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 cyclonically- 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 /18Z TAFS THROUGH 00Z THURSDAY/

Issued at 1226 PM CDT Tue Apr 28 2026

Key Aviation Messages for the current TAF period:

- Scattered low/no impact showers overnight. Possibly again on Wednesday

There is some lingering high MVFR cigs over northeast Illinois and northwest Indiana this afternoon; however, they are slowly eroding away. Perhaps they linger just a little longer than in the TAF, but with many obs already reporting VFR conditions giving confidence in a VFR forecast this afternoon. Winds remain out of the west, though occasionally flirting with a southwest direction, and around 10 knots with occasional gusts to 20 knots early this afternoon.

Wind direction trends are a little challenging through the overnight as weak surface troughs pass over the area. Winds are expected to be less than 10 knots, but there is lower confidence in specific wind directions overnight. Models continue to suggest rain showers overnight. But with weak forcing probability of showers at any one specific terminal is too low for TEMPO/prevailing mention presently. The better forcing is to the south, so perhaps areas closer to KGYY have the best chance, but for now maintained VCSH as showers are possible.

Northeast winds are expected to start the day tomorrow with a chance they flip over to west of north in the afternoon. As another wave of moisture pushes in overhead and increases cloud cover, there is another chance for a shower to pop-up (especially near the lake), but confidence was too low to add into the TAF. While thunder is not expected with any of the rain chances, if a shower does develop there could be a brief window of high MVFR cigs with the rain.

DK

LOT WATCHES/WARNINGS/ADVISORIES

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