TL;DR

The NOAA Storm Prediction Center (SPC) has issued a Marginal Risk for severe thunderstorms today across eastern Illinois, Indiana, western and central Kentucky, and middle Tennessee. Storms will develop this afternoon and evening along a cold front, with isolated hail, damaging winds, and perhaps a tornado possible between 1 pm and 8 pm CT. A separate Marginal Risk covers central and southern Florida for sea-breeze storms this afternoon.

Where is the Marginal Risk today?

The primary Marginal Risk area stretches from eastern Illinois southeastward through Indiana, western and central Kentucky, and into middle Tennessee. This includes the greater Indianapolis, Louisville, and Nashville metro areas.

A second, smaller Marginal Risk zone covers the central and southern Florida Peninsula, where Atlantic Coast sea-breeze thunderstorms may produce small hail and gusty winds this afternoon.

What's driving today's storms?

A mid-level shortwave trough is digging southeastward over the Upper Midwest toward the mid-Mississippi Valley, according to the SPC Day 1 outlook issued at 06:55 am CT Wednesday. A weak surface low will track from southeast Iowa this morning to the Indiana vicinity by late afternoon, dragging a cold front east-southeastward across the region.

The main limiting factor today is shallow low-level moisture. Surface observations this morning show dewpoints generally in the 30s to mid-40s across the Midwest and lower Ohio Valley—far from the humid conditions that fuel stronger severe weather. Somewhat richer moisture is present farther south across the Ozarks and lower Mississippi Valley, but west-northwesterly low-level flow is expected to prevent that moisture from surging northward in any meaningful way.

Still, some mid-level moistening and modest boundary-layer destabilization should occur this afternoon as the front advances. The SPC notes that most guidance appears "a bit moist" with dewpoint forecasts, so actual instability may fall short of model output.

When will the storms hit?

Surface-based thunderstorms are expected to initially develop over eastern Illinois between 1 pm and 4 pm CT (18–21Z). Additional development is possible along and ahead of the front in western and central Kentucky through the afternoon and early evening as the boundary layer gradually destabilizes.

Storms will spread quickly eastward through the evening, with the severe threat diminishing after dark as instability weakens with the loss of daytime heating. The window for severe weather across the lower Ohio Valley should close by late evening.

What hazards are expected?

The combination of weak but potentially sufficient mixed-layer CAPE (convective available potential energy) and 30–40 knots of deep-layer shear oriented largely perpendicular to the cold front should support organized updrafts. The SPC expects "a mix of clusters and marginal supercells" capable of producing:

  • Isolated hail, aided by cold mid-level temperatures and modestly steepened low-level lapse rates
  • Damaging wind gusts with the stronger cores
  • Perhaps a tornado, especially if a supercell can develop and sustain itself in the modestly enhanced low-level shear environment

However, the lack of stronger low-level moisture and instability should limit the overall severe threat. The SPC also notes that low-level flow will tend to veer to westerly through the late afternoon and evening, reducing effective storm-relative helicity and lowering tornado potential with time.

What about Florida?

Isolated to scattered thunderstorms should form along the Atlantic Coast sea breeze this afternoon across the central and southern Florida Peninsula. Glancing large-scale ascent associated with a weak mid-level perturbation over the Southeast will aid convective development, but multiple inversions aloft noted in the 12Z observed Miami and Tampa Bay soundings should limit updraft intensity.

Small hail and gusty winds could occur with the stronger cores, given the presence of moderate instability and 30–35 knots of deep-layer shear. The threat will be confined to the afternoon and early evening hours.

Tomorrow's outlook

The SPC Day 2 outlook shows no severe weather risk for Thursday across the continental United States outside of a continued low-end threat for isolated storms in Florida. The cold front responsible for today's activity will push offshore, and no organized severe weather is expected behind it.

Staying alert in a Marginal Risk

Marginal Risk days can be deceptive. While the overall severe weather threat is low, isolated strong to severe storms can still develop—and a tornado, though unlikely, remains possible. If you're in eastern Illinois, Indiana, Kentucky, or middle Tennessee today, keep an eye on the sky this afternoon and have a way to receive warnings.

Third-party weather apps are silenced by Do Not Disturb, though Wireless Emergency Alerts for tornado warnings still break through on most phones. If you want an extra layer of protection—especially if you'll be indoors, at work, or otherwise distracted—consider a service that can call your phone when a warning is issued for your exact location.

VORTEX is a free web app at vortexintel.app that monitors severe weather nationwide. Pro ($4.99/month) places phone calls to your phone when a tornado or flash flood warning is issued for a location you care about — calls can ring through Do Not Disturb once you enable Emergency Bypass for the number, unlike most third-party app notifications.