textproduct: Chicago

KEY MESSAGES

- A band of lake effect snow will migrate eastward from NW IN through early afternoon. There is a low to medium (40 to 60%) chance for a slushy 1 to 3 inches of snow to accumulate with the band of snow, mainly in northwestern Indiana inland from the lake by at least 5 to 10 miles.

- A period of dangerous and intense lake effect snow is expected tonight into Monday in northeastern Illinois and northwestern Indiana. The peak coverage of lake effect snow should be between 12 AM and 10 AM Monday. Travel is not advised during periods of lake effect snow.

- A gradual warming trend, possibly accompanied by periods of breezy winds, is expected toward the middle of next week.

UPDATE

Issued at 1107 AM CST Sun Nov 9 2025

Not planning any major changes with the morning update this morning, as I would like to run through the incoming 12Z guidance a bit more.

Currently, some lingering light snow continues to fall across the immediate Chicago area and points southward into Iroquois County. Farther east, a lake effect band of rain and snow has organized within the convergence zone along the spin of Lake Michigan. This activity is currently oriented into Porter and parts of northern Jasper Counties in NW IN. Surface observations and web cams indicate that this activity is mainly in the form of rain right near the lake shore and along the I94 corridor, where temperatures are hovering around 40 degrees. Several miles inland (10-15 miles), slightly cooler temperatures are supporting all snow, with webcams in and around Valparaiso, IN showing some slushy accumulations starting on paved surfaces. While slushy accumulations up to an inch or two will remain possible on grassy surfaces inland from the lake in northwestern IN through early afternoon, we are not anticipating any significant impacts through the daylight hours today. Expect this area of lake effect precipitation to gradually shift east of Porter County this afternoon as the flow over the lake backs northwesterly.

The forecast for tonight into Monday morning remains largely unchanged from previous thinking. A robust lobe of the Polar Vortex continues to drop southward into the Upper Midwest this morning, and all indications continue to point at it shifting right over IL this evening and overnight. Of note, is the 12 RAOB out of KINL, which sampled a 500 mb temp of -35C. Similarly, KMSP and KGRB sampled -33C and -31C, respectively. As this impulse drops south into our area this evening, expect steepening lapse rates to foster snow showers and flurries across much of the area. However, of most concern continues to be the intense lake effect snow expected with, and in the wake of, a southward moving mesolow on the Lake tonight. Expectations continue to support the initial band of snow with this mesolow to come inland over northeastern IL and northwestern IN in the 10PM-12AM timeframe. Lake effect parameters continue to be exceptional, and will thus support heavy snow rates (2-3+" per hour) within this band. Thunder and lightning is also likely to accompany this activity, and the combination of locally strong winds could result in periods of very low VSBY (<1/4 mile).

This initial arcing band of snow should penetrate well inland, particularly across eastern IL along and east of I-55 and I-355, but potentially as far inland as the western suburbs into the Fox Valley. Following this initial arcing band of snow, which may last a couple hours overnight, it appears likely that a trailing north-northeast to south-southwest single intense band of lake effect snow will set up and point right into the northeastern IL shore overnight. The most intense part of this band should become more localized, likely resulting in significant variability in weather conditions over just a few miles. There also seems to be support for this band to become somewhat progressive into early Monday morning, as it shifts eastward along the southwestern shore of the lake, and likely into northwestern IN by mid to late morning on Monday. While this movement may help limit total snow accumulations from reaching the higher end amounts forecast, the heavy nature of the snow could still result in near double digit total amounts in some local areas that fall under the most intense bands. These extreme totals just end up being of a lower foot print then our current deterministic forecast advertises.

While we are not planning any major changes to the forecast or headlines this morning, we will continue to consider if Lake County IL needs to be converted to a warning.

KJB

DISCUSSION

Issued at 310 AM CST Sun Nov 9 2025

Through Monday:

Through this evening:

First thing is first. The low pressure system responsible for bringing rain and snow to the area will continue shifting eastward this morning allowing for precipitation to end from west to east. The exception will be near Lake Michigan as a band of lake effect snow (already extending across Cook County) gradually pivots east- southeastward through the morning hours. As low-level convergence increase down the spine of the lake after daybreak, the intensity of the lake effect snow band should similarly increase (roughly as the band is moving from northeastern Illinois into northwestern Indiana). As a result, do have concern that a quick 2-3" of snow may fall across parts of Lake and Porter counties between 8 and 11 AM CT. We will issue an SPS for this period to cover the threat for slick spots developing on untreated roadways.

By early afternoon, the lake effect snow band should be east of our area. Cooling low-level thermal profiles within pockets of stratocumulus clouds look to be supportive of occasional flurries throughout the day, with highs only topping off in the low to mid 30s. A few bonafide snow showers may develop by early evening as well, though little to no snow accumulation is expected. Attention then turns toward the threat for unusually intense and dangerous lake effect snow tonight and into Monday.

DANGEROUS LAKE EFFECT SNOW EXPECTED TONIGHT INTO MONDAY: Water vapor imagery depicts the leading edge of an early- season lobe of the Polar Vortex diving southward out of western Ontario and into the Upper Mississippi River Valley. Of particular interest as an embedded shortwave anchored by unusually cold 500mb temperatures of -40C, which is moving into far northern Minnesota at press time. Ensemble and deterministic model guidance has converged in the path of the embedded shortwave diving almost due southward from central Wisconsin and into central Illinois this evening (much in line with what the EPS has advertised all along).

As a direct consequence of the path of the shortwave moving along and just west of the Lake Michigan shoreline and the unusually cold low-level airmass spilling into the Great Lakes, a mesolow originating off Lake Superior currently traversing the eastern Upper Peninsula of Michigan will intensify while dropping into northern Lake Michigan later this morning. Once it reaches the Manistee/Ludington areas this afternoon, it will further intensity and cause the band of lake effect snow from earlier this morning positioned near the western Michigan shore this afternoon to pivot back southwestward, reorienting in shape to almost like a southwestward-moving squall line. Increasing northeasterly flow across Lower Michigan will then cause the northwest-to-southeast band to accelerate and crash into northeastern Illinois and northwestern Indiana tonight, reaching the shoreline sometime in the 9 pm to 12 am timeframe. Note that this leading band of snow will be spatially wide and penetrate far inland, spreading southwestward possibly as far west as I-39 and as far south as central Illinois during the pre-dawn hours of Monday.

After the leading band of lake effect snow, a trailing north- to-south oriented band of intense lake effect snow is expected to move into northeastern Illinois (should be established by daybreak Monday). Perhaps the only lingering item of forecast uncertainty is exactly where the trailing band of lake effect snow will begin, with guidance depicting scenarios of the band starting as far north as Waukegan to as far south as Calumet Harbor. With that said, there is very strong consistency in HRRR/RAP/EPS data that the band will impact Cook County. With time, the band of lake effect snow should shift southeastward in response to gradually developing northwesterly steering flow from Wisconsin, moving down the lakeshore into northwestern Indiana by mid-Monday morning. The band should then pivot eastward toward Michigan City by Monday evening. The main time windows for the intense lake effect snow band are from daybreak to 12 PM in Illinois, and then 10 AM through 6 PM in northwestern Indiana. Note that the degree of lake instability (more on this soon) will allow the band to maintain intregity surprisingly far inland, with accumulating lake effect snow reaching as far south as Iroquois, Newton, and Jasper counties (behind the initial burst).

As has been discussed in previous forecast discussions, the magnitude of lake instability available tonight into Monday is incredible and more typical of environments over Lake Erie. Forecast BUFKIT profiles over the southwestern shore of Lake Michigan tonight depict 850/700mb temperature differentials of 25/35C, lake-induced CAPE in excess of 1,500 J/kg, and effective equilibrium levels nearing 25,000 feet. As a result, the expectation is for the lake effect snow bands to be unusually intense with peak snow rates of 3 to locally 5 inches per hour, frequent lightning and thunder, and pockets of hail. Even with the expectation for the snow bands to be relatively transitory, the unusually intense snow rates will render snow clearing measures ineffective leading to dangerous travel conditions. If any band were to park over the same area for several hours (which may very well happen in Cook county at around daybreak Monday), travel may become impossible with the potential for stranded cars during the rush hour commute. Needless to say, this may become a very dangerous and life threatening situation beneath the most intense and stationary lake effect snow bands. Travel is not advised beneath the lake effect snow bands.

By the time the lake effect ends on Monday, total snow amounts will vary considerably across short distances. Where the snow bands are most intense, total snow accumulations of 12 to 18 inches are expected with a ceiling of 2 to 4 feet if snow bands end up stationary. This threat appears to be most pronounced over Cook county, but may extend to Lake County Illinois and Lake County Indiana, as well. Outside of Cook County, the highest snow totals are expected east of I-355 in the Chicago area, east of I-57 south of the Chicago area, and north of the Kankakee River Valley in northwestern Indiana.

With all of this said, and in deference for the potential for locally life-threatening travel conditions, opted to go ahead and upgrade the Winter Storm Watch to a Winter Storm Warning. Winter Weather Advisories have been issued in counties bordering the Winter Storm Warnings. Finally, did issue a Winter Storm Watch for Lake County, Illinois, to cover the scenario where the trailing lake effect snow band starts further north than currently expected. The start/end times of the products do have buffers in case the lake effect snow arrives early/stays late.

Borchardt

Monday Night through Saturday:

With persistent NW flow, lake effect snow showers may linger in northwest Indiana through Monday Night, but generally taper off toward daybreak. Cloud cover will spread in Tuesday morning as an upper level ridge builds to the west. A weak short wave trough is expected to move across the western Great Lakes. Given the wave's expected transect farther northeast, most of the forecast area should receive no precipitation, but there is a slight chance for areas near the lake expedience snow showers. Surface to 850 mb winds are expected to slowly turn to the southwest and allow for better warm air advection to raise temperatures through the morning. This could allow for a change in precip type to sleet and rain through the late morning. Given model wet bulbs are still below freezing and some uncertainty in temperature trends/timing, no changes were made to messaging the chances for snow that morning but will need to be looked into in later forecasts. Additionally, winds look to remain fairly breezy on Tuesday and again on Wednesday.

Dry and quiet conditions are expected for the end of the work week. Temperatures will slowly return to seasonable norms, if not slightly above. Models are still suggesting the potential for another wave to move over the area next weekend. Given the uncertainty on timing no changes were made to the chance PoPs that the NBM provided.

DK

AVIATION /18Z TAFS THROUGH 00Z TUESDAY/

Issued at 1148 AM CST Sun Nov 9 2025

Forecast concerns include...

Heavy Thundersnow late this evening/early overnight. Band of heavy lake effect snow early Monday morning. Blowing snow overnight/Monday morning. Gusty north/northwest winds through the period.

A band of light snow has developed across far northeast IL late this morning, including at ORD. This is expected to be short lived but flurries and a passing snow shower will remain possible through early this evening.

A period of heavy lake effect snow is expected late this evening into the early overnight hours and the latest guidance suggests this will arrive a little faster than previously expected and have adjusted timing accordingly. Instability over Lake Michigan will be extreme with a few hour period of very heavy snow expected along with the potential for thunder. This snow will move inland and weaken in intensity with visibilities still under 1sm possible at DPA. Light snow will likely remain prevailing behind this first burst of lake effect snow, but then a longer, more narrow band of lake effect snow will develop and may move across ORD and then MDW. Confidence is only medium for this second period/band of lake effect snow and opted to maintain prob mention at ORD and tempo mention at MDW. Further refinement and changes can be expected for this time period with later forecasts. The lake effect snow is expected to move east into northwest IN by mid morning Monday and east of GYY by late Monday morning.

North/northwest winds will remain gusty through the period, in the mid/upper 20kt range, slowly diminishing Monday afternoon with gusts then ending Monday evening. Wind directions may turn more to the north or north/northeast overnight at the Chicago terminals, after the first period of snow. Confidence is low for specific wind directions, but opted to include this more northerly shift with this forecast.

The snow is expected to become drier through the night into Monday morning and the gusty northwest winds should allow for some blowing/drifting snow, especially when snow is falling. cms

MARINE

Issued at 310 AM CST Sun Nov 9 2025

Periods of north to northwesterly gale force winds to 35 kt are expected through Monday as winds funnel on the western side of periodic bands of lake effect snow. A Gale Warning is in effect until Monday evening. Waterspouts are possible where bands of lake effect snow are prevalent.

NWS Chicago

LOT WATCHES/WARNINGS/ADVISORIES

IL...Winter Storm Watch from 9 PM CST this evening through late tonight for ILZ006.

Winter Weather Advisory from 9 PM this evening to noon CST Monday for ILZ013-ILZ106-ILZ107.

Winter Storm Warning from 9 PM this evening to noon CST Monday for ILZ103-ILZ104-ILZ105-ILZ108.

Winter Storm Warning from midnight tonight to noon CST Monday for ILZ023.

Winter Weather Advisory from midnight tonight to noon CST Monday for ILZ033.

IN...Winter Storm Warning from 9 PM this evening to 6 PM CST Monday for INZ001-INZ002.

Winter Weather Advisory from midnight tonight to 6 PM CST Monday for INZ010-INZ011.

LM...Gale Warning until 6 AM CST Monday for the IL and IN nearshore waters.