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 /00Z TAFS THROUGH 06Z FRIDAY/

Issued at 655 PM CDT Wed Mar 25 2026

Aviation forecast concerns for the 00Z TAFs:

- Gusty southwest winds 20-25 kts diminish quickly with sunset this evening. LLWS this evening appears weaker than earlier, and have removed from TAFs.

- Winds eventually become somewhat light/variable after midnight tonight, then become northeast mid-late morning Friday and eventually become blustery with gusts 20-25 kts in the afternoon and 25-30 kts Thursday evening.

- SHRA and isolated TS develop across the area Thursday afternoon. Greatest TSRA threat/coverage looks to be after 20Z at RFD and after 22Z at Chicago sites lasting through about 00Z, though additional embedded TS may persist into late Thursday evening.

Early evening surface analysis places a weak low pressure wave over northeast WI, with a cold front trailing across southern MN into the Plains. Breezy southwest winds ahead of the front across northern IL/northwest IN will diminish quickly with sunset this evening. A modest 35-40 kt low-level jet will transit the area in the 00-06Z time frame, though this is weaker than earlier indicated and makes for a more marginal LLWS setup with surface winds still near 10 kts. Therefore, have removed the LLWS mention from the forecast for this evening. Winds are expected to decrease further overnight, and become more variable in direction as the frontal trough sags toward the area and another weak surface wave propagates across the region. Winds eventually turn northeasterly Thursday morning, with an increasing trend through the day as the frontal zone (now south of the terminals) strengthens. Northeast winds will become gusty in the 20-25 kt range by early afternoon and are expected to strengthen into the 25-30 kt range Thursday evening as the surface pressure gradient tightens.

Patchy mid-high level VFR cloud cover will persist across the area tonight. Lower (IFR/MVFR) bases are eventually expected to spread into the area off of Lake Michigan on the aforementioned northeast low-level winds Thursday. Confidence is somewhat low on timing arrival of these lower conditions, though confidence becomes fairly high in low-MVFR/IFR ceilings by Thursday evening. Shower/thunderstorm chances also increase across the area from midday onward, as a mid-level disturbance tracks across the region. High-res CAM guidance indicates thunderstorm potential ramps up by about 20Z for RFD, and by 22Z for the Chicago metro terminals. Though these storms will be elevated above a cool, stable surface layer, some storms could be strong to severe, with large hail and isolated strong winds. Greatest coverage of TSRA over the terminals looks to be through about 00Z, though additional embedded TS will remain possible into late Thursday evening.

Ratzer

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 4 PM Thursday to 7 AM CDT Friday for the IL and IN nearshore waters.