textproduct: Chicago

KEY MESSAGES

- A pair of storm systems may bring accumulating snow to parts of the area this weekend, favoring areas near and north of the Wisconsin state line Saturday afternoon, and areas well south of Interstate 80 Sunday night. Much of the area may miss out on snow. - A pattern change toward warmer and wetter conditions is expected through the first few weeks of March.

DISCUSSION

Issued at 115 PM CST Fri Feb 27 2026

Through Sunday Morning:

Recent satellite imagery augmented by surface observations depicts a vigorous upper-level shortwave and associated surface low moving through central Ontario. Continued low-level pressure falls ahead of the system is supporting aggressive warm air advection across the Lower Great Lakes, which with mostly sunny skies is resulting in temperatures climbing into the lower 60s. The combination of the warm and dry conditions atop dormant vegetation will continue to support a threat for the spread of brush and grass fires through the afternoon (have noted several fires already in radar and satellite data as of press time).

As the low pressure system moves toward Quebec tonight, a dry cold front will move across the Lower Great Lakes and usher in reality with falling temperatures and breezy northwest winds. Lows will fall toward the upper 20s (northwest) to lower 30s (southeast) as the baroclinic zone stalls across central Illinois and Indiana.

By tomorrow morning, deep cyclonic upper-level flow will be established across the Upper Great Lakes. Ensemble guidance remains in agreement that an upper-level jet streak will nose into the central Great Lakes by mid-morning and excite a frontogenetical circulation within the low-level baroclinic zone. With residual dry low-level air remaining in place across northern Illinois and northwestern Indiana as well as the positioning of the jet to move almost directly overhead (keeping upper-level diffluence maximized to our north), a majority of guidance keeps any measurable snow north of the Wisconsin state line through much of the day on Saturday. So, will cautiously trim PoPs to focus along the Wisconsin border. Highs will vary quite a bit across the area tomorrow, with readings near the freezing mark near the Wisconsin state line to around 40 degrees along I-80 to the lower 50s along US-24.

Tomorrow afternoon and evening, an embedded shortwave embedded within the cyclonic flow aloft will induce the development of a low pressure system within the baroclinic zone somewhere in central Illinois. HRRR/RAP guidance has trended far more aggressive with the speed and strength at which the low deepens, leading to a corresponding increase in flow along the backside of the system as it moves into Indiana after dark. While the upper-level cyclonic shear axis will remain well to our northeast tomorrow evening, the increase in low-level flow and associated shoreline convergence in tandem with falling 850mb temperatures may be able to support low-topped lake effect snow showers Saturday evening especially if HRRR/RAP solutions were to verify. With an easterly component to the wind behind the system, any snow showers may be prone to moving into northeastern Illinois before shifting toward northwestern Indiana Sunday morning. Given the marginal thermodynamics and a lack of support from other guidance, am not quite ready to go all-in on the idea of snow showers developing quite yet but will nevertheless introduce low (20-30%) chances for snow showers along the Lake Michigan shoreline Saturday night through Sunday morning.

Borchardt

Sunday Afternoon Onward:

An expansive Canadian high will slide across the Upper Midwest into the Great Lakes on Sunday. As it does, the next mid-level short wave impulse will track east-southeastward along its southwestern periphery, and along the southern periphery of the low-level baroclinic zone, right into the Mid-Mississippi Valley region by Sunday night. Overall, conditions continue to be quite favorable for this to support a somewhat narrow corridor of notable accumulating snowfall late Sunday into early Monday somewhere across portions of the Midwest eastward into the Ohio Valley. The primary uncertainty for our area is how far north any accumulating snow will extend, especially considering it will be fighting a progressively much dryer airmass with northward extent. Recent ensemble trends have favored areas farther south across central IL for this area of accumulating snow. If these trends continue, little to no snow will fall across northern IL and northwestern IN.

Otherwise, following a chilly period Sunday into Monday (highs in the 30s Sunday and low 40s Monday), a significant large scale weather pattern shift is expected this coming week. The pattern will transition to the negative phase of the Pacific North American (PNA) teleconnection pattern, which is characterized by western CONUS mid and upper level troughing and eastern CONUS ridging. While the day-to-day specifics of this pattern remain somewhat unclear, there is above average forecast confidence that this pattern will favor periods of active weather, including notable rainfall amounts and thunderstorms, along with above average temperatures through next weekend.

Our current forecast does advertise at least some lower end chances for rain each day Tuesday through next weekend. In spite of this, do not expected to be raining constantly, as there will be periods of dry time. Our first favored period of rain is late Monday night into Tuesday in association with increasing warm air advection (isentropic upglide) setting up across a surface frontal boundary downstream of an impulse traversing the CO Rockies. There is a strong signal for rainfall during this period, with roughly 70 to 90+ percent of the ensemble members producing rain across a good portion of the area. Thereafter, ensemble spread increases, particularly with the speed and timing of the the next impulse expected to shift into the lower Great Lakes region sometime Wednesday into Thursday. If the slower solutions advertised by main of the ensemble suites pan out, a good chuck of Wednesday could be dry before rain chances increase again Wednesday night into Thursday.

Regardless of exactly how the mid to late week period evolves, precipitable water of 250-300% of average/normal for early March could certainly spell corridors of heavy rainfall and an associated flooding threat. The extent (or lack thereof) of embedded thunderstorms will likely play a role as well. Finally, the exceptionally mild and moist cyclone warm sectors in the upcoming pattern may also feature a threat for organized strong to severe convection within in the general region.

KJB

AVIATION /00Z TAFS THROUGH 06Z SUNDAY/

Issued at 536 PM CST Fri Feb 27 2026

Forecast concerns include...

Wind shift to northwest this evening. Wind shift to northeast Saturday morning. Low chance for light snow/flurries Saturday. Possible mvfr cigs Saturday afternoon into Saturday evening.

Southwest winds are steadily diminishing with sunset currently and will shift to northwest this evening. Some gusts into the 15-20kt range will still be possible both ahead and behind the front with speeds/gusts gradually diminishing in the predawn hours. Winds will then shift northeasterly Saturday morning. Speeds will increase into the 10-12kt range by afternoon with gusts into the 15-20kt range expected to develop by early Saturday evening.

Snow is expected to remain mainly north of the terminals on Saturday with a low chance for light snow or flurries along and north of a RFD/ORD line. Maintained prob mention at RFD for midday but confidence continues to decrease for the rest of the terminals and removed mention elsewhere.

Cigs are expected to trend down to low vfr by late Saturday morning with at least patchy mvfr cigs possible Saturday afternoon and then a better chance for mvfr cigs Saturday evening, especially for the Chicago terminals and near Lake Michigan, though only medium confidence. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 9 PM CST this evening for Winthrop Harbor IL to Burns Harbor IN.

Small Craft Advisory from 2 PM Saturday to 3 PM CST Sunday for Winthrop Harbor IL to Burns Harbor IN.

Small Craft Advisory until 3 PM CST Sunday for Burns Harbor to Michigan City IN.