May 17th, 2020 - Severe Thunderstorm Outlook

Good morning everyone, a decent start to the weekend has once again turned into wet weather as we move into a new week. Two to four inches of rain fell Thursday across the region with another two to five anticipated through early Monday. For more information about that please reference this article - www.illinoisstormchasers.com/forecasts/sat-mon-may-16-18th-heavy-rain-flood-threat-update-1

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Let's take a look at what's going on above us tomorrow from left to right.  An upper level low across the central U.S. is slowly sending multiple disturbances out into the plains while a deepening surface low evolves over Nebraska/Iowa. Winds will respond to the deepening low and back winds across most of eastern Illinois through the afternoon and evening. Bulk shear is more than supportive for severe weather. It's not your classic severe weather set up by any means, but any amount of wind shear in May generally can cause a couple of problems across the region. 

What's needed for a more substantial severe weather severe weather episode is heat and instability. We have more than enough moisture in place (above right) and if model data is correct (above left) we will sufficiently warm to create an unstable air mass (below). 

High-res convective allowing models show numerous showers and thunderstorms across central through northern Illinois through the day Sunday. What intrigues me is the overall lack of convection across eastern Illinois during peak heating. While I do not think it will be completely clear, any breaks in cloud cover to allow surface heating could cause trouble. Temperatures in the mid to upper 70's progged by models lead me to believe that there is some opportunity for a break in cloud cover. 

Much of the convection shown [HRRR - left; Nest - right] is within a larger shield of precipitation across the I-39 corridor. Severe weather threat here is still possible, but I suspect that would be more in the way of gusty winds versus tornadoes. What I am eyeing is the pre-frontal convection developing over eastern Illinois. This would be the "instability" based convection with the greatest potential for producing a few tornadoes. 

As a result we are introducing a minimal risk for severe thunderstorms across the eastern half of the state. Trends will be monitored if an upgrade to limited will be needed. 

DNEAL