Springlike Severe Weather Risk in the Midwest on Sunday | Hail in the AM, Wind in the PM, Isolated Tornado?

Unseasonably deep moisture and a dynamic storm system will have an environment supportive of severe thunderstorms lifting into portions of the Midwest on Sunday, Dec 28.

I'll be totally honest, my head is not in the game right now, it's the holidays, I've got a million things going on that aren't severe weather and storm chasing focused, but the risk for organized severe storms is significant enough that I wanted to make a quick post and take an opportunity to try out some new weather forecast model graphics I'm working on for 2026.

The "Conditional Storm Severity" index is a parameter that I came up with a few years ago but just got plotting with the HRRR in the last few days. Similarly simple like 'supercell composite' and 'significant tornado parameter', this index attempts to look at an environment and say "if a storm were to form, how severe could it become based on the ingredients in place?". Unlike supercell composite and significant tornado parameter I'm trying NOT to mask elevated, warm air advection driven environments often focused north of a warm front and in the presence of a stout near-surface inversion where you're still getting severe hail.

Cold season or early spring events are going to be prone to setting this index off, but deservedly so in a lot of instances. Yes, it's going to be December 28th, but the forecast presence of mid 60s dew points, off-the-charts wind shear, and a deepening area of low pressure you're not going to need much in the CAPE department to generate potentially big hail with the morning round, and/or transfer significant 65+ mph wind gusts to the surface with the later afternoon round, and an isolated tornado can't be ruled out.

"Conditional Storm Severity" with reflectivity and surface winds with contoured echo tops at 8 AM via the 18z HRRR Sat Dec 27.

"Conditional Storm Severity" with reflectivity and surface winds with contoured echo tops at 3 PM via the 18z HRRR Sat Dec 27.

Dew points with 10 m winds, reflectivity at 11 AM via the 0z HRRR.

Based on coverage and confidence, this definitely should only be a marginal/slight risk day, and I wouldn't scoff at the SPC if they didn't hatch their wind or hail corridors, but I do think the environment from northern Missouri into southern Michigan, but especially focused on portions of Illinois and Indiana could support a risk of by-definition significant wind gusts or hail.

Here's a quick summary of the afternoon Day 2 outlook from the SPC:

There is still considerable uncertainty in how Sunday’s storm system evolves as a developing surface low moves from the lower Missouri Valley toward the Great Lakes. Small differences in how quickly and where this system deepens could have a meaningful impact on when and where thunderstorms develop, as well as how much severe weather ultimately occurs. In general, a moist warm sector is expected to spread northward ahead of the system, with dew points near or above 60°F reaching into parts of the Mississippi Valley and southern Great Lakes by Sunday afternoon. Thunderstorm development is most likely near and east of the surface low and along a warm front, though limited instability and warm air aloft should keep the overall severe threat relatively constrained in many areas.

That said, there may be a narrower window for stronger storms Sunday afternoon from parts of central and eastern Missouri into central Illinois, just ahead of the advancing cold front. In this corridor, modest surface-based destabilization combined with strengthening wind shear could support a few strong thunderstorms before activity shifts east into the Ohio Valley and weakens Sunday evening. Low-level wind profiles may briefly become favorable for low-topped supercells across central Illinois into north-central Indiana, where a tornado or two cannot be ruled out late in the afternoon. Farther southwest along the cold front, storms are more likely to organize into a narrow line capable of producing strong, potentially damaging wind gusts. Even after thunderstorms diminish, gusty winds behind the cold front are expected to continue across much of the Mississippi and Ohio Valleys.