textproduct: Chicago

KEY MESSAGES

- Next chance of thunderstorms as early as Tuesday afternoon, but mainly Tuesday evening near/south of I-88.

- Warming trend through Thursday (except cooler near Lake Michigan on Wednesday).

- More widespread showers and thunderstorms with another cold front Thursday night into Friday.

DISCUSSION

Issued at 235 PM CDT Mon Apr 20 2026

Early afternoon surface analysis depicts high pressure drifting off to the east of the forecast area, centered near Lake Erie as of 18Z. Farther to the west, the surface pressure gradient was tightening from the Plains into the upper Midwest/northern Lakes, with southerly winds expected to gradually increase tonight as a low-level jet develops across the region. Our typically warmer urban core of the Chicago metro will likely see at least occasional gusts in the 20-25 mph develop overnight, with breezy southwest flow area-wide after sunrise Tuesday. By Tuesday afternoon gusts of 30-35 mph appear likely especially south of I-80. Within the broader region of southwest flow, another EML plume is forecast to spread atop a relatively dry boundary layer during the day Tuesday, with fairly warm temps of +10/12C noted at the base of the plume around 800-850 mb. This looks to provide a cap with respect to surface-based convection through much of the morning/midday hours. Forecast soundings do indicate some cooling of the column ahead of a low-amplitude short wave during the afternoon and evening however, which along with the northeastward advection of a narrow axis of low-level moisture may be sufficient to erode the cap. Various CAM guidance (4km NAM, RAP, HRRR) does develop isolated to scattered showers and thunderstorms roughly along and south of I-88 by late afternoon. Greater coverage and confidence however remains along/south of I-80 toward/during the evening hours as the surface cold front pushes southeast. Despite the relatively limited moisture return, west-northwest 40-45 kt winds aloft would support some modest deep-layer shear, which may provide enough updraft longevity to produce a sub-severe small hail and gusty wind threat. The orientation of the mid-level flow may also support slow storm motions/back building for a very localized training/heavy rainfall/flash flood potential as the front continues to sag south and southeast Tuesday night.

Rising heights/subsidence then develops Wednesday, with surface high pressure drifting across the Great Lakes region supporting a period of dry, quiet weather mid-week. After temperatures in the 70s on Tuesday (right up to the Lake Michigan shore given the breezy southwest winds), Wednesday looks equally mild in many areas, though onshore winds near the lake will likely hold temps in the 50/60s closer to the shore. Breezy southwest low- level flow then develops again Thursday, warming temperatures into the lower 80s in most areas even near the lake.

Guidance is in good agreement in developing a deep long-wave trough from the western CONUS eastward later in the week. This looks to spread more widespread shower and thunderstorm chances back into the area Thursday night into Friday, ahead of a cold front currently depicted to cross the area Friday afternoon/evening. Models continue to indicate stronger instability and greater severe threat would reside upstream (MO- central/southern IL) from the WFO LOT cwa ahead of the cold front, but we'll have to monitor details in further trends as this end of the week period approaches.

Ratzer

AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/

Issued at 1230 PM CDT Mon Apr 20 2026

Aviation forecast concerns for the 18Z TAFs:

- Breezy south-southwest winds develop tonight (gusts 20-25 kts at urban metro sites) and persist Tuesday.

- LLWS away from the metro sites late evening/overnight with 45 kt low level jet around 2 kft AGL.

- Low (~20-25%) chance for scattered SHRA/TSRA development late Tuesday/Tuesday evening. Greater potential appears to be just off to the south of terminals, but will need to monitor trends.

Quiet VFR aviation weather across the terminals this afternoon, with surface winds gradually becoming south-southwest while high pressure drifts away to the east. Pressure gradient is forecast to tighten up late this evening/overnight, with southwest winds increasing and likely becoming at least occasionally gusty in the 20-25 kt range around/after midnight for ORD/MDW/GYY. Farther west, where surface winds more likely to decouple, a 45 kt southwesterly low level jet will support LLWS conditions at DPA/RFD. Surface winds then become breezy southwest at all sites after sunrise Tuesday.

A surface cold front will approach the area from the northwest late Tuesday afternoon and evening. Some guidance depicts scattered convective SHRA/TSRA across parts of the area, with high-res ensembles favoring development just south of the terminals after 21-23Z. While a few model runs do show some potential across the terminals. probability of that currently appears to be less than 30 percent and thus have not mentioned in the ORD/MDW 30 hour forecasts. Will need to continue to monitor trends into Tuesday for any increase in potential across the terminals.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

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