textproduct: Chicago

KEY MESSAGES

- A generally cool pattern will prevail with occasional opportunities for scattered showers.

DISCUSSION

Issued at 128 AM CDT Thu Apr 30 2026

Broad upper-level troughing remains in place across the Great Lakes. Meanwhile, a broad surface high pressure system is currently meandering through the middle Mississippi River Valley. The combination of the weak pressure gradient and modest upper-level subsidence in the wake of a departing upper-level shortwave has allowed for mostly clear skies and nearly calm winds early this morning. With temperatures falling into the mid to upper 30s by daybreak, suppose there could be patchy frost especially near the Wisconsin state line where readings will be coolest. Also, will have to watch for patchy shallow ground fog toward daybreak, particularly across northeastern Illinois where dew point depressions are within a degree or two.

Today looks pretty similar to yesterday. Partly cloudy skies a northwesterly breeze will lead to highs in the mid to upper 50s. Another lake breeze will hold lakeshore locations to the upper 40s. The next upper-level shortwave pinwheeling around the broad cyclonic flow is evident in GOES-19 water vapor imagery moving into the Dakotas, and will move into the region this evening. Steepening mid- level lapse rates by virtue of mid-level cold aid advection will support scattered showers from the evening through the first half of the overnight period with a higher coverage expected today compared to yesterday. With forecast soundings depicting the centroid of instability from 0C to about minus 15C, would not be surprised to see a few instances of graupel and perhaps evening a lightning strike in the deepest cores.

Tonight looks similar to tonight with clearing skies and chilly temperatures. With clearing taking place largely during the overnight hours (as opposed to before), overnight lows will be coldest and in the mid 30s across the northwest and modestly warmer in the lower 40s southeast. Again, some patchy frost may develop where temperatures are coldest (near the Wisconsin state line).

Another shortwave (or two) may move into the region on Friday, supporting another diurnal flare of showers. At this point, indications are that instability will be more muted on Friday compared to this evening, so will hold with "silent" 10 percent chances for showers. With a large onshore component to the wind, highs on Friday will be the chilliest of this stretch and range from the upper 40s lakeside to mid 50s well inland (Mendota to Gibson City).

Ensemble guidance continues to indicate that the aggregate upper-level troughing will shift eastward this weekend and beyond, placing the Great Lakes in broad upper-level northwesterly flow. As a result, a line of shortwaves will be poised to parade over the region leading to periodic opportunities for showers and a few thunderstorms. Modest warming may take place as well, particularly in the warm advection regimes ahead of each wave. Looking well beyond, am seeing a growing signal (particularly in EPS data) for another shot for somewhat chilly air in the Thursday/Friday timeframe of next week followed by a more definitive warming trend into the middle of May.

Borchardt

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/

Issued at 628 AM CDT Thu Apr 30 2026

Forecast concerns include...

Chance of showers this afternoon into this evening. Lake breeze/wind shift to northeast this afternoon. Chance for mvfr cigs overnight/Friday morning.

Areas of fog mainly south of the terminals and near GYY will quickly dissipate this morning.

Light west/northwest winds will slowly increase this morning and will shift northeasterly this afternoon with a lake breeze. Only medium confidence for timing and trends will need to be monitored. Winds are expected to turn back to light northwest later this evening. There is another potential shift back to north/northeast with a frontal boundary Friday morning. Added the northeast directions to the ORD/MDW 30 hour tafs, but low confidence at this time.

A weak disturbance will move across the area this afternoon into this evening bringing scattered showers. Expected coverage continues to increase and have changed prob to tempo mention. Showers may linger into this evening before ending.

Guidance is trending lower with cigs overnight into Friday morning with low mvfr cigs possible. Confidence is low but introduced scattered mention for now. cms

LOT WATCHES/WARNINGS/ADVISORIES

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