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KEY MESSAGES

- Friday will be unseasonably warm and breezy with a threat for brush fire spread.

- A narrow band of snow may develop near the Illinois and Wisconsin state line on Saturday.

- A swath of accumulating snow may fall south of Interstate 80 Sunday night. - There is a growing signal for a period of spring-like warmth and periods of showers and thunderstorms during the first half of March.

DISCUSSION

Issued at 109 PM CST Thu Feb 26 2026

This afternoon through Sunday:

A low-amplitude upper-level wave continues to move across the region this morning, and has contributed to broken cloud cover and even a few sprinkles. Temperatures have hence been slow to warm today, though the expectation remains for most areas to reach the lower 40s by mid afternoon. With our area on the western side of a surface high pressure system centered over southern Ontario, winds will remain out of the southwest with speeds generally near or below 10 mph. Clouds should clear this evening as the upper-level wave departs.

Tonight looks fairly quiet. The low-level pressure gradient will gradually reoriented in response to pressure falls across the Upper Great Lakes ahead of a vigorous upper-level clipper system. Have noted a few CAMs hint at the threat for shallow ground fog to develop toward sunrise, though suspect the increasing low-level flow should provide just enough entrainment of dry air into the nocturnal stable layer to prevent any saturation. Overnight lows should fall toward the upper 20s to lower 30s.

Tomorrow, the aforementioned clipper system will race across central Ontario. Mostly sunny skies in tandem with aggressive low-level warm air advection into the Great Lakes ahead of the system (850mb temperatures rising some 6 to 8 degrees C in 24 hours) will boost highs into the low to mid 60s. Upward mixing into the base of a strong low-level jet will also support southwest winds gusting in the 30 to 35 mph range. If more aggressive mixing scenarios play out (HRRR/RAP), gusts could approach 45 mph. With antecedent dry conditions, the more aggressive mixing scenarios would support a threat for brush fire spread. Will forgo the issuance of a Fire Weather Watch given conditions looks like they may flirt, but not definitely meet, Red Flag Warning criteria anywhere for more than a few hour period (conditions currently look less impressive compared to last Wednesday). A cold front will race across the area after sunset, driving temperatures downward toward the low to mid 30s by daybreak Saturday.

Ensemble model guidance is in agreement that the leading clipper system will establish deep cyclonic upper-level flow along the US/Canadian border into the weekend. There remains a signal for an expansive region of upper-level diffluence in the right entrance region of an upper-level jet streak to develop atop the low-level baroclinic zone draped across the Lower Great Lakes on Saturday. As a result, would expect a deep layer of frontogenesis to develop somewhere within the baroclinic zone, supporting an elongated west-to-east band of snow. With a residually dry low-level airmass across the region, snow may struggle to reach the ground except where frontogenesis and lapse rates are maximized. Taken together, the stage will be set for a narrow (perhaps one to two-county wide) band of plowable snow in the general region on Saturday. At this point, ensemble guidance favors the band of snow materializing near the Illinois and Wisconsin border. Also, cannot rule out additional bands of snow, perhaps mixed with rain, developing elsewhere across the region where secondary frontogenetical circulations interact with a plume of steep mid-level lapse rates. So, will paint a gradient in PoPs across the region ranging from likely (>55%) near the Wisconsin border to slight chance (15%) near US-24.

Sunday looks comparatively quieter as a surface high pressure system builds into the Great Lakes. Increasing high-level cloud cover in advance of the next storm system materializing in the central Plains will stunt highs to the lower 30s.

Borchardt

Sunday night onward:

The pattern next week and into the second week of March is starting to look much more active as a parade of weather systems traverse across the CONUS.

The first of these systems is expected to arrive Sunday night into Monday in the form of a shortwave trough that is progged to be moving across the mid-Mississippi Valley. While a surface low is expected to develop with this wave, the low is currently forecast to develop over the southern Plains closer to the surface and mid-level baroclinic zones that are expected to be draped over the Ohio and Tennessee River Valleys. What this means for northern IL and northwest IN is that an arctic high over the Ontario and the eastern Great Lakes will be pushing drier low-level air into the region which may limit the coverage of precipitation locally. That said, the combination of the shortwave moving through and mid-level moisture still looks to be sufficient to generate at least some precipitation, especially over the southern half of the CWA. With temperatures expected to be in the 20s precipitation is expected to fall as snow and result in some accumulations. Given that there is some uncertainty in snow coverage, it appears the better chances for accumulations (40-50% chance of at least 1") will be south of I-80 with lesser amounts expected north. Though, locally higher amounts are possible especially closer to the aforementioned baroclinic zone.

Snow is expected to taper Monday morning as the shortwave and associated surface low progress eastward into the Mid-Atlantic. However, the arctic surface high is expected to remain in place which will keep temperatures on the seasonably cool side with highs in the mid to upper 30s. Heading into Tuesday the more active pattern will start to take hold as the next shortwave trough digs into the western CONUS. While this shortwave is also projected to eject into the mid-Mississippi Valley late Tuesday into Wednesday, guidance varies on how far north the aforementioned baroclinic zone forecast to be over the Ohio River Valley will get. Therefore, there is a scenario in which the bulk of any precipitation Tuesday night and Wednesday remains well to our south in central and southern IL/IN but there is also a chance for at least the far southern part of our CWA to get brushed by some precipitation. For now the 20-40% POPs offered from WPC centered mainly south of I-80 seems reasonable at this range. Furthermore, depending on where this baroclinic zone is may give us some mixed precipitation with this shortwave mainly in the form of rain and snow.

From Wednesday onward the forecast will begin to trend warmer and wetter as broader troughing establishes over the western CONUS. This in turn will generate southwesterly flow across IL and the Great Lakes allowing temperatures to warm above seasonal values (typical highs for early March are in the lower 40s) and closer to temperatures seen in April. In addition to the warmer temperatures, this flow pattern will also favor any shortwaves that break off the main trough to eject towards the Great Lakes which would favor more weather systems in our area. That said, guidance does continue to vary the exact track and timing of any potential weather systems which does make this part of the forecast more uncertain especially when it comes to precipitation coverage and intensity. Regardless, given the ensemble signal for this period of more stormy weather, we recommend keeping a close eye on the forecast as we get closer.

Yack

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 1113 AM CST Thu Feb 26 2026

Forecast concern for the terminals...

- Gusty (25-30 kt) southwest winds Friday afternoon.

Generally quiet weather conditions are expected through tonight with VFR conditions and semi-light winds. While skies are presently BKN to OVC around 5000-7000 ft, these clouds will begin to scatter out from west to east after 19z with mostly clear skies thereafter. Winds will reside around 5-10 kts through tonight out of the southwest before increasing Friday morning as mixing deepens. Expect wind gusts Friday afternoon to be around 25-30 kts with locally higher gusts possible if deeper mixing is achieved.

Yack

..Threat for brush fires on Friday

On Friday, a vigorous storm system will race across central Ontario causing strong southwesterly winds to develop across the Lower Great Lakes. At this point, our forecast favors sustained winds increasing toward 15 mph and gusts ranging from 30 to 35 mph. However, if more aggressive mixing scenarios pan out, sustained winds could exceed 20 mph and gusts could exceed 45 mph. When paired with unseasonably warm temperatures in the low to mid 60s and afternoon minimum relative humidity values of 25 to 30 percent, conditions may approach Red Flag Warning criteria on a localized basis.

At this point, confidence in Red Flag Warning criteria being met across a large area for a prolonged period of time is too low to warrant the issuance of a Fire Weather Watch. However, burning is still discouraged throughout the day on Friday to prevent the uncontrolled spread of brush fires.

Borchardt

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Gale Watch from Friday morning through Friday afternoon for Winthrop Harbor IL to Gary IN.