textproduct: Chicago

KEY MESSAGES

- A clipper system may produce a few inches of powdery snow accumulation in areas mainly south of Interstate 80 on Saturday.

- Very cold temperatures and bitterly cold wind chills are expected this weekend with wind chills below zero.

- There is a growing signal for a pattern change toward milder (and less snowy) conditions next week.

UPDATE

Issued at 851 PM CST Thu Dec 11 2025

Recent radar imagery augmented by surface observations and webcams depict the northern edge of snow associated with the much advertised clipper system extends from La salle toward Cissna Park, IL. North of this line, a wedge of dry low-level air has relegated all radar returns (including a separate band from northeastern Iowa into southwestern Wisconsin) to virga (snow not reaching the ground). Expansive cloud cover is once again holding temperatures in the low to mid 20s.

Snow is expected to continue south of a line from La Salle to Cissna Park through the next 2 to 4 hours, perhaps accumulating up to an inch or two. Will have to watch for snow associated with the aforementioned secondary band currently across northeastern Iowa and southwestern Wisconsin to reach the ground in northern IL over the coming hours, though suspect the wedge of dry low-level air will continue to chew up all the snowflakes before the reach the ground. So, will go ahead and tighten up the gradient in PoPs and snow accumulations to focus on our far south and maintain dry conditions elsewhere through the night.

After midnight, both observational and model trends suggest that at least partial clearing will take place from northwest to southeast. As a result, do suspect temperatures have a better shot of falling tonight compared to last night, especially with northwestward extent. The inherited forecast already has this idea handled well, so no other changes to the gridded database were needed.

Updated products are out.

Borchardt

DISCUSSION

Issued at 156 PM CST Thu Dec 11 2025

Through Friday:

The primary forecast focus continues to center on the next quick moving clipper system expected to produce a 100 to 150 mile wide swath of accumulating snow across portions of the region tonight. Forecast guidance, in conjunction with recent regional radar and satellite observations, continue to focus this swath of snow well southwest of the Chicago metro area, roughly from southeastern IA southeastward through central IL and southern IN. Across this region, a rather robust mesoscale frontogenetic response is expected to develop for a few hour period late this afternoon into this evening along the tightening lower level baroclinic zone. This is likely to result in an even narrower band of moderate to briefly heavy snow rates (0.5 to 1" per hour) for a couple hours early this evening. This will particularly be the case across central portions of IL just to the southwest of my area where a Winter Weather Advisory is in effect.

Snow rates outside of the main band will be much lighter (generally a 0.5" or less per hour), particularly with northeastward extent across LaSalle, Livingston, Ford and Iroquois counties. In these areas, 1 to as much as 3" are possible (highest in the southern portions of the counties). Snow amounts will then tapper quickly with northeastward extent, with little or no snow accumulation anticipated across most of northeastern IL and far northwestern IN. Of the accumulations that occur in my southwest counties, much of them will occur this evening, with rates quickly tapering off overnight. Some occasional flurries and/or light snow showers may persist across portions of the area very into early Friday morning, particularly near the IL Lake Michigan shore. However, lake effect parameters are not expected to be overly impressive, so little in the way of additional accumulation is anticipated.

Conditions on Friday will continue to be largely mostly cloudy. Temperatures will also remain chilly, with highs generally in the 20s to low 30s. Some occasional light snow or flurries may also occur at times, particularly across northern IL during the day as another weak impulse passes over WI. Little if any accumulation is expected with this activity.

KJB

Friday Night through Thursday:

Cold front will move across the CWA Friday evening with much colder air spilling south into the area in the wake of the front. There could be a few flurries lingering into Friday evening, but not looking for any organized or measurable precipitation. Confidence in just how cold temperatures will get Friday night is a bit on the lower end of the spectrum. NBM and global guidance are quite cold with lows ranging from near 0 northwest CWA to around 10 southeast CWA. The heart of the arctic air mass doesn't look to arrive until later Sat or Sat evening, so it's plausible that those low temps are a bit too cold. Didn't make any changes to the NBM at this point since even if the ambient temp is 4-5F warmer it is still going to feel very cold and really won't change the message.

An Alberta Clipper is progged to dive southeast into the Midwest and phase with the tropospheric polar vortex over the Great Lakes Saturday. Medium range guidance the past couple of days has generally been on a gradual shift southward with the axis of accumulating snow with this feature. Despite the trends in GFS/GEFS and ECMWF/EPS, medium range guidance often struggles with phasing and its quite plausible that there could still be some sizable latitudinal changes in where the axis of accumulating snow lays out Saturday. Current guidance would favor our southern CWA south into central IL which is where NBM has the highest pops and accums. Given the potential for this to shift north or further south still, felt chance pops offered up by the NBM up to the WI border looked reasonable.

Secondary push of arctic air is expected in the wake of that clipper with the coldest air mass expected over the area Saturday night into Sunday morning. Surface high looks to remain well west of the region Saturday night, so despite clearing skies, anticipate we'll maintain some wind (10-15 mph). This wind combined with what looks to be widespread subzero air temps should push wind chills into the 15 to 25 below zero range across the area.

Winds will gradually ease during the day Sunday and skies should be clear, but only offering up deceptive sunshine as temperatures should remain very cold. Many areas could struggle to get out of the single digits above zero for highs. Even with easing winds, wind chills during the day Sunday will probably remain near or below zero.

Sunday night's low temps are tricky and will likely depend significantly on the exact location of the sfc high. If the high is overhead or nearby, then a rapid evening temp drop to well below zero could occur, however if the high is already moving off to the east by evening, then developing southerly winds could temper the evening temp fall. Guidance currently suggests that the high will move well east of the area by early Monday morning, so southerly winds should pick up overnight and result in rising temps. The rise in temps will likely be partially offset by the strengthening winds resulting in wind chills remaining bitterly cold.

Medium range guidance continues to suggest a pattern change to a more zonal flow across the country next week. This would tend to keep the colder air bottled up to our north Canada with a return to normal and probably eventually above normal temperatures.

- Izzi

AVIATION /06Z TAFS THROUGH 12Z SATURDAY/

Issued at 1100 PM CST Thu Dec 11 2025

Forecast concerns for the terminals...

- MVFR ceilings to return Friday afternoon.

- Increasing west-northwest winds Friday evening with 20-25 kt gusts expected.

A clipper system continues to pivot through IL this evening which has lead to a band of snow across south-central IL and IN. While the accumulating snow will remain well south and west of the terminals, there is a chance for a period of light snow showers and flurries to occur tonight along and south of a La Salle, IL to Fowler, IN line.

Otherwise, expect light and variable winds to persist through tonight before winds settle into a westerly direction Friday afternoon. Wind speeds will gradually increase into Friday evening as a cold front moves through the area with 20-25 kt gusts expected behind the front. As for ceilings, the 2500-3000 ft MVFR ceilings earlier this evening have continued to erode beneath an area of 10000 ft clouds with most sites now reporting VFR conditions. While a period of MVFR clouds may still ooze in off of Lake Michigan at the Chicago area terminals, it appears VFR conditions should prevail through tonight. However, MVFR ceilings will return ahead of the aforementioned cold front Friday afternoon and should linger through the conclusion of the TAF period.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

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