textproduct: Chicago

KEY MESSAGES

- Isolated non-severe thunderstorms may persist into portions of northern Illinois through mid-morning, mainly north of I-88.

- Potential for strong to severe thunderstorms this afternoon into early evening along a cold front, with the greatest severe threat near and east of a Paxton, IL to Valparaiso, IN line.

DISCUSSION

Issued at 330 AM CDT Wed Aug 26 2026

While a narrow ribbon of warm advective showers/sprinkles managed to develop briefly in northern Illinois early this morning, insufficient MUCAPE locally has prevented more robust showers or isolated storms from developing. Meanwhile, a cluster of thunderstorms in southwest Wisconsin is drifting toward northern Illinois. While currently near severe limits, these storms have surged well ahead of the cold front and may soon outrun the residual elevated instability axis and better shear, and gradually weaken as they approach. A lightning threat may still persist into at least portions of northern Illinois this morning, mainly near and north of I-88. Will continue to monitor trends in the event that a stronger storm manages to persist this far south.

The parent mid-upper low continues to slowly drift southeast across western Ontario and northeast Minnesota early this morning with the attendant surface cold front currently draped along a line from near La Crosse, WI into the western UP of Michigan. The low center will drift slowly east southeast across Lake Superior into southern Ontario through the day. In tandem, the surface cold front will sweep southeast across the area, the timing of which will be a key factor in the thunderstorm potential this afternoon.

Admittedly, the timing and coverage of thunderstorms later today along with the severe thunderstorm threat remains highly uncertain. Hi-res guidance continues to trend later and farther southeast with thunderstorm initiation along the cold front, likely owing to an apparent disconnect between the surface front location and the mid-level forcing with the parent shortwave. Residual cloud cover this morning and any showers/storms that persist into northeast Illinois may also delay destabilization, especially across northern Illinois. However, model soundings indicate that the area may still be largely uncapped ahead of the front this afternoon, particularly near and east of I-55. So it may not take much to initiate at least an isolated storm or two earlier than guidance suggests as the front moves through. If storms manage to initiate, instability (2000-2500 J/kg MLCAPE) and shear (35-45 kt) will be sufficient for supercells and multi-cell clusters capable of localized damaging winds and hail. The potential for higher storm coverage and greatest severe threat still favors areas mainly south and east of a Paxton to Valparaiso line when the mid-level forcing begins to catch up to the surface front.

A few additional showers and non-severe storms may develop in the wake of the front closer to the mid-level trough axis late afternoon and have held onto 20-30% precip chances through 7 PM CDT for portions of the Chicago metro near Lake Michigan to account for this.

Looking ahead, a trailing weaker wave rounding the upper trough over eastern Canada which could support a few sprinkles or showers on Thursday (15% chance), especially near the lake breeze, but expect mainly dry conditions with seasonably cool temperatures in the upper 70s to around 80 and comfortable humidity. Dry weather continues through the end of the week as upper ridging briefly expands east into the area Friday into Saturday paired with expansive surface high pressure. The upper levels then transition toward a more active pattern across the northern Plains and Upper Midwest as a series of disturbances move atop the upper ridge and across the broader region early next week. This will bring periodic shower and storm chances with temperatures also trend warmer with highs in the mid-upper 80s.

Petr

AVIATION /06Z TAFS THROUGH 12Z THURSDAY/

Issued at 113 AM CDT Wed Aug 26 2026

Forecast concerns for the terminals...

- Scattered showers and thunderstorms are possible on Wednesday (20-40% chance) ahead of a cold front.

- Breezy westerly winds Wednesday afternoon with gusts around 20-25 kts.

A shortwave trough is pivoting into the northern Great Lakes with an associated cold front stretching from northern WI to eastern MN and northern IA. Ahead of these features a band of showers and embedded thunderstorms have developed in WI and southeastern MN and these showers/storms are drifting east- southeast. The expectation is for these showers and storms to continue to develop overnight as they progress towards northern IL, but as they near the IL-WI should begin to weaken as they start to outrun the better instability and moisture. Therefore, it seems that a broken (isolated to scattered) line of showers with perhaps some embedded lightning should drift across northwest IL after 10z and may fully erode before reaching the Chicago area terminals. Given this expectation have opted to maintain PROB30s for SHRA at RFD, ORD, and DPA for a few hours early Wednesday morning to account for a low end chance of a passing shower should this line hold together, though the overall thunder threat at these sites looks low (10-15% chance).

By mid-morning on Wednesday, the cold front and shortwave should be over northern IL and should lead to the redevelopment of showers and thunderstorms across portions of northeast IL and northwest IN towards the morning and early afternoon hours. Depending on where exactly the front is MDW and GYY could be in play to see a couple hour period of thunderstorms (20-40% chance) before the activity shifts south and east into central IL and north-central IN. Furthermore, MDW and GYY could also see a period of showers Wednesday morning before the redevelopment occurs if the aforementioned band of showers holds together. With the forecast for these sites being a bit more complicated have opted to adjust the PROB30 timing to try to better capture thunderstorm window and have removed the PROB30 for the earlier showers as thinking is these should erode prior to reaching MDW and GYY. Since this forecast is lower confidence this will obviously be monitored closely. Lastly, there is also a threat for strong to severe thunderstorms Wednesday afternoon with damaging winds and hail the main threats, but this threat looks to largely be east of the TAF sites in northern IN and southern MI.

Any showers/storms that develop Wednesday should shift away from the terminals by mid-afternoon and the rest of our area by late afternoon/early evening. Therefore, the rest of Wednesday will feature VFR conditions with breezy westerly winds gusting around 20-25 kts ahead of the front. Behind the front winds will turn northwesterly and diminish in speeds (around 5-10 kts) by Wednesday evening where they are expected to remain for the rest of the TAF period.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

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