textproduct: Chicago

KEY MESSAGES

- Showers and perhaps a thunderstorm will move across the area in scattered fashion this afternoon.

- After a relatively cool Wednesday, temperatures will trend upward Thursday onward.

- The May 16 through 19 timeframe may feature several rounds of showers and storms in the general region.

DISCUSSION

Issued at 125 PM CDT Tue May 12 2026

Rest of Today:

The center of a low pressure system is currently over southwestern Lake Superior and is moving eastward toward the Upper Peninsula of Michigan. A cold front extends southward through western Wisconsin and across central Iowa away from the center of the low pressure system. Meanwhile, a narrow band of showers with embedded thunder associated with low-level isentropic ascent continues to fester across far northern Illinois and southern Wisconsin. Ahead of the region of showers, the boundary layer is deep and well-mixed with air temperature and dew point spreads pushing 20 to 25 degrees. Southwesterly winds continue to increase with recent gusts of 30 to 35 mph noted near the Mississippi River. While upper-level clouds will spread over the rest of the area through the afternoon as the band of showers tries to shift southeastward, the continued feed of warm air from the southwest will help high temperature reach the upper 70s to around 80 in the next few hours. The exception will be near the Wisconsin state line where showers will hold temperatures some 10 to 15 degrees lower.

As the band of showers shifts southeastward through the area this afternoon, attention will turn northwest toward the cold front for signs of convective development. Cooling mid-level temperatures ahead of the parent trough shifting atop a plume of surface dew points in the upper 40s to around 50 will allow for around 250 J/kg of MUCAPE to materialize ahead of the front by late afternoon. Provided the instability materializes, largely uncapped mixed profiles should allow for at least isolated but probably scattered high-based but low-topped convection. Relatively shallow echo depths should tend to limit the development of large hail. With that said, the deep dry profile beneath the cloud bases will promote locally gusty winds with the heartiest precipitation cores. In all, cannot rule out a locally damaging gust this afternoon, mainly between 6 and 9 PM. Clouds will clear quickly behind the cold front overnight.

Not to be forgotten, still am expecting at least patchy but possibly areas of blowing dust to develop this afternoon along and south of Interstate 80 where gusty southwesterly winds will overlap with working agricultural fields. A plethora of personal weather stations are showing gusts in excess of 35 mph south of I-80 right now, so would expect the blowing dust threat to only increase through the afternoon. The threat should end toward sunset as boundary layer winds relax and rain arrives.

Tomorrow:

Deterministic guidance remains in agreement that a secondary upper- level trough (currently racing into central Manitoba) will move into the Great Lakes tomorrow morning. While the timing will not be favorable to maximize diurnal heating, steepening low-level lapse rates (by virtue of low-level cold air advection to lower 850mb temperatures toward 0C) should support the quick development of a stratocumulus deck by mid- morning. Provided the depth of the cloud deck grows sufficiently deep, cannot rule out instances of sprinkles across the area from mid-morning to mid-afternoon. Highs will be noticeably cooler than today and in the upper 50s along the lakeshore and in northwestern Indiana to the mid 60s across central Illinois.

Clearing skies and the slackening wind field will set the stage for temperatures to tumble tomorrow night. While the outgoing forecast will feature overnight lows in the upper 30s to lower 40s, would not be surprised if actual lows verify even colder and in the low to mid 30s. If such temperatures were to verify, frost would be a definite threat.

Thursday onward:

Toward the end of the week and into the weekend, a well- advertised change in the upper-level pattern will take place with predominant cyclonic flow along the US/Canadian border turns to quasi-zonal and eventually southwesterly. The net effect will be a notable upward trend in temperatures as well as a return of regular opportunities for springtime thunderstorms. Note that the pattern will be supportive of severe thunderstorms in general region, particularly Friday through Monday.

Borchardt

AVIATION /18Z TAFS THROUGH 00Z THURSDAY/

Issued at 109 PM CDT Tue May 12 2026

Key Messages:

- A band of showers will move through the area this afternoon with a few isolated lightning strikes possible (~20% chance).

- A secondary round of showers and possibly storms (~25-30% chance) may occur just ahead of a cold front this evening.

- Strong southwesterly winds are expected this afternoon with 30+ kt gusts likely at times, though some uncertainty exists with the magnitude and frequency of these stronger gusts.

- A northeasterly wind shift behind a lake breeze may occur at ORD and MDW tomorrow afternoon.

At press time, a band of rain was entering northern Illinois and gradually progressing east-southeastward. Dry sub-cloud air was causing a lot of this precipitation to evaporate before reaching the ground, but these showers were still making it to the ground in spots. Would expect a similar theme to continue through this afternoon as this rain band slides across the terminals, and a brief period of MVFR visibility could not be ruled out wherever the rain falls steadiest. Also noted a few isolated lightning strikes near DBQ earlier, and additional isolated lightning strikes may occur this afternoon. However, the probability of one occurring near any particular TAF site remains too low to warrant a formal VCTS or TSRA mention in the TAFs through 00Z.

Additional showers may attempt to develop closer to an incoming cold front this evening, possibly very near or directly over the terminals. If this convection were to grow tall enough, then it may also produce lightning. Confidence in this remains low, but have maintained the inherited PROB30 groups for TSRA this evening at the Chicago metro terminals.

Southwesterly winds are also expected to strengthen this afternoon, though extensive mid- and upper-level cloud cover may limit how high and how frequent the stronger (30+ kt) gusts will be. Occasional gusts near 40 kts may nevertheless occur near and during the showers/storms this afternoon and evening. Winds will then shift to a northwesterly direction and subside this evening behind the aforementioned cold front.

Late overnight into tomorrow morning, high-end MVFR ceilings may be observed, but confidence in this is only low-medium and have kept the TAFs VFR for now. A lake breeze is expected to develop tomorrow afternoon and will likely cause a northeasterly wind shift at ORD and MDW at some point. Confidence in the timing of this wind shift remains low at this time.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

IL...Blowing Dust Advisory until 7 PM CDT this evening for ILZ019- ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ107.

IN...Blowing Dust Advisory until 7 PM CDT /8 PM EDT/ this evening for INZ010-INZ011-INZ019.

LM...Small Craft Advisory until 7 PM CDT Wednesday for the IL nearshore waters.

Small Craft Advisory until 1 AM CDT Thursday for the IN nearshore waters.