854
FXUS63 KLOT 232343
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
543 PM CST Sat Nov 23 2024

.KEY MESSAGES...

- Low clouds will continue overnight with areas of fog,
  potentially dense, developing by midnight.

- Sunday will be noticeably warmer with highs in the 50s, though
  showers and drizzle will develop Sunday night and continue
  into Monday morning.

- Potential for travel impacts from accumulating snow in the
  Tuesday night to Thanksgiving Day period appears to be
  increasing.

- Below normal temperatures will prevail Monday night through
  Thursday, followed by much below normal (true mid-winter
  cold) Friday-Saturday and beyond.

&&

.DISCUSSION...
Issued at 228 PM CST Sat Nov 23 2024

Through Monday Morning:

We remain stuck under a low-level stratus deck.

Neutral to modestly positive mid-level height tendencies and
associated subsidence are forcing the inversion to lower and cloud
layer to thin. Indeed, recent AMDAR soundings from aircraft
arriving/departing MDW indicate that the base of the inversion has
lowered some 1000 feet in the past 3 hours, and forecast soundings
suggest the cloud depth is a mere 1000 feet deep. It is thus not
surprising that recent satellite imagery across eastern Iowa and
southwestern Illinois (where inversion heights/cloud depths are even
lower/shallower) is depicting an erosion of the stratus layer thanks
to the mechanical entrainment of dry air via the warming boundary
layer. As steering low-level flow become southwesterly this
afternoon (thanks to the center of a surface high positioned just to
our southeast), the clearing line should sneak into northwestern
Illinois and central Illinois by this evening.

Where the clearing line is at sunset may be fairly important to how
the night will go. As low-level warm-air advection gradually
increases, the strength/depth of the inversion will correspondingly
increase. With a lack of mixing beneath the inversion to erode
stratus, cloud bases along the southwestern edge of the stratus
layer will be encouraged to quickly "build downward." Now, it`s
always tough to know if dense fog or just very low clouds will be
favored anytime low-level flow is non-negligible in these regimes.
For now, felt there was enough justification to introduce a
northeastward-moving area of low clouds/fog across our area tonight.
Overnight lows should settle in the low to mid 30s.

Tomorrow, aggregate zonal upper-level flow with consolidating
shortwaves traversing the Rocky Mountains should induce the
development of a broad surface low in the central Plains. As a
result, southeasterly winds will increase locally after daybreak and
erode any lingering fog/low clouds. Robust warm-air advection (950mb
temperatures rising ~7C in 24 hrs) will facilitate to a jump in
afternoon temperatures to the low to mid 50sF in spite of overcast
skies (upper-level clouds).

Tomorrow night, the surface low will lift northeastward toward the
Lower Great Lakes. The passage of a low-level warm frontal zone and
aggressive warm-air advection facilitated by an intensifying 925-
850mb low-level jet will set the stage for widespread drizzle and
rain showers. (Forecast BUFKIT profiles exhibit a classic
"crosshairs" signature of lift within a deepening low-level stratus
layer and a double-inversion temperature profile). Accordingly,
opted to aggressively increase PoPs and collaborate the introduction
of QPF with WPC tomorrow night to offset the deficient QMD bias
correction scheme from which the NBM suffers. Drizzle and rain
showers will be poised to continue unabated until the cold front
associated with the surface low surges through after daybreak on
Monday. As a result, Sunday night through Monday morning look rather
dreary.

Borchardt


Monday Afternoon through Saturday:

In the wake of the cold front, Monday afternoon will feature
blustery winds (westerly gusts up to 30 mph) and falling
temperatures. The guidance continues to exhibit run to run
variance with the extent of post-frontal light rain Monday
afternoon and early evening. Over the past few cycles, the GFS
has been the most consistent in sufficient moisture for a
footprint of QPF to traverse southeast in response to short-wave
impulse crossing the mid and upper MS Valley. PoPs in the 30-40%
range continue to appear reasonable given the lingering uncertainty.
Skies might *finally* clear out in the continued blustery cold
advection Monday night into Tuesday. Despite chilly temps only
topping out in the mid to upper 30s, Tuesday may have the only
real chance for an all too fleeting glimpse of the sun through
Thanksgiving Day.

For the all important busy Thanksgiving travel period, there are
a few features of interest. The past few model cycles have shown
an increasing signal for warm advection and frontogenesis driven
period of snow (modest accumulations) and associated travel
impacts late Tuesday night into/through the Wednesday morning
commute. In collaboration with neighboring offices, adjusted
PoPs into the (30%) chance range across the area.

Even more notably, a fairly moisture laden southern stream wave
is likely to emerge from the southern Rockies during this period.
Strongly confluent flow over northeastern North America caused
by a deep PV lobe near James Bay will tend to cause more zonal
flow less prone to strong amplification of the southern stream
wave. With that said, one of the key sensitivities in the
forecast is the potential for a northern stream wave crossing
the northern Rockies to phase with the southern stream mid-level
wave. This constructive interaction (or lack thereof) has major
implications on the track, strength, and wintry impacts caused
by the surface low pressure system passing to our south later
Wednesday into Thanksgiving Day.

Given the noteworthy increase in ensemble membership for a
stronger system tracking farther north near the Ohio River
Valley, increased PoPs into the 30-50% range area-wide Wednesday
afternoon into Thanksgiving. The higher PoPs are still indicated
south of I-80 to account for the slightly more suppressed
solutions that comprise a sizable ensemble membership of their
own. Lower-level thermal progs appear cold enough for a
primarily wet (due to temps generally near/above 32F) snow
event, barring a much stronger solution which would bring the
rain/snow line farther north. For this reason, did hold onto a
chance of rain primarily south of I-80 Wednesday evening/night.
Northeast winds off the mild waters of Lake Michigan may pose
some p-type and/or accumulation efficiency issues along the
lakeshore, including Chicago, if the stronger/wetter outcome
comes to fruition. Nevertheless, the Wednesday afternoon into
Thanksgiving period bears close watching for accumulating snow
and travel impacts in the general region. Regardless of how
Wednesday PM-Thanksgiving Day plays out, another northern stream
trough could bring a chance of light snow/snow showers
Thanksgiving night into early Friday.

In a stark change from recent cool seasons, looking ahead into
the beginning of December, the decidedly cold and wintry
pattern will persist. Pronounced high latitude riding over the
northeast Pacific near Alaska will allow for plenty of cold air
to surge southward from our source region (Canada and the
northern Plains) later week and beyond. Friday looks to be
blustery and cold (highs only in the upper 20s-lower 30s!) for
the busy holiday shopping day. The lake effect machine will
undoubtedly get going in this pattern, though likely focused
primarily east of our CWA on Friday-Friday night due to west-
northwest flow. Following lows in the teens to lower 20s (Brr)
Friday night, highs on Saturday may not get out of the 20s, with
even colder air waiting in the wings for the rest of the weekend!
We`ll have to keep an eye on any low amplitude clipper-type
waves in the upcoming pattern for additional opportunities for
periods of accumulating snow.

Castro

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 543 PM CST Sat Nov 23 2024

Key Messages:

- Challenging forecast period tonight into Sunday morning with
  overall low confidence in cig and vsby trends. Unscheduled TAF
  amendments will probably be necessary at times tonight.

- MVFR cigs expected to build down to IFR this evening. BR/FG
  and LIFR/VLIFR conditions remain possible, particularly late
  tonight into Sunday morning, but confidence is too low for a
  mention in the TAFs.

- Winds turn southeasterly Sunday morning.

- Threat for low clouds and drizzle increases towards the end
  of the extended ORD/MDW TAF windows.

Low confidence aviation forecast tonight into Sunday morning.
Prevailing low MVFR cigs at the Chicago-area terminals should
slowly build down to IFR this evening. However, the back edge of
the stratus deck is making slow but steady northward progress
out of central Illinois. If this continues at the current rate,
VFR conditions could even return after 9-10z. With increasing
warm advection expected tonight however, redevelopment of low
stratus remains on the table, so elected to continue with the
trends established by the previous TAFs and have carried IFR
through mid-morning Sunday.

The addition of a thick plume of incoming mid and high-level
cloud cover also complicates the forecast for br/fg tonight as
this generally works to stunt visibility reductions to some
degree. If conditions continue to clear out across central
Illinois, it`s conceivable that additional low clouds and fog
will develop and then surge northward towards the c90 late
tonight and early Sunday morning as the low-level flow becomes
more southerly with time. Confidence in this outcome remains
under 40-50 percent, however, precluding a formal mention in
the TAFs. With that said, the period of greatest concern for
possible LIFR/VLIFR conditions is during the roughly 9-15z time
period.

Winds will turn southeasterly through Sunday morning with any
lingering br/fg and low cloud cover scattering. VFR conditions
are expected into the afternoon before clouds begin to build
down again into the evening in advance of the next weather
system. Another round of low clouds and drizzle will likely
develop towards the very end of the current ORD/MDW extended
TAFs.

Carlaw

&&

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

&&

$$

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