textproduct: Chicago

KEY MESSAGES

- Scattered showers and isolated storms are possible this evening through Sunday morning, with the greatest chances of thunderstorms around and west of I-39 this evening and east of I-55 Sunday morning.

- Above normal temperatures are expected through early next week, though cooler conditions will occur near Lake Michigan each day.

DISCUSSION

Issued at 222 PM CDT Sat May 23 2026

An upper-level low over southern Manitoba will shift NNE through tonight as a trough axis extending from the Arrowhead of Minnesota to the Missouri River Valley drifts eastward. Meanwhile, an ongoing occlusion of the associated low-level warm sector is ongoing across eastern Iowa into southwest Wisconsin.

As low-level moisture increases via advection across western Illinois (evident in the low 60s dew points and growing cumulus early this afternoon), a scattered to broken axis of showers with some embedded thunderstorms is expected to develop later this afternoon over eastern Iowa and drift eastward across eastern Illinois into this evening. Coverage of thunderstorms remains questionable as modest mid-level warming should inhibit deeper surface-based convection through sunset. Additionally, weak flow of under 25 knots throughout the potential convective layer would yield short-lived, slow-moving cells. That said, decent low-level CAPE below the mid-level warming indicates showers could produce locally heavy rain even in the absence of thunder through this evening. Expectation is for the highest chances to reside around and west of I-39 very late this afternoon into mid-evening before shower and storm chances wane with eastward extent late this evening into the early overnight hours.

Continued low-level moisture advection ahead of the slow-moving front tonight will then interact with a trio of forcing mechanisms to increase shower coverage generally around and east of I-55 late tonight and especially from sunrise to mid-morning Sunday. Coverage may increase quickly as loosely coupled upper jet streaks align with a secondary mid-level trough axis and mid-level convectively enhanced wave currently emanating from Oklahoma. Thunder chances again appear relatively low, but with PWATs increasing to over 1.5 inches and the bulk of CAPE supportive of low-centroid cells, efficient precip and swaths of heavy rainfall appear likely. Also cannot rule out brief funnels under any growing convection over the far southeast CWA mid-morning as surface-based instability increases, though the best chance appears over southern IL/IN closer to more favored surface vorticity.

High pressure will then bring quieter conditions to the area late Sunday through Memorial Day and possibly through Tuesday. An attempt at Rex blocking is becoming more evident in guidance mid to late week as ridging over the central and northern Great Plains is undercut by a southwestern trough cutting off over Texas and the Lower Mississippi Valley. Expansive Hudson Bay high pressure Tuesday and Wednesday will drift southward across the Great Lakes region late in the week, which should stave off higher precip chances during this period. Between these windows above, a backdoor front associated with the approaching wave may bring a period of showers and some storms to the area Wednesday before precip clears south by Thursday.

Kluber

AVIATION /00Z TAFS THROUGH 06Z MONDAY/

Issued at 636 PM CDT Sat May 23 2026

Forecast concerns include...

Ifr/lifr cigs/vis near Lake Michigan this evening. Chance of thunderstorms around daybreak Sunday morning. Mvfr, possible ifr, cigs early Sunday morning. East wind shift with a lake breeze Sunday afternoon.

Fog and low clouds over far southwest Lake Michigan continue to slowly move inland east of MDW and across GYY. This fog has been slowly dissipating from north to south as well as over parts of the lake. If these trends continue, the fog and low clouds may dissipate over the next few hours but confidence is low and trends will need to be monitored.

Isolated showers will be possible across parts of north central IL this evening, including along the lake breeze as it moves west, away from the Chicago terminals. Currently only expecting a low chance of showers at RFD this evening. These showers may slowly dissipate after sunset with a brief lull around midnight and then additional showers are expected to develop overnight and as an upper trough swings across the area in the predawn hours, showers are expected to increase in coverage with at least isolated thunderstorms possible. Some of the most recent guidance would suggest a broken line of thunderstorms possibly developing right over the Chicago terminals around daybreak Sunday morning. Only medium confidence from this distance and opted to include tempo thunder mention for this potential. Showers will end during the mid morning hours and then dry weather is expected for the rest of the period.

Cigs overnight into Sunday morning will likely be dependent on precipitation coverage with mvfr cigs expected along with the possibility for ifr cigs for at least a few hours.

Easterly winds around 10kt can be expected with the current lake breeze as it continues moving west until it dissipates later this evening. Easterly winds will become light and variable overnight, then shift to the north/northwest Sunday morning as a cold front moves across the area. Winds will shift easterly at the Chicago terminals Sunday afternoon as a lake breeze moves inland. Timing will likely needed some adjusting with later forecasts. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Dense Fog Advisory until 10 PM CDT this evening for the IL and IN nearshore waters.