textproduct: Chicago

KEY MESSAGES

- Breezy and colder conditions through the weekend into early next week.

- Potential (20-40%) for some gusty showers Sunday afternoon.

- Frost/freeze conditions are expected many areas Sunday and especially Monday morning.

DISCUSSION

Issued at 119 PM CDT Sat Apr 18 2026

A very cold upper trough (with 500 mb temperatures at an impressive - 38C) will continue its trek across the Great Lakes region tonight into Sunday. After the diurnal increase in cloud cover, expect rapid clearing this evening. As mentioned for the past few discussions, the northern periphery of a modest mid 1020s mb high will be centered southeast of the region across the ArkLaTex, thus outside of a few outlying locations, we'll hold onto at least some surface flow through the night. Additionally, there will be an increase in higher cloud cover late tonight in the northwest the coldest temperatures are forecast, but this would probably come in a little late to prevent the temperature fall. While there are some factors that would tend to limit the expansiveness of the frost threat, areas of frost certainly may occur in more sheltered locations to warrant a Frost Advisory tonight away from the urban core.

We will maintain the chances for showers on Sunday. A shortwave on the back side of the upper trough will eventually get absorbed into the bottom of the trough axis with time, however there will be a period of moderately strong lower level forcing. Moisture is not overlying plentiful, but cold temperatures aloft will support a significant steepening of the low level lapse rates and lead to scattered showers. The top of the cloud layer, while relatively shallow, may get just close enough to the charge separation layer to ring out a lightning strike or two, but the bulk of calibrated thunder guidance from both the HREF/REFS suggest this would be isolated. With the steep lapse rates, breezy conditions may be augmented by any showers with a few gusts to 35 mph certainly plausible.

There is much higher confidence in frost/freeze conditions on Sunday night with a surface high to be in place overhead. There is the potential with onshore flow that there could be some lower cloud cover near the lake to keep things in check in both the IL/IN near lakeshore areas. A Freeze Watch will get hoisted to cover this.

There is also a strong ensemble signal for a bounce back to early summer-like temperatures Tuesday through Thursday. The only fly in the ointment for an overall nice mid week period will be on Tuesday. Some guidance (mainly the GFS) squeezes out some showers--likely under an incoming EML plume--ahead of a cold front Tuesday afternoon and evening. Forecast soundings at this time look too capped for thunderstorm development, although very steep lapse rates aloft suggest we'll need to keep an eye on this. CMC/EC ensemble suites are not biting yet on this solution quite yet and few any members wring out any precipitation.

Thereafter, a closed large upper low off the Pacific coast will sweep through the Upper Midwest towards the end of the week. While there are some uncertainties as to how this system will evolve, it will send several synoptic shortwaves in our direction late week through the weekend. The first will arrive in the Friday timeframe with a robust cold front. This will bring the next thunderstorm chances to the area late frame. 1" + PWAT values will accompany each of these waves, and with potent forcing we may see some re- aggravation of hydrologic issues during this time. Confidence is also quite high that temperatures will drop back below normal.

KMD

AVIATION /00Z TAFS THROUGH 06Z MONDAY/

Issued at 623 PM CDT Sat Apr 18 2026

Forecast concerns include...

Strong/gusty west/northwest winds early this evening. Gusty northwest winds Sunday. Chance of showers Sunday.

West/northwest winds gusting to 30kt early this evening will quickly diminish with sunset with prevailing speeds under 10kts expected overnight, when directions are expected to turn more west/southwest. Winds will turn back to west/northwest Sunday morning with gusts into the mid 20kt range expected. Winds may turn more to the north/northwest late Sunday afternoon into early Sunday evening and then some of the guidance is suggesting winds may shift to the northeast for the Chicago terminals. Only medium confidence for this shift but did include a north/ northeast direction at ORD/MDW Sunday evening.

A weak disturbance will move across the area from mid morning Sunday through mid/late afternoon, allowing for a chance of showers. Overall confidence is low and opted to include only prob vfr shower mention for now. Some additional showers are possible over/near Lake Michigan Sunday evening. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...Frost Advisory from midnight tonight to 9 AM CDT Sunday for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012- ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039- ILZ106-ILZ107-ILZ108.

Freeze Watch from late Sunday night through Monday morning for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012- ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039- ILZ103-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Frost Advisory from midnight CDT /1 AM EDT/ tonight to 9 AM CDT /10 AM EDT/ Sunday for INZ001-INZ002-INZ010-INZ011- INZ019.

Freeze Watch from late Sunday night through Monday morning for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...Small Craft Advisory until 10 PM CDT this evening for the IL and IN nearshore waters.