textproduct: Chicago

KEY MESSAGES

- A cooler and drier pattern starts today and will last into early May.

- There is a threat for frost Friday night across the region.

DISCUSSION

Issued at 109 AM CDT Tue Apr 28 2026

Early this morning, a 994 mb surface low was located over the Upper Peninsula of Michigan with a cold front stretching southward through the Great Lakes and into the lower Mississippi River Valley. Expansive low-level stratus is present in the wake of the cold front, stretching from northern Wisconsin through northern Missouri. The departing low pressure system is just one piece on the board this morning with aggregate troughing across the northern United States, with several embedded shortwave-length troughs propagating within. The overarching pattern is setting the stage for the well- advertised pattern change toward predominantly cooler and drier conditions across much of the northern United States.

Today will be noticeably chillier than yesterday. A robust push of low-level cold air advection (driven by northwesterly winds gusting 30 to 35 mph overnight) will drive temperatures down to the upper 40s to lower 50s by daybreak. With low stratus expected to be prevalent across the region through the morning, temperatures will be slow to warm with most areas remaining in the 50s through early afternoon. Areas that experience clearing this afternoon will have the best opportunity to jump into the lower 60s, with current indications favoring locals north of I-80 and west of the Fox Valley. Northwesterly winds should gradually ease throughout the day.

Tonight, an upper-level shortwave embedded in the broad troughing aloft will propagate through the Ohio River Valley. Occasional flares of frontogenesis acting upon the cyclonicly- curved baroclinic zone across the Lower Great Lakes will support scattered showers through the overnight hours. REsidually dry low-level air may act to limit the overall duration and footprint of showers tonight, with overall rainfall amounts favoring less than one quarter inch at any given location. Overnight lows will fall to the 40s.

Tomorrow morning, the upper-level shortwave will move away from our area. With a camp of model guidance advertising broad low- level convergence along the Lake Michigan shoreline (almost an inverted low-level trough-like structure), showers may overstay their welcome across northeastern Illinois and northwestern Indiana through early afternoon. Decreasing cloud cover through the afternoon should allow an opportunity for highs to warm toward 60 degrees with cooler conditions near Lake Michigan due to onshore flow. Tomorrow night looks relatively cool as surface ridging and clearing skies sets the stage for overnight lows in the upper 30s to lower 40s.

Toward the end of the week and beyond, the pattern will be more or less the same with aggregate upper-level troughing and the occasional passage of shortwaves to support a few showers. The coolest day of the pattern looks to be Friday in the wake of an upper-level shortwave anchored to a pool of cooler 850mb temperatures, leading to highs in the low to mid 50s (if not a smidge colder). Friday night hence looks to be a standout for frost potential as a surface high moves into the region allowing for calm winds and overnight temperatures in the low to mid 30s. At least some moderation in temperatures may occur by the weekend as the aggregate upper-level troughing drifts eastward.

Borchardt

AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 1227 AM CDT Tue Apr 28 2026

Forecast concerns include...

Mvfr cigs through late morning/early afternoon. Gusty westerly winds through early this afternoon. Chance of showers late tonight/early Wednesday morning.

A few remaining showers currently over Lake Michigan will continue moving away from the terminals with dry conditions expected until later tonight when scattered showers are possible across the entire area, which will continue into Wednesday morning.

Mvfr cigs are expected through late this morning, then scatter from west during the late morning, to the east during the early/ mid afternoon. There may be an hour or so of ifr cigs ahead of a cold front that will move across the area in the next 1-2 hours and shift winds westerly. Gusts into the mid 20kt range can be expected through mid morning, then speeds/gusts will slowly diminish with gusts ending during the afternoon. Winds are expected to become light northerly this evening and may become light and variable for most areas. Winds under 10kts are expected early Wednesday morning as directions turn to the northeast. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 10 AM CDT this morning for the IL and IN nearshore waters.