621
FXUS63 KLOT 231949
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
149 PM CST Mon Dec 23 2024

.KEY MESSAGES...

- Some potential for patchy fog development tonight across
  portions of the area.

- Tuesday night through Friday morning look damp and gray with
  persistent clouds and periods of drizzle, mist, and showers.
  Temperatures will be above freezing.

- Friday through the weekend will be marred with periods of rain
  showers (30 to 60% chance for any given 6-hour period).

&&

.DISCUSSION...
Issued at 149 PM CST Mon Dec 23 2024

Through Tuesday:

Late this afternoon, a fairly potent short-wave was tracking
near the Illinois-Wisconsin state line. However, due to lacking
moisture near and south of the state line, most of the radar
echoes have been virga, with meaningful precipitation from
Madison WI and points north. Expectation is perhaps a few
spotty sprinkles or light showers at times until sunset or a bit
after as the short-wave works eastward to southwest lower MI by
the mid evening. A cold front trailing the weak surface low
reflection of the aforementioned short-wave will progress
southeast tonight, shifting winds to light northerly. The main
forecast question mark tonight pertains to cloud cover trends
and the potential for any patchy fog development.

The southwest half or so of the CWA (ie. near and west of a
Durand IL to Fowler IN line) may have a long enough period
without stratus tonight to enable lower temp/dew point (Td)
spreads, with both temps and Td settling into the low-mid 30s.
On the other hand, mid and high clouds streaming overhead will
serve to limit radiational cooling. In addition, much drier air
aloft above the strong low level inversion should be another
limiting factor for fog. Farther north and east, low stratus to
ooze southward as low-level flow turns north-northeasterly. If
there are any pockets of clearing prior to this occurring, a
marginally drier air mass advecting south from the center of
1030 mb high pressure over Wisconsin should preclude fog
development. The evening shift will be able to assess
observational trends, though all in all, we`re not currently
anticipating conditions becoming more favorable for impactful
dense fog development.

Surface high pressure (still around 1030 mb) passing just to
our north will otherwise result in tranquil conditions through
the daytime hours of Christmas Eve. Assuming mostly cloudy skies
hold, which is our current expectation, temperature rises
tomorrow afternoon will be limited. Most areas will be in the
mid to upper 30s, with localized 40F readings south of Highway 24.

Castro


Tuesday Night through Monday:

Tomorrow afternoon, an upper-level shortwave moving across the Texas
and Oklahoma borders will induce a northeastward surge in low-level
moisture into the Mississippi River Valley. As a secondary upper-
level shortwave dives into the southwestern US tomorrow night, the
lead shortwave will dampen, shear apart, and stall over the Lower
Great lakes atop similarly-stalling low-level moisture. While there
remains some spread among model guidance on the depth of the low-
level moisture, confidence is increasing that a prolonged period of
thick clouds accompanied by periods of showers, drizzle, mist, and
even fog will materialize starting Tuesday night and lasting through
through early Friday. Heavy clouds and weak advection points toward
similarly stagnant temperatures, with readings generally from the
mid 30s to lower 40s through the entire period. With all of this in
mind, Christmas Eve, Christmas Day, and the day after Christmas are
shaping up to be decidedly gray. (The last white Christmas, which is
defined by 1" of snow on the ground, was in 2022 at both Rockford
and Chicago).

On Friday, the secondary upper-level shortwave will pivot
northeastward into the Great Lakes quickly followed by several
additional Pacific shortwaves within an unusually broad zonal upper-
level jet streak encompassing much of the western US. Waves of
showers will accompany each wave warranting continuous broad-brushed
chances (30 to 60%) for rain Friday through the weekend. Depending
on the exact evolution of each upper-level shortwave and associated
lower-level mass response, temperatures Friday through Sunday may
reach the lower 50s across parts of the area. Outside of hard-to-
predict holes in varying levels of clouds, the pattern doesn`t
look favorable for seeing the sun anytime soon!

Next week and beyond, successive runs of ensemble model guidance
ubiquitously advertise a shift in the pattern toward colder
temperatures. With colder temperatures will probably be
opportunities for snowfall, as well. Note that January is
typically the coldest month of the year in our area, so a
transition toward a colder and potentially snowier pattern would
be hardly unusual!

Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1145 AM CST Mon Dec 23 2024

The primary aviation weather concerns for the 18Z TAF period
include:

* Continued MVFR cigs throughout the period with IFR possible
  this evening and tonight

* Potential vsby restrictions overnight due to fog, primarily at
  RFD

MVFR cigs are hanging over all sites this afternoon and are
expected to prevail into this evening. We may see a brief
period of scattering to VFR during the early-mid evening before
MVFR fills back in. Cigs may drop to IFR late in the evening
which could hang on through the night. Confidence in timing and
overall presence of IFR cigs is on the lower side, but the
seemingly highest IFR potential exists very late this evening
through the early overnight hours. We look to return to MVFR by
dawn and remain as such through the day on Tuesday.

Additionally, areas of fog may materialize overnight into early
Tuesday. As of this afternoon, the greatest potential for seeing
any impactful vsby restrictions exists out near RFD. Meanwhile,
SW winds gusting as high as 20 kt early this afternoon will
gradually subside to under 10 kt by this evening. Direction will
veer through the evening to near-northerly before flopping over
to NNE early Tuesday. Expect NNE winds below 10 kt during the
day.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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