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 low-end 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. Additionally, top-of-channel flow around 30 knots suggests the potential for occasional 35 mph 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 /06Z TAFS THROUGH 12Z SUNDAY/

Issued at 1227 AM CDT Sat Apr 18 2026

Forecast concerns include...

Low mvfr, possible ifr, cigs overnight into Saturday morning. Strong/gusty northwest winds Saturday. Scattered showers overnight.

Scattered showers will continue for a few more hours over the terminals with showers extending through daybreak just southeast of the terminals. There is a small chance for thunder over the next hour or so, but any additional thunder potential is expected to remain southeast of the terminals.

Low mvfr cigs and patchy ifr cigs have developed across northwest IL and southern WI along and behind an approaching cold front. These lower cigs are expected to spread east across the rest of the area overnight. Only medium confidence for cig trends, but prevailing low mvfr cigs look on track for at least a few hours. Cigs will likely be slowly lifting into low vfr after daybreak with cigs possibly scattering out during the afternoon and most likely by Saturday evening.

Southwest winds gusting into the lower 20kt range will shift to the west/northwest behind the cold front overnight with gusts into the mid 20kt range. Gusts will increase further, perhaps into the 30kt range mid/late Saturday morning. Speeds and gusts will diminish quickly with sunset Saturday evening and turn to the southwest. cms

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.