textproduct: Chicago

KEY MESSAGES

- Isolated to perhaps widely scattered showers/storms this afternoon and early evening mainly across parts of northwest Indiana.

- Fog possible near the lake tonight.

- Warm, breezy, and dry areawide Thursday, with minor lakeshore cooling perhaps hanging on near the Lake Co. IL shore.

- More widespread showers and scattered thunderstorms with another cold front very late Thursday night into Friday.

- Seasonable and dry conditions over the weekend, then shower and thunderstorm chances return Sunday night and Monday.

DISCUSSION

Issued at 244 PM CDT Wed Apr 22 2026

Through Thursday:

Scattered thunderstorms took off earlier over southeast Lake Michigan and then spread into northern Indiana and southern lower Michigan. As of this writing, the lake assisted cold front extends from McHenry County IL to the Kankakee River Valley of Indiana. Given the proximity of this boundary and upper 50s to around 60F dew points east of I-57 in eastern Illinois and interior northwest Indiana, 500-1000 J/kg of MLCAPE may be realized by mid to late afternoon. We'll need to keep an eye on isolated to widely scattered shower and non-severe thunderstorm development. If thunderstorms develop, slow storm motions would portend locally heavy rainfall. Finally, as noted in the previous discussion, conditional to convection initiation, favorable parameters near the boundary for weak lower level spin could conceivably yield a funnel cloud or two.

Once any showers and storms fade diurnally this evening, the main focus through tonight is the potential for fog to bleed inland from the lake and/or fog development in northeast Illinois and far northwest Indiana. There's a window from the evening into the early overnight when flow just off the deck will be quite light, especially closer to the lake. With the existing bank of fog out over the lake this afternoon, as the marine influenced boundary layer quickly cools this evening, onshore flow could help drive fog and very low stratus inland. Any patchy dense fog should tend to favor the late evening until a few hours after midnight. Thereafter, increasing southwesterly flow off the deck should result in the fog and stratus retreating northward. Overnight lows will be mild inland, in the mid to upper 50s, and in the mid to upper 40s near the lake, prior to rising prior to daybreak Thursday.

The surface pattern over the eastern CONUS will hold off appreciable moisture return until late in the day Thursday. Strong surface low lifting into the Canadian Prairies and its approaching cold frontal trough will tighten up the pressure gradient. Expect a good deal of sunshine until late day high level cloud cover arrival. These ingredients point to a breezy (south-southwesterly gusts up to 25-35 mph) and summer-like warm day, with highs near to likely above 80F for much of the area. The possible exception may be the immediate Lake County IL shoreline if winds back a bit due to lake influence. Dew points are expected to mix out into the low-mid 50s during peak heating, which may contribute to localized mid 80s highs, especially in parts of central Illinois.

Thursday Night through Friday:

Showers and thunderstorms will develop well to our west through Thursday afternoon on the aforementioned cold front and will then track eastward into our area late Thursday night into early Friday morning. Given the unfavorable diurnal timing and the progged instability/shear parameter space remaining unimpressive locally, the severe threat appears low. Can't completely rule out strong/gusty winds from a bit earlier arriving outflow associated with decaying convection, particularly west of the Fox Valley. The current setup does not appear favorable for widespread convective maintainenance and an appreciable heavy rainfall threat. However, given the heightened sensitivity to additional rainfall in areas hard hit by river flooding, we'll certainly keep a close on trends in this regard.

The southeastward progression of the cold front on Friday is a bit of a question mark, with the faster solutions likely limiting renewed convective development late Friday morning into early Friday afternoon to our southeast CWA. Meanwhile, slower solutions would bring the threat for renewed scattered storms farther west. Barring stronger than expected destabilization, the threat for strong to severe storms should remain south of out area. The late day into the evening hours should feature a quick improving trend (especially I-55 and NW) as much drier air punches in behind the front crossing our IL counties.

Castro/Carlaw

Friday night through Wednesday:

Surface cold front should be exiting the forecast area Friday evening, with any lingering showers southeast of I-55 ending as the primary mid-level short wave wraps northeastward around the closed low over southern Saskatchewan. While height rises are fairly minimal behind this wave, models and their ensembles do depict strong drying through the column in both soundings and precipitable water forecasts. Thus despite the persistence of fast west-southwesterly flow aloft, organized precipitation chances appear minimal from Friday night through Sunday, while weak surface high pressure ridge drifts east across the region.

There is decent ensemble agreement in depicting another strong mid-level short wave out of the southwestern CONUS and across the Plains Sunday, which induces renewed low-level southerly flow and associated warm/moist advection into the region Sunday night into Monday, ahead of a deepening surface low which lifts into the upper Midwest/western Great Lakes region by Monday afternoon/evening. Based on the track of the surface low, a strong upper level jet and tapping of low-level moisture off of the western Gulf (12Z ECMWF forecast precipitable water values in excess of 1.50-1.60"), this will be a period to monitor for potential severe weather and locally heavy rainfall threat in or near the forecast area. Current guidance is then in decent agreement in dry weather returning for Tuesday and Wednesday of next week, as surface high pressure builds in behind the departing deep low and surface cold front which looks to move east of the area Monday night.

Temperatures appear seasonably mild Saturday into Monday, with upper 60s to mid-70s for highs (except cooler 50s along the lake with onshore flow Saturday-Sunday). Cooler 60s (still about average for late April) are then expected behind the cold front Tuesday into Wednesday.

Ratzer

AVIATION /18Z TAFS THROUGH 00Z FRIDAY/

Issued at 1224 PM CDT Wed Apr 22 2026

Aviation forecast concerns for the 18Z TAFs:

- Scattered TSRA should remain east/southeast of terminals this afternoon across LM/MI/IN.

- Improvement from MVFR ceilings early-mid afternoon for MDW.

- IFR/LIFR ceilings and MVFR/IFR vis possible near Lake Michigan this evening/overnight. Proximity to lake of GYY and MDW make these sites most likely to have a period of LIFR.

- Breezy south winds with gusts 20-25 kt develop by mid-late morning Thursday.

Lake-assisted cold front has pushed through all TAF sites except DPA at midday. Extensive MVFR deck is gradually scattering out from north to south, and should see improvement even into MDW/GYY early-mid afternoon. Surface winds will be east- northeast in lake breeze enhanced flow.

Concern increases for a period of low stratus/fog near Lake Michigan this evening, as low-level flow remains onshore. Various model guidance depicts an expansion of IFR/LIFR cigs and MVFR/IFR vis several miles inland early-mid evening, with GYY and MDW most at risk due to proximity. ORD should be a little less at risk farther inland. Guidance focuses on the mid- evening through midnight or a little after for the lowest conditions, after which a gradual veering of low-level winds to southeast appear to decrease impacts (though some MVFR/VFR fog may linger through daybreak). Confidence is medium in GYY/MDW impacts, and lower where ORD has been kept VFR in the TAF.

Increasing southerly flow Thursday morning should lift the current weak cold/lake breeze front back north early, with winds becoming breezy from the south-southwest with gusts 20-25 kt by mid-late morning.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.