textproduct: Chicago

KEY MESSAGES

- Scattered thunderstorms diminish early this morning across northern IL. Thunderstorm chances then increase again this afternoon, mainly southeast of a Peru IL to Valparaiso IN line. A few strong storms are possible.

- Summer-like warmth and breezy winds today will give way to cooler and dry conditions for the middle of the week.

- Intermittent shower/thunderstorm chances return late in the week and into the weekend, though with a return of warmer weather by Memorial Day.

DISCUSSION

Issued at 345 AM CDT Tue May 19 2026

Early morning surface analysis places 1005 mb low over northwest Wisconsin, with a cold front trailing through central Iowa into southeast Kansas. Strong/severe thunderstorms that had developed Monday afternoon ahead of the front across the mid- Missouri Valley and elsewhere ahead of the boundary, have since evolved into another linear MCS, which currently stretches all the way from the central upper peninsula of Michigan to northwest/western Illinois, then southwest to Oklahoma. Aloft, GOES vapor imagery indicates a deepening mid-level short wave trough propagating northeastward from the northern/central Plains toward the northern Great Lakes. Strong large-scale ascent, depicted by mid-level height falls of 60-80 meters across the upper Mississippi Valley, is aiding in maintaining a strong southwesterly low-level jet across our region, which latest RAP soundings/mesoanalysis depict at 60 kts across western IL.

While low-levels have diurnally stabilized, the strong low level jet and associated warm/moist advection has maintained an elevated MUCAPE axis of 1500-2000 J/kg per RAP soundings. This, combined with low-level forcing provided by the eastward- propagating outflow boundary along the leading edge of the aforementioned MCS has allowed scattered convective cells to continue to initiate atop the outflow boundary across western and northwest Illinois. Forecast soundings do indicate a weakening and veering of the low-level jet over the next few hours however, along with a decrease in elevated instability. Combined with the stable boundary layer conditions, this should continue to result in weakening trend to the convection as the outflow continues to shift east across northern IL this morning, which is already evident in recent radar and lightning data. Stable outflow and lingering scattered morning showers, combined with a stream of high-level cloud cover off of stronger convection well to our southwest should limit local convective redevelopment for the balance of the morning.

The surface cold front is progged move into the forecast area early this afternoon, pushing southeast across the area into this evening. Forecast soundings are generally capped across the area initially, though become conditionally unstable with diurnal warming later this afternoon ahead of the front mainly across the southeast half of the forecast area (and especially southeast of a Pontiac-Valparaiso line). Guidance has generally backed-off with coverage of afternoon convection across our area, though frontal forcing and favorable diurnal timing especially in our far southeast should support an increasing thunderstorm threat mid-late afternoon. Forecast 1000-1500 J/kg MLCAPEs and up to 35 kts of deep-layer shear (generally parallel to the frontal zone) would support some storm organization with some wind/hail potential. Note SPC has moved earlier slight risk out of the WFO Chicago area with the new Day 1 outlook, leaving a marginal (level 1 of 5) risk in our far southeast.

A few showers may linger across our southeastern tier of counties this evening until the front exits the region, with a period of dry and cooler weather then expected Wednesday through Thursday. After several recent days of highs in the 70s and 80s, daytime temperatures in the 60s are expected mid-late week with 50s closer to Lake Michigan where breezy northeasterly winds will persist around an area of high pressure building across the Great Lakes region. Nighttime temps both Wednesday and Thursday night will dip into the 40s in most areas.

Rain chances then return Thursday night into Friday, as guidance depicts an area of low pressure tracking from Missouri into Illinois and Indiana in association with an upper level trough transiting the region into the weekend. Some detail and timing differences remain within the global guidance suite, though rain chances appear to be highest on Friday, then more spotty from Saturday into Monday. It does appear that warmer temperatures will return later in the weekend, but likely with cooler conditions near Lake Michigan.

Ratzer

AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 631 AM CDT Tue May 19 2026

Forecast concerns for the terminals...

- Low chance for an isolated shower or storm this afternoon, mainly south of a PNT to VPZ line.

- Breezy west-southwest winds through this afternoon with 20-30 kt gusts.

- Period of MVFR ceilings this afternoon with otherwise VFR conditions.

A line of scattered showers continues to meander across northern IL and will continue to do for the next 2-3 hours. While the showers may dissipate prior to reaching northwest IN, a period of light showers/sprinkles cannot be ruled out here. Otherwise, mostly dry conditions are expected for today at the terminals. However, there is still a low chance (15-20%) for an isolated shower or storm to develop this afternoon ahead of the cold front but the better coverage of any showers/storms should be south and east of a PNT to VPZ line. That said, did opt to introduce a PROB30 at GYY for SHRA this afternoon given their closer proximity to the better forcing and instability.

Winds will continue to increase out of the southwest this morning with 25-30 kt gusts expected through early afternoon. A brief period (1-2 hours max) of 180-170 wind directions may occur this morning in the wake of the showers but overall southwest winds should prevail. Gusts are forecast to ease a bit into the 20-25 kt range this afternoon as a plume of MVFR ceilings drifts overhead behind the aforementioned cold front with directions becoming westerly as well. The MVFR ceilings are only expected to last a few hours before the scatter back to VFR this evening where conditions will remain for the rest of the TAF period. Gusts will also diminish further after sunset with generally 10-12 kt winds expected overnight out of the northwest. Though, some occasional upper teen to lower 20 kt gusts could linger through the night. Finally, winds will turn northeasterly early Wednesday morning and remain as such for the rest of the period.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 4 PM CDT this afternoon for the IL and IN nearshore waters.