textproduct: Chicago

KEY MESSAGES

- A few additional rounds of showers and thunderstorms today and Tuesday, some of which could be strong to severe mainly during the afternoon hours. - Summer-like warmth and breezy winds at times will prevail through Tuesday, then cooler temperatures return for the middle of the week.

- Milder weather returns heading into the weekend, though with at least intermittent shower/thunderstorm potential.

DISCUSSION

Issued at 347 AM CDT Mon May 18 2026

Regional radar mosaic shows a linear MCS moving across the Mississippi River into western WI/IL early this morning. There has been a notable weakening trend to the convection over the past few hours as the complex propagates into a region of decreasing deep- layer shear and low-level instability, though storms should continue to spread eastward this morning along a composite outflow boundary at the leading edge of the convection. In fact, recent radar trends indicate a few new cells developing ahead of the line near the Quad Cities, perhaps aided by enhanced ascent associated with one or more an apparent MCVs near the IA/IL/MO border. Gusty winds of 30-40 mph with the outflow and some small hail are possible with these storms early this morning, though the overall threat of organized severe weather appears low for the next several hours. Later this morning however, diurnal warming and associated destabilization along the east/southeast periphery of the complex remnants and outflow boundary may support renewed development/strengthening of convection with more of a severe threat into this afternoon. Though depending on how far the outflow boundary makes it this morning, this would likely be across the south/far east portions of the forecast area. Farther north/northwest, lingering stratiform rain and cloud cover this morning looks to limit destabilization and recent CAM trends have been consistently dry there during the day/evening.

Farther to the west, early morning GOES vapor imagery indicates another mid-level short wave rounding the base of a western CONUS long-wave trough. Widespread strong convection is progged to develop late this afternoon and evening across KS/NE/IA once again in response, eventually growing upscale into another eastward- propagating linear MCS. Similarly to this morning, CAM runs continue to depict a similar weakening of this system as it reaches the Mississippi River and spreads into WI/IL early Tuesday morning with associated relatively low severe potential. The lingering MCS cold pool footprint and residual cloudiness will likely delay destabilization into midday Tuesday, though additional storm development appears possible across the southeast half of the forecast area Tuesday afternoon/evening as the cold front finally pushes through the forecast area. Stronger mid-level flow on the southeastern periphery of the aforementioned short wave will likely support enough storm organization for a hail/wind threat during this time.

Following the cold front, surface high pressure is progged to build east into the western Great Lakes region Wednesday and Thursday. Dry, but breezy and cooler weather is expected both days, with temps mainly in the 60s, but only in the 50s near Lake Michigan with northeast-east flow off the lake. Nighttime lows in the 40s are expected. The high then drifts off to the east Thursday night, with the upper level pattern favoring a series of mid-level disturbances tracking through the area Friday into the weekend. This will support milder temperatures, but also some occasional shower/thunderstorm chances into the holiday weekend.

Ratzer

AVIATION /06Z TAFS THROUGH 12Z TUESDAY/

Issued at 100 AM CDT Mon May 18 2026

Forecast concerns for the terminals...

- Line of showers and embedded thunderstorms to move through northern IL and northwest IN late tonight into Monday morning. A few storms could be strong with a threat for locally gusty winds.

- Breezy south-southwest winds Monday afternoon with gusts around 25 kts.

- Another period of showers and possible storms late Monday night into Tuesday morning.

Latest radar imagery continues to show a line of showers and thunderstorms stretching from MN to KS that is racing towards northern IL and eventually northwest IN. Arrival time of this line continue to be around 09z at RFD and between 10-12z for the Chicago area terminals. While lightning trends within the line segment heading towards the terminals has been decreasing, recent AMDAR sounding out of MDW continue to show steep mid- level lapse rates (around 7-8 C/km) which could still support an occasional lightning strike as the line moves through. Additionally, a few stronger showers/storms are possible and could result in localized gusty winds and maybe some small hail.

Once the line of showers/storms arrives, it should move through within 3-4 hours at each site. However, with the expected arrival time around daybreak there continues to be a chance that some renewed development could occur within the line as it traverses the area, especially south of I-80. If this occurs as guidance suggests then shower/storm activity could persist through the morning before things fully exit the area. It was for this reason that the long PROB30 was maintained, but hopefully we can refine this window as the line arrives.

After the line exits the area around midday, breezy south- southwest winds will develop and persist through the afternoon. Gusts during this period look to generally be around 25 kts but locally higher gusts (upwards of 30kts) are possible. While there is still a non-zero chance (15-20%) for additional shower and/or thunderstorms to develop this afternoon, latest guidance trends have continued to back off on this scenario given that the better forcing for storms will be centered to our west in IA. That said, another round of showers and thunderstorms is expected to develop in IA this afternoon and move into northern IL and northwest IN late Monday night into Tuesday morning. Though, the coverage of these storms into our area looks much more limited (especially the thunder coverage) so have opted to introduce a PROB30 for SHRA in the 30-hour TAFs for now.

Otherwise, expect VFR conditions to prevail through the period.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 4 AM CDT early this morning for the IL and IN nearshore waters.