textproduct: Chicago

KEY MESSAGES

- Tonight will be very cold with overnight lows ranging from -5 to +5F. Typical cold spots may make a run for -10 to -15F.

- Areas of dense freezing fog may develop tonight, especially in open or low-lying areas and particularly along/east of I-55. Where freezing fog develops, hoar frost may lead to slick spots on untreated surfaces.

- Friday will start cold, before breezy southwesterly winds develop by mid afternoon. Isolated blowing/drifting snow cannot be ruled out.

- A mix of freezing drizzle and snow may materialize north of I-80 Friday night (20-30% chance).

- A clipper-like pattern will become established this weekend into early next week with regular opportunities for snow in the general region.

UPDATE

Issued at 758 PM CST Thu Dec 4 2025

Main focus this evening remains on the potential for freezing fog development overnight.

Clear skies and calm winds have allowed temperatures to crater across our Illinois locales and outside of the urban Chicago core, with a more muted temperature fall across northwest Indiana where dewpoints are higher. While dewpoint depressions are only running a handful of degrees in most locations, there are some competing aspects at play which may curtail the dense freezing fog threat across parts of NW Illinois in particular. Here, the surface pressure gradient will slowly tighten as high pressure drifts east into Indiana which should allow surface winds to slowly inch upwards overnight. At the same time, a region of 1500-3500 foot stratus marching across north central Iowa may build into NW Illinois late tonight. Both of these aspects may end up limiting the fog threat or, if fog develops, result in improving conditions late tonight into early Friday morning.

Locally greater concern for areas of dense freezing fog exists across northwest Indiana and near/east of I-55 where dewpoints are higher and forecast soundings look notably more decoupled compared to farther northwest. Have reorganized freezing fog in our grids to start initial development across NW IN and then expand it north and west from there. Will continue to hold onto patchy wording towards Rockford, but again suspect any threat here may remain isolated. One other wildcard in the fog development department is the expanding area of stratus across downstate Illinois. If this accelerates northward, this could also end up limiting the fog threat across parts of Ford - Benton counties.

With all of this in mind, no plans for any Dense Fog Advisories at the moment, but we'll be keeping a close eye on observations through the evening and overnight.

Carlaw

DISCUSSION

Issued at 145 PM CST Thu Dec 4 2025

Tonight through Friday night:

The center of a surface pressure ridge will continue to slide through the region this afternoon and settle across the Ohio River Valley tonight. Clear skies and a healthy snow pack at sunset will set the stage for quick decoupling and ideal conditions for efficient radiational cooling tonight, supporting overnight lows dropping like a rock. (It's probably not a good sign that some areas already have dew points ranging from 5 to 10 below). Will go ahead and explicitly forecast overnight lows ranging from -5 to +5F, keeping in mind that typical cold spots (Rochelle, Aurora) may make a run for -10 to -15F. The record low temperature for the calendar date of December 5 will be threatened at Chicago (4F set in 2005) and Rockford (-5F, also set in 2005).

Gradually increasing warm air advection along the backside of the high pressure system in tandem with the very cold surface temperatures will set the stage for the development of a strong low-level inversion tonight. With forecast soundings depicting trapped low-level moisture beneath the inversion (partially a consequence of sunshine facilitating sublimation at the very top of the snowpack today), concern is rising that shallow but dense freezing fog will develop tonight, especially in sheltered areas and near rivers. The main limiting factor for fog appears to be some 20-25kt of flow right at the top of the inversion, which may mechanically mix dry air downward toward the surface. However, given the very cold forecasted overnight low temperatures, do think that the near surface layer may end up so stable it resists the downward entrainment of any dry air. So, will introduce patchy freezing fog across the entire area, save for near Lake Michigan. If freezing fog does develop and become dense, the deposition of ice (hoar frost) would be likely on all surfaces (trees, fences, roadways/bridges/overpasses). If/where temperatures verify on the colder end of the spectrum, a saturated DGZ may develop literally at ground level, perhaps supporting suspended ice crystals (ice pillars). In all, tonight will be more typical of the middle of winter in the northern Plains than early December in the Great Lakes.

Tomorrow, a surface low will move across northern Wisconsin and act to tighten the low-level pressure gradient across the Lower Great Lakes. Mixing into the strengthening low-level wind field will be stunted initially by the strong morning inversion, though by early afternoon, do expect southwesterly winds to gust 20 to 25 mph. In scenarios where mixing is most efficient, gusts in the 30 to 35 mph range cannot be ruled out by early evening. If such strong winds were to materialize, would have to watch for areas of blowing and drifting snow in open areas (a quick check outside confirmed the top layer of snow has become quite powdery). Highs will rise toward the mid to upper 20s.

Tomorrow night, weak isentropic ascent ahead of an approaching cold front will induce the development of low stratus across the region. Forecast soundings indicate that stratus thickness may become sufficient to produce precipitation at some point early Saturday morning, mainly near and north of Interstate 80. With in-cloud temperatures appearing to be warmer than -8C (unsupportive of cloud ice), precipitation type may tend to favor freezing drizzle of snizzle. So, will tuck in a mention of freezing drizzle into the forecast, ending by daybreak Saturday. Temperatures look to hover in the mid to upper 20s all night, so any freezing drizzle would be prone to sticking to any untreated surface.

Borchardt

Saturday through next Thursday:

This weekend through the middle of next week, we'll find the general region sandwiched between a blocking upper high off the central Pacific coast and a persistent upper low spinning over Canada's Hudson Bay region. This will result in a series of upper trough axes training across the central CONUS and lock the Midwest into a NW to zonal upper flow pattern. And with a rich atmospheric river feeding onshore the PNW and Alberta coasts, such a setup will drive a procession of mid-level impulses off the lee of northern US and Canadian Rockies and down across the Midwest presenting frequent opportunities for snow. As my colleague puts it, "hop aboard the clipper express!"

The first in this series of waves comes Saturday evening into Sunday morning. Low pressure will track across central IL and likely bring a good clip of snow showers to the local area. Forecast soundings from around the CWA feature saturation and decent forcing for ascent through and on either side of the DGZ, which should support a widespread push of snow north of the storm track. Heavy banding doesn't appear to be a big concern with this system, but with the mid-level jet immediately to our north, there is a signal for some moderate shearing deformation on the storm's northern flank which could offer a narrow corridor of relatively higher totals north of the storm track. This signal is much stronger out in IA before weakening into our area. Accordingly guidance strongly favors higher totals toward and west of the Mississippi and lesser totals locally. Ensemble and deterministic guidance likes a largely 1 to over 2" range around a majority of our CWA with fair support for a little less than that, and almost no support for anything more than 3".

Bands of lake effect snow look to develop in the storm's wake Sunday afternoon into early Monday, but it's unclear how this potential will impact us locally. The ENS maintains more BL moisture into Sunday night and has the low level wind field more veered making it more bullish on the lake-effect potential, particularly for the IL side, than the GEFS. Latest deterministic Euro even resolves an explicit mesolow around the bend of the lake which would also help steer snow toward the Chicago. Probs from the GEFS are lower around the lake and are focused toward northwest IN. Meanwhile, the GEPS gives a look closer to that of the ENS. This doesn't appear to be an overly impactful lake-effect setup, but the potential is there for localized areas to maybe see upwards of an additional inch or two, likely much less than that on a more widespread basis.

Guidance then resolves a one-two punch of clipper systems tracking through the region in quick succession early next week; the first being Monday night into Tuesday and the other slated for Wednesday. Uncertainty grows quickly from this point as these systems will be in the process of phasing with the Hudson Bay upper low making them especially tough to get a good handle on. A lot of run-to-run model variability exists with these systems, and even individual members of any given ensemble system offer a wide variety of outcomes following this weekend.

Monday night-Tuesday, the clipper and highest PoPs are favored to track just to our north with a surface cold front moving across our local area. Moisture and forcing may be tightly wound around the low to the north and ensemble PoPs drop off pretty quickly with southern extent through the CWA. Surface high pressure to the SE may also impinge on precip chances into our CWA; the GFS is most bullish on this drier outcome. Snow showers appear likely for at least northern portions of the CWA.

Wednesday's clipper is also favored to track to our north across WI, although there is even more variability with this storm. Medium range guidance agrees that enough warm air will infiltrate the system to offer rain or a wintry mix to those roughly near and south of the storm track, and there's a good chance that includes at least a portion of our CWA. Additional opportunities for lake-effect snow may present themselves on the backsides of the these systems next week, although details are very fuzzy for the time being.

Despite the uncertainties in precip expectations, there is fair agreement on temperatures through the middle of next week, particularly among daytime highs where there is more spread in low temperature guidance. Below-normal conditions will persist with daytime temperatures in the upper teens to mid 20s Sunday and Monday. Sunday night into Monday morning looks particularly cool, similar to what we saw this morning. There's a bit of spread, but single digits appear likely outside of the city and interior Chicago metro, with single digits below zero very attainable, especially across outlying areas. Winds look rather light which should keep the wind chill factor from getting out of hand. We'll trend warmer for Tuesday and Wednesday, but likely still below normal with lower and middle 30s favored.

Doom

AVIATION /00Z TAFS THROUGH 06Z SATURDAY/

Issued at 553 PM CST Thu Dec 4 2025

Surface high pressure centered over northern Illinois early this evening will drift east overnight calm/VRB winds to settle southwest while gradually increasing through the night. Winds will remain southwest through the period, with gusts over 20 knots late Friday morning through the evening.

Generally clear skies combined with the light winds and existing snow pack will allow a strong and shallow nocturnal inversion to develop this evening. BR is expected to initially develop at the outlying sites by the early overnight hours, with patchy FZFG and shallow stratus also possible. As weak warm-air advection ensues with the increasing southwest winds overnight, any FZFG and/or IFR/LIFR stratus will begin advecting northeast toward the Chicago terminals. Mixing of very dry air atop the inversion (roughly at 1kft) will somewhat impede the northeastward progress of stratus, so do not have a lot of confidence that IFR (or lower) ceilings and visibility will reach ORD/MDW. Will need to monitor upstream trends south and southwest of the Chicago metro this evening to determine if sub 1kft ceilings and lower visibility need to be included in the TAF.

Moisture and warm-air advection ahead of an approaching cold front Friday evening will promote MVFR ceilings at the end of the TAF period. IFR ceilings with flurries or light freezing drizzle cannot be ruled out, particularly late Friday night beyond the current TAF period, but chances remain too low to include at this time.

Kluber

CLIMATE

Issued at 1126 AM Thu Dec 5 2025

Here are the current record low temperatures for Friday, December 5:

Chicago Low Friday 12/5 4 (2005)

Rockford Low Friday 12/5 -5 (2005)

- NWS Chicago

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from midnight tonight to midnight CST Friday night for Winthrop Harbor to Wilmette Harbor IL.

Small Craft Advisory from noon Friday to midnight CST Friday night for Wilmette Harbor IL to Michigan City IN.