503
FXUS63 KLOT 250942
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
342 AM CST Mon Nov 25 2024

.KEY MESSAGES...

- On and off drizzle expected this morning through early evening.

- Turning cold and blustery late tonight into Tuesday morning
  with wind chills in the teens.

- Potential remains for a winter storm in the vicinity of the
  Great Lakes and Ohio River Valley during the Tuesday night
  through Thursday timeframe.

- Pattern shift towards colder temperatures and perhaps snowier
  conditions heading into the first week of December.

&&

.DISCUSSION...
Issued at 340 AM CST Mon Nov 25 2024

Through Tuesday:

Drizzle has yet to develop early this morning. While there are
patchy weak echos visible via the KLOT radar, given cloud bases
are still 5000+ ft this is likely not reaching the ground yet.
While still too fast with the onset timing, models continue to
suggest cloud saturation depths eventually become sufficient for
greater drizzle coverage after daybreak as the center of the
weak surface low moves overhead. There may be a relative lull in
drizzle coverage late morning/early afternoon before an axis of
additional light rain and/or drizzle currently across eastern
NE/western IA is forecast to drift through during the afternoon
into early evening coincident with the trailing mid-level short-
wave. Temperatures during any rain/drizzle today are expected
remain above freezing.

Winds will turn breezy out of the northwest by mid afternoon
which should help dry off any lingering damp surfaces before
temperatures drop below freezing. With clearing skies expected
overnight in the wake of today`s cold front expect a much colder
night with lows in the 20s. With continued gusty winds expected
overnight into Tuesday morning, wind chills are forecast to
drop into the teens! Continued cold air advection into the
region should limit warming to mainly the mid to upper 30s in
the afternoon.

Petr


Tuesday Night through Sunday:

The main focus of the extended period continues to be the
potential for a winter storm in the vicinity of the Great Lakes
and Ohio River Valley midweek followed by much colder
temperatures over the holiday weekend and beyond.

Up first will be the potential winter storm which will be
driven by the evolution of an upper-low/trough just of the coast
of the Pacific northwest and a shortwave trough centered over
the boarder of British Columbia and Alberta. Both of these
features are expected to move into the northwest CONUS overnight
tonight and through the day on Tuesday, but with the Canadian
shortwave forecast to be over northern MT by Tuesday night while
the Pacific trough lags over the Pacific northwest. From this
point is where the uncertainty grows as guidance continues to
struggle on the speed and interaction (or not) of these two
features heading into Wednesday. However, still only two
scenarios continue to highlighted within the latest guidance
suite.

The first scenario which is supported by the ECMWF and several
of its ensemble members, is that the two features phase (or
merge) into a broad trough as they traverse across the central
Plains on Wednesday. If this is to occur then a more neutrally
tilted trough would pivot across the Great Lakes and Ohio River
Valley Wednesday night into Thanksgiving resulting in the
development of a surface low along a stout baroclinic zone near
the Ohio River. This baroclinic zone would also serve as the
focus for some hefty snowfall accumulations along itself and to
its north which could lead to travel impacts for Thanksgiving
somewhere in the southern Great Lakes and Ohio River Valley
regions. This scenario would be more problematic for us locally
as the main snow axis would be much closer if not somewhere in
our forecast area.

The second scenario which is supported by the GFS, GEM, and
their respective ensembles as well as a handful of the ECMWF
ensemble members; is that the two features do not phase and
pivot across the central CONUS as two separate waves. If this
scenario is to occur then the Canadian wave would traverse
across the Upper Midwest and Great Lakes resulting a perhaps a
few snow showers as it races by Wednesday night. The Pacific
trough on the other hand, would then traverse into the central
and southern Plains becoming more positively tilted as it does
so. This positive tilt would focus the troughs forcing into the
southern Ohio and Tennessee Vallies where guidance shows a weak
surface low developing along the stalled baroclinic zone. While
precipitation still occurs along and north of the baroclinic
zone in this scenario, it would be focused well south of our
area over southern IL and IN. Meaning that locally we wouldn`t
have much aside from the aforementioned spotty snow showers and
any minor accumulations with those.

Unfortunately, which of these two scenarios will come to
fruition remains anyones guess at this point. While there
continues to be subtle shifts within the ECMWF ensemble towards
the lesser phased solutions (the second scenario described
above), there is still a strong signal supporting the first
scenario that makes this forecast highly uncertain. That said,
the latest forecast this morning will continue to advertise the
potential for some snow in the area starting Tuesday night with
a leading subtle frontogenetic band of showers followed by the
main system Wednesday through the day on Thursday. Given the
continued uncertainty we continue to recommend people stay
updated on the forecast, especially those with travel plans for
Thanksgiving, and we will continue to closely watch forecast and
observational trends with these features in hope that a more
unified solution materializes soon.

Regardless of which scenario pans out, the phased trough or two
separate ones will exit the area Thursday night and become
intertwined with a broad upper low that is forecast to develop
over northern Ontario and Quebec. Initially this upper low looks
to support the development of some lake effect showers into
northwest IN and Lower MI, but as winds turn more westerly on
Friday the bulk of the lake effect should pivot into northern IN
and Lower MI. Additionally, the upper low will also usher in
much colder air (compared to what we`ve been experiencing) for
the holiday weekend and into at least the first week of
December. Therefore, expect high temperatures during this time
to be in the 20s with overnight lows in the teens to near single
digits resulting in wind chills near zero at times.
Furthermore, guidance does show some shortwaves pivoting around
the upper low and across our area at times during this period
which could result in additional periods of snow. While
confidence is much higher (around 80-90%) on the cold,
pinpointing when and where the aforementioned shortwaves with
the upper-low will be at this range is difficult so stay tuned.

Yack

&&

.AVIATION /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1120 PM CST Sun Nov 24 2024

The main aviation weather concerns for the 06Z TAF period
include:

* Light rain and drizzle tonight into Monday

* MVFR cigs beginning late tonight with IFR possible during the
  morning

Scattered pockets of drizzle and perhaps a light shower or two
will overspread the area tonight after 06Z which may continue
through the better part of Monday morning. It`s during the
morning that we see the greatest potential for a period of MVFR
vsbys associated with the drizzle, although this signal has come
down recently. A second wave of showers may move across during
the afternoon as well.

Cig guidance through the period has been rather inconsistent
leading to low confidence in trends. Cigs will gradually sink
during the night and are expected to hit MVFR territory late
tonight, possibly holding off until closer to daybreak. MVFR is
then expected through the day on Monday. Model guidance has
recently backed off quite a bit on the IFR potential for
tomorrow, especially at the Chicago sites. While a period of IFR
cigs remains possible during the morning hours, the stronger
signal now favors holding at MVFR. VFR looks to return later in
the evening.

Meanwhile, light near-southerly winds tonight will likely go
variable at times. We`ll see westerly winds accelerate during
the mid to late morning to feature 20 to 25 kt gusts into the
afternoon, building to 25 to near 30 kt during the latter part
of the afternoon. Winds will remain breezy through the night and
into Tuesday.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 2 PM this afternoon to 9 AM CST
     Tuesday for Winthrop Harbor IL to Gary IN.

     Small Craft Advisory from 2 PM this afternoon to 3 PM CST
     Tuesday for Gary to Burns Harbor IN-Burns Harbor to
     Michigan City IN.

&&

$$

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