textproduct: Chicago

KEY MESSAGES

- A clipper-like system will lead to a coating of snow tonight through Wednesday morning. Untreated surfaces may become slippery.

- Another clipper system may deliver rain showers on Thursday.

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

DISCUSSION

Issued at 134 AM CDT Tue Mar 17 2026

Outside from one last narrow (1-mile wide) convective-like band of snow moving through northern Illinois and northwestern Indiana early this morning, the blowing snow event has largely come to an end. Gradually clearing clouds from the north, continued low- level cold air advection, and a fresh snowpack (especially across northwestern Illinois) will set the stage for a chilly morning. Temperatures at daybreak are expected to range from the low single digits northwest to the low to mid teens southeast. When combined with the wind, it will feel more like minus 10 to around 0 out there. Bundle up when heading out the door this morning!

Today looks relatively quiet in the weather department. A surface pressure ridge currently centered west of the Mississippi River will shift eastward today, leading to winds deceasing in magnitude and shifting in direction to be out of the southwest. While a few clouds may linger south of I-80 at sunrise, most areas should have a sunny start to the day. A relatively cold start and an increase in upper- level clouds this afternoon will set the stage for an unseasonably cool day with highs generally in the 20s.

Tonight into Wednesday the first half of Wednesday, a clipper- like system will race from the northern Plains to the Great Lakes. A region of low-level warm air advection on the south side of the clipper system will support the development of a broad region of snow, which will track more or less directly over our area tonight. Relatively modest forcing (peak omega of minus 4 to 5 ubar/sec) points toward a 4 to 8 hour period of relatively modest snow rates (0.1 to 0.2"/hr), leading to one half to perhaps just over an inch of snow across the area by the time snow tapes between daybreak and noon. Snow character will start somewhat fluffy with ratios near 20:1 before the warming low-level thermal column trends ratios closer to 10:1 through the event. In the absence of meaningful wind (southerly gusts perhaps intermittently hitting 20 mph), the snow tonight through early Wednesday should largely be a slippery travel threat on secondary or untreated roads. Clearing skies, warming temperatures to the upper 30s to mid 40s, and the increasingly strong March sun angle should help melt most of the snow during the afternoon.

Thursday carries above average forecast uncertainty as ensemble guidance advertises another clipper system moving through the general region though with spread on the track and quality of moisture. Outcomes in which the clipper moves directly overhead with quality moisture are characterized by rain showers and highs limited to the upper 30s to perhaps lower 50s, warmest southwest. Meanwhile, outcomes in which the clipper misses our area entirely to the north would support mostly sunny skies and highs racing into the mid 60s. We'll take the middle ground approach for now and advertise low (10- 30%) chances for rain showers, and highs in the mid 40s to low 60s.

Toward the end of the week and into the weekend, the incredibly anomalous upper-level ridge responsible for record- breaking heat across the southwestern United States will shift eastward, leading to a warming trend in the Great Lakes. Ensemble mean highs Friday and Saturday soar into the 60s and even lower 70s (though we'll have to keep an eye on a backdoor front which may drop temperatures in northeastern Illinois and northwestern Indiana Friday afternoon). Thereafter, still am seeing a signal for a strong upper-level shortwave to drive a cold front through the Great Lakes on Sunday, leading to cooler temperatures (upper 40s to lower 50s?) to start next week. With that said, ensemble guidance supports temperatures returning to above- average values by the middle of next week.

Borchardt

AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 646 AM CDT Tue Mar 17 2026

Key Messages:

- Another period of snow with IFR/MVFR visibilities and ceilings is expected late tonight/early Wednesday morning.

Precipitation-free and VFR conditions are expected to prevail during the daytime today. Still couldn't rule out that MVFR ceilings are briefly attained at the Chicago metro terminals as cumulus clouds initially develop this afternoon before cloud bases lift above 3000 ft AGL, but it's more likely that cloud bases will begin and stay above this level or that low-based cloud coverage ends up remaining FEW or SCT. Winds will gradually back from a west-northwesterly to a south-southwesterly direction over the course of the day and into the evening before prevailing from a southerly direction overnight into Wednesday morning.

Another period of snow is expected late tonight into early Wednesday morning as an upper-level disturbance drops through the region. IFR visibilities are likely to be observed during the steadiest periods of snowfall, and ceilings are likely to drop down to MVFR levels as well. Cold temperatures in the 20s and upper teens will allow for snow to accumulate on all surfaces, though only around 0.5 to 1.5 inches of snow accumulation is expected in total before the snow tapers off around sunrise Wednesday morning.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 10 AM CDT this morning for Winthrop Harbor IL to Gary IN.

Small Craft Advisory until 3 PM CDT this afternoon for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.