textproduct: Chicago

KEY MESSAGES

- Untreated surfaces may be slippery this morning owing to a coating of snow.

- Another clipper system may deliver rain showers Wednesday night into Thursday morning.

- Temperatures will warm toward the 60s to locally lower 70s Friday and Saturday. A cold front will move through the area at some point Saturday night or Sunday.

DISCUSSION

Issued at 108 AM CDT Wed Mar 18 2026

"Streaky" bands of snow continue to stream over the area within the warm-air advection regime of a low-amplitude wave propagating toward the Ohio River Valley. Pockets of dry low- level air have been chewing up a majority of snowflakes on their journey to the ground thus far. With that said, the expectation is for snow to eventually make it down through the early morning hours. Fairly modest forcing (peak lift of only minus 3 to 4 ubar/sec) will limit overall snow accumulations with this system, favoring generally a coating where snow is most persistent.

Snow should end from northwest to southeast toward and especially by daybreak. An initially cloudy start to the day will transition toward partly cloudy (to even mostly sunny) skies. Highs today should top off in the upper 30s near the Wisconsin state line to around 50 near US-24.

Clouds will increase tonight as another low-amplitude waves approaches the Great Lakes. Another bout of low-level warm air advection may act to counteract diurnal cooling (if not encourage temperatures to warm a degree or two though the night), with lows expected to settle in the lower 30s. The next wave will be accompanied by a pocket of modestly steep mid-level lapse rates (near 7K/km), which should allow for scattered showers to develop through the early morning hours of Thursday. Coverage of showers is currently favored north of I-80, though any adjustment in the track of the wave would be subject adjustments to the area of highest shower coverage.

At this point, it appears shower coverage should wane by daybreak Thursday, setting the stage for clearing skies and a warm early spring day. With 850mb temperatures expected to settle near +4 to +5C, afternoon highs on Thursday will have the potential to shoot into the upper 50s to lower 60s. With a weak pressure gradient in place and lake Michigan temperatures in the upper 30s, a strong lake breeze should develop by early afternoon and drive lakeside temperatures downward back into the 40s. (It is the season for "cooler by the lake," after all).

This weekend, the monster upper-level ridge across the southwestern United States will shift eastward, leading to quasi-zonal flow across the Great Lakes. Interestingly, ensemble model guidance is somewhat split with how fast the low-level thermal ridge will expands eastward, with the GFS/GEFS camp decidedly slower than the ECMWF/EPS camp. Such differences have fairly large consequences on the temperature forecast, including the arrival time of a cold front at some point this weekend associated with an upper- level wave propagating within the zonal flow aloft. Regardless, with 850mb temperatures poised to rocket toward +15C, current thinking is at least one day of the weekend (Saturday or Sunday) will have a shot of having high temperatures in the upper 60s to lower 70s before a cold front brings high temperatures back into the 40s and 50s to start next week.

Borchardt

AVIATION /06Z TAFS THROUGH 12Z THURSDAY/

Issued at 1254 AM CDT Wed Mar 18 2026

Key Messages:

- Snow will continue through around sunrise, resulting in IFR/MVFR visibilities.

- A chance for rain is forecasted towards the end of the TAF period.

Snow has spread over the terminals at the start of the TAF period and is expected to continue through the overnight hours before ending from northwest to southeast closer to sunrise. IFR visibilities will occur during the periods of steadier snowfall, and MVFR ceilings will be observed as well. Total snow accumulations are still expected to end up being around 1" or a little less. Still couldn't completely rule out a brief period of freezing drizzle right as the snow comes to an end, but the most likely outcome (~90% chance) is that no freezing drizzle occurs.

Another chance for precipitation will come towards the end of the current TAF period as the next upper-level disturbance dives into the region. Thermal profiles should have warmed up enough by this point for most or all of this precipitation to come down as rain, but it is possible that air temperatures may still be cold enough for this rain to freeze on colder surfaces. The track that the bulk of this precipitation will take is still somewhat uncertain, with about a 35-45% chance at this time for it to occur at ORD and MDW prior to the end of the 30-hour TAF period there. Thus, have elected to highlight this precipitation potential in a PROB30 group for starters.

Southerly winds with gusts to 20-25 kts will gradually diminish over the course of the daytime today while turning southwesterly before becoming light this evening into Thursday morning.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

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