textproduct: Chicago

KEY MESSAGES

- Waves of showers and storms are expected in the general region tonight through much of the week.

- While some areas may not see much rain at all, others may experience repeated downpours with a threat for localized flooding.

- The next few days will also be marked by high humidity.

- A gradual pattern change toward cooler, less humid, and drier conditions is favored by next week.

DISCUSSION

Issued at 129 PM CDT Mon Sep 14 2026

A 1024mb surface high pressure system continues to move through the central Great Lakes leading to a generally quiet day. Increasing cloud cover is muting the diurnal temperature rise, though highs should still top off in the mid 70s most places amidst a southeasterly breeze. Meanwhile, an early-autumn storm system is moving along the US/Canadian border driving a large area of warm air- advection driven showers and thunderstorms across the Upper Mississippi River Valley. With a residually dry airmass in place across the Lower Great Lakes on the southwestern side of the departing high pressure system, expectations remain that our area will remain dry through sunset. With that said, cannot rule out a random sprinkle or two this afternoon mainly near/west of I-39 and north of I-88.

Tonight, the northern Plains low will drive a surface warm front northward into the Lower Great Lakes in advance of a cold front sliding southeastward across the Upper Midwest. Strong theta-e advection driven by an intensifying southwesterly low- level jet will usher in a warmer and more humid airmass into our region, pushing atmospheric column moisture (PWATs) near 2 inches. Although large- scale forcing will be modest through the overnight hours, continued isentropic upglide in advance of the cold front will support a blossoming region of showers and thunderstorms later tonight from IA east-northeastward across parts of northwestern IL into southern WI. This activity is expected to shift over areas along and mainly north of I-80 after midnight tonight and will likely linger in the area through Tuesday morning. The primary threat with these showers and storms will be torrential downpours. Fortunately, the steering layer flow tonight will favor faster storm motions (~40 mph), so any potential flash flood threat would likely remain localized and tied to any potential areas of training storms.

Widespread cloud cover and lingering morning showers will limit diurnal warming on Tuesday especially with northward extent leading to the reinforcement of an outflow-driven stationary front across the Lower Great Lakes. Highs should range from the upper 70s near the Wisconsin state line to perhaps the mid 80s near and south of the stationary front south of I-80. By mid- afternoon, decreasing convective inhibition may trigger renewed scattered thunderstorms near the front primarily south of I-80. Access to 30-35kt of mid- level flow may promote localized clustering and associated threats for water-loaded downbursts where convective coverage is highest consistent with the SPC Level 1/5 threat level for severe weather tomorrow.

Tomorrow night through Thursday, the stationary front is expected to stall or modestly retreat back northward as a subtropical ridge strengthens across the southeastern United States. Subtle impulses parading around the perimeter of the ridge along the stationary front will support episodes of showers and thunderstorms in the general region, though forecast confidence on exact timing and locations is lower than average given the overarching pattern. Will have to watch for focused corridors of torrential downpours with an associated threat for localized flooding in this timeframe given PWATs hovering near or just above 2". Also, could envision heat index values poking toward 100 degrees near and south of the stationary front at times. Unfortunately, this is a pattern that'll have to be forecasted day by day as individual waves and the location of the stationary front can be parsed out. Finally, note that the next 2 to 4 days do look wet for many but not everyone in our area. In the absence of persistent large-scale forcing as well as the presence of modest mid-level height rises, downpours may be relegated to scattered coverage rather than enjoyed by all on a widespread basis.

Extended guidance indicates weak mid-level blocking as a subtropical low over the Gulf slows south of a weakening upper- level ridge. Meanwhile, a upper-level trough moving across the northern Great Plains on Friday may dig far enough south to bring showers and thunderstorms to the area on Saturday.

Birk/Borchardt

AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 659 PM CDT Mon Sep 14 2026

Main aviation forecast concerns for the 00Z TAFs:

- Breezy southeast winds with gusts near 25 kts for Chicago terminals this evening/early overnight.

- LLWS conditions for the same period for DPA/RFD where prevailing gusts less likely.

- Timing of TSRA/convective SHRA later tonight into Tuesday.

Deep surface low pressure was lifting from the Northern Plains into the Canadian prairies early this evening, with a trailing cold frontal trough to our west across the Plains. Strong surface pressure falls (near 5 mb/3 hours) into the Mississippi Valley were helping to tighten the gradient and increase southeast surface winds across the region, and this is expected to continue this evening/overnight. Many locations across eastern IA have been reporting peak gusts around 30 kts. Breezy conditions are expected to continue, though prevailing gusts around 20 kts are more likely with sunset and some loss of mixing. Chicago terminals, which tend to remain a little warmer/more deeply mixed, will likely see gusts around 25 kts however, and potentially some sporadic higher gusts closer to 30 kt later tonight as a 50 kt southerly low-level jet develops. Gusts are expected to be more occasional at DPA/RFD which tend to decouple more strongly at night, leading to more of a LLWS scenario. The jet is forecast to eventually weaken during the predawn hours with decreasing LLWS threat and diminishing gust speeds.

The aforementioned cold front will make it's way into the region on Tuesday, with increasing SHRA/TSRA chances after midnight tonight through mid-morning or so Tuesday. Some high-res model runs have indicated the brief potential for a few isolated elevated SHRA/TS developing over northern IL as early as this evening, though this appears to be a low-probability and likely a low-coverage situation if it were to occur. A prolonged period of convective SHRA/TS is more likely roughly in the 09Z-16Z time frame for the Chicago terminals (08Z-15Z for RFD). Looks like this will support more than a PROB30 mention, so went with a longer duration VCTS with targeted TEMPO periods for TS. Additional scattered SHRA may linger into the midday/afternoon hours Tuesday, though the greater TS threat appears to shift south of the terminals.

Those breezy southeast winds should diminish and turn more south-southwest after 08Z-10Z, and eventually more westerly Tuesday afternoon. The cold front is expected to push southeast across the area late in the forecast period with a shift to northwest winds by evening.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement through Tuesday morning for ILZ006- ILZ103-ILZ104.

IN...None. LM...Small Craft Advisory until 9 AM CDT Tuesday for the IL nearshore waters.

Small Craft Advisory until 7 AM CDT Tuesday for the IN nearshore waters.