DAY TO WATCH: Thursday, May 15 Bi-Modal Severe Storm Event Possible

Two separate zones of severe weather potential exist on Thursday, May 15th, 2025.

I spent the weekend unplugging from the weather and celebrating my wife's birthday / Mother's Day in Chicago. We were born ten days apart (me late April, her early May) and this year has featured the near-perfect evolution of "storms to chase for my birthday weekend, followed high pressure and sunny skies for her birthday weekend". It was nice to get away and celebrate her and enjoy some sunshine and warm spring weather all the while knowing the peak of the 2025 storm observation season is just around the corner.

Even though we got home pretty early on Sunday evening I managed to avoid really diving into anything weather related until I woke up and poured my first cup of coffee this morning. The potential for Thursday, May 15 severe weather in the Midwest has been rather consistent since the middle of last week, and the Storm Prediction Center has been highlighting the day/region for a couple of Day 4-8 outlooks already.

The signal for severe weather in the Midwest on May 15th has been loud, but nuanced. There are two distinct waves ejecting into the region, including a sub-tropical jet component. You have a northern short-wave that gets some cyclonic curvature and lifts into the Great Lakes, with a southern streak that punches into Illinois and Indiana.

Lift will be the greatest associated with the northern shortwave and surface low in Minnesota and Wisconsin. Wind shear and ambient vorticity in the vicinity of the low should be quite high and efficient and encouraging rotating thunderstorms, but stronger forcing along the Pacific front / dry punch should also encourage numerous thunderstorms and higher tendency for upscale growth and outflow dominance in some storms. The ceiling in this northern zone feels like a swath of wind damage reports across Minnesota and Wisconsin with a few tornado reports where sunshine and pockets of greater 0-3 km CAPE values manifest. It's also possible a longer-lived discrete supercell or two could develop on the tail-end of this activity near the Wisconsin/Illinois border.

Further south across Illinois and Indiana a more questionable evolution exists, but with a potentially higher ceiling for a couple of intense supercells along with the potential for a strong tornado or two along with damaging hail and wind production.

Even with some diurnal mixing, the atmosphere across central and eastern Illinois is likely to become moderate to very unstable with surface temperatures approaching 90 deg F and dew points near 70 deg F resulting in MLCAPE values of 4000-4500 j/kg. Capping and weaker surface forcing could, and should limit overall thunderstorm coverage compared to the northern zone, but given the volatile thermal environment and just enough lift from the approaching jet stream and Pacific front/dryline feature I expect an intense supercell or two to develop.

While this southern zone has bigger questions regarding storm development, a more favorable window for long-lived supercells exists from central and eastern Illinois into northern and central Indiana on Thursday afternoon and evening. A long-track, significant tornado would not surprise me, and I say that realizing we're still 3-4 days out.

For my own storm observation planning purposes, it makes obvious sense to prioritize the closer-to-home target that favors a more significant supercell/tornado risk, but I'll happily drift north if the evolution of the day demands.

Holding off on getting too detailed at this range, but it was time to get some early thoughts down.