textproduct: Chicago

KEY MESSAGES

- Winter storm watch has been issued for late Friday night through Saturday night for all of northern and central IL as well as extreme northwest Indiana.

- Impactful snow is likely late Friday night through Saturday night. It remains too early for specific snowfall amounts and locations, but there is >60% chance for 6"+ amounts and higher end travel impacts in portions of the area.

- Well below normal temperatures will persist into or through next week, perhaps with additional chances for snow (Monday night).

DISCUSSION

Issued at 314 AM CST Thu Nov 27 2025

Through Friday:

Extensive stratus deck is blanketing the area this morning with plenty more stratus upstream. As is so often the case, guidance is doing an exceptionally poor job handling this wrap around cold air advection stratus, and for this reason have trended toward a much cloudier forecast today into this evening. There will probably be a few breaks in the overcast, but overall mostly cloudy conditions are expected.

GOES IR and low level water vapor imagery both depict a vort max over northwest MN early this morning. RAP seems to have initialized this feature decently and has it zipping southeast to southern WI by this afternoon. Slightly colder cloud tops are evident on IR imagery with this feature and quite a few sfc obs reporting flurries, so certainly seems plausible that there could be some flurries leaking out of the stratus deck this afternoon across northeast IL and northwest IN.

Speaking of northwest Indiana, guidance does show boundary layer winds veering from a west-northwesterly to more of a northwesterly direction this evening in the wake of a weak sfc trough. This could allow lake effect snow showers to build back west into northeastern Porter County tonight with a chance of some minor accumulation even over far northeast Porter County.

Otherwise, look for some subsidence tonight in the wake of the shortwave which could *maybe* encourage some breaking up and dissipation of the stratus deck overnight. If stratus doesn't clear out tonight, it likely will Friday morning with some sunshine expected Friday.

Temps today into this evening should largely flatline under the stratus with only a 2-4F diurnal swing. Assuming stratus does indeed break up tonight, colder temperatures are expected. Sunshine should be pretty ineffective at warming things up Friday with highs still only expected to reach into the low to mid 30s.

- Izzi

Friday Night through Wednesday:

In collaboration with neighboring offices, we have opted to hoist a Winter Storm Watch for all of northern IL and far northwestern IN from late Friday night through early Sunday morning.

The typical uncertainties that go with the forecast of accumulating snowfall at a 48+ hour lead time persist. Nevertheless, confidence continues to grow that much of the area will experience an impactful accumulating snowfall late Friday night through Saturday night. At the present time, there is a 60%+ chance of snow amounts exceeding 6" across a vast majority of the area. Admittedly, even if snow amounts underperform with this event, the threat of hazardous travel is still very likely during the busy holiday weekend. Accordingly, we have opted to pull the trigger on a watch.

A strong short-wave trough off the WA/OR coast this morning will come ashore later today into tonight. Thereafter, in response to amplifying ridging into the Gulf of Alaska, this impulse will dig into the central and southern Rockies and foster lee surface cyclogenesis near the OK Panhandle into Friday evening. The surface low will then track east- northeastward across the lower MO Valley and over the the mid MS Valley region Saturday into Saturday evening.

The combination of increasingly diffluent upper-level flow within the entrance region of an upper-level jet over the Great Lakes and the onset of robust and deep low-to mid-level WAA/isentropic upglide will foster the development of a sizable precipitation shield (primarily in the form of snow) across the Midwest Friday night into Saturday. Across our area, the onset of snow should occur sometime after midnight Friday night. Once it begins, widespread accumulating snow is a good bet through the day Saturday. Peak snow rates during this event could end up around, or a bit over an 1" per hour. At the present time, these higher rates look to be most favored sometime from late morning through early Saturday evening. Ultimately, however, this time window will be dependent on the actual speed of the system, which is still not entirely clear, so this general timeframe could change. Snow rates should gradually diminish Saturday night into early Sunday morning as the main storm system shifts into the the eastern Great Lakes.

As can be expected, assessing total snowfall amounts remains a challenge with this system owing to the magnitude of QPF and variability in snow ratios (exact track of the low), and the potential for some areas (particularly in my far south) to mix with rain Saturday evening. Overall, snow ratios continue to favor around the typical 10-13:1. However, across my far southern areas (Livingston, Ford, Iroquois, Newton, Jasper and Benton counties) there remains a play where enough warm air gets driven northward ahead of the surface low into Saturday evening to allow the snow to mix with or change to rain, or at very least result in lower liquid to snow ratios for a period. If this occurs, total snow amounts in this region would be limited (at least to some degree) compared to areas that remain all snow farther north. It is recommended to monitor for forecast updates and, if necessary, consider adjusting travel plans.

The main snow shield should quickly exit to the east Sunday morning, leaving a period of strong CAA and flurries from shallow stratocumulus. Conditional on the presence of the fresh snowpack over the weekend, clearing skies and diminishing winds with an approaching surface ridge will promote a rather chilly Sunday night with lows potentially at or below zero across interior northern Illinois. Very cold conditions will persist through Tuesday as a reinforcing shot of colder air arrives on Monday. A mid-level wave crossing the Ohio River Valley late Monday afternoon and Monday night may brush southern portions of the forecast area with light snow. Sub-zero low will again be possible Monday night if the aforementioned system remains solidly south of the area.

KJB

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/

Issued at 534 AM CST Wed Nov 26 2025

- Gusty WNW winds will continue, with occasional gusts to 30kt possible today.

Gusty WNW winds will continue today and probably into this evening with occasional gusts to around 30kt today. Expecting mostly BKN-OVC conditions to continue into this evening, though a hole in the stratus has opened up recent to the west of the terminals which could result in a period of partial clearing this morning. Can't rule out a few flurries this afternoon into early this evening, but not expecting any meaningful impacts to VSBY. Stratus is expected to break up tonight and once CIGs break up, winds should respond by decreasing and losing their gusts.

- Izzi

MARINE

Issued at 314 AM CST Thu Nov 27 2025

Winds have eased some over the lake since earlier Wednesday evening, however gales are still occurring over the open lake and into the Indiana nearshore waters, especially east of Gary. With the Chicago Crib no longer reporting gales and with winds expected to very slow ease today, seems the gale threat for the IL waters has diminished and planning to downgrade to a small craft advisory there. For the Indiana water with a longer fetch, no changes planned to the gale warning, but certainly possible the day shift may be able to transition to a SCY earlier than the current 03z Friday expiration time.

LOT WATCHES/WARNINGS/ADVISORIES

IL...Winter Storm Watch from late Friday night through late Saturday night for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008- ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023- ILZ032-ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107- ILZ108.

IN...Winter Storm Watch from late Friday night through late Saturday night for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...Small Craft Advisory until 4 AM CST Friday for the IL nearshore waters.

Gale Warning until 10 PM CST this evening for the IN nearshore waters.