textproduct: Chicago

KEY MESSAGES

- Waves of showers and storms are expected in the general region 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.

- Another period of showers and storms is favored in the general region Friday through the weekend.

DISCUSSION

Issued at 134 PM CDT Tue Sep 15 2026

Per regional surface analyses charts, a stout 996mb surface low is currently moving through central Ontario. A cold front extends south of the low through central Wisconsin through eastern Iowa. Meanwhile, showers and thunderstorms continue to fester within a low- level warm air advection regime ahead of the cold front from the central Plains through northern Illinois and central Lower Michigan. Broad low-level confluence across the Lower Great Lakes ahead of the front is leading to pooling of low-level moisture as confirmed by surface dew points rising into the upper 60s to lower 70s early this afternoon. A stout temperature gradient is also present across the area (thanks to morning clouds with northern extent), with current temperatures ranging from the lower 70s near the Wisconsin state line to lower 90s near the US-24 corridor. It definitely feels more warm and humid out there today compared to yesterday.

Current expectations are that the continued warming of the moistening boundary layer will support the redevelopment of showers and thunderstorms by mid afternoon. Initial activity may originate out of a remnant mid-level boundary currently stretching just northwest of the I-55 corridor before expanding near and south of I-80 by sunset as the aforementioned cold front arrives and stalls across the area. Some 35 to 40 kt of upper-level flow atop the instability axis (on the southern side of an upper-level jet focused in the Upper Great Lakes) will provide some means for localized clustering or even brief supercellular structures this afternoon and evening supporting a low-end (level 1 out of 5) threat for localized damaging winds or even a funnel cloud.

After dark, a somewhat broad and unfocused but nevertheless persistent low-level jet will develop and override the stalled frontal boundary across the middle Mississippi River Valley. At the same time, a broad region of mid-level warm air advection across the central US ahead of a diving Pacific trough will facilitate the northward expansion and strengthening of a mid- level ridge into the Great Lakes, tightening the upper-level geopotential gradient and associated jet (including upper-level diffluence in the region). The net result should be an increase in coverage of showers and storms along the stalled frontal boundary across northern Missouri, southern Iowa, central Illinois, and perhaps central Indiana overnight. It's difficult to say for sure where the primary axis of storms will be as often observed activity often verifies some 25 to 50 miles further south than depicted in CAM-guidance. With that said, persistent low- to upper-level forcing within the 2.0"+ PWAT regime will support torrential downpours and a threat for flash flooding tonight (especially near the IA/MO border where low- level convergence will be maximized). Will broad-brush mid-range likely (55-75%) PoPs generally along and south of I-80 for the overnight period.

Coverage of showers and storms should begin to wane after daybreak as the low-level jet weakens and mid-level heights rise (indicative of subsidence) along the northern flank of the strengthening mid- level ridge. With that said, do see a signal in CAM guidance for some sort of MCV or convectively augmented wave to propagate eastward along the stalled frontal zone during the late morning and early afternoon hours south of I-80. Should that occur, showers and storms may continue unabated through the afternoon hours (again, mainly near and south of I-80). Confidence in the temperature forecast is hence pretty low tomorrow. Will simply stick with the inherited forecast calling for low 70s (north) to low 80s (south).

Conceptually, do think there may be another round of storms in the general region tomorrow night through Thursday morning near the terminus of yet another low-level jet and beneath the right entrance region of the semi-perminent upper-level jet centered over the Upper Great Lakes. Based on the forecasted location of the terminus of the low-level jet across central Iowa, the main focus for storms may end up extending from eastern Iowa overnight and across northern Illinois by daybreak Thursday. Will broad-brush mid-range chance (45-55%) chance PoPs across the area in favor of refinement in later forecast packages. Like tonight, there will be a threat for flash flooding tomorrow night given the continued high PWAT environment.

By Thursday afternoon, ensemble guidance supports the idea that the effective front will be well south of our area (potentially aided by north to northeasterly flow on the southern side of a a surface high pressure system moving across the Upper Great Lake). Chances for showers and storms will return by Friday as a lead shortwave propagates into the area ahead of the aforementioned Pacific tough. At this point, can only offer broad-brushed chance to low-end likely (40-60%) PoPs through the weekend for what may end up as several rounds of storms in the general region. Will note a heightened threat for flooding given another period of PWATs approaching 2 inches overlapping strong synoptic scale forcing.

Borchardt

AVIATION /06Z TAFS THROUGH 12Z THURSDAY/

Issued at 1249 AM CDT Wed Sep 16 2026

Main Concerns:

- A period of -RA early this morning.

- Lower flight category trends, including potential for low MVFR to IFR tonight into early Thursday morning.

- Additional SHRA and embedded TS tonight into early Thursday, though better potential may focus south of the terminals.

Spotty showers may occur near the terminals any time early this morning. However, the most favored timing is from the pre-dawn hours to mid morning on the northern edge of activity over western/west central IL as of this writing. It appears any embedded TS will remain primarily south of the terminals. Best chance for reduced VSBY with the rain is at GYY.

CIGs are likely to deteriorate to MVFR today, with lower confidence in exact details, and mostly maintained previous TAF timing. Overnight into early Thursday, some of the guidance suggests CIGs and VSBY lowering to IFR and even LIFR in spots. The northern extent of SHRA and TS over the region is again uncertain, though latest thinking is that TS chances are higher south of the terminals. Given the overall uncertainty, confidence in rain is low, but opted for a PROB30 at ORD and MDW to also highlight the lower CIGs and VSBY. Expect generally northeasterly winds for most of the period, becoming light/VRB to calm in spots early Thursday.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

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