textproduct: Chicago

KEY MESSAGES

- Warm, dry, and breezy conditions will lead to a continued elevated risk for brush fires across northern Illinois this afternoon.

- Cooler conditions are in store Sunday into Monday with showers and storm chances returning on Tuesday.

DISCUSSION

Issued at 300 PM CDT Sat May 9 2026

Low pressure centered over Ontario is slowly progressing east today with a trailing cold front extending southwest across the lower Great Lakes, currently located near the Illinois and Kankakee River valleys here locally. Deep mixing in the wake of this boundary has allowed dewpoints to mix down quickly into the lower 30s. In tandem, winds have increased this afternoon with frequent gusts into the 25-35 mph range. Sporadically higher gusts over 40 mph can't be ruled out for the next few hours. This paired with temperatures in the low to mid 70s have resulted in relative humidity values dropping into the 20s. While recent rains have allowed many areas to green up, an elevated threat of fire spread will exist through the rest of this afternoon. Also can't rule out patches of blowing dust near any recently worked over farm fields.

Ahead of the front an axis of marginal MLCAPE exists (~250-500 J/Kg). Some cumulus has started to bubble up south of the Kankakee River early this afternoon with showers and isolated storms having developed farther east across northern Indiana. Can't rule out a stray shower developing prior to sunset as the cold front continues to progress south across the area. Think that the notable dry air within the vicinity of the front should hinder more robust shower development and hence keep the thunder potential east of the area.

Winds will steadily ease with sunset with temperatures falling into the low to mid 40s as skies clear out overnight. A separate disturbance moving across southeast Nebraska may bring a narrow axis of light showers across the far southern CWA and across central IL overnight and have accordingly held onto low precip chances (15-25%) south of a Pontiac to Rensselaer line.

Mother's Day and Monday will feature cooler temperatures, with highs in the 60s (50s near the lakeshore). While a closed mid- level low continuing to linger across the Great Lakes region could lead to spotty light showers/sprinkles development at times within the vicinity of any vort lobes, dry conditions are favored here locally given dry low-levels and overall weak forcing mechanisms.

Attention then turns to Tuesday when a shortwave dives across the area. A band of showers and thunderstorms may develop along an associated cold front northwest of the area and then progress southeast across the area. Ample shear will be in place amidst strong northwest mid-upper jet and steady southwesterly low- level flow. Marginal instability owing to poorer mid-upper lapse rates may inhibit more robust thunderstorm development/maintenance into the local area and accordingly the severe weather threat. Still plenty of time to iron out the details though. The strong southwesterly surface winds (gusts to 35-40+ mph) may also lead to some blowing dust concerns, especially in rural and open areas near recently worked over farm fields.

Beyond Tuesday, temperatures briefly cool down once more in the wake of the cold front, with high temperatures on Wednesday back in the 60s. We then warm up gradually each day through the end of the week as upper level ridging builds across the central CONUS into the Upper Midwest by the weekend with additional shower/storm chances (20-30%) late Thursday into Friday.

Petr

AVIATION /00Z TAFS THROUGH 06Z MONDAY/

Issued at 624 PM CDT Sat May 9 2026

Aviation forecast concerns for the 00Z TAFs:

- Lake-assisted wind shift to northeast possible with boundary currently pushing south along the IL Lake Michigan shore. Low confidence just how far inland northeast winds will push against otherwise gusty NNW flow. Much higher confidence in a period of NNE flow at GYY this evening.

- Breezy NW winds develop again Sunday midday/afternoon, with gusts 20-25 kt.

Latest surface analysis depicts 997 mb low pressure tracking north of Lake Huron early this evening, with a cold front trailing through lower MI into IN and southeast IL. A secondary cold front was pushing southeast across southern WI, with a lake-enhanced portion pushing more quickly southward across Lake Michigan. This boundary is easily noted in ORD TDWR imagery pushing inland into eastern Lake and northeast Cook counties in IL. This may bring a wind shift to NNE into ORD/MDW early this evening, though confidence in it moving fully across both airfields is somewhat low given gusty NNW flow across southern WI/northern IL. Have elected to indicate a TEMPO for NE winds at both ORD and MDW from 01Z, and if boundary pushes inland more strongly may have to consider a several hour period of NE flow. Wind gusts initially around 25 kt should gradually diminish after sunset, with winds then favoring light North-northwest overnight into Sunday morning. After sunrise, winds are expected to shift more northwest, with gusts in the 20-25 kt range (not as gusty as today).

Otherwise, VFR conditions are expected through the period, cloud bases generally 6-8 kft or higher.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 7 PM CDT this evening for Winthrop Harbor IL to Gary IN.