textproduct: Chicago

KEY MESSAGES

- Dense fog around parts of the area this morning, especially around Lake Michigan.

- Another round of showers and thunderstorms is expected later this evening into tonight. Some of the storms could be severe. Locally heavy rainfall could result in flooding issues.

- Windy and unseasonably warm weather today will be replaced by much colder weather over the weekend into early next week.

- Frost/freeze conditions are expected many areas Sunday and especially Monday mornings.

DISCUSSION

Issued at 334 AM CDT Fri Apr 17 2026

Mesoscale high pressure centered over southern Lake Michigan is pushing a bank of fog and low stratus inland off the lake early this morning. Half mile or less visibilities are spreading around the interior Chicago metro area and farther west up near the WI state line. A Dense Fog Advisory is in effect through 10AM around the northeastern quadrant or so of our CWA. Southerly winds will ramp up quickly beyond sunrise as the high moves away and pressure falls begin ahead of an advancing low. Strong warm advection ahead of the wave will allow for highs in the lower 80s around most of the area this afternoon. Relatively dry BL air today will promote deep mixing and encourage strong southerly winds with gusts to 30 to 35 mph, occasionally near 40 mph, during the afternoon.

Main focus for the forecast is the potential for severe thunderstorms this evening with the passage of a strongly-forced cold front. As a large upper trough scoots eastward from the Rockies into the central Plains, an EML will get advected into the region. The heart of the EML will focus just west of the Mississippi, but forecast soundings still depict a deep column of low to mid level lapse rates greater than 8 K/km allowing for anywhere from 1,500 to 3,000 Joules of afternoon MUCAPE. It's not entirely clear how much capping we'll maintain into the afternoon with some camps suggesting we may be largely uncapped. However, dry low level air and weak ambient forcing in the open warm sector should do well to inhibit freely convecting storms even in a weakly capped environment. Only a couple of CAMs resolve isolated convection immediately ahead of the storm's cold front later in the day, but chances for free convection in the warm sector throughout the afternoon are seemingly low. Should any afternoon free convection develop, they would have the ingredients necessary to quickly become severe and large hail and damaging winds would be the primary threat. Can't rule out a tornado threat with anything that might spawn during the afternoon either given elongating, looping low level hodographs, but high LCLs and low instability through the low levels are not favorable for tornadogenesis.

The primary window of concern for severe convection is this evening as the aforementioned low's cold front gets dragged across the area. A cluster of supercells is favored to form along the front across eastern IA during the afternoon before congealing into a strongly- forced line of convection across northern IL during the evening. Storms look to approach the I-39 corridor during the early evening and exit to the east early in the overnight, possibly with additional showers lingering late into the night. Guidance generally depicts storms on a weakening trend as they move into a thermodynamically less favorable environment across northern IL (and worsening with time via diurnal cooling). However, the environment will remain plenty supportive of strong to severe convection across our CWA, especially given the strong dynamic forcing at play with the front. The biggest concern with storms is the damaging wind threat which recently prompted the Storm Prediction Center to drag the Day 1 Enhanced Risk (Level 3 of 5) east to the Indiana state line, including the city of the Chicago. BL CIN will be increasing through the evening, but can't rule out a spin up tornado threat along this line either given the strong forced ascent and impressive low level wind field.

Another big concern with rain and storms later today is the heavy rain and flooding threat. Area soils are especially moist from recent rains making them more susceptible to flooding and many local rivers and creeks, especially the Rock, Fox, and Des Plaines, are already well into flood stage. Near or over 1.5" of PWAT amid deep dynamic forcing and instability favors periods of heavy rainfall with the frontal passage. There is also a growing signal in model guidance for a shortwave impulse to track across the area and provide an additional period of moderate to heavy rain into the overnight in the wake of line of storms. Can't say for certain where this wave will track, but guidance likes near the I-80 corridor and near and south of I-80 is where the highest QPF is generally favored. Probs for 0.5" of rain from last evening's HREF range from 60 to nearly 100% around the entire CWA and you won't find a piece of recent deterministic guidance without a swath of 1-2" across at least a small portion of the area. As little as 0.5" of rain could easily spell trouble for parts of the CWA from a flash flooding perspective, especially those north of I-80. Given the uncertainties and the fact that the highest amounts are generally favored outside of the more flood-sensitive areas, decided to hold off on issuing a Flash Flood Watch on this shift, but this is something the day shift will have to reconsider. A Hydrologic Outlook has been issued for these flooding concerns.

Behind the cold front, a much cooler airmass will settle into the region for this weekend. In fact, we'll likely see Saturday's high temperatures occur at midnight with cooler conditions expected during the day. Daytime highs are forecast in the middle 40s to lower 50s, a whole 30+ degrees cooler than today's high temperature forecast. Similar highs are expected for Sunday. Low temperatures are forecast in the 30s Saturday night into Sunday morning with some spots slated to get down to freezing. A modest westerly breeze may help mitigate frost/freeze impacts some, but we may be headed for our first frost/freeze headlines of the growing season this Saturday night. Better frost/freeze conditions will set up Sunday night/Monday morning with low temperatures forecast in the lower and middle 30s with calm winds and clear skies as surface high pressure centers over the region. Beyond tonight's rain chances which could stretch into early Saturday morning, no precip is anticipated over the weekend. In fact, conditions looks to remain largely quiet through early next week as we trend back toward unseasonably mild conditions.

Doom

AVIATION /12Z TAFS THROUGH 18Z SATURDAY/

Issued at 617 AM CDT Fri Apr 17 2026

The key aviation messages are:

- Rapid improvement to VFR cigs/vsbys through 14-15z.

- SSE to S winds becoming strong and gusty through the morning and afternoon. Brief period where winds could gust around 35 kts roughly 19-23z.

- Line of thunderstorms will impact all terminals this evening. Potential for localized wind gusts over 50 knots. - Cold front arrives late tonight with a gusty northwest wind shift and MVFR cigs.

Cigs and vsbys will improve quickly over the next few hours with VFR expected by 14-15z. SE winds will gradually become gusty later this morning, with gusts increasing further during the afternoon. There is a potential for occasional gusts near 35 kts in the 19-23z timeframe--approaching some cross wind thresholds at ORD. Additionally, wind directions could end up 170V190 this afternoon, with a propensity to become more SSE early this evening ahead of approaching thunderstorms.

No significant changes to TEMPO TSRA timing this evening, with earlier arrival at RFD, and latest at GYY. Some storms will be severe with a threat for wind gusts over 50 knots, although confidence in these gusts at individual terminals is too low to include a formal mention of gusts that high at this time. Lingering showers and embedded storms will diminish through about 06z. Thereafter, a cold front will deliver a gusty northwest wind shift and MVFR cigs which should prevail into Saturday morning.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

IL...Dense Fog Advisory until 10 AM CDT this morning for ILZ005- ILZ006-ILZ012-ILZ013-ILZ103.

IN...Dense Fog Advisory until 10 AM CDT this morning for INZ002.

LM...Dense Fog Advisory until 10 AM CDT this morning for the IL and IN nearshore waters.

Small Craft Advisory from 1 PM this afternoon to 10 PM CDT Saturday for the IL and IN nearshore waters.