textproduct: Chicago

KEY MESSAGES

- Cold temperatures will be observed this morning before south- southwesterly winds become blustrier later today. Isolated blowing/drifting snow could not be ruled out in open areas this afternoon and evening.

- A mix of snow and patchy freezing drizzle may materialize tonight, primarily in areas north of I-80 (20-40% chance). - A clipper-like pattern will bring regular opportunities for snow to the general region beginning Saturday night into Sunday and continuing through the upcoming week.

DISCUSSION

Issued at 350 AM CST Fri Dec 5 2025

Through Saturday:

It's a cold morning across the region today with most observation sites in our forecast area reporting single digit temperatures. Still monitoring the potential for freezing fog development, but thus far, outside of (interestingly) some narrow streaks of cooling pond-induced low clouds/fog, there hasn't been much else in terms of fog development as dew point depressions have generally remained too high for that. There's still some time for fog to develop in southeastern portions of our forecast area, but the window for that is slowly shrinking with a stratocumulus deck rolling in from the west and the expectation that surface winds will pick up towards daybreak. Growing cautiously optimistic that we'll be able to get away without needing a fog headline, but will continue to monitor satellite and surface observations over the next few hours to be sure.

The south-southwesterly winds will creep upward even further and even begin gusting later on today as the surface pressure gradient continues to tighten between the departing surface high pressure and the incoming surface low pressure trough/cold front. A strong 900-950 mb temperature inversion and the existing deep snowpack across the area should temper gust magnitudes enough to keep the potential for blowing/drifting snow limited during the daytime today. The strongest gusts (and thus the greatest potential for blowing/drifting snow) today may actually occur this evening as warm air advection discourages the decoupling of the planetary boundary layer at a time when the overhead low-level jet will be strengthening. However, even during this time, BUFKIT momentum transfer output using several different models isn't too keen on gusts greater than 25 kts occurring at ground level from the resultant downward mixing of stronger low-level flow, so not planning to highlight the potential for blowing/drifting snow any more at this time.

Anyways, the south-southwesterly winds will help push air temperatures up into the mid to upper 20s during the daytime today. While a dip in temperatures may occur after sunset, it appears that the strengthening warm air advection should allow for many areas to warm towards and into the low 30s during the late evening and overnight hours ahead of the cold frontal passage.

Of greater note tonight will be the potential for a brief period of wintry precipitation along/ahead of the front that may include freezing drizzle. Low-level isentropic ascent along this frontal corridor should help sustain a stratus deck that appears will be deep enough to support precipitation. Model guidance remains mixed on just how cold the top of this stratus deck will get, which will dictate whether the presence of ice nuclei will favor poor quality snowflakes as the dominant or sole precipitation type, or whether the lack thereof will keep any precipitation limited to drizzle (which would freeze upon contact with most untreated surfaces given the presence of sub- freezing air temperatures both during and in the lead-up to this potential precipitation event).

In general, it appears that most of the 00Z and 06Z hi-res model solutions have cloud tops reaching -10C, which would point to snow occurring over freezing drizzle. However, some sounding progs still have at least one hour when the stratus deck is entirely warmer than -10C while still being deep enough to support precipitation, which would favor freezing drizzle or snizzle (a mix of freezing drizzle and snow) as the prevailing precipitation type -- at least for a brief period of time. If the aforementioned model guidance ends up being a degree or two too cool with regard to cloud top temperatures, then that could also be enough to tip the scales in favor of freezing drizzle. There's also the possibility that none of this matters because the stratus ends being too shallow for precipitation to occur. Since it still looks like we'll be toeing the line between having cloud ice or not (and between having precipitation or not), maintaining a mention of both snow and freezing drizzle in the grids while keeping PoPs capped in the chance range still seems like the way to go until upstream observational trends become apparent later today.

Any overnight precipitation should end by sunrise on Saturday. Daytime temperatures will likely struggle to rise much in the wake of the frontal passage, and accordingly are largely forecasted to remain in the 20s with many areas likely setting their high temperatures for the day during the early morning hours tomorrow.

Ogorek

Saturday Night through Thursday:

Focus in the extended forecast period remains the continued unseasonably cold temperatures for early December and the beginning of a likely active clipper-like pattern setting up (upper ridging across the western CONUS and upper troughing across the Great Lakes). This will allow a series of disturbances to move within the resultant northwest flow overhead leading to multiple opportunities for snow somewhere in the general region.

The first of these systems arrives late Saturday evening through Sunday morning. Moisture with this system is not especially high, but a period of modest ascent through the depth of the column (and within the DGZ) overnight would support widespread steady light snow (rates up to around 0.25"/hr) and generally 1-2" totals. Localized amounts up to 3" will be possible, especially west of the Chicago metro where the forcing is slightly more pronounced and for a bit longer duration.

While the recent bitterly cold weather has helped cool the lake by several degrees (now upper 30s to lower 40s), the unseasonably cold airmass overhead in the wake of the weekend system will lead to modest lake induced instability supportive of lake effect snow Sunday PM. There remains a lot of variability in how this plays out, however, from the band focusing mainly into northwest Indiana to potentially drifting back to the west into the Chicago metro before ending Sunday night. Confidence accordingly remains quite low in the details. Stay tuned!

Beyond the weekend, additional waves move within the upper level flow beginning Monday night and continuing off and on through the end of the week. These generally favor a slightly more northerly track compared this weekend with snow chances increasing with northward extent, particularly into Wisconsin. Given we are still several days out this could certainly change with each subsequent disturbance! It is worth noting that ensemble guidance remains fairly consistent in developing a stronger surface low that tracks across the Upper Mississippi Valley Tuesday into Wednesday (albeit with notable differences in just how strong). Nevertheless, this could help lift warmer air into the area (low-mid 30s) and support more of a rain/snow mix scenario.

After the aforementioned potential warmer temperatures in the 30s midweek, temperatures then fall again in the wake of that system, with forecast highs back in the teens and 20s through the end of the week and lows in the single digits and teens, paired with the continued periodic snow chances (20-40%).

Petr

AVIATION /12Z TAFS THROUGH 18Z SATURDAY/

Issued at 558 AM CST Fri Dec 5 2025

- Brief period of -SN possible tonight (30% chance)

Light south to southwesterly winds this morning will steadily increase through the morning with gusts in the 20-25kt range then expected through the afternoon and evening.

Ceilings lower to MVFR ahead of an approaching cold front late this evening and overnight paired with a low chance for a brief period of light snow/flurries. Have introduced a PROB30 for 5SM -SN and IFR CIGs to account for this from 4-9Z (3-7Z RFD). There is also a non-zero chance (15%) that patchy freezing drizzle mixes in with the poorer quality snowflakes, but confidence remains too low to include in the TAF with this update and coverage would be very patchy if it were to develop.

Winds turn west and northwest behind the front late tonight with lingering MVFR CIGs eventually scattering out by late Saturday morning.

Petr

CLIMATE

Issued at 335 AM Fri Dec 5 2025

Here are the current record low temperatures for Today, 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 until midnight CST tonight for Winthrop Harbor to Wilmette Harbor IL.

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