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 /12Z TAFS THROUGH 18Z TUESDAY/

Issued at 632 AM CDT Mon May 18 2026

Forecast concerns for the terminals...

- Line of showers and storms will move through the terminals this morning. Strongest storms may produce locally gusty winds around 30 kts.

- Period of gusty southeast winds behind the rain this morning before winds turn back southwest with gusts around 25 kts.

- Another period of showers and possible thunderstorms late tonight into Tuesday morning.

A line of showers and embedded thunderstorms continues to advance eastward across northwest and western IL this morning. While the line has been gradually weakening, newer showers/storms have been developing ahead of the outflow from the main line and that trend is expected to continue. Therefore, expect a 2-3 hour period of thunder at the terminals this morning which will result in some MVFR ceilings and visibilities. Additionally, the stronger storms will be capable of producing locally gusty winds upwards of 30 kts and maybe some small hail.

Behind the line are a couple of small scale areas of low pressure. The low over northeast MO is expected to lift northeast into north-central IL and northwest IN later this morning which should help in either redeveloping or maintaining any ongoing showers and thunderstorms. Given the low's trajectory it appears the greater potential for any thunderstorms with the low will be south of I-80 but cannot fully rule out an isolated strike or two as far north as ORD and DPA. Therefore, have opted to introduce a VCTS for this potential at the Chicago terminals. Once this area of showers/storms begins to exit there is also a wake low that has developed in IA. While this low shouldn't result in more rain, it does look to bring a period of gusty southeast winds around 25-30 kts but higher gusts are certainly possible.

After the wake low moves through around midday/early afternoon, winds will return from the southwest with persistent gusts around 25 kts through the rest of the afternoon. Overall dry conditions should prevail at the terminals through this afternoon, but there remains a 15-20% chance for some isolated showers or storms to redevelop. Since the confidence in any afternoon activity remains low have opted to forego any formal TAF mentions for now.

Heading into tonight, gusts will diminish after sunset but will still remain breezy with upper teen to lower 20 kt gusts. At the same time more showers and thunderstorms are forecast to develop across IA along a cold front and eventually will move into northern IL and northwest IN towards daybreak. Given that these showers/storms should be arriving in a decaying state confidence on thunder reaching the Chicago terminals is low so for now have just maintained PROB30s for showers here. That said, did opt to add a PROB30 for thunder at RFD late tonight into Tuesday morning where confidence is slightly higher.

Otherwise, expect mostly VFR conditions to prevail through the TAF period.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

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