcing. What about energy?
This map below indicates available energy for storms. With our temperature set-up, anything shaded in green is “enough” for strong storms.
That map above is valid around Thursday evening. You can see the higher energy 750-1000 value (CAPE, or Convective Available Potential Energy) stretching from southern Indiana/Illinois to the Gulf Coast.
So, we have energy.
What about moisture? Let’s check out dewpoint temperatures. You need robust moisture both at the surface and aloft for a larger severe weather day.
Here is the moisture at 6pm Thursday:
60 degree dewpoints are a good “rule of thumb” for what could be termed “enough moisture.” That changes with any given weather set-up, but in this case it seems to be enough. See those orange shaded areas in the map above? Those are dewpoint temperatures that are 60° or higher.
So, we have moisture.
Finally – we need wind shear. That is typically the hardest ingredient to find in a storm, hence why we don’t see hail and tornadoes in most storms. Wind shear (changes in wind speed and/or direction with height) is what allows an otherwise weak or benign storm to become self-sufficient, instead of running out of steam and fizzling out.
For an example of wind shear I pulled up the following image from central Tennessee valid around 6pm Thursday:
While this isn’t the “greatest” profile for severe storm development, one thing that sticks out to be is the wind direction shift with height. In the bottom mile of the atmosphere you’re talking about a 50-60 degree shift. That could be enough for at least large hail in spots, and possibly an isolated tornado or two.
This was the Storm Prediction Center’s outlook for severe weather Thursday, as of Wednesday morning:
We’ll be watching very closely – and then also watching Friday as our cold front continues eastward.
WeatherNation Meteorologist Aaron Shaffer @ashafferWNTV