Deer Park, IL Weather

Key Messages

- A cold front will move across the area tonight leading to low clouds and cooler temperatures tomorrow, especially north of Interstate 80.

- The relative cool down will be short-lived as high temperatures rebound to the low to mid 70s by the middle of the week.

- A pattern change toward more opportunities for regular rainfall is set to occur toward the end of the week and into the first week of November.

Discussion

Issued at 1229 PM CDT Sun Oct 25 2026

The upper-level pattern remains benign and is characterized by a narrow ridge extending from the Gulf toward the Lower Great Lakes, a cut-off low pressure system over the Four Corners region, and aggregate troughing across the Pacific northwest. Closer to the surface, a cold front stretches across the Upper Mississippi River Valley southeastward from a low pressure system traversing Hudson Bay. Ahead of the cold front and indeed across our area, it's another seasonably beautiful day with a light southwesterly breeze, partly cloudy skies, and temperatures climbing toward the lower 70s.

The cold front will drop into northern Illinois and northwestern Indiana toward daybreak leading to a north to northeasterly wind shift and gusts up to around 15 to 20 mph. Modest ascent behind the front interacting with the cool and moist environment should support low clouds and even pockets of mist/fog immediately behind the front, making way for a fall scene to start the workweek. With the parent low pressure system remaining so far removed from our area and the aforementioned southwestern cut-off low expected to lift northeastward toward the northern Plains (inducing low-level warm air advection into the middle Mississippi River Valley), the cold front appears poised to stall somewhere between the I-80 and US-24 corridors. Taken altogether, the stage is set for a tight gradient in temperatures across the area tomorrow with highs in the low 60s near the Wisconsin state line to the mid 70s south of US-24.

Tuesday into Wednesday morning, a surface high pressure system will settle across the northwestern United States as aggregate surface troughing becomes established across the Plains. A series of mid- level impulses originating from northern Mexico will propagate northeastward toward the Great Lakes atop a somewhat parched low- level airmass (backside of the surface high). While a sprinkle or two cannot be ruled out, suspect we'll just have to deal with prevalent mid to upper-level clouds leading to a muted diurnal temperature response (highs on Tuesday perhaps only a few degrees warmer than Monday). Clearing skies Wednesday afternoon should help highs warm back into the low 70s areawide.

Ensemble guidance continues to advertise a pattern change toward the end of the week and beyond as the subtropical and polar jets begin to interact, leading to deep long-wave troughing across the western US and shortwave shedding eastward into the central US. As a result, regular chances for rainfall will return to the region perhaps as early as Thursday but more likely toward and after next weekend.

Borchardt

Aviation

Issued at 1229 PM CDT Sun Oct 25 2026

Key aviation forecast messages:

- A N to NE wind shift is expected behind a cold front early Monday AM followed by the potential for a brief period of IFR CIGs and MVFR VSBYs.

VFR conditions are forecast through the early overnight hours with filtered sunshine through SCT to BKN thin high clouds. Winds will remain generally light (under 10 kt) mainly out of the SSW. Winds then ease after sunset, becoming light and variable ahead of an approaching cold front which is expected to move through the area from north to south during the early morning hours Monday. Have moved up the wind shift timing by an hour given a trend toward a slightly earlier arrival (now in the 9-11Z timeframe, earliest at ORD).

There is also an increasing signal for the potential for a brief period of IFR (or lower) ceilings moving across the area in the wake of the front, potentially paired with reduced visibility in BR and patchy fog (VSBYs of MVFR or lower). Have accordingly introduced TEMPOs to account for this, though there remains the possibility that coverage ends up more scattered. Expect winds to increase into the 10 to 15 kt range behind the front gradually turning more ENE farther inland.

Petr

Lot Watches

IL...None. IN...None. LM...Small Craft Advisory from 7 AM Monday to 7 AM CDT Tuesday for Winthrop Harbor to Wilmette Harbor IL.