textproduct: Chicago

KEY MESSAGES

- Tuesday will be warm and start dry with showers and perhaps a thunderstorm during the afternoon and evening hours.

- Cooler conditions are expected on Wednesday before a longer- lasting warm up arrives to close the week.

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

DISCUSSION

Issued at 138 PM CDT Mon May 11 2026

A large surface high pressure system continues to serve as the main factor of influence today, and is leading to sunny skies, an east to northeasterly breeze, and cool temperatures in the upper 50s to lower 60s.

Tonight, the surface high will shift eastward causing winds to gradually veer southeasterly. A gradient in overnight lows is expected with partly cloudy skies holding values in the upper 30s to lower 40s across Illinois and largely clear skies allowing for temperatures to drop into the mid 30s across northwestern Indiana. Do expect at least patchy fog to develop across northwestern Indiana overnight, but will hold off on a Frost Advisory given the expectation for temperatures to remain above freezing.

Tomorrow, an upper-level shortwave (currently propagating along the Montana, North Dakota, and Saskatchewan borders) will dive southeastward into the Great Lakes. Ahead of the wave, southwesterly flow will increase markedly leading to breezy surface winds through the afternoon hours. With a very dry start to the day (dew points in the upper 20s), continue to lean on the more "mix-y" guidance which advertises gusts peaking near 40 mph by early afternoon. Consequently, a plume of warm temperatures (850mb values near +12C) will advect into the region via the strong southwesterly flow, which with the dry profile will support highs in the upper 70s to lower 80s areawide.

During the afternoon hours, gradually increasing DCVA ahead of the approaching trough will support an area of southeastward- moving showers ahead of a southeastward-moving cold front. With the residual dry airmass, it may take time for top-down saturation to allow for raindrops to survive to the ground through the afternoon hours. In fact, local enhancements to the wind field may occur as evaporation leads to more efficient downward mixing. With that said, suspect that saturation will finally be archived with southeastward extent, supporting a gradual increase and broadening of PoPs through the afternoon hours with time. With effective equilibrium levels expected to terminate right around minus 20C, chances for thunder continue to look on the lower end of the spectrum and around 15 to 20% at any given point.

With respect to the threat for severe weather along the cold front tomorrow evening, am not impressed with the exceedingly marginal thermodynamics (MUCAPE of 100-200 J/kg) and apparent lineage to preceding rainfall (to moisten the low-level moisture profile) to take advantage of what would otherwise be a supportive forcing/kinematic regime. In fact, suspect little to no convection will be sustained along the front, favoring a dry evening and overnight period. So, will withhold any mention of severe weather in any outgoing products.

Quickly rising surface pressure in the wake of the cold front will facilitate the efficient advection of cool air in the wake of the front Tuesday night into Wednesday. A trailing shortwave arriving in the morning hours may actually bolster cold air advection through the day. Falling 850mb temperatures toward 0C will set the stage for steep low-level lapse rates leading to an expansive stratocumulus deck by mid-morning. In fact, would not be surprised in the least to see sprinkles or a few showers throughout the day (have manually introduced "silent" 10 PoPs for now). When taken altogether, Wednesday is shaping up to feel a bit more like fall than spring.

Toward the end of the week, the upper-level pattern will undergo a transition from predominant toughing to quasi-zonal flow along the US-Canadian border. As a result, the much- anticipated transition toward warmer weather remains on track to arrive by this weekend. Will also note an ensemble signal for rounds of showers and storms in the general region in the May 16-19 timeframe.

Borchardt

AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 1238 AM CDT Tue May 12 2026

E/ESE winds under 10 knots at TAF issuance will steadily increase and veer this morning. While there may be a period of SSE gusts to 20 knots around and after sunrise, winds are expected to shift just W of S by 14-15Z at ORD/MDW. Gusts will continue to increase through the afternoon as deep diurnal mixing develops. SW winds will frequently gust over 30 knots through the afternoon, including a period of the highest gusts around 40 knots mid afternoon. Winds will slightly diminish early this evening before shifting NW with gusts around 25 knots behind a cold front late evening and overnight.

While conditions will remain VFR through the period, there could be a few exceptions. Substantial virga from high-based SHRA this afternoon may become heavy enough to reach the ground and produce brief bouts of MVFR visibility and additional strong gusts. There is also a 10 to 20 percent chance of TSRA with the cold front mid- evening. Maintained a dry forecast as moisture availability appears too low for coverage to warrant a mention in the TAF at this time. Finally, patches of high-end MVFR ceilings may brush the area to the northeast overnight.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 9 AM this morning to 7 PM CDT Wednesday for the IL nearshore waters.

Small Craft Advisory from 9 AM this morning to 1 AM CDT Thursday for the IN nearshore waters.