textproduct: Chicago

KEY MESSAGES

- Potential for strong to severe thunderstorms returns Wednesday afternoon/evening along a cold front, with greatest severe threat (level 2 of 5) east of I-55 and south of I-80.

DISCUSSION

Issued at 330 AM CDT Tue Aug 25 2026

All is quiet weather-wise across the area early this morning with temperatures in the 50s to lower 60s. Only concern is the potential for shallow ground fog development through daybreak, especially along river valleys and other typical fog prone low- lying spots. Temperatures today will be slightly warmer than yesterday for some areas, in the upper 70s to lower 80s, as winds return to a southwesterly direction. The lake breeze will also mainly hug the Illinois lakeshore rather than surge farther inland and allow warmer temperatures to reach areas closer to the lake this afternoon.

A closed upper low currently over southern Manitoba and Saskatchewan will very gradually drift southeast toward the northern Great Lakes today and tonight. An attendant cold front is then forecast to drift across the area through the day on Wednesday. Increasing low to mid level moisture ahead of the front will help set the stage for potential thunderstorm development into the afternoon and evening hours.

Prior to the front's arrival, there remains a signal for warm advective showers and non-severe thunderstorms developing in southwest Wisconsin and northwest Illinois late tonight which may drift southeast into portions of the area through early Wednesday morning. A stray shower or rumble of thunder may linger through mid-morning across northern Illinois (20% chance).

While the center of the vort is expected to remain well to our north, steepening mid-level lapse rates along the trough axis paired with the increased low-level moisture ahead of the surface front should be sufficient for a pool of increasing instability (upwards of 2000-2500 J/kg MLCAPE) over portions east central Illinois and northwest Indiana Wednesday afternoon. This paired with a 35-45kt 500mb jet overhead suggests any storms that develop will have the potential to become strong to severe. Storm mode will favor semi-discrete supercells initially then potentially trending toward merging multi-cell clusters with southeast extent. Primary hazards include localized damaging winds, large hail, and localized downpours. While a tornado can't be ruled out with the strongest storms (owing to supercell mode paired with slight low-level hodograph curvature), it does appear that the locally higher potential is just east of the area into north central Indiana.

Given the local area will be slightly displaced from the larger vort (and mid-level forcing) to our north and northeast, questions remain regarding thunderstorm coverage with southwest extent along the front. Residual cloud cover or lingering showers through morning may also limit destabilization for at least portions of the area and especially west/northwest of the Chicago metro. The majority of CAM guidance does indicate there will be sufficient lift along the front to support storms initiating generally near the I-55 corridor in the afternoon with increasing coverage occurring along the front into east central Illinois and northern Indiana through early evening. Accordingly, the Storm Prediction Center has introduced a level 2 out of 5 threat for severe weather for areas near and east of I-55. Expect some refinement to this area as the timing and location of storm initiation becomes more clear.

Looking ahead, a trailing weaker wave rounding the upper trough over eastern Canada could support a few sprinkles on Thursday 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 zonal flow pattern across the northern Plains and Upper Midwest which could bring a series of disturbances across the broader region early next week paired with periodic shower and storm chances. Temperatures also trend warmer with highs in the mid-upper 80s.

Petr

AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 622 AM CDT Tue Aug 25 2026

No major forecast concerns are expected for this TAF period. High pressure will remain overhead through this evening which will keep tranquil weather conditions at the terminals. So expect SCT to BKN VFR cirrus overhead today with light southwest winds around 5-10 kts.

Heading into tonight, a cold front and associated upper disturbance will be pivoting towards northern IL. While some showers/storms may be ongoing in MN and WI as the front approaches, shower/storm coverage does look to be in a weakening trend and may erode before reaching northern IL. That said, there has been a notable signal in some hi-res guidance that a period of showers could drift over the terminals late tonight into early Wednesday morning so have opted to toss in a PROB30 for SHRA at ORD, MDW, DPA, and RFD where confidence is slightly higher. A better coverage of showers and thunderstorms are forecast for Wednesday afternoon which will need to be addressed in future TAF packages since the onset should be after 18z Wednesday. Regardless, winds will become breezy out of the west-southwest on Wednesday with gusts peaking around 20-25 kts.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

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