textproduct: Chicago

KEY MESSAGES

- Elevated grass/brush fire threat continues 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. There is a level 3 out of 5 severe threat in the region with large, possibly destructive hail and damaging winds the main threats.

DISCUSSION

Issued at 215 PM CDT Wed Mar 25 2026

Today and Tonight...

An area of somewhat enhanced mid-level moisture (near 12-15 kft agl) across Iowa is steadily translating eastward and will arrive in our southwest locales through mid-late afternoon. Water vapor loops and model analyses clearly reveal a low amplitude wave centered near 850- 700 mb which will continue to slide southeast across the forecast area through early evening. Given the presence of steep mid-level lapse rates (at least in a narrow zone aloft), there is at least a low potential for high based convection to develop as this activity scoots across the area. With significant sub-cloud dry air in place, if more substantial shower (and isolated thunderstorm) activity materializes, some strong wind gusts around 40 mph can't be ruled out, mainly across the southwest third of the CWA. Have introduced some slight chances (mainly for showers) to cover this potential through early evening.

More robust convective development will be possible later this evening and into the early overnight period as a plume of enhanced 925-850 mb moisture pushes east-northeastward out of central Illinois. Intensifying isentropic ascent at the nose of a southwesterly LLJ should allow at least widely scattered elevated convection to blossom. Precisely where this occurs and how much MUCAPE is available remains in question due to some lingering model spread. The latest NAM has trended a bit farther south and less robust with the degree of 850 mb moistening and thus MUCAPE, more in line with the rest of the guidance suite. In turn, have gently pushed the highest thunderstorm chances a bit farther south closer to the Kankakee River vicinity and points south. Given at least modest skinny CAPE profiles and respectable deep layer shear with strong veering through the column, some instances of marginally severe hail can't be ruled out with the strongest cores mainly across our far south and southwest locales after about 9-10 PM, but this potential appears limited in both scope and location at this time.

Any elevated convection should diminish and push east of the region late tonight. This will set the stage for a very active period of weather Thursday afternoon and evening.

Thursday...

One lingering question mark which will ultimately have a large bearing on the precise location, timing, and general evolution of the severe weather threat is the speed/location of a very sharp backdoor cold front. Today's guidance still remains somewhat split on this, and in particular how quickly the front initially rolls off the lake and across northern Illinois during the late morning and early afternoon. The latest ECMWF remains a reasonable middle ground solution, but a faster solution like the 12z HRRR/WRFARW/GFS remains very much in play. As such, the high temperature forecast across northern Illinois and lake- adjacent portions of NW Indiana remains uncertain, with a very large "bust" potential. If the front ends up slower, temperatures in Chicagoland could surge into the upper 70s before falling rapidly into the 40s.

By about 1 PM/early afternoon, there is decent agreement that the front will be somewhere in the vicinity of a Rochelle to Aurora to Valparaiso line and will continue to move southward at a decent clip. To the south of the boundary, temperatures will warm very quickly into the 80s (possibly mid to upper 80s in spots). Deeper mixing may facilitate the development of stronger southwesterly wind gusts and have boosted these a bit with this forecast. The base of a warm EML will effectively cap this warm sector to convection until at least 2-3 PM, if not even a bit later than this. To the north of the front, markedly colder conditions with gusty northeasterly winds will develop. Increasing upper jet divergence and the arrival of a 700 mb shortwave may allow initial elevated convective clusters to develop across northern Illinois through mid afternoon.

Through mid-late afternoon, the cold front will continue to press southward, likely in the vicinity of the Kankakee River Valley by about 4 PM or so. Gradually-increasing large scale ascent will eventually erode what lingering capping exists to the south of the boundary and moistening/cooling atop the frontal inversion, resulting in what should be generally explosive thunderstorm development across the region around this time period. The primary severe hazard during this time and into Thursday evening will be from large, possibly VERY large hail, particularly for elevated cells atop the shallow frontal inversion. Deep layer shear is forecast to be extreme, with 1-7 km shear values approaching 60 to 80 knots. Additionally, modest MUCAPE values and very strong storm- relative inflow winds (35+ knots) will be supportive of very wide/large updrafts, facilitating large hail growth. Even though most storms will remain elevated, there will be a localized damaging wind gust threat (high res guidance depicts some degree of evenly- spaced convective elements and associated 50+ knot gust output, suggesting the potential for gravity wave damaging wind gusts).

Regarding the tornado potential: It remains unclear how many storms will fire in the lingering free warm sector. The HRRR/RAP/RRFS (which could be a bit overmixed) do not appear to convect aggressively south of the front, possibly due to a warm/dry PBL. However, if storms develop in this area and remain discrete or semi-discrete, there would be a tornado potential before the cold front rolls south of the forecast area. At this time, the greatest tornado threat (based on the latest cold front timing) appears to be relegated to areas south of about a Streator to Rensselaer line, and this area would become increasingly small with time. Even aggressively rightward-moving supercells would also likely eventually get undercut by the surging cold front (augmented by thunderstorm outflow to the north).

Some localized training of west-east moving storms may result in locally heavy rainfall, but this does not appear to be a set up favoring widespread flash flooding.

Activity will eventually end as the main elevated instability axis is shoved well south of us through the late evening. Right now, it looks like precip will end prior to the arrival of sub- freezing temperatures and do not have any snow mention in the gridded forecast Thursday night.

Friday and beyond...

Broad cyclonic flow will develop across the Upper Great Lakes on Friday as a sprawling mid 1030s mb high spreads across the central CONUS. An embedded perturbation will drift across northern Wisconsin and across northern lower Michigan on Friday, and some guidance hints at the potential for some rain/snow shower development during the afternoon across northern Illinois. Boundary layer moisture looks pretty minimal in our area, so have left a dry forecast at this time. Lingering northwesterly breezes will quiet down Friday night and Saturday morning as the surface high builds east of the Mississippi River. After a cool start, Saturday looks like a decent day with temperatures pushing back to around 50. Warmer and breezy conditions are expected on Sunday as the surface pressure gradient tightens in advance of the next system.

Forecast confidence diminishes for next week, but the large scale pattern generally looks active in the region as broad longwave troughing to our west will likely guide repeated disturbances across the area. At the same time, an expansive elevated mixed layer (EML) will continually expand and advect eastward on increasing southwesterly mid-level flow. As a result, shower and thunderstorm chances look like they'll persist intermittently through much of next week.

Carlaw

AVIATION /18Z TAFS THROUGH 00Z FRIDAY/

Issued at 1227 PM CDT Wed Mar 25 2026

Key Aviation Messages for the current TAF period:

- Gusty southwest winds this afternoon to 30 knots

- Period of low-level wind shear this evening

- A strong cold front will push through tomorrow. Lower confidence on exact timing, but a wind shift from southwest to northeast with the front can be expected, with increasing gusts in excess of 20 knots in the afternoon

- Shower and thunder chances increase with the passage of the cold front.

Relatively quieter conditions in the near term compared to what is expected tomorrow. Dry and VFR conditions are expected through the rest of today. The main impacts will be from the increasing gusty southwest winds. Gusts 20 to 25 knots can be expected, but maximum gusts to 30 knots are possible this afternoon. There is moderate confidence in wind gusts diminishing around and after sunset. A strong low level jet is expected to increase around and after sunset leading to low level wind shear to be possible through around midnight.

Thursday will start with VFR conditions and southwest winds to 10 knots. However, a stout cold front will come racing southward over northern Illinois tomorrow. Ground crews can expect a stark temperature drop from the day's high temperatures in the 60s down into the 40s in under 2 hours or less. High res guidance has been developing a weak pressure trough tomorrow morning ahead of the front, allowing winds to switch from the southwest to the northeast. Winds will then remain out of the northeast and increase with gusts in excess of 20 knots with the passage of the front in the afternoon. Scattered showers and the chances for storms can be expected along and behind the front. Some of these storms may be strong to even severe with the potential for damaging hail and wind. While confidence was high enough to include thunder mention in the TAF, confidence remains lower on the overall timing of the wind switch and thunder arrival and will need to be continually re-evaluated in later forecast packages.

DK

FIRE WEATHER

Issued at 215 PM CDT Wed Mar 25 2026

Southwesterly winds will gust up to 30 mph this afternoon. Additionally, relative humidity values will settle into the 15 to 25 percent range, with the lowest readings occurring generally near and east of I-55. This combination will result in an increased threat for rapid grass and brush fire spread. After 7 PM, RH values will increase quickly and the strongest gusts will ease. A special weather statement has been issued to highlight the increased fire weather threat.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 8 PM CDT this evening for the IL and IN nearshore waters.

Gale Warning from 5 PM Thursday to 7 AM CDT Friday for the IL and IN nearshore waters.