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000
FXUS63 KLOT 062331
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
531 PM CST Wed Dec 6 2023

.SHORT TERM...
Issued at 253 PM CST Wed Dec 6 2023

Through Thursday night...

Key Messages:

* Dry conditions through the period, with warmer temperatures
  expected on Thursday.

After sunset this evening, a robust low-level jet (45-50kt
1500-2000 ft AGL) will briefly drift across the region tonight.
While most of this momentum will not reach the surface, we should
have a period of increased southwesterly breezes later this
evening, with some occasional gusts up to 25 mph. Expect these
winds to ease prior to daybreak Thursday, with early morning
temperatures bottoming out around freezing.

Southwesterly winds on Thursday will transport a rather stout
lower-level thermal ridge northeastward into the area.
Temperatures at 925 mb of +6 to +8C should support surface highs
around, or just above, 50 degrees under mainly sunny skies
Thursday afternoon. Temperatures are expected to remain
seasonably mild (in the 40s) Thursday night as southwesterly winds
turn increasingly breezy again.

KJB

&&

.LONG TERM...
Issued at 253 PM CST Wed Dec 6 2023

Friday through Wednesday...

Key Messages:

* Unseasonably mild Friday into Saturday morning then turning much
  colder Saturday PM-Sunday with breezy west-northwest winds

* Periods of rain showers (60-80+% chance) late Friday night-
  Saturday morning

Stratus with 1500-3000 ft bases is expected to blossom on Friday
from robust low-level moisture advection. Despite this, the very
mild starting point to the day and air mass in place along with
breezy southwesterly boundary layer flow gusting up to 25-30 mph
will result in well above normal temperatures. Maintained forecast
highs in the mid to upper 50s. If there is more sun than anticipated,
the chance for 60+F readings will increase (currently 5-10% chance
of 60+F highs based off the NBM).

As the moist layer deepens Friday evening in concert with
increasing large scale ascent out ahead of a short-wave and
associated surface low, isolated to widely scattered lighter
showers and patchy/areas of drizzle (15-40% chance of measurable
precip Fri. evening). The model guidance has come into very good
agreement into Saturday that the ejecting short- wave will
consolidate rain focus near the surface low and its trailing cold
front. The late Friday night- Saturday morning timeframe is when
we have our highest PoPs (~60-80%+ as noted in key messages) for a
3-6 hour window of showers capable of producing occasional
downpours. Temperatures Friday night through early Saturday out
ahead of the front will remain well into the 40s to lower 50s,
5-10F or more above normal *highs* for the date.

The pronounced faster trend with the cold front approach and
passage on Saturday resulted in two additional changes to the
forecast message: 1) Instability will likely be too meager for any
notable convection given unfavorable timing at an already less
favorable time of year; 2) Saturday afternoon should dry out
across much of if not the entire area. With the ensemble PoPs
coming down into the 10-30% range, our already trended down PoPs
Saturday PM will likely need to be tweaked down farther given
guidance trends, should these trends continue.

Strong pressure rises and cold advection Saturday afternoon-
Saturday evening will yield breezy west-northwest winds gusting
up to 30-35 mph and steadily falling temps. Lows Saturday night
will be about 20 degrees colder than Friday night`s mild readings.
Cold advection will persist through Sunday and with cyclonic flow
aloft, some flurries or light snow showers can`t be ruled out, but
for now trended to a dry forecast with the ensemble mean PoPs
primarily dry. Highs will only be in the mid to locally upper 30s
with northwest winds gusting up to 20-25 mph adding to the chilly
feel vs. the mild stretch we`ll have come out of.

Next work week will be dominated by a series of high pressure
systems at the surface and split flow aloft likely both acting to
keep us atypically quiet for this time of year. Temperatures will
be near normal, though may trend milder by or especially beyond
current day 7 (Wednesday).

Castro

&&

.CLIMATE...

Issued at 253 PM CST Wed Dec 6 2023

Unseasonably warm temperatures are expected Thursday through
early Saturday and here are the record warm temps for Rockford and
Chicago.

Rockford...
Friday, December 8th.....60 in 1980

Chicago...
Friday, December 8th.....64 in 1946

The record warmest low temperatures for the calendar day, December
8th are,

Rockford...37 in 1987

Chicago....53 in 1946

&&

.AVIATION...
Issued at 530 PM CST Wed Dec 6 2023

For the 00Z TAFs...

* Threat for LLWS for about a 6 hour period mid to late evening into
  the overnight

* Another potential window for LLWS Thursday night

VFR conditions and southwesterly winds are expected through the
TAF period. A strong low-level jet, 45 to 50 knots, expected
tonight between 1500-2000 ft AGL. While some surface gusts up to
25 kts, gusts may be more occasional through the evening.Therefore
the TAFs maintained the risk for LLWS from 03-09z as confidence
remained with that 6-hour window, though a couple hours earlier
for KRFD. Winds aloft diminish overnight; however the potential
for another risk for LLWS on Thursday night after 00Z.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...IL and IN nearshore waters until 3 AM
     Thursday.

&&

$$

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