513
FXUS63 KLOT 242352
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
552 PM CST Sun Nov 24 2024

.KEY MESSAGES...

- A surface low will move over the region providing periods of
  rain showers and drizzle starting tonight through monday
  evening.

- Cold morning temperatures return starting Tuesday morning
  after the cold front

- There remains a chance (33% or so) for a winter storm to
  impact the region in the Tuesday night through Thursday
  timeframe.

- There is high confidence (80-90% chance) in much colder and
  perhaps snowier conditions arriving for the start of
  meteorological winter (next weekend onward).

&&

.DISCUSSION...
Issued at 158 PM CST Sun Nov 24 2024

Through Monday Night:

High clouds dominate the skies for areas north of I-88, but
better clearing to the south has allowed for sunshine to break
through this afternoon. However, as the next system moves in,
expect cloud cover to increase this evening.

A surface low, currently out over the Great Plains, will move
east to northeast over the region tonight and tomorrow providing
the next chance for rain. Guidance came in a little slower to
the low`s movement. Moist low levels and warm air advection out
ahead of the low are expected to produce in patchy drizzle out
ahead of the low through the evening. Better chances for
scattered showers to develop will occur after midnight through
daybreak with better forcing as the center of the low approaches
the forecast area.

A surface cold front will move eastward over the area following
the low`s path. Lingering drizzle/light rain is possible along
the front as it sweeps through. High temperatures will still
reach the upper 40s and low 50s on Monday, but steadily drop in
the mid to late afternoon as winds switch to the northwest and
better cold air advection filters in. Wind gusts will become
breezy Monday night into Tuesday morning as temperatures drop to
below freezing. Overnight lows will fall to the upper 20s east
of I-55, with low to mid 20s north and west. Luckily, the
showers will have ceased (maybe a lingering shower in Porter Co
IN?) before then to not have concerns about frozen
precipitation.

DK


Tuesday through Sunday:

The main focus in the long term period is (1) the threat for wintry
weather in the general region Tuesday night through Thursday, and
(2) a sharp drop in temperatures at the end of November and early
December.

GOES-West imagery depicts a positively-tilted upper-level trough off
the northern coast of California and a narrow upper-level shortwave
along the central borders of British Columbia and Alberta. Over the
next 48 hours, the evolution of both features will be paramount
to how the weather evolves in the central US during the middle
of the week including during the all-important pre-Thanksgiving
travel period with the key question being whether they phase
together or not.

If the two features phase together, a tightening of the baroclinic
zone draped across the Lower Great Lakes beneath coupled upper-level
jet streaks would enable the rapid development of a surface low in
the Ohio River Valley. Along the northwestern side of the low would
be a shield of accumulating snow across the Lower Great Lakes
including in our area with associated impacts to travel, primarily
in the Wednesday afternoon through early Thursday morning timeframe.
This is the scenario with travel impacts in our area in the pre-
Thanksgiving period.

If the two features fail to phase together, the Canadian shortwave
would be poised to propagate quickly through the Great Lakes in
the Tuesday night-Wednesday timeframe with perhaps non-
impactful snow showers (dry sub-cloud layer). Meanwhile, the
Pacific trough would track much further southeast, favoring the
development of a surface low over the Appalachian Mountains with
little to no snowfall (cold air too far displaced to the
northwest). In other words, this is the scenario with little to
no travel impacts in our area in the pre- Thanksgiving period
with mostly dry weather and seasonable temperatures.

So, which scenario is most likely? From the perspective of a
statistical approach, either scenario are equally likely. With
that said, the scenario where the upper-level troughs phase is
being almost ubiquitously advertised by only EPS members, as
opposed to broad support by members within all three ensemble
systems (EPS, CMCE, and GEFS). In addition, the number of
individual EPS members showing the phased/impactful outcome have
been steadily decreasing over the past 24 hours, with 29/51
(around 57%) of members on board with the 12Z run yesterday
compared to only 13/51 (roughly 33%) of members with the 12Z run
today. Meanwhile, from a meteorological perspective, the upper-
level pattern being largely dominated by a broad upper-level low
centered over eastern Canada will force largely zonal upper-
level flow across the central United States which is typically
not overly favorable for phasing upper-level troughs. With these
factors in mind, would have to think the unphased/not impactful
scenario may be slightly more favored at this point, though
that does not mean we are out of the woods. All things
considered, will maintain the inherited low to mid-range chances
(20- 50%) for snow in our area in the Tuesday to Thursday
timeframe, and continue messaging the need to watch the forecast
if planning to travel through the general area next week.

Regardless of what happens in the middle of the week, ensemble model
guidance continues to advertise a remarkably strong signal for the
coldest air so far this season to spill into the Great Lakes this
weekend. Conceptually, such a shift in the pattern makes sense as
the zonal flow midweek will cause all the shortwaves propagating
eastward across the country to collect and merge with the
aforementioned Ontario upper-level low. As a result, the broadening
upper-level cyclonic flow across the northeastern US will open the
door for seasonably cold air in western Canada to spill
southeastward into the Great Lakes. Ensemble mean highs in the 20s
and lows in the teens early next week hence appear on target.
Considering our local area will likely be beneath upper-level
northwestern flow to the southwest of the center of the Ontario
upper-level low, would also have to think that clipper-like systems
may parade close to or over our area next weekend onward, too. So,
grab those winter jackets since meteorological winter (which starts
December 1) will start undeniably winter-like.

One final note, confidence in the pattern shift next weekend and
beyond is quite high (80-90% chance for below average temperatures,
per the CPC 6-10 day temperature outlook), unlike confidence with
the system during the middle of this week (call that a 33% chance).

Borchardt

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 552 PM CST Sun Nov 24 2024

The main aviation weather concerns for the 00Z TAF period
include:

* Light rain and drizzle tonight into Monday with associated
  vsby restrictions possible

* MVFR cigs late tonight dropping to IFR early Monday morning

Scattered light rain showers will overspread the area late this
evening and remain through the night. The showers are expected
to change over to patchy drizzle by daybreak Monday which may
continue through the better part of the morning. It`s during the
morning that we see the greatest potential for MVFR vsbys
associated with the drizzle. Additional pockets of rain and
drizzle will be possible into the afternoon, although there is
lower confidence in the afternoon period.

Cigs will gradually sink this evening and are expected to hit
MVFR territory not long after 06Z. MVFR is then expected
through the night, but cigs will continue to fall and should
drop to IFR near dawn. We should spend most of Monday morning
beneath IFR cigs before lifting to MVFR around late
morning/early afternoon and remaining as such through the rest
of the day.

Meanwhile, ESE winds near or below 10 kt this evening will go
light for the overnight. 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
evening.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 2 PM Monday to 6 AM CST Tuesday for
     Winthrop Harbor IL to Gary IN.

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

&&

$$

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