Joliet, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

389
FXUS63 KLOT 031750
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1150 AM CST Tue Dec 3 2024

.KEY MESSAGES...

- Monitoring a low (less than 20 percent) chance for freezing
  drizzle east of a Pontiac to Valparaiso line late tonight into
  Wednesday morning.

- Scattered gusty snow showers/squalls late Wednesday afternoon
  and evening.

- Conditions turn sharply colder with sub-zero wind chills
  behind an arctic front Wednesday night.

&&

.DISCUSSION...
Issued at 255 AM CST Tue Dec 3 2024

Through Wednesday night:

Conditions are quiet but cold this morning with air temperatures
generally in the low to mid teens outside of the slightly "warmer"
urban core of Chicago. Temperatures should crawl their way through
the 20s to near 30 degrees this afternoon, although there`s some
uncertainty regarding stratus/cloud cover trends which may have an
impact on highs today (more on that in a bit). Otherwise, the
main item we`re watching is an impressive, curling lake effect
snow band off the lower Michigan shoreline associated with a meso-
low embedded within a subtle surface trough. Radar scans over the
last few hours show the western terminus of this band starting to
push in a more easterly direction with stout northwesterly flow
continuing off of Wisconsin and northeast Illinois. The LES
parameter space is impressive (cloud tops near -25C/15 kft have
been noted at times), so we`ll continue to keep a close eye out
for any unexpected westward wobbles but at this time, it looks
like this should be a non factor in our weather today.

Nighttime microphysics RGB imagery depicts a slowly-expanding
region of stratus with cloud bases near 3 kft oozing out of
Wisconsin, with some more aggressive southward expansion over the
last hour. In typical fashion this time of year, guidance hasn`t
been particularly helpful and is having a difficult time with this
fairly shallow (moist layer). With northwesterly cloud-bearing
flow, increased cloud cover a bit this morning across northern
Illinois to account for recent trends. It`s difficult to say how
things trend this afternoon, but with increasing upper-level
moisture in advance of the next system, partly cloudy/partly sunny
wording seemed like the appropriate way to play things in the
grids this afternoon.

Warm advection will increase late this afternoon and evening in
advance of a broad and intense disturbance pushing southeast out
of Manitoba. While the mid and upper levels will saturate overhead
through Tuesday night, low-level dry air looks to preclude any
precipitation of consequence (light snow) reaching the ground.

Very late tonight through Wednesday morning: a semi-consistent
signal for freezing drizzle has been evident within the NAM
guidance in particular (NAM and NAMNest) as robust low-level warm
advection drives rapid saturation below about 800 mb. Recent
trends over the last 3-4 runs indicate the corridor of deepest
low-level moisture may end up focusing more into ILX and IND`s
forecast areas with successive runs stairstepping south and
eastward. For this reason, continue to indicate precip-free
conditions, but we`ll obviously need to monitor this closely as
any drizzle falling onto the very cold ground could quickly cause
travel issues. The main region of interest is south of I-80 and in
particular east and south of a roughly Pontiac to Valparaiso line.

Warm advection and breezy west/southwesterly winds should help
temperatures rise into the mid/upper 30s into the afternoon,
although ground temperatures may not be as quick to respond to
this exceptionally brief "warm up". A reinforcing arctic front
will plow through the region late in the afternoon and evening,
driving temperatures swiftly through the teens along with sharply
falling wind chills as northwesterly winds gust into the 35 to 45
mph range. Given steepening surface-850 mb lapse rates with 40+
knots at the top of the mixed layer, some potential we`ll be
flirting with wind advisory gusts (45+ mph) for a time Wednesday
evening as well.

While boundary layer moisture doesn`t look incredible, intense
DCVA as the main vort lobes arrive Wednesday afternoon and evening
look sufficient to start cranking out snow showers--perhaps
initially as a rain/snow mix, although the low-level column is
forecast to cool very rapidly through the early evening. Any
stronger snow showers will likely have squall-like
characteristics, with brief visibility reductions and strong wind
gusts. Have increased PoPs into the chance category across the
northern 2/3rds of the area. "Streaky"/isolated accumulations
near a half to perhaps locally one inch appear plausible into
Wednesday evening. Depending on how quickly stratus clears out,
flurries could linger through the overnight hours with the cloud
layer planted deep within the dendritic growth zone.

Carlaw


Thursday through Monday:

As the exiting system moves to the east, the pressure gradient
will gradually weaken allowing winds to diminish. Temperatures
will remain cold with highs expected only in the mid to upper
20s. Persistent northwest flow over the lake could keep lake
effect showers in northwest Indiana (primarily over Porter
County) through the day and gradually diminishing in the
evening.

Northwesterly flow aloft will remain through the weekend.
However, surface winds will gradually turn to the southwest on
Friday allowing for some meager warming to temperatures, with
the potential of daytime temperatures getting back above
freezing (and above seasonal normals) over the weekend. When it
comes the sensible weather forecast, there is lower confidence
in system tracks. It seemed models were suggesting over the last
several days a weak system would come down the broader cyclonic
flow either Friday for Saturday. Technically it is still in the
suite of ensemble members, but it is much weaker and most
likely farther to the northeast. Maybe it could provide another
weak round of lake effect snow in northwest Indiana overnight
Friday into Saturday, but if current models play out impacts
would not be expected and most of the area should remain dry.

Despite not handling the Friday/Saturday timeframe particularly
well, ensembles are still suggesting a somewhat active pattern
in the midlevels returning next week. While the models are
still bouncing around and not agreeing on time and location, it
seemed okay to leave the slight chance to chance PoPs that the
National Blend was suggesting. As an upper level wave arrives,
colder temperatures are expected to come with it returning the
area to more chillier seasonal norms.

DK

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1150 AM CST Tue Dec 3 2024

Aviation Forecast Concerns for the 18Z TAFs:

- SW winds become breezy with gusts 20-25 kt tonight, then
  strong WNW/NW with gusts 35-40 kt Wednesday afternoon.

- Patchy MVFR stratocu with bases ~2000 ft this afternoon. MVFR
  ceilings likely Wednesday.

- Potential for convective snow showers later Wednesday
  afternoon with localized IFR vsbys and minor accums.

Surface low pressure currently over the Canadian prairies will
steadily deepen as it passes north of Lake Superior through the
period. Surface winds across the Midwest will turn southwest
and gradually increase later this afternoon and tonight, in
response to the tightening gradient south of the low. By early
this evening, winds will likely be at least occasionally gusting
into the 20-25 kt range, and will further strengthen and veer
west- northwest behind a sharp cold front Wednesday afternoon.
Winds will likely gust to 35+ kts by late afternoon as cold
advection steepens low-level lapse rates.

In the near term, patchy MVFR stratocu has developed across
northern IL late this morning, and will spread across the
terminals by early this afternoon with bases around 2000 ft and
scattered to broken coverage. More persistent and widespread
MVFR ceilings appear likely on Wednesday. Scattered, gusty
convective snow showers are expected to develop Wednesday
afternoon as instability develops behind the aforementioned
cold front. Localized IFR visibilities are possible within
these snow showers with some minor accumulations, in addition to
briefly higher wind gusts around 40 kts.

Ratzer

&&

.MARINE...
Issued at 255 AM CST Tue Dec 3 2024

Key Maritime Messages:

- Gale force winds are expected Wednesday afternoon through
  Thursday morning

A strong system is projected to descend out of Canada and surge
southeastward over the Great Lakes Wednesday into Thursday. A
strengthening pressure gradient in the wake of a sharp cold
front will substantially increase winds and wind gusts Wednesday
afternoon through Thursday morning. A Gale Watch has been
issued from late Wednesday afternoon through Thursday morning.
Some higher-end gales to near storm force gusts are possible
Wednesday night.

In addition, with temperatures dropping into the teens
Wednesday night into Thursday morning and winds remaining
strong, there is the potential for freezing spray. Northwest
winds will slowly diminish beyond Thursday morning, though will
remain gusty with choppy waters that would be hazardous to
smaller crafts.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 6 PM this evening to 3 PM CST
     Wednesday for the IL and IN nearshore waters.

     Gale Watch from Wednesday afternoon through Thursday morning
     for the IL and IN nearshore waters.

&&

$$

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