March 9th-10th (Sat-Sun) Impressive Weekend Storm System to Bring Threat of Severe Storms and Strong winds to Illinois

Numerical model guidance continues to remain fairly consistent on a potent storm system coming ashore the west coast tomorrow and ejecting out into the plains late tomorrow night and into Saturday. This would then lead to a strong area of low pressure system developing across the plains and moving into portions of the Midwest during the heart of the weekend. Given the good agreement on the low passing to our west and northwest, this would put Illinois on the warm side of the system and now that we are in March, this typically means increased chances of thunderstorms given there is a better chance of more ingredients coming together...

I dig into the details below...outlining the highest threat area for strong storms, the timing of them and then very windy conditions Saturday night into Sunday. 

As you'll see below, this system will move eastward through southern California during the day tomorrow (image valid at noon tomorrow) and then continue off to the east and northeast over the next 24 hours into early Saturday afternoon when it will be off to our west across the plains. 

Associated with this system will be a very strong mid-level jet stream, one that you instantly see and think increased thunderstorm chances if both moisture and instability are in place (which will be talked about below). The loop below runs from Friday evening through 3pm on Saturday, showing the progression of a very strong 80-100kt jet streak (strongest winds in red) moving from the plains into the Midwest. 

Favorable moisture trajectories across the Gulf of Mexico and southern U.S. along with strengthening low pressure across the plains will help advect deeper moisture (higher dew points in dark green and gray colors) further northward into the southern Midwest during the morning and afternoon hours on Saturday. 

So above, we have plenty of shear and sufficient moisture along with strong forcing from the wave and front itself. Lastly when looking for strong to severe thunderstorm potential, we need to see the atmosphere be unstable and for that we can look at CAPE values (Convection Available Potential Energy). The values below are more than enough to support thunderstorms, some of which could be strong to severe. 

Now looking at some high resolution model guidance, it currently looks like about the southern half of the state (central and southern Illinois) will stand the best chance for strong thunderstorms, some of which could be severe across downstate. Timing wise, it looks like current guidance would favor the afternoon and evening hours on Saturday. 

​Lastly, as the area of low pressure moves off to our north and northeast Saturday evening into Sunday, the pressure gradient will tighten on the backside of the system, leading to a period of potentially very strong wind gusts across portions of the state. The values below could be a bit overdone but there could easily be a period Saturday night where wind gusts are reaching 40-50mph. 

Below are our TAM severe weather and high wind forecasts for this storm system.

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