textproduct: Chicago

KEY MESSAGES

- Breezy to windy conditions today with colder weather over the weekend into early next week.

- Potential (20-30%) for some gusty showers Sunday afternoon.

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

DISCUSSION

Issued at 301 AM CDT Sat Apr 18 2026

A strong cold front is shifting across the region early this morning. In its wake, blustery northwest winds will gust to 30 mph today. Persistent cold advection will hold highs generally in the upper 40s and lower 50s. Lingering stratus will erode and thin through the afternoon, yielding mostly clear skies overnight into Sunday morning. Since we'll remain on the northern periphery of a modest mid 1020s mb high across the ArkLaTex, we'll likely hold onto at least some surface flow through the night. This should tend to limit the expansiveness of the frost threat, although with temperatures expected to dip to near freezing west and north of the Fox Valley, it's possible we may need to consider a small Frost Advisory for parts of the area. Will defer this decision to the dayshift.

On Sunday, a fast-moving and fairly energetic/robust wave embedded within broad cyclonic flow will drop across the region. As this approaches through the afternoon, surface-700 mb lapse rates are forecast to increase significantly--towards 9 C/km. With the increasing in attendant forcing, forecast soundings reveal sufficient saturation through the 700-800 mb layer and the development of just enough instability to support perhaps widely scattered diurnally-driven showers and have added some precip chances to the forecast to account for this. Given the low wet bulb zero heights and presence of 0-3 km CAPE, suppose it wouldn't be entirely out of the question for some graupel if robust convective elements develop, although no accumulations would result with air temperatures in the 40s/50s.

Also, based on the progged the depth of the PBL and forecast equilibrium level temperatures, it's also not out of the realm of possibility that some bonafide low-topped thunderstorms materialize. Given the long, straight nature of hodographs with 0-4 km shear near 35 knots, some ephemeral updraft organization would be possible, leading to a potential for some more robust/gusty convective elements. Finally, top-of-channel flow around 30 knots suggests the potential for occasional 35+ mph gradient wind gusts (particularly near and south of I-80) if this level of mixing is realized.

Surface high pressure will then build nearly overhead Sunday night into Monday morning. This will set the stage for more robust radiational cooling and somewhat more widespread near to sub- freezing temperatures outside of Chicago. Will likely need a combination of frost advisories and freeze warnings for this.

Broad warm advection will re-establish across the area on Monday and Tuesday with temperatures expected to push back into the 70s 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. Thereafter, there's a signal for a more robust synoptic wave and associated cold front sweeping through the Upper Midwest towards the end of the week, which could bring the next thunderstorm chances to the area.

Carlaw

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/

Issued at 605 AM CDT Sat Apr 18 2026

Lingering MVFR/IFR cigs will lift to VFR through 14-15z at GYY. Thereafter, while some patchy MVFR cigs will be possible today, largely VFR conditions are expected with gusty WNW winds. Winds will ease sharply this evening before increasing again mid-late Sunday morning. Beyond the ORD/MDW extended TAFs: there will be a chance for scattered showers later Sunday afternoon, as well as intermittent NW gusts over 30 knots.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

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