textproduct: Chicago

KEY MESSAGES

- Potential for strong to severe thunderstorms mid to late afternoon along a cold front east of a Paxton, IL to Valparaiso, IN line.

UPDATE

Issued at 1024 AM CDT Wed Aug 26 2026

The cold front that will drive the main thunderstorm potential this afternoon is quickly shifting eastward across northern Illinois, with the front currently analyzed from northwest McHenry to eastern Lee County. Meanwhile, decent subsidence in the wake of this morning's initial wave that initiated isolated elevated showers has left little cloud cover over the area (beside an area of stratus over northeast Illinois). The environment will still need several hours of destabilization to erode an existing nocturnal cap and weak inversion from the aforementioned subsidence. By that time, the front will have cleared the I-57 corridor, effectively ending the conditional severe thunderstorm threat for areas to the west. While there is still a potential for convective initiation along the front in our area east of I-57, erosion of the remaining cap may not occur until the front is east of the forecast area. Will continue to highlight the conditional severe thunderstorm risk east of a Valparaiso to Paxton line mid to late afternoon, but chances continue to dwindle with time.

With all that said, will continue to message isolated showers and perhaps a rogue non-severe thunderstorm across northern Illinois and northwest Indiana behind the front mid to late afternoon as a weak trailing mid-level trough drags across the area. Surface-based mixing and modest mid-level warming should begin to reduce instability with time, but broader dry-air advection will lag the front by several hours.

Kluber

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, and low-level convergence increases.

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 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 /18Z TAFS THROUGH 00Z FRIDAY/

Issued at 1216 PM CDT Wed Aug 26 2026

There are no significant aviation weather concerns anticipated at the terminals through Thursday morning.

A surface cold front continues to shift eastward across the terminal airspace early this afternoon. Fortunately, it appears the favored development for isolated to scattered afternoon thunderstorms will be east-southeast of the terminals, roughly from east central IL northeastward into northern IN and MI. Accordingly, we have trended to a dry forecast for all TAF sites. Otherwise, expect VFR conditions to prevail through the period. The only item of not for Thursday's weather is the likelihood of an afternoon lake breeze, which looks to turn the winds east- northeasterly at MDW and ORD late in the afternoon.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

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