Marine Weather and Tide Forecast for Gurnee, IL

UPDATE
Issued at 837 PM CST Fri Dec 12 2025

Recent radar imagery across northern Illinois depicts subtle regions of weak reflectivity (generally 20 dBZ or less) ebbing and flowing with corresponding drops in visibility to 4 to 8 miles.
The areas of reflectivity are within a modest plume of relatively higher low-level moisture (dew points in the lower 20s) and beneath a pocket of relatively lower clouds (bases between 800 and 1200 ft), altogether ahead of an arctic cold front approaching the Mississippi River. The 00Z DVN RAOB was nicely timed and placed to sample the modest moisture plume ahead of the arctic cold front, and depicted a roughly 4,0000 ft deep low-level stratus layer with nearly isothermal in-cloud temperatures around -6 to -7C and shear of about 10kt. With the marginal depth of and shear within the stratus layer to support precipitation, have been happy to only see occasional reports of very fine mist across the area this evening considering the relatively warm in-cloud temperatures would otherwise support freezing drizzle.

Going forward, the expectation is for patchy fine mist to continue spreading eastward across the area ahead of the arctic cold front. Even though temperatures are in the 20s across the area, this type of set-up rarely leads to accumulations of ice on surfaces. So, barring any unexpected intensification of the mist, really don't expect much impact this evening wherever fine mist occurs outside of having to flip the windshield wipers a few times if traveling. Once the arctic front sweeps across the area (taking place in the next 3 to 4 hours), in-cloud temperatures will fall quickly leading to a transition of any fine mist to flurries. So, all in all, any precipitation this evening should be non-impactful.

For the clipper system tomorrow... The incoming 00Z guidance is largely unchanged both with the strength and track of the system as described in the Short Term Discussion below. The general idea remains for snow to spread across the area from west to east after daybreak and continue at a steady rate through the evening. Lift with the system won't be particularly impressive (on the order of 10 to occasionally 15 ubar/sec), though the arctic airmass will facilitate a surprisingly deep DGZ (occasionally exceeding 15,000 in depth) affording fluffy snow ratios in excess of 20:1. So, the relatively meager QPF amounts should easily translate to snow rates ranging from 0.25"/hr to locally 1"/hr, lowest near I-80 and highest near US-24. When taken together, the inherited forecast calling for 1" of snow near I-80 and 4 to locally 6 inches of snow near US-24 appears right on the money.

All this talk of snow amounts is a bit of a red herring, as the real problem tomorrow will be the cold temperatures hovering in the single digits to lower teens as snow is falling. Snowfall occurring during such cold temperatures often leads to surprisingly impactful travel conditions, as cold temperatures limit the melting ability of chemicals. Just a few tenths of an inch of snow with air temperatures in the single digits can lead to slick travel conditions, while a few inches of snow can lead to hazardous conditions. While the inherited Winter Weather Advisory area across La Salle, Livingston, Ford, Iroquois, and Benton counties captured where the snow totals should be highest tomorrow, felt it would be prudent to add another tier of counties northward where even just an inch or two of snow in concert with the cold temperatures could lead to major road impacts. So, have added Grundy, Kankakee, Newton, and Jasper counties to the Winter Weather Advisory.

Just to reiterate, the snow amounts tomorrow aren't really the message we want to convey with this upcoming clipper. The problem will be the combination of the cold temperatures and falling snow, which will make travel conditions hazardous. This will be especially true in the now-larger Winter Weather Advisory area.

One final note - am noting incoming HRRR/RAP guidance trending much colder for air temperatures Saturday night with locations along and north of I-88 away from the urban core of Chicago dropping below -10F. Not quite sure how much stock to put into such an idea, given such models have been too aggressive with dropping temperatures in arctic airmasses in the past.
Regardless, the message would remain more or less unchanged with an expectation for bitterly cold wind chills below -20F Saturday night. So, the Cold Weather Advisory is in excellent shape.

Updated products are in the process of being sent.

Borchardt

DISCUSSION
Issued at 305 PM CST Fri Dec 12 2025

Through Sunday night:

An active, wintry period of weather is expected to affect the forecast area through the weekend, with accumulating snow Saturday and very cold temperatures and bitterly cold sub-zero wind chills.

The upper-level pattern continues to feature broad troughing across eastern North America, downstream of an upper ridge along the west coast. An upper level low was also evident over central Manitoba, north of a 150+ kt 300 mb jet max translating from the Canadian Rockies to the Northern Plains. This jet streak was embedded within the main axis of the jet stream, which extends along the southern periphery of the upper trough into the Midwest and mid-Atlantic regions. The evolution of these features and their surface reflections over the next 60 hours will be the main drivers of our period of wintry and colder weather through the weekend.

In the near term, a more subtle mid-level short wave was analyzed at midday within the fast northwest flow aloft across MN/IA. This is accompanied at the surface by a strengthening cold frontal trough stretching from western WI into IA. 12Z RAOBS from MPX and TOP indicate shallow low-level moisture pooling ahead of the front, with surface dew points in the low-20s. Forecast soundings indicate somewhat deeper moisture farther to the north, where regional radar mosaic depicts an area of light snow/snow showers across southeast MN/western WI.
Forecast shallower moister depth into northern IL suggests mainly flurries or spotty light snow showers here toward evening, though shallower moisture also raises the question of lack of cloud ice at onset (moist layer >10C) which could support some low-confidence brief, very light patchy freezing drizzle. WFO ARX has noted some of this in their area late this morning. Eventually by early this evening the column cools behind the cold front such that cloud ice is likely (and present in thickening mid-cloud deck which may provide feeder-seeder for snow flakes), which should support just flurries. Intrusion of arctic air behind the cold front tonight will send overnight temperatures into the single digits and low teens across the area and wind chills in the negative single digits and teens by morning.

By late tonight/early Saturday, a stronger mid-level short wave (currently coming ashore over British Columbia) is forecast to track quickly southeast across the Northern Plains. Warm advection develops quickly into the region in advance of this disturbance Saturday morning, impinging on a fairly strong west- east oriented baroclinic zone from the IA/MO border region into central IL/IN, and beneath the divergent right entrance of the upper level jet across northern IL/IN/LM/MI. Guidance is in generally good agreement in depicting snow blossoming into the forecast area from the west during the morning and persisting through the afternoon before ending, with the primary axis of highest accumulations (5-6") along and south of the WFO LOT/ILX border. Impressively deep DGZ noted in forecast soundings, though the strongest forcing for ascent appears to be focused near/above the top of this layer. Given the cold column behind this evening's cold front, SLRs approaching and even exceeding 20:1 are likely, supporting a low-density, fluffy snow. This looks to yield amounts in the 3-5" range across our southern tier or two of counties (locally higher along the ILX border), tapering off into the 1-3" range farther north into the I-80 corridor and lesser amounts north of that. With the high ratios however, can't rule out some slightly higher amounts up through the I-88 corridor especially if any (even weaker)
frontogenetically driven banding develops that far north. Based on these expectations, have elected to issue a Winter Weather Advisory for the southern tier of LOT counties (included LaSalle but mainly for the far southern part) from 8 am through 8 pm Saturday, for hazardous travel conditions with the accumulating snow.

As snow ends late Saturday/early Saturday evening, very cold temperatures and bitterly cold wind chills become the main story. Persistent advection of cold arctic air combined with 10-15 mph winds is expected to produce wind chills as low as 20 to 25 below across most of the forecast area overnight and into Sunday morning. Therefore have also issued a Cold Weather Advisory for these wind chills for all but Porter county from 8 pm Saturday evening through noon Sunday. Overnight air temperatures will sink into the negative single digits, and will only recover into the positive single digits midday/afternoon on Sunday despite sunshine. For Porter county, a slight wind component off the lake and lake effect clouds are expected to keep readings a little higher and wind chills above advisory criteria. A period of lake effect snow showers is also likely for northern Porter county Saturday night into Sunday morning, with some additional accumulations possible there.

Winds will ease Sunday afternoon and evening as strong arctic surface high pressure builds into the region. Temperatures will likely tank after sunset with the ridge axis over the area, then become steady or slowly rise somewhat overnight as the ridge shifts east and south-southwest winds develop with associated warm advection.

Ratzer

Monday through Friday:

No changes to previous discussion.

Strengthening warm advection on Monday will help send temperatures back into the teens and 20s. Thankfully, not seeing any signs of this intensifying WAA and attendant ascent driving any meaningful mid-level saturation, with precip-free conditions to start the week.

Medium range guidance, to varying degrees, depicts a low- amplitude disturbance in the general region Tuesday night into Wednesday. Intensifying southwesterly flow may help drive an increase in low-level moisture, with some guidance (GFS and CMC in particular) looking like they want to squeeze out some drizzle at times. At this range, this signal remains a bit nebulous, but air and dewpoint temperatures look to be warming to near/above freezing.

An abrupt transition to quasi-zonal flow is advertised towards the end of next week/next weekend with a strong signal that the deep arctic airmass will--at least briefly--get shoved north of the region.

Carlaw

AVIATION /06Z TAFS THROUGH 12Z SUNDAY/
Issued at 1057 PM CST Fri Dec 12 2025

Forecast concerns for the terminals...

- Breezy northwest winds tonight with gusts in the 20-25 kt range.

- MVFR ceilings tonight lifting to VFR Saturday morning.

- Accumulating snow Saturday resulting in MVFR to IFR visibilities.

A cold front continues to swing through the area this evening which is resulting in some isolated flurries. While the flurries are causing some 3-6 SM visibilities, they are not leading to any accumulation and should be coming to a conclusion by 07z at the latest. Otherwise, the main forecast item of note tonight will be the breezy northwest winds gusting 20-25 kts tonight.

Heading into Saturday, the gusts should subside by daybreak but 10-12 kt winds are expected to persist through Saturday afternoon. Ceilings are also expected to lift to VFR Saturday morning before falling back to MVFR as the clipper system arrives. The clipper will move into the area by mid to late morning Saturday and bring with it accumulating snow to the terminals. Given that the better forcing will be south of a VYS to VPZ line, accumulations at the terminals look to be generally less than an inch but locally higher amounts up to 2 inches may be seen near MDW and GYY. Regardless, a period of IFR visibilities is expected to materialize late morning through early afternoon.

Snow will taper by 00z Saturday evening with skies quickly scattering out as snow ends. Though, winds will once again become breezy with northwest gusts once again in the 20-25 kt range through Saturday night.

Yack

LOT WATCHES/WARNINGS/ADVISORIES
IL...Cold Weather Advisory from 8 PM Saturday to noon CST Sunday for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011- ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033- ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

Winter Weather Advisory from 8 AM to 8 PM CST Saturday for ILZ019-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039.

IN...Cold Weather Advisory from 8 PM CST /9 PM EST/ Saturday to noon CST /1 PM EST/ Sunday for INZ001-INZ010-INZ011- INZ019.

Winter Weather Advisory from 8 AM CST /9 AM EST/ to 8 PM CST /9 PM EST/ Saturday for INZ010-INZ011-INZ019.

LM...Small Craft Advisory until noon CST Sunday for Winthrop Harbor IL to Gary IN.

Small Craft Advisory until 9 PM CST Sunday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.