Joliet, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

282
FXUS63 KLOT 050508
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1108 PM CST Thu Dec 4 2025

.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 /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1108 PM CST Thu Dec 4 2025

Southwest winds under 10 knots at TAF issuance will gradually
increase this morning as a surface high exits eastward. Expect
gusts over 20 knots late this morning through this evening. A
cold front will then shift winds WNW with gusts around 20 knots
overnight.

Generally clear skies combined with the light winds and
existing snow pack have allowed a strong and shallow nocturnal
inversion to develop. BR is becoming more evident across
northwest Indiana, with the potential for IFR (or lower)
visibility and ceilings to form and expand/advect toward the
southern Chicago metro through the early morning hours. Mixing
of very dry air atop the inversion (roughly at 1kft) will
somewhat impede the northward progress of fog or stratus, and
VFR stratus shifting eastward across northwest Illinois will
also limit BR/FG development farther north over the main Chicago
terminals. Have maintained a mention of VFR BR at MDW/ORD, but
will need to continue to monitor for inclusion of lower
ceilings/visibility if fog quickly develops to the south and
southeast over the next few hours.

Moisture and warm-air advection ahead of an approaching cold
front this evening will likely promote MVFR ceilings mid-evening
through the overnight hours. IFR ceilings with flurries or
light freezing drizzle cannot be ruled out for a few hours
centered around midnight/06Z just ahead of the cold front, 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 until 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.

&&

$$

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