textproduct: Chicago

KEY MESSAGES

- There is a 20 to 30% chance for isolated thunderstorms south of a Gibson City to Rensselaer line late tonight. IF a storm forms it would be capable of producing hail.

- There is a low (5 to 15%) chance for severe thunderstorms on Thursday, primarily during the afternoon and early evening hours.

- Temperatures will tumble tomorrow night into Friday. A rain/snow mix may graze the Illinois and Wisconsin state line.

- Another period of strong winds is expected on Friday, with southwesterly wind gusts in excess of 40 mph.

UPDATE

Issued at 1008 PM CST Wed Feb 18 2026

Hi-res model guidance over the past several runs continue to develop isolated thunderstorms overnight tonight near and south of a Gibson City to Rensselaer line during the ~2-5 AM CST timeframe coincident with an axis of elevated instability that lifts into those areas. Given the persistent signal, opted to introduce 20-30% shower and thunderstorm chances to the forecast. Confidence in storms initiating remains on the lower side given rather subtle forcing mechanisms for ascent. However, if storms do manage to develop they would be capable of producing hail in addition to lightning.

Looking ahead to tomorrow there has been a noted southward trend with the track of the surface low compared to previous model runs. This could support a southward shift in the greater severe weather threat farther south as well, closer to central IL/IN. In fact, shower and isolated embedded thunderstorm development could hold off until late afternoon/early evening for areas north of I-80. Confidence is not high enough to make big messaging changes with that just yet though and will let the overnight shift get one final look at things in case things trend back north.

Petr

DISCUSSION

Issued at 245 PM CST Wed Feb 18 2026

Through Friday:

A stout 990 mb surface low continues to traverse across MN and the northern Great Lakes which has been generating gusty west- southwest winds over northern IL and northwest IN. As the core of the low moves into WI over the next couple of hours winds should peak with gusts around 45-50 mph especially for areas along and north of I-88 where a Wind Advisory remains in effect through 6 PM this evening. In addition to the gusty winds, dew points have continued to tank behind a dry-line resulting in 15-25% RHs across the area. These conditions are making conditions very favorable for rapid grass and brush fire spread which is why the Red Flag Warning has once again been expanded to include all of northern and northeast IL counties through 8 PM this evening. Regardless if your area is under a Red Flag Warning any burning should be postponed and extreme caution should be used when discarding any burning materials until conditions improve.

Heading into this evening, the gusty winds will subside after sunset and become light and variable as a lull in the pressure gradient moves overhead. With the lack of cloud cover, temperatures will cool rather quickly and dip into mid 30s to lower 40s which will further improve RHs for tonight. That said, our attention will turn towards the shortwave trough that is ejecting out of the Four Corners and into the Central Plains. The shortwave is expected to be lifting into the mid-Mississippi Valley by Thursday morning with another surface low forecast to develop over northern MO that will then lift across eastern IA and northern IL Thursday afternoon. As the shortwave begins to enter the Mississippi Valley a couple of subtle impulses look to break off the wave and drift across IL and IN early Thursday morning. While moisture locally continues to look limited, a rather stout EML is forecast to be overhead as these impulses move through which could be sufficient to kick off some isolated to widely scattered showers and thunderstorms Thursday morning. Given that confidence on any shower and/or storm developing in the morning is low, have opted to largely maintain a dry forecast for this period aside from some 15-20% POPs east of I-55 where a plume of slightly better moisture is forecast. However, if any storms due materialize the 40-50 kts of deep layer shear and 7-8C/km lapse rates could result in a strong storm or two capable of hail and maybe some gusty winds.

The better coverage of showers and thunderstorms on Thursday are expected to develop in the afternoon and evening as the aforementioned surface low pivots overhead and allows a surge of moisture to spread into the area. Though exactly how far north this moisture will get remains a point of uncertainty and therefore the coverage of storms with northward extent also remains uncertain. Regardless, an arc of scattered showers and storms should develop and lift across the southern half to 2/3rds of the area Thursday afternoon. With deep layer shear still expected to be around 40-50 kts, a few of these storms could become strong to severe especially south of the US-24. Any severe storms will be capable of damaging winds and hail with some possibly even being able to produce tornadoes if sufficient low-level instability is realized. As a result the Storm Prediction Center has expanded their marginal (level 1 of 5) risk across most of northern IL and northwest IN with a sliver of the slight (level 2 of 5 risk) along US-24.

Showers and storms will quickly move east of the area Thursday evening as the system's dry slot moves into the region. However, some precipitation associated with the deformation band on the backside of the low is forecast to drift across portions of northern and northwest IL late Thursday night into Friday morning. With temperatures expected to be quickly cooling behind the cold front precipitation should transition to snow by Friday morning. That said, surface temperatures are forecast to be generally in the lower to mid-30s which should limit overall snow accumulations to maybe a half inch with the highest amounts forecast to occur near the IL-WI line. Outside of the precipitation, winds will also become gusty as the low lifts into MI with west-northwest gusts around 30-40+ mph expected. Since the NBM gusts were only around 20-25 mph did opt to increase them into the 30-35 mph range for now, but suspect our current forecast is still too low and will need to be adjusted up closer to the 40-45 mph range with time.

Yack

Friday night Onward:

This weekend, somewhat disorganized upper-level cyclonic flow (with an embedded orphaned ridge "bubble") will be in place across the Great Lakes region. With all embedded shortwaves more or less far from our area on Saturday, relatively benign weather is in store locally with seasonable highs in the mid to upper 30s, northwesterly winds of 10 to 15 mph, and partly cloudy skies.

On Sunday, several upper-level shortwaves will merge and dive southeastward across the Great Lakes. Tumbling 850mb temperatures toward the minus 10 to minus 12 range will lead to a corresponding drop in temperatures, with highs only expected to top off near the freezing mark. Increasing over-water instability will support lake effect snow showers as well, with predominantly northwesterly flow favoring northwestern Indiana. Sunday night looks like it'll be relatively chilly with a continuation of breezy northwesterly winds and overnight lows in the teens contributing to wind chills falling toward the single digits.

Lake effect snow may continue into early Monday morning before the cyclonic shear axis of the upper-level cyclonic flow slides overhead. Ensemble guidance continues to exhibit a signal for broad quasi- zonal flow to become reestablished next week allowing for a steady feed of relatively mild Pacific flow to return to the area. Accordingly, the expectation is for temperatures to warm into next week (back into the 40s). Ensemble 24-hour meteograms from the EPS and GEFS favor the next opportunity for widespread precipitation materializing sometime toward the end of next week.

Borchardt

AVIATION /06Z TAFS THROUGH 12Z FRIDAY/

Issued at 1053 PM CST Wed Feb 18 2026

Forecast concerns include...

Chance of thunderstorms Thursday afternoon/evening. Ifr/low mvfr cigs possible Thursday. Ifr, possible lifr cigs, Thursday night. Chance of snow predawn hours Friday morning.

Overall, no significant changes to the previous forecast. Scattered showers and a few thunderstorms are expected to develop over east/central IL overnight and move northeast across northern IN through mid/late Thursday morning, mainly southeast of a BMI/IKK/VPZ line. There is increasing confidence with this area and thus maintained a dry forecast for all the terminals.

Showers and isolated thunderstorms are expected to develop Thursday afternoon and continue into Thursday evening, lifting mainly from south to north across the area. While confidence for prevailing showers is increasing, there remains uncertainty for thunder coverage, especially across the terminals. Have maintained prob thunder mention with this forecast, but further refinement can still be expected.

Cig trends remain uncertain for Thursday and Thursday night. There remains the potential for ifr/low mvfr cigs Thursday and this chance appears to be increasing for areas south of the terminals. A surface low pressure will move across the area Thursday night and will likely produce low cigs and possibly fog along its path, which could be near or over the terminals. There is too much uncertainty at this time and maintained the previous forecast for mvfr vis/cigs with scattered ifr cigs.

The track of the surface low will also have large impacts on the wind directions. Light southerly winds will become light and variable overnight then northeast by daybreak and remain east/northeast Thursday. If the low passes north of the terminals, winds will turn southeast then southwest, as is currently expected. However, a more southern track, as a few models now show, would maintain northeast winds. Eventually, winds will turn westerly early Friday morning and steadily increase mid/late Friday morning. cms

LOT WATCHES/WARNINGS/ADVISORIES

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