textproduct: Chicago

KEY MESSAGES

- Breezy and much warmer today with a somewhat elevated grass/brush fire risk this afternoon.

- Isolated to scattered thunderstorms expected south of I-80 tonight, some may produce hail up to around 1" in diameter.

- Powerful cold front will move across the area Thursday with temps in the 70s/80s ahead it, falling sharply into the 40s/50s behind it.

- Thunderstorms are likely to develop near the front Thursday afternoon, with the highest chances of thunderstorms near and south of I-80. Some thunderstorms will probably be severe.

DISCUSSION

Issued at 255 AM CDT Wed Mar 25 2026

The weather is going to get pretty active the next couple of days around here with dramatic temperature swings!

Today, looks breezy and unseasonably mild with gusty southwest winds likely to help push high temps up to near, if not into, the lower 70s. There will high cloudiness around again today, but less prominent than Tuesday, with indications that there could be some modest decrease in cloudiness this afternoon. This is likely to help deepen the boundary layer which should mix into some extremely dry air off the surface. Made no real changes to forecast dewpoints for today, keeping them 5F+ lower than NBM, which could still be too high if we see a good deal of sunshine this afternoon.

Low level moisture return is underway across the southern Plains with 850mb dewpoints around 10C observed on soundings Tuesday evening. This moisture will advect northward today, then eastward into the Midwest tonight, driven by a 50kt westerly low level jet. Low amplitude shortwave currently approaching the northern Rockies will race eastward and is progged to reach the mid-Mississippi Valley tonight. Strong moisture convergence near the terminus of the low level jet aided by some large scale ascent from the aforementioned shortwave should result in scattered elevated thunderstorms developing this evening.

There is some spread among guidance with respect to the magnitude of elevated instability tonight, but most guidance depicts an axis of 500-1000 J/kg of MUCAPE nosing into Illinois along the leading edge of a strong elevated mixed layer (EML). Hodographs in the effective shear layer are long and straight with effective bulk shear of 40kt+. Assuming elevated CAPE of 500-1000 J/kg is realized, then there would be concern for some elevated supercells with the potential for isolated severe t-storms capable of large hail across the Illiana region tonight, mainly affecting our southern CWA.

Lots to talk about with Thursday's weather...

First, guidance is in good agreement on a strengthening surface front becoming established across northern IL or far southern WI Thursday morning. Unfortunately, there are some very meaningful differences in models with respect to where this boundary sets up. The HRDPS and NAM are farther north with the boundary developing north of the WI border with the ECMWF farthest south with sfc frontogenesis taking place near I-80 Thursday morning, with the GFS in the middle. Climo this time of year would certainly favor the ECMWF's farther south solution given the still very cold lake, but confidence is pretty low at this point. Where the front develops will have huge implications on temperatures and where the eventual severe threat will materialize Thursday afternoon.

South of the strengthening warm front, temperatures are likely to climb into the upper 70s to mid 80s, while north of the boundary temperatures may struggle to get out of the 50s. That tight temperature gradient opens to the door to huge temperature forecast busts for Thursday. For now, leaning toward the compromise solution of the GFS in the grids, which would allow most of the Chicago area and most of northern IL to at least spend the first half of the day in the warm sector with temps climbing into the 70s. If the ECMWF solution pans out, high temps near and north of the I-88/I-290 corridor could end up being easily 25 degrees colder than currently forecast, so stay tuned!

During the afternoon, a weak surface low is progged to ripple eastward across northern IL/southern WI into southern lower MI or northern IN by later in the afternoon. In the wake of that low, the front should begin to surge southward resulting in a dramatic temperature drop. If areas near Lake Michigan do warm well into the 70s early tomorrow, then this could end up being a pneumonia front with a rapid temp drop greater than 30F during the afternoon.

An usually strong EML for this time of year is forecast to overspread the Midwest into the lower Ohio Valley Thursday. Very steep mid level lapse rates within this EML combined with moderate low level moisture should result in an MLCAPE axis of 1500-2000 J/kg along and ahead of the cold front. The EML will also provide for a strong capping inversion near its base, but ascent associated with the approaching shortwave trough, heating of the boundary layer, some modest pooling of low level moisture ahead of the front, and convergence near the front should be enough to breach the cap during the mid-late afternoon.

The fairly strong instability for this time of year will be juxtaposed beneath very strong kinematic field with 0-6km bulk shear likely to be near/above 60kt, creating a potentially explosive severe weather set-up. Forecast hodographs are long and straight, suggesting a potential for supercells with large to very large hail as the initial convective mode. Most CAMs suggest a fairly quick evolution to linear mode, which given the shear vectors fairly parallel to the front and strong frontal forcing would make sense. The transition to linear mode would likely lead to a transition to more of damaging wind threat.

There's still a good deal of uncertainty about how far north convective initiation takes place as well as how quickly the transition to linear mode takes place. The greatest overall severe risk in our CWA looks to be near/south of I-80 and particularly south of the Kankakee River Valley. There looks to be a couple hour window of supercells capable of producing hail greater than 2" in diameter with the transition to more a damaging wind threat potentially not taking place until storms are out of or nearly out of our CWA.

Precipitation should end quickly Thursday night, likely ending before the column is cold enough for any wintry precipitation. Friday looks quite chilly with highs in the 40s, except only in the 30s near and immediately downwind of Lake Michigan. It still looks as though a cold snap will be short with a quick moderation in temperatures taking place over the weekend.

- Izzi

AVIATION /12Z TAFS THROUGH 18Z THURSDAY/

Issued at 635 AM CDT Wed Mar 25 2026

Expect VFR conditions with periods of BKN mid to upper-level clouds filtering over the area through at least this evening. Elevated TS forming on a modest LLJ late this evening into the early morning hours Thursday may reach as far north as the Chicago metro. Have included PROB30 TS for GYY/MDW roughly in the 04-07Z window, with the potential ORD/DPA will also need TS mention with future TAF cycles. Any TS tonight has the potential to produce hail, as well as gusty winds due to initially dry low-level air. MVFR ceilings also cannot be ruled out early Thursday morning as low-level moisture surges northward.

S winds up to 10 knots at TAF issuance will quickly settle SSW and gust over 20 knots as daytime mixing into the LLJ commences 13 to 14Z. Frequent SW gusts of 25 knots are then expected this afternoon, with sporadic gusts to 30 knots possible late in the afternoon.

LLWS was maintained for this evening as gradual decoupling leads to SW/SSW winds at or below 10 knots (sporadic gusts still possible) beneath a developing 40-45 knot LLJ. After the LLJ exits southeast overnight, surface winds will back S overnight, then veer SW Thursday morning.

Kluber

FIRE WEATHER

Issued at 302 AM CDT Wed Mar 25 2026

Gusty winds and dry conditions will likely result in a heightened fire danger this afternoon. Some decrease in the coverage and thickness of the high cloudiness this afternoon is expected to allow near surface moisture to mix out this afternoon with min afternoon RH in the 20-30% range, likely lowest near and southeast of I-55 (including all of northwest Indiana). If cloudiness is more prominent than currently forecast, it is possible that min afternoon RH could be higher than forecast and temps a bit lower than forecast.

Southwest winds are expected to increase and become gusty today with gusts of 25-30 mph likely and occasional gusts over 30 mph possible. While winds will increase, sustained 20ft winds are likely to remain safely below the 20 mph red flag threshold, so no red flag headlines appear necessary or are planned for today.

- Izzi

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Gale Watch from Thursday afternoon through Friday morning for the IL and IN nearshore waters.