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 /06Z TAFS THROUGH 12Z MONDAY/

Issued at 1213 AM CDT Sun Apr 19 2026

Light southwesterly winds will become WNW/NW through the morning, with increasing gustiness expected into the early afternoon. A fast-moving disturbance will result in thickening VFR cloud cover and the development of scattered showers towards midday through the afternoon. Deep mixing will promote the potential for stronger wind gusts (potentially 35+ knots) near any deeper showers. Confidence in showers and attendant stronger winds impacting individual sites remains around 30 percent at this time, and have continued PROB30 mentions as a result.

A front will push off the lake during the evening, resulting in a northeast wind shift at the Chicago area terminals. There is a small threat for MVFR cigs with the front, but chances are too low to include a cloud layer in the TAFs. Winds may eventually trend light and variable overnight.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

IL...Frost Advisory until 9 AM CDT this morning for ILZ003-ILZ004- ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019- ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ106-ILZ107- ILZ108.

Freeze Watch from late tonight 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 until 9 AM CDT /10 AM EDT/ this morning for INZ001-INZ002-INZ010-INZ011-INZ019.

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

LM...Small Craft Advisory until 3 AM CDT early this morning for the IN nearshore waters.