Low-Topped Supercell and Tornado Risk in Illinois/Indiana on Thu, Feb 19

Looks like the first storm observation of the season is near!

The Storm Prediction Center has maintained a 'slight risk' and 5% tornado probability draped across central and southern portions of Illinois and Indiana on their latest Day 2 Severe Weather Outlook. Their thoughts are very much aligned with my own, and you can read their latest discussion here.

I mostly wrote this off as a moisture-starved storm system several days ago, but as it's drawn closer the potential for a few low-topped supercells and an attendant risk for a couple of tornadoes has emerged across the Ohio Valley.

Behind today's moisture scouring frontal passage in Illinois and Indiana, an ejecting trough over the Southern Plains today will begin to lift into the Ozarks tonight with renewed moisture return across the target area.

Lead trough #1 across the Great Lakes does help get moisture return started but an associated front and dry punch will knock deeper moisture south toward the Ohio River by Wednesday evening. Ejecting trough #2 across the Southwest U.S. right now will lift into the Ozarks early Thursday morning helping rejuvenate moisture return through the day on Thursday.

High-resolution model guidance continues to depict dew points at least in the middle 50s, potentially approaching 60 deg F up to Interstate 70 from St. Louis to Indianapolis by late Thursday morning. A pre-frontal wave is expected to ignite a cluster of low-topped convection in the St. Louis area mid-day, and as this increasingly saturated and unstable environment surges north and "meets" the developing storm cluster, one or two sustained supercells may emerge.

Below is a look at my "Conditional Storm Severity" index from the 12z HRRR, with surface winds, MSLP, and simulated reflectivity. Note the deeper orange and red shaded colors lifting northward and "meeting" the storm cluster in south-central IL - there's your target storm. These plots are valid 12 - 1 PM CT on Thursday:

There may be a sweet spot early in the afternoon, between 12-4 PM CT where low-level buoyancy is the highest and low-level hodograph curvature is sufficient combined with a vorticity rich environment ahead of the pre-frontal wave to support a tornado and severe hail risk with any intense updrafts. 500 mb heights will begin to rise, along with mid-level temperatures as the trough continues to eject which may lead to an overall messy or muted evolution, especially by late afternoon and evening as storms push into central and southern Indiana.

With those potential flies in the ointment, it will probably be important to maximize insolation, or sunshine over the priming target area to hit the ceiling on potential 0-3 CAPE values and low-level lapse rates to stretch the vorticity being ingested by these shallow updrafts. Low-topped storms can be a ton of fun to chase, but they can also really struggle to do anything without that low-level juice.

It's hard to find an uncontaminated forecast sounding ahead of the early afternoon storm cluster, but this HRRR forecast sounding might be just enough for a low-topped supercell and tornado risk. I love the very steep near-surface lapse rates, though the hodograph and wind profile could be a little more loopy.

Secondary risk zones exist as well - it's not uncommon to see a "what are you doing there??" tornado producing updraft displaced to the north and west of the primary target area in the warm sector. Should the air mass recover along the occluded front across central or northwestern Illinois on Thursday afternoon, a spinny updraft or two may develop. It's also possible a severe storm or two could develop further south along the cold front into northern Kentucky, or that the risk could continue east into southwest Ohio by Thursday night.

I've got the afternoon free to play with what should be a rather local first chase of the season on my end. I'll probably keep an eye on things from home base in Champaign-Urbana before drifting south toward Interstate 70 and the Effingham vicinity... they've got a QuikTrip I love to visit there, though I've had bad luck in the two most recent chase days that I refueled here so superstition may drive me elsewhere. Like I hypothesized a little above... I want to hug the area closest to the warm front and those big loopy hodographs, so the northernmost edge of the expanding 0-3 km CAPE plume is probably where you'll find me.

Highway 36 corridor in south-central Illinois? Interstate 70 a little further south? To be determined, but while a lot of questions won't be confidently answered until Thursday morning a lot of guidance continues to hone in on that St. Louis to Indianapolis corridor for a couple of early-season supercells and tornadoes.