For several days I've been kind of loud on social media about the potential for organized severe thunderstorms across the Midwest on Thursday, August 15th. We're getting to within 24 hours of the event and I'm still lacking quite a bit of clarity with respect to when and where the most intense storms might end up. For that reason, I'll probably end up regretting drawing up a map of the vague vision that's emerging in my brain.
We'll see an area of thunderstorms push out of Missouri and Iowa into Illinois late tonight into Thursday morning, that much is certain. How intense these storms are is still kind of up in the air, but I wouldn't be surprised to see at least sporadic damaging wind reports across portions of northern and central Illinois into the early morning hours on Thursday. These morning storm clusters have a bad habit of carrying on hours longer and hundreds of miles further east than we expect them.
I'm intrigued by the potential for an area of interest to emerge along the potential outflow boundary in south-central Illinois during the late afternoon and evening hours. This could be roughly between St. Louis and Vincennes, IN between the I-70 and I-64 corridors in south-central Illinois. If the boundary is there, and if we get some sunshine cooking the boundary by mid-day, we could see a modified outflow regime emerge where there's an area of surface convergence and a pocket of vorticity. The amount of sunshine that this area sees would largely determine how significant the risk for a supercell or tornado is. Eventually the entire area along and south of the outflow boundary across southern Missouri and southern Illinois could see scattered storms erupt into the evening.
I'm also intrigued by the potential for significant severe weather across portions of Iowa and northern Missouri into northern and central Illinois. In this zone it seems like things just phase a little late - the main upper-level wave sweeps into this area very close to sunset, with what should be a hot and muggy environment that's ready to pop. The HRRR in particular has been rather gungho with the potential for scattered storms to erupt in this area perhaps congealing into an MCS that races eastward across northern and central Illinois between 7 PM and 1 AM. Nothing is a given with these late-summer setups, but anecdotally the HRRR is usually pretty good at verifying with significant severe weather when it starts modeling intense storms with 85 knot wind gusts.
This area will depend on the quick exit of cloud debris following morning storms. Surface convergence is rather diffuse in this northern zone until late in the day, so while this area may end up producing the most intense storms, I wouldn't be surprised to see the majority of it occur after dark.
Like I said - I'm trying to get a little momentum going with this tricky forecast, but I'll probably regret this entire map by this evening. From a storm observation standpoint I'm probably going to just clear the day and go where my heart takes me. I don't feel confident in much yet, but it's probably the last opportunity for a while locally.