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000
FXUS63 KLOT 062053
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
253 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 breeze 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. 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 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 trended down PoPs Saturday PM will likely
need to be tweaked down farther given guidance trends.

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).

&&

.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...
For the 18Z TAFs...

Forecast concerns for the terminals...

* MVFR ceilings persist into this afternoon before improving
  prior to sunset

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

MVFR ceilings with bases as low as 1800 feet will continue to
blanket northeast Illinois and northwest Indiana into this
afternoon. We likely are not to see much improvement until after
21z this afternoon. Shortly thereafter, VFR conditions are
expected to return for the remainder of the forecast period.

Westerly winds this morning will become southwesterly through the
afternoon as a surface high settles in across the Tennessee
Valley. Wind speeds should generally remain around 10 kts through
the remainder of the day. Speeds are expected to increase this
evening as a strong 45 to 50 kt low-level jet develops about
1500-2000 ft off the surface. While some surface gusts of 20 to 25
kts are possible as this low-level jet develops through the
evening, the gustiness may be more occasional in nature. For this
reason, we have opted to add a formal mention of LLWS to the TAFs
for about a 6 hour period from 03-09z tonight for the Chicago
area terminals, and a couple hours earlier at KRFD. The threat of
Low- level wind shear and gusty surface winds will wind down
after 09z, with a light (up to around 10 kt) southwesterly
surface wind expected through the day on Thursday.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...IL and IN nearshore waters...6 PM
     Wednesday to 3 AM Thursday.

&&

$$

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