textproduct: Chicago

KEY MESSAGES

- Showers and a few isolated thunderstorms expected this evening, mainly along/south of the I-80 corridor.

- 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.

UPDATE

Issued at 750 PM CDT Tue May 12 2026

Minor updates to the near term forecast this evening, mainly for pop trends and associated temp/wind impacts.

Early evening GOES vapor imagery depicts a deep mid-level trough propagating across the upper Midwest early this evening, with an embedded closed mid-level circulation over eastern Lake Superior as well as a secondary vort wrapping around its base across the MN/WI/IA/IL border region. 00Z surface analysis places 1003 mb low pressure just north of Manitowoc, WI, with an attendant cold front trailing southwest through far northwest IL and eastern IA to the Kansas City area. An area of rain and embedded heavier showers (but little/no lightning) was occurring from west-central IL into northwest IN, within a region of moisture transport/convergence on the nose of a 40-45 kt low level jet. While surface dew points had risen into the low-mid 50s within the area of rain across the forecast area, low-level moisture return was otherwise fairly limited which along with poor low-mid level lapse rates associated with thick cloud cover and rain-cooled boundary layer temps has yield only marginal low-level instability (generally 250 J/kg or less).

For the remainder of the evening hours, additional shower and isolated thunderstorm potential appears to be greatest along and south of the I-80 corridor across the forecast area (and farther north across eastern WI/LM), where the best moisture transport and some MUCAPE will be focused. In fact, latest GOES IR and radar trends indicate some new convective updrafts developing in the VYS- JOT-GYY corridor. Severe weather threat appears fairly minimal, though any stronger updrafts will be capable of mixing some stronger winds to the surface. Can't completely rule out a few isolated showers farther north ahead of the advancing cold front, though conditions appear much less favorable north of I-80. All of the showers and isolated storms will gradually shift southeast through late evening as the cold front pushes through the area.

Gusty northwest winds behind the cold front have been impressive across MN/northern IA with some 45-50 mph gusts noted there. While it will be breezy overnight here, the loss of deeper mixing after sunset should temper those higher wind speeds as they arrive later tonight.

Ratzer

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 /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 638 PM CDT Tue May 12 2026

Forecast concerns for the terminals...

- Scattered showers to linger across north-central IL this evening.

- Breezy west-southwest winds this evening with gusts upwards of 30-35 kts. Winds then turn northwesterly behind a cold front after 03z.

- Period of MVFR ceilings Wednesday morning, before skies scatter back to VFR by midday.

- Breezy northwest winds on Wednesday with gusts around 20-25 kts. A lake breeze is also forecast to move through ORD, MDW, and GYY Wednesday afternoon resulting in a northeast winds shift.

A surface low continues to drift across northern WI into the southern UP and northern Lower MI this evening with a cold front stretching from this low in WI into central IA. A plume of showers has developed ahead of the cold front and continues to slowly sag southward into north-central IL. While these showers are expected to remain south of the terminals this evening, there is still a low chance (15-20%) for an isolated shower to develop along the cold front as it moves through northern IL and northwest IN prior to 03z. Given the low confidence in new shower development and lack of cloud growth on satellite have opted to keep the TAFs dry.

Outside of the showers, winds this evening will remain breezy out of the west-southwest with gusts generally in the upper 20 to lower 30 kt range through the evening. However, upstream observations do show some locally higher gusts (upwards of 35+ kts) which may materialize over the terminals as the front gets closer. If these gusts occur they should only persist for a couple hours at most before they begin to wane. Once the front moves through, winds behind it will turn northwesterly and will decrease in speed with winds expected to be around 10 kts overnight.

While generally VFR conditions are expected tonight, a plume of MVFR clouds has developed in eastern MN with an upper-level disturbance that will be pivoting into northern IL and northwest IN Wednesday morning. Forecast soundings continue to show the moisture depths to be rather shallow and with the lower clouds not forecast to arrive until closer to daybreak confidence on the longevity of MVFR ceilings is still somewhat low. Therefore, have opted to include a TEMPO for a period of MVFR ceilings (around 2500 ft agl) for a few hours Wednesday morning. Regardless, any MVFR ceilings that do materialize should scatter out by midday Wednesday as diurnal heating and increasing winds erode the moisture plume resulting in VFR conditions to close out the TAF period.

As mentioned, winds on Wednesday will once again be on the breezy side with northwesterly gusts peaking around 20-25 kts. Though, a lake breeze is forecast to develop Wednesday afternoon and move inland through ORD, MDW, and GYY. Given the breezy winds prior to the lake breeze development, confidence on when exactly the lake breeze will move through is low so expect some shifts in the forecast to occur as we get closer. Nevertheless, winds will become more light and variable Wednesday evening as a surface high moves overhead.

Finally, as the aforementioned upper disturbance pivots through on Wednesday there is a non-zero chance (10-14%) for an isolated sprinkle or two to get squeezed out of the mid-level moisture plume. With the dry sub-cloud layer noted in forecast soundings confidence on these sprinkles occurring is very low and thus have foregone a formal TAF mention.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

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