textproduct: Chicago

KEY MESSAGES

- Southerly winds will become breezy (gusts 30 to locally 40 mph) amidst scattered showers and isolated thunderstorms tonight.

- The combination of dry conditions, gusty west winds, and warm temperatures will lead to a threat for brush fires on Wednesday, especially toward the Wisconsin state line. A Fire Weather Watch is in effect for areas north of I-80 in IL.

- Another storm system will arrive on Thursday accompanied by arcs of showers and thunderstorms. Another push of breezy winds will follow into Friday.

- Temperatures will trend cooler toward or below the average for this time of year Friday and through the weekend.

DISCUSSION

Issued at 228 PM CST Tue Feb 17 2026

Though Wednesday Night:

A broad upper trough continues to spiral over the western CONUS with a leading shortwave trough starting to eject into the central Plains. Ahead of the shortwave a surface low is developing over the Dakotas and northern NE with an associated frontal boundary stretching from the low eastward to southern Lower MI. Presently the weather across northern IL and northwest IN is rather tranquil with just some mid-level clouds and mild temperatures in the mid to upper 50s. However, that will change through the evening as the aforementioned shortwave and surface low pivot eastward into the Upper Midwest and Great Lakes. As this occurs the frontal boundary in far northern IL will begin to lift into WI with as an area of rain develops along it. While this rain will likely stay well north and east of our area tonight, some scattered rain showers are expected to develop after 9 PM ahead of an approaching dry-line and linger through the early morning hours on Wednesday. Forecast soundings across northern IL and northwest IN tonight continue to show some steep lapse rates developing overhead (6-8C/km) which could result some embedded thunderstorms especially after midnight. Though, with moisture expected to be a limiting factor (dew points only around 50F) coverage of any thunderstorms looks to be very hit and miss (20% chance of occurrence). Outside of the rain, winds will also increase through the night with gusts around 25-30 mph expected.

Heading into Wednesday, the aforementioned dry-line will move through the area and allow any lingering rain to quickly conclude. In the wake of the dry-line dew points will also quickly tank into the lower 20s (possibly even the upper teens in places) which in combination with temperatures warming into the mid 60s will result in RH values around 15-25%. These conditions in combination with gusty west-southwest winds around 30-40 mph will result in an elevated fire danger Wednesday afternoon. While meteorologically these conditions are very close to our local Red Flag Warning criteria, recent observations from fire partners indicate that fuel moistures vary across the area and may be more of a limiting factor. Therefore, we have opted to issue a Fire Weather Watch for our IL counties north of I-80 where confidence is highest in the highest winds and lowest RH values being observed for now. Regardless, any burning Wednesday should be postponed and extreme caution should be used when using or discarding any burning materials.

The gusty winds will gradually subside after sunset Wednesday evening as mixing ends. Dew points will also slowly recover Wednesday night as another shortwave ejects into the central Plains with cloud cover expected to increase as well. While the clouds should limit the cooling of temperatures Wednesday night, overnight lows are still forecast to dip into the upper 30s and lower 40s.

Yack

Thursday into Early Next Week:

Wednesday night, a series of upper-level shortwaves (currently dropping southward along the Pacific Coast) are expected to eject east-northeastward across the Rocky Mountains and induce the development of a low pressure system in the central Plains. Ensemble model guidance remains in fair agreement that the center of the low pressure system will then track from near Kansas City, Missouri Thursday morning toward Davenport, Iowa Thursday evening. Provided coverage of any early showers and thunderstorms is kept to a minimum (more on this in the next paragraph), afternoon high temperatures in our area on Thursday will be poised to rocket back into the mid 60s, some 20 to 25 degrees above the average for this time of year.

The continued eastward advection of an EML plume emanating from the high plains, increasing low-level warm air advection, and the arrival of the first of several shortwaves may spur the development of isolated to scattered showers and thunderstorms as early as daybreak Thursday across parts the area. Coverage of showers and storms should increase within northeastward-moving arcs during the afternoon and evening hours as the primary shortwave arrives. As can be expected 48 to 72 hours in advance, it's tough to have confidence in the exact structure, location, and arrival time of what will probably be a somewhat narrow surface-based warm sector to the east of a rapidly-advancing dryline and/or pre-frontal trough along the southeastern flank of the low pressure system. With that said, any overlap of the incoming upper-level jet (to support deeply-sheared kinematic profiles) and really any instability warrants watching for supercell structures (including during the morning hours). Stay tuned.

Behind any convective activity Thursday evening, an initial push of southwesterly winds will be overcome by the cold front toward daybreak Friday and an associated increase in westerly winds. A building 1020mb surface high pressure system in the central Plains will facilitate respectable pressure rises on the order of 10mb/6hr, supporting gusts in the 30 to 40 mph range during peak heating on Friday. If higher-end scenarios within the ensemble envelope were to materialize (deeper low pressure system, stronger high pressure system), will have to increase gust magnitudes beyond our current forecast (EPS mean gusts are closer to 45 mph). Stout cold air advection will also mark the (temporary?) end of the recent stretch of much above-average temperatures, with afternoon temperatures on Friday only poised to reaching the upper 30s (northwest) to mid 40s (southeast).

Forecast confidence lowers from this weekend onward as the door opens for the complex interaction of several upper-level shortwaves across the general Great Lakes region. With that said, temperatures this weekend look to remain near or below average. Ensemble model guidance hints at the cool-down being somewhat short-lived as an upper-level low stalls off the Pacific Coast and establishes another southwest flow regime across the central US to close the month.

Borchardt

AVIATION /18Z TAFS THROUGH 00Z THURSDAY/

Issued at 1124 AM CST Tue Feb 17 2026

As a synoptic-scale cyclone moves into the Upper Mississippi River Valley this evening, increasing low-level warm air advection will advect a region of IFR to MVFR (800 to 1200ft) cigs currently across southern Illinois northward and over the terminals (arrival time in the 02-04Z window). Further intensification to the isentropic ascent should eventually support scattered showers as well, with perhaps the best opportunity tied to the eastward advection of a plume of steep mid-level lapse rates (focused on the 04-10Z window). With forecast equilibrium levels for elevated parcels expected to exceed minus 20C, the expectation is for isolated lightning strikes within the scattered convective activity though confidence in thunder being observed at any given terminal remains lower than 30% (so no formal mention will be introduced in the outgoing TAF package). Coverage of showers should taper by daybreak and cigs should erode from west to east by mid- morning as the dry slot of the cyclone races into the area.

In terms of winds, generally light (10kt or less) southeasterly flow will continue through the afternoon hours. Wind direction should turn toward the south-southeast toward sunset as both sustained and gust speeds increase in tandem with the aforementioned intensifying warm air advection. Gusts of 25 to 30 kt should be common during the overnight hours. After daybreak, the rapid drying of the column in tandem with rising mixing heights will tap into the strong low-level wind field on the southern side of the cyclone, turning winds southwesterly and further increasing speeds. An ensemble of mixing profiles supports prevailing gusts in the 30 to 35kt (locally 40kt at RFD) from mid-morning through the remainder of the TAF period.

Borchardt

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from midnight tonight to 6 PM CST Wednesday for the IL and IN nearshore waters.