textproduct: Chicago

KEY MESSAGES

- Scattered showers and thunderstorms are expected today, mainly during the afternoon and evening hours. A few storms could become severe in the evening with damaging winds and hail the main threats, but an isolated tornado cannot be ruled out.

- Dangerous swimming conditions are expected at Porter County, Indiana Lake Michigan beaches Saturday, then all Illinois and Indiana Lake Michigan beaches late Saturday night and Sunday.

DISCUSSION

Issued at 132 PM CDT Fri Aug 21 2026

Early this afternoon, a decaying MCS is pushing south across far northern IL along an north-south oriented instability gradient draped very roughly along the I-39 corridor. Additionally, scattered showers are moving east near and north of the Kankakee River valley as a subtle leading shortwave works across. This MCS has been on a very gradual weakening trend, but it managed to hold together much better than any recent guidance had thought. It's still generating a fair amount of lightning and pockets of heavier rainfall near its leading edge which will likely persist as it continues to drop south across the Chicago metro through the afternoon. No strong or severe storms are expected through mid-afternoon with this system, but embedded downpours will likely remain.

Farther upstream, a more potent shortwave trough axis is swinging into the upper Midwest coincident with a surface cold front stretching from western WI southwest into northeast and central IA. It's along and ahead of the front this evening where we expect a chance for severe convection. The first showers have just started to go up along the front near La Crosse. Thunderstorms are expected to blossom along the boundary as it continues southeastward and eventually crosses into northern IL near the end of the afternoon. SPC mesoanalysis suggests instability is building quickly west of I- 39 in a region of efficient theta-e advection ahead of the front. CAM guidance has been rather consistent on a tongue of 2,500 to 3,000 Joules of MLCAPE building into our western and northwestern CWA by late afternoon. It's not clear the quality of the thermo environment farther east this evening between this MCS, added daytime cloud cover, and eventually diurnal cooling into the evening. All along, guidance had been resolving notably less instability with eastward extent largely owing to a drier low level airmass and diurnal cooling through the evening. However, the added rain and cloud cover could be enough to greatly inhibit much of a severe threat east of the Fox Valley and especially into northwest Indiana, but there's not great confidence there.

Effective shear this evening is marginal to fair at about 25 to 35 kt, highest in our west. However, models do resolve a veering low level shear profile which would support a low-end tornado threat where the low levels can destabilize sufficiently. Even when discounting this ongoing MCS, CAMs had been consistently hanging onto some low level capping roughly east of the Fox Valley through the afternoon, and would imagine that's even more likely now, before being further reinforced by diurnal cooling. For that reason, this evening's marginal tornado threat will be focused west of the Fox Valley and north of I-80 where the more supportive low level thermo environment is anticipated. This is also where the severe wind and hail threat will be maximized given the greater instability and generally more support expected from the left exit region of a 700 mb jet max out to our west. Severe hail and damaging winds are on the table area-wide but it's not clear the eastward/southeastward extent of deep convection this evening and it's very possible much of the area is spared from severe weather.

Areas of very heavy rainfall are expected as well, in addition to the ongoing pockets of heavy rain within the remnant MCS. The storms look progressive enough in nature to avoid many widespread flooding concerns, but scattered pockets of 1-2" with the evening coverage alone is a strongly favored signal among guidance which would likely lead to localized flooding impacts, mainly in low-lying and other flood prone areas. The severe threat looks to exit the area to the southeast shortly after midnight.

The upper trough will continue digging into the Great Lakes region through tomorrow and drive a vort max across WI and Lake Michigan during the day tomorrow. Isolated to widely scattered showers and storms will attempt to develop on the back side of the system and fall on parts of the area tomorrow afternoon. With the cooler mid level air spilling over, as many as 1,500 to 2,000 Joules of MUCAPE is favored to develop over NE IL and NW IN for the afternoon. Nothing too strong or severe is anticipated, but if one or two more robust updrafts can develop and survive through significant dry air aloft, it may be enough to spit out some locally heavy rain and gusty thunderstorm winds. Most of the area will be staying dry tomorrow with highs forecast in the upper 70s to around 80.

Strong northerly winds will develop on Lake Michigan tomorrow and generate high waves and dangerous currents at northwest Indiana beaches leading to hazardous swimming conditions. Such conditions will work west to include the Illinois lakeshore for Sunday. High pressure will dominate the Midwest and Great Lakes regions early next week keeping conditions largely quiet. Daily highs are forecast to range from the middle 70s to lower 80s with rain chances looking to return closer to midweek.

Doom

AVIATION /00Z TAFS THROUGH 06Z SUNDAY/

Issued at 626 PM CDT Fri Aug 21 2026

Forecast concerns include...

Chance of thunderstorms this evening. Possible isolated thunderstorms Saturday afternoon. Possible mvfr level cloud cover early Saturday morning. Gusty northwest winds Saturday.

There will be a chance of thunderstorms across the Chicago terminals this evening, but the better chance of thunderstorms is expected to be across northwest and west central IL where there is better instability. While coverage is expected to be better west of the Chicago terminals, there is still enough uncertainty to maintain tempo thunder for now and will monitor trends. All of this activity is expected to move south of the terminals by late evening.

There may be a few showers and isolated thunderstorms Saturday afternoon but confidence is very low and maintained a dry forecast. Any activity that may develop would likely be over southern Lake Michigan into northwest Indiana, possibly just east of the Chicago terminals.

There may be some low mvfr level cloud cover in the predawn hours into mid morning Saturday, but confidence is very low for any possible bkn/ovc low mvfr cigs. Appears the best chance for any mvfr cigs will be east of the Chicago terminals.

Southerly winds generally under 10kt this evening will turn more westerly overnight, northwest by daybreak and then gusts into the 20kt range are expected through sunset Saturday evening. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...Beach Hazards Statement from Saturday morning through Sunday evening for INZ002.

LM...None.