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000
FXUS63 KLOT 050856
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
256 AM CST Tue Dec 5 2023

.SHORT TERM...
Issued at 256 AM CST Tue Dec 5 2023

Through Wednesday...

Key Messages:

* Snow this morning with slushy accums mainly on grassy and the
  coldest surfaces.

* Transition to drizzle in spots early this morning, although
  spatial extent of any freezing drizzle will remain low and of
  minimal impact owing to temperatures at or above freezing.

* Lingering rain/drizzle through late-morning/early-afternoon

* Scattered lake effect precip (mainly rain) develops later this
  afternoon and tonight.

Have not made significant changes to the inherited forecast for
today. A robust but narrow wing of warm advection-driven
precipitation continues to press across the region early this
morning. The attendant dryslot is also rapidly surging eastward,
with the window for deeper saturation only about 4 hours or less
in any one location. With the entire system gradually
pivoting/acquiring a more southeastward motion, precip will end
quickly south of the Kankakee River this morning, while lingering
through the balance of the morning farther to the north.

Precip/snowfall rates are currently in line with earlier
expectations. As the aforementioned dryslot impinges on our
region through daybreak, a narrow corridor of somewhat enhanced
rates may set up on the steepened lapse rate/deeper moisture
interface, but this continues to look like a tenuous relationship
with a steady loss of deeper saturation area-wide through the
morning hours. This in concert with slowly-rising temperatures
will lead to a transition to poor-quality snow and/or drizzle with
time. With air temperatures at or above freezing, no significant
roadway impacts are expected. Rain looks to be the main p-type for
locales south of a Streator to Rensselaer line.

Starting to see more signs of fog development across eastern Iowa
where the low-level flow is easing. Will continue to monitor this
progression, with some mention of fog possibly needed for later
this morning, particularly south of I-80 as the surface low
trundles overhead.

This afternoon and evening: spotty lake effect precipitation is
expected to develop. The parameter space is pretty modest although
equilibrium levels are forecast to briefly peak near 7 kft. This
should be enough to support disorganized showery lake effect,
initially focusing into NE Illinois before pivoting into NW
Indiana as winds veer through the evening. Looking like
predominantly rain as the p-type, although some mixing or
ephemeral changeovers to all snow are possible this evening under
more convective/deeper activity.

The cloud cover--and hence temperature--forecast is a bit unclear
tonight and into Wednesday. Mix of signals across the guidance
suite from completely scouring stratus out to expansive overcast
conditions lingering through Wednesday afternoon. Suspect that
while we might start to poke holes in things tonight that we`ll
mix back into a strato-cu deck on Wednesday although this is lower
confidence. Another stout upper wave will traverse the region
late in the afternoon which will chuck some increasing mid and
upper cloud cover our way. Currently, looks like the deepest
saturation will remain just north of the region so no precip
chances at this time with this warm advection wing.

Carlaw

&&

.LONG TERM...
Issued at 256 AM CST Tue Dec 5 2023

Wednesday night through Monday...

Key Messages:

* Unseasonably warm temperatures continue to look likely Thursday
  and Friday where highs could flirt with 60 degrees especially on
  Friday.

* Breezy southwest winds develop Thursday into Friday with gusts
  upwards of 25 to 30 mph, though higher gusts are possible.

* Active weather returns looks to return to the area this weekend
  into early next week.

An amplified upper ridge is expected to in place across the
central CONUS at the start of the period and be building into the
Great Lakes during the day on Thursday. As the ridge moves in,
southwesterly flow will develop and advect in much warmer air not
just at the surface but in the mid-levels as well. Guidance
continues to show that 850 mb temperatures of 6 to 10 C will move
overhead which in combination with mostly clear skies should
generate unseasonably warm temperatures for both Thursday and
Friday. Thursday is forecast to be the slightly cooler day of the
two despite the ridge axis being virtually on top of us with highs
in the low to mid-50s, but the strengthening warm advection
should allow temperatures to flirt with the upper 50s and possibly
some 60s Friday afternoon. While our current forecast is below
the daily records for both Chicago (64 for both Thursday and
Friday) and Rockford (64 for Thursday and 60 for Friday) it is not
entirely out of the realm of possibilities that we could get
close especially if temperatures over perform on Friday.

While we are enjoying the warmth, a pair of shortwave troughs are
forecast to be working their way on shore in the Pacific
Northwest. The first of these waves looks to track along the
northern periphery of the ridge as it moves eastward across the
far northern CONUS and southern Canada. As this occurs, guidance
suggests that a rather deep surface low should develop across the
Upper Midwest and northern Great Lakes late Thursday into Friday
which should generate a stout pressure gradient across the rest of
the Great Lakes region. Thus, breezy southwest winds are expected
to materialize late Thursday through the day on Friday where
gusts of 25 to 30 mph look like a safe bet. Though, forecast
soundings do show that a strong low-level jet should be overhead
on Friday that could be tapped into if diurnal mixing is deep
enough which in turn may allow for stronger gusts to be realized.

As for the second shortwave, guidance continues to forecast it
deepening into a broad trough as a 120 kt jet pours into its base.
Continued intensification is forecast as the trough ejects into
the central CONUS on Friday and eventually into the Great Lakes
over the upcoming weekend. In doing so, the trough should overrun
a surface frontal boundary that will be advancing eastward across
the southern Great Lakes during this time which looks to serve as
the breading ground for precipitation and potentially a more
organized storm system. Where exactly where this front will be
when this occurs remains very uncertain among guidance and thus
not much can be said for the coverage and/or type of precipitation
expected locally. Therefore, we will have to keep an eye on this
system through the week as trends evolve but at the very least we
should expect a more active period and seasonable temperatures to
make a return this weekend into next week.

Yack

&&

.AVIATION...
For the 06Z TAFs...

Aviation concerns:

 * Snow moves in tonight and continues after daybreak with
   associated IFR vis/cigs (potentially LIFR) ending by mid-late
   morning

 * Lake effect showers develop Tuesday afternoon into early
   evening

An area of snow is beginning to move across northwest Illinois
late this evening and will continue to shift east the next couple
of hours. Ceilings and visibility will accordingly steadily
decrease to IFR as the column saturates, earliest at RFD. Snow
is expected to persist after daybreak as the system pivots over
northern Illinois. Continue to hold off on a formal LIFR mention
in the TAFs due to lower confidence on timing and placement of a
locally heavier snow band expected to set up after daybreak
Tuesday morning, currently favored to be just south of the
terminals. If confidence increases in such a band impacting the
terminals, a targeted TEMPO for reduced vis would become
warranted. Forecast snowfall amounts are generally up to around
half an inch for the Chicago area terminals and up to around an
inch at RFD.

Snow will begin to taper from west to east mid-late morning,
transitioning to a light rain and/or drizzle. While RFD/DPA mainly
clear out by early afternoon, isolated to scattered rain/snow
showers develop near Lake Michigan that could persist into the early
evening, especially at GYY. This is accounted for with VCSH at
ORD/MDW and -SHRA at GYY (where confidence is highest) where
snow may mixed in at times. Ceilings remain IFR to low-end MVFR
through Tuesday afternoon, then gradually lifting in the evening.

Winds will be light overnight through Tuesday morning as winds back
from SW to SE to NE by late morning. Winds pick up out of the N to
NNW in the wake of the system in the afternoon around 10 kts.

Petr

&&

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

$$

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