textproduct: Chicago

KEY MESSAGES

- Threat for isolated to widely scattered thunderstorms this evening, mainly south/southeast of Chicago. Severe threat is low, but a few strong/gusty cores possible, particularly south of I-80 and east of I-55.

- Warming trend through Thursday (except cooler near Lake Michigan on Wednesday).

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

DISCUSSION

Issued at 230 PM CDT Tue Apr 21 2026

Through Friday:

Early afternoon surface analysis depicts a weak surface low pressure wave over central WI, along a cold-frontal trough which trails southwest across IA and southern NE. This surface wave is associated with a mid-level short wave trough evident in GOES water vapor imagery tracking east-southeastward across the region. Breezy southwest flow was in place across the warm sector southeast of the front/wave, with gusts in the 30-35 mph range (highest southeast cwa), helping to advect a narrow axis of low-level moisture (surface dew points upper 40s/low 50s) into the area. Despite this modest increase in low-level moisture, soundings (including 18Z DVN RAOB) indicate low-level capping beneath an inversion at the base of an EML plume. Subtle moistening aloft at the base of the EML however, has allowed for elevated convection to develop across parts of southern WI and far north/northeast IL over the past few hours. Various high-res CAMs have struggled with the general handling of the elevated moisture axis and resulting footprint/coverage of this convection, though empirically it appears that coverage into the Chicago metro may be relatively isolated this afternoon. While coverage may be limited, the steep lapse rates within the EML plume and relatively dry sub-cloud base air mass may support localized hail/gusty wind potential, especially from the Chicago area south/southeast where CAMs continue to indicate slightly higher coverage of storms. A few showers may linger across our far south/southeast cwa into tonight.

The surface cold front to our northwest is progged to sag slowly southeast across our northern cwa after midnight tonight, eventually stalling in the vicinity of the I-80 corridor. High- res RAP and HRRR runs have maintained a signal for fog/low stratus to develop over Lake Michigan behind the front, which may spread inland into adjacent northeast IL/northwest IN as winds turn northeast off the lake. Not currently seeing much evidence of this trend in satellite over the lake, this signal has been consistent for several runs and thus have maintained the mention of fog over the lake and patchy fog inland several miles across the Chicago metro and I-80/94 corridors of northwest IN.

On Wednesday, upper level ridging builds slowly east from the Plains in the Mississippi River Valley ahead of a deep long-wave trough and closed upper low developing across the western CONUS. This induces southerly low-level flow across the Plains and Mississippi Valley which eventually lifts the stalled front back north as a warm front later Wednesday and Wednesday night. Onshore flow near the lake will keep temperatures cooler (50s to near 60) near the lake, while inland areas warm back into the mid and upper 70s in the afternoon. With pooled moisture on the warm side of the front, HRRR/RAP/ARW indicate some potential for some isolated showers/thunderstorms across mainly our southeastern cwa, where afternoon SBCAPE values increase to 500-750 J/kg. However, other than the presence of the boundary, larger-scale forcing appears absent with rising heights associated with the upper ridge and no notable short wave or upper jet support. Thus have only slight (<20 percent) pops during the late afternoon. The warm front continues to lift north Wednesday night as the upper low over the western CONUS lifts northeast across MT and broad southwest low-level flow continues to edge eastward into the Mississippi Valley.

The warm front shifts north of the forecast area Thursday, placing the forecast area within a breezy warm sector ahead of the western trough. While some low-level moisture return is noted, forecast soundings once again depict another EML plume with moderately steep laps rates aloft, and an inversion based around 800 mb providing capping through the day. Thus daytime hours of Thursday appear to be precipitation-free at this point, with breezy and warmer conditions as temperatures warm into the lower 80s in most areas. Shower and thunderstorm chances then look to increase Thursday night, as a mid-level short wave rounds the base of the western upper trough, and approaches/shifts across the forecast area. GFS/ECMWF MUCAPE is greatest Thursday night, likely supporting a better thunderstorm potential than during the day Friday, when a slow-moving cold front pushes slowly east across the forecast area. Even so, relatively limited instability, poor diurnal timing and weak mid-level flow suggests the severe threat would remain fairly low, especially considered to areas farther southwest of the forecast area.

Ratzer

Friday night through Tuesday:

As an upper level trough exits to the east-northeast, rain showers may linger through the overnight, but eventually dissipate through the early morning hours. The surface cold front (mentioned in the discussion above) will continue to move through the area Friday night through Saturday morning, dropping morning low temperatures into the 40s. A cooler air mass and persistent north-northeast winds will help limit high temperatures through the weekend to the 60s north of I-88, and the 50s along the lakeshore. Weak upper level ridging over the weekend should make for drier conditions on Saturday and Sunday.

As shortwave trough over southern California on Sunday morning is expected to move northeastward and phase with the larger upper level low parked over Alberta early next week. There is pretty decent model agreement in surface cyclogenesis developing in the Colorado front range Sunday night before moving northeast across the Plains toward the Great Lakes early next week. While confidence in timing is still limited at this distance, there is a growing signal for the likelihood for showers and storms to pass over the forecast area early next week. The exact track and depth of the low will also have to be monitored in the coming model runs. That can help influence the duration and how strong the showers and storms may become. Seasonable temperatures are forecasted through the middle of next week behind the system.

DK

AVIATION /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 627 PM CDT Tue Apr 21 2026

Forecast concerns include...

Ifr/Lifr cigs/vis with fog overnight/Wednesday morning. Wind shift to northeast overnight.

Scattered showers and thunderstorms currently along and south of an IKK/SBN line will continue moving east/southeast and away from the terminals this evening.

Southwest winds may gust into the 20kt range for the next 1-2 hours and will quickly diminish after sunset, possibly becoming light and variable later this evening. Winds will shift to the northeast with a weak cold front overnight and then remain northeast or easterly Wednesday for the Chicago terminals, more southeasterly elsewhere.

The cold front will allow ifr/lifr cigs and possibly dense fog to form over Lake Michigan. Some of this is expected to move inland across the Chicago terminals but only medium confidence and opted to make no changes to the previous forecast. Trends will need to be monitored for possible lower vis at ORD/MDW. Vis/cigs should steadily improve through mid/late morning over land but the fog is expected to persist over Lake Michigan and possibly move back inland Wednesday evening. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 7 PM CDT this evening for the IN nearshore waters.