textproduct: Chicago

KEY MESSAGES

- There is another threat of severe storms mainly south of I-80 later this afternoon, and across the area this evening and overnight. Isolated large hail and damaging winds are the main threat, though an isolated tornado threat will exist late this afternoon and early evening in the south.

- A transition toward a quieter, but cooler, pattern is on track to arrive Sunday through early next week before milder and more active weather returns for the second half of the week.

DISCUSSION

Issued at 353 AM CDT Fri Apr 3 2026

Through Thursday:

Surface low pressure was analyzed over the Straits of Mackinac early this morning, with a trailing cold front extending through northwest IN and east-central IL. Only a few isolated showers linger across our far southeastern cwa in advance of the front at this hour, and these will move out of the area shortly as the cold front continues to push off to the east. Behind the front, blustery west surface winds were pulling cooler air into the region (low-mid 40s upstream in eastern IA) along with an extensive area of stratus. Surface ridging noted across central IA will move east into the area later this morning in response to strong subsidence (indicated by 80-100 meter H5 height rises) in the wake of the departing mid-level short wave and surface low, allowing winds to quickly diminish as well as likely eroding some holes in the shallow stratus deck with time today.

Farther west however, another strong mid-level wave/closed low was evident in GOES vapor imagery over WY. This disturbance will propagate east across the High Plains by midday, inducing renewed low-level return flow and warm advection across the mid- Mississippi Valley and western Great Lakes by this afternoon ahead of a new surface low expected to develop into southwest IA/northwest MO by this evening. In response, the trailing portion of the aforementioned cold front will lift back north into our southern cwa (to the south of a Pontiac-Rensselaer line) as a warm front late this afternoon. As the warm front lifts north this afternoon, CAM guidance indicates a chance of isolated to scattered thunderstorm development along the boundary as warm/moist/conditionally unstable low-level air mass returns. Deep-layer shear of 35-40 kts and modestly-steep (~7 C/km) mid-level lapse rates appear to support storm organization including some supercellular modes, while backed south-southeast low-level winds and 0-2 km speed/directional shear in the vicinity of the boundary may support an isolated tornado threat near the warm front late this afternoon into early this evening across our southern tier of cwa counties or so. SPC has nudged the Day 1 Slight (level 2 of 5) severe and 5% tornado risk into our far south/southwest cwa in association with this threat.

The upper low/short wave is progged to lift northeast across the upper Midwest overnight into Saturday morning, with its attendant surface low currently progged to track across the IL/WI border region around daybreak Saturday. The warm front thus lifts across much of the forecast area tonight, followed closely by the eastward moving cold front early Saturday morning. Scattered thunderstorms are expected to increase in coverage ahead of this system this evening/overnight, with more of a large hail/isolated damaging wind threat (SPC marginal, or level 1 of 5 severe risk). Unseasonably high PWATS ~1.50" will support locally heavy rainfall with some of these storms as well, and we'll have to keep an eye on localized flooding potential especially across portions of northern IL where MRMS estimated rainfall of 2.50-3.50 inches fell Thursday/Thursday night. Lingering showers should move out of the area shortly after daybreak Saturday morning, as the cold front continues to march to the east, though a few wrap-around light showers or sprinkles may develop beneath the upper trough axis in the afternoon.

In response to the surface low developing west of the area today, surface winds will turn easterly across far northern IL. This will set up a significant temperature gradient with highs only around 50 along the IL lake shore, and lower 60s well- inland. Farther south where the warm front returns this afternoon, temperatures will likely warm into the low-mid 70s. Temperatures will likely warm from south to north overnight (especially across northeast IL and northwest IN) as the warm front continues to lift north ahead of the low. The temp roller coaster then continues, as temps gradually fall into the 50s and upper 40s behind the cold front Saturday. A period of more tranquil weather conditions appears to be in store for the region Sunday through early next week, as an expansive area of surface high pressure gradually builds east through the area. It will remain cooler than normal (with blustery northwest winds lingering Sunday), before trending milder and more active again mid-late week.

Ratzer

AVIATION /06Z TAFS THROUGH 12Z SATURDAY/

Issued at 1232 AM CDT Fri Apr 3 2026

Key Aviation Messages for the current TAF period:

- MVFR/IFR cigs are possible through the overnight and lingering into the day on Friday.

- Gusty southwest winds through daybreak. Then switching to the east by midday.

- Another round of showers is expected Friday evening and through the overnight. There is a 30 percent chance for storms Friday evening.

This past evening's line of showers and storms is moving east over Indiana, allowing for a brief period of dry weather. VFR conditions are currently around the Chicago terminals, but MVFR cigs are slowly creeping into northern Illinois. There is some uncertainty with how low cigs will drop overnight, though the best chances for IFR remain around KRFD.

Winds remain out of the southwest presently between 10 to 15 knots. Gusts around 20 knots are possible through around 09Z before diminishing toward daybreak. As a warm front starts to move northward through central Illinois around midday tomorrow, winds are expected to turn to the east, with some lake enhancement pushing it farther inland through the afternoon. While KRFD might entertain a brief window in the afternoon of VFR cigs, opted to trend the Chicago TAF sites more cynically with more guidance suggesting MVFR cigs move inland with the lake enhanced easterly winds.

As that warm front approaches after 00Z, showers and gusty easterly winds are expected across the airspace and remain through the overnight. There is the potential for embedded thunder along the warm front, so that was added to the PROB30 group for the area. MVFR cigs are expected with this front with a chance for IFR during heavier downpours. As a surface low enters western Illinois toward daybreak, winds will diminish and directions become variable depending upon the track of the surface low, as showers continue through daybreak. As a cold front moves through after 12Z (and the current TAF period), showers are expected to diminish and winds switch to the southwest.

DK

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 4 AM CDT early this morning for the IL and IN nearshore waters.