textproduct: Chicago

KEY MESSAGES

- Scattered thunderstorms are possible this evening (50% chance), some of which may be capable of damaging wind gusts and heavy downpours which may lead to localized flooding.

DISCUSSION

Issued at 130 PM CDT Tue Aug 18 2026

An area of showers, with a few embedded storms, have been developing across parts of northwestern IL and far southern WI over the past couple hours. It appears this activity is tied to a corridor of warm air advection just in advance of a remnant MCV shifting into northeastern IA. We anticipate this isolated to widely scattered activity to shift eastward across far northern IL (mainly north of I-80) this afternoon well in advance of an approaching cold front over the Upper Midwest. Lightning activity is expected to remain isolated, and likely focused closer to the WI state line the next few hours as the instability gradient begins to slide southeast. We do not anticipate much of a severe threat with these initial showers and storms this afternoon.

The cold front currently extends from southeastern MN east- northeastward into northeastern WI. Additional strong to severe thunderstorms are expected to develop near the front late this afternoon as the prefrontal airmass moistens/destabilizes. Deep-layer shear will support organized severe weather, driven by 45-50 kt mid-level westerly flow overspreading the warm sector at the base of a digging mid-level trough. Initial supercells along the front are expected to upscale into a southeastward-propagating clusters into southern/southeastern WI late this afternoon, potentially reaching northern IL this evening. The primary local threats include torrential rainfall, frequent lightning, and damaging wind gusts exceeding 60 mph. While the highest threat is currently near the WI state line, localized damaging winds may persist across the Chicago metro area this evening, roughly between 7 and 11 PM.

On Wednesday, the primary threat for convection shifts toward central IL and IN, well south of I-80. However, lingering low- level moisture in northeastern IL may support isolated afternoon showers and storms, particularly along the lake breeze boundary. No severe weather is expected Wednesday afternoon, and most areas should remain dry. Given recent trends, a 20% chance of afternoon storms has been included for northeastern IL. High temperatures will reach the low 80s inland and mid-70s along the lakeshore.

Surface high pressure will move overhead Thursday, bringing light winds, mostly sunny skies, and lower humidity. Temperatures in the upper 70s to near 80 will make for a pleasant day. Forecast confidence decreases late in the week as additional waves move through the northwest flow pattern, bringing a return of shower and storm chances on Friday. While some storm threat could linger into Saturday, Sunday currently looks dry with pleasant temperatures in the 70s.

KJB

AVIATION /18Z TAFS THROUGH 00Z THURSDAY/

Issued at 1245 PM CDT Tue Aug 18 2026

Forecast concerns include...

Chance of thunderstorms this afternoon into this evening. Possible mvfr/ifr cigs and fog overnight/Wednesday morning. Wind shift to northeast Wednesday morning.

Scattered showers and isolated thunderstorms continue to slowly increase in coverage across far eastern IA/southwest WI and far northwest IL early this afternoon. This activity is expected to continue moving east across southern WI and far northern IL this afternoon, though there is quite a bit of uncertainty for coverage. This current activity may end up only being showers for the Chicago area terminals, while thunder remains north of the terminals. The better chance for thunderstorms appears to be late this afternoon into this evening ahead of a cold front that will move across the terminals tonight. Adjusted thunder timing slightly later based on these expectations, but some short duration thunder may be needed this afternoon for the current activity and will need to monitor trends.

Depending on precip coverage this evening, there may be mvfr or ifr cigs across the area overnight into Wednesday morning. Low confidence at this time, so have not included any prevailing cigs, but trends will need to be monitored. There does appear to be an increasing signal for fog early Wednesday morning, mainly across northwest IL and have included tempo mention at DPA/RFD.

Southwest winds may gust into the 15-20kt this afternoon and through sunset, when gusts will end and speeds will slowly diminish through the evening, possibly becoming light and variable overnight into Wednesday morning. Most of the guidance shows north/northeast winds Wednesday morning then becoming more easterly in the afternoon with a lake breeze. Have followed these trends with this forecast, but there is still uncertainty for wind directions Wednesday morning, prior to the expected afternoon lake breeze.

There is a low chance for a few showers, possibly an isolated thunderstorm Wednesday afternoon, perhaps mainly south of the terminals. Confidence is too low to include any mention with this forecast. cms

LOT WATCHES/WARNINGS/ADVISORIES

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