Downers Grove, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

163
FXUS63 KLOT 031130
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
530 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 /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 530 AM CST Tue Dec 3 2024

Key Aviation Impacts for the current TAF period:

- Gusty southwesterly winds increase tonight through the end of
  the TAF period

- There is a chance for LLWS tonight, but was left of the TAFs
  due to low confidence

It is another morning of stratus clouds oozing southward out of
Wisconsin. Luckily today, cloud bases are above 3000 feet, so
VFR conditions should remain through the TAF window. Winds are
expected to be out of west this morning. Winds will slowly pivot
to the southwest with frequent gusts just below 20 knots.

Tonight a strong jet between 45 to 50 knots around 3000 feet is
expected to develop. As models have trended toward a stronger
jet aloft, so has confidence towards surface winds remaining
coupled to the winds aloft allowing wind gusts at the surface to
increase between 20 to 25 knots tonight. If surface winds were
to decouple, there could be a period of LLWS overnight at area
terminals. Mention of LLWS was left out of the TAFs given the
lower confidence, but may need to be re-added depending on
trends.

Winds will remain gusty and drift more to the south Wednesday
morning ahead of the next system arriving in the afternoon (and
outside of the current TAF period).

DK

&&

.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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