Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 01/05/23

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 912 PM CST Wed Jan 4 2023

.UPDATE...
Issued at 912 PM CST Wed Jan 4 2023

Periodic light precipitation will continue tonight as a series of
lead vort lobes embedded within broad cyclonic flow traverse the
region. VAPOR AMDAR soundings this evening continue to show the
top of the cloud deck is hovering right around -10 C, and this is
really right on the cusp of supporting cloud ice. While GOES IR
loops show ephemeral deeper saturation and attendant colder cloud
tops, significant pockets of warmer cloud layers exist, suggesting
p-types will probably oscillate between all snow and drizzle
(freezing drizzle where temperatures fall under 32 F). Have
attempted to reflect this in the grids through the early overnight
hours. After recent heavier rainfall, concern is that road
treatment has likely been effectively washed away, and with road
sensors showing surface temperatures near 30-31 degrees across
northern Illinois, there is some threat for icy/slick spot
development tonight. Since precipitation remains pretty light and
not entirely drizzle, thinking is this will help curtail
potential travel impacts, but have re-issued a Special Weather
Statement to highlight the threat for some slick travel overnight.

A notably stronger vort lobe will arrive late tonight and into
Thursday morning. Upstream radar looks pretty impressive, with
decent coverage of 20-25 dBZ returns across central Iowa. Have
boosted pops after 4 AM when DCVA looks to maximize into our
western counties. P-type by this time should be all snow as the
column will have cooled markedly. Expecting pretty good coverage
of snow through Thursday morning, and think there could be some
pockets of heavier bursts, especially along and south of I-88.
Some isolated "streaky" accumulations over an inch seem quite
plausible along with resultant slippery travel due to snow-coated
elevated and less-traveled surfaces. Snow intensity is expected to
ease during the afternoon.

Carlaw

&&

.SHORT TERM...
Issued at 300 PM CST Wed Jan 4 2023

Through Thursday night...

Main forecast messages and concerns:

* Patchy freezing drizzle potential this evening through early
  overnight, with slick spots possible on some roads

* Waves of light snow/snow showers/flurries tonight, with minimal
  if any accumulation

* Waves of light to moderate snow/snow showers on Thursday with
  minor accumulations, periodic reduced visibility, and slippery
  travel on some roads

Another expansive well defined closed mid-upper level low pressure
will affect the area through Thursday evening. This evening, at
least to start, the issue is marginal saturation into the DGZ
(dendritic/snow growth zone), as low as -10C. As of this writing,
multiple observation sites are showing BR with reduced visibility,
with a few spots of explicit drizzle being reported. These obs at
non- manned automated sites can be indicative of drizzle, with
freezing drizzle where temps are at or below 32F. This is despite
sufficiently cold IR temps on GOES-East.

With the marginal saturation depth, it can be a fine line between
mainly light snow/flurries, snizzle (a mix of light snow and
drizzle), and primarily drizzle/freezing drizzle. With
temperatures in the Fox Valley and points west already down in the
32-33F range and temperatures expected to slowly fall tonight,
this is the primary of concern for slick spots due to patchy
freezing drizzle. Another factor to keep in mind is that prior
road treatments were likely washed away by the waves of rain the
past few days. Will have to keep a close eye on observations
through this evening.

Expecting the column to trend to more snow supportive with time
(deeper saturation in the DGZ and isothermal near -10C), so if
freezing drizzle or a mix with light snow occurs this evening, it
should go to primarily snow. Regarding the snow through Thursday,
our feature of note is a stout vort max circulating around the
upper low gyre, currently centered near the IA/MN border and
producing decent coverage of snow in that area and points north.
There are liable to be pockets of modestly enhanced snow showers
later this evening and overnight pending sufficient DGZ saturation.

Then on Thursday, the core of the aforementioned vorticity maxima
will translate eastward and produce good forcing for ascent amidst
a by then saturated DGZ and cold advection. Snow/snow shower
coverage should increase fairly quickly after sunrise, peaking in
the mid- late morning through mid afternoon (~8am-3pm CST), and
then easing to scattered coverage after sunset as the daytime wave
shifts east. Snow showers should taper to flurries and end late
evening and overnight following one last mid-level short-wave.
With relatively steep mid-level lapse rates paired with the peak
of snow coverage, envision occasional moderate snow rates with
visibility as low as a bit under 1 mile. Temperatures will
generally be near to a couple degrees below freezing with the
snow, so pavement accums will be tied to prior treatment, pavement
temps, and how heavily traveled the roads are. Thus, would appear
bridges, overpasses, less traveled roads and side streets would
be most susceptible to being coated with snow. Minor accumulations
generally of a coating to a few/several tenths are forecast,
though localized 1" totals wouldn`t be surprising where duration
of higher snow rates maximizes.

Cloudy skies will be the rule until perhaps late Thursday night
after the flurries end, so should be another limited diurnal
range, though a bit colder (mid-upper 20s northwest CWA) to around
30F east.

Castro

&&

.LONG TERM...
Issued at 244 PM CST Wed Jan 4 2023

Friday through Wednesday...

The first workweek of the new year will wrap up on a quiet note
Friday with higher pressure building in at the surface behind the
departing storm system. Steady low-mid level dry air advection on
the lee side of the approaching low-amplitude ridge should introduce
a fair amount of blue sky for the latter half of the daytime with
highs expected around or just above freezing. Behind the
aforementioned ridge, a trough will be digging into the central
Plains and is expected to become a closed upper low by Friday night.
A half-decent jet max is anticipated to develop around the trough
base and translate to an organized surface low which will be
trekking across the Midwest through Sunday. The exact track of this
low, however, is not well-agreed upon, though it will be a
determining factor in our weekend`s snow chances.

Guidance has been shying away from snow chances this weekend over
the past day or so. The GFS remains the most excited with the snow
potential given its outlier of a storm track for this low. The GFS
and Euro are friendly through about 18Z Saturday with the low
centered over SE MO before the two split the storm off in completely
different directions. The GFS has the low jetting to the northeast
and cutting right across the CWA. This is a result of the GFS
showing a deeper upper low on Saturday leading to southwesterly flow
on its lee side guiding the surface low up into the Great Lakes. As
far as precipitation goes, this solution would likely bring snow for
most and rain for some in the southern CWA who manage to sneak into
the storm`s warm sector Saturday afternoon through night followed by
additional snow showers on the backside of the low through a good
deal of Sunday. Most other guidance, however, favors more zonal flow
on the leeward side of the upper trough leading the surface low
across the lower Midwest. This solution results in a much drier
profile and little forcing that would suggest little to no chances
for snow, at least for most of the CWA. The NAM, Canadian, and, most
recently, the Euro have hopped on this train. Additionally, few
members of the GEFS have snow making into the CWA and snow chances
are dropping off of the Euro ensemble members as well. The NBM
continues to bring chances for snow to the whole CWA for Saturday
evening and night. Given that the Euro continued to show snow area-
wide through its 06Z run, decided to leave chances in for the time
being. However, if little changes over the next few runs, I would
imagine those chances will be dropping off, or at least demoted to
slight chances before long.

Temperatures will be somewhat steady through the long term period
with highs in the 30s expected to hang on through the weekend and
the earlier half of next week. 40s may sneak in across the southern
CWA Monday and Tuesday with some modest low level WAA taking place
behind the trough. A good deal of sunshine is forecast for Monday
before more clouds move in with a low level jet advecting moisture
into the area ahead of an elongated low level trough. Models show a
weak cold front moving across on Tuesday. As of now, the majority of
guidance remains dry along the front. However, a few ensemble
members are showing a signal for some snow developing within the
surface trough. With ample moisture in play, snow appears certainly
possible looking at forecast soundings, though this is something we
will continue to watch. As of now though, the forecast for this
period is dry.

Doom

&&

.AVIATION...
For the 00Z TAFs...

Deep cyclonic flow will remain locked across the Upper
Mississippi River Valley leading to generally steady-state
conditions through the TAF period. Areas of drizzle this evening
will transition to on-and-off snow grains with visibility
generally above 6 miles. Occasional periods of snow associated
with vort maxes embedded in the upper-level cyclonic flow will
present opportunities for visibility reductions to 1-2SM. The
first wave is now crossing the Mississippi River and is on track
to reach RFD toward 02Z. Should it hold together, it would reach
ORD/MDW toward 05Z (will withhold mention for now in favor of
watching trends). Thereafter, the next signal for snow is from
mid-morning to mid-afternoon Thursday (14-20Z or so), which
warrants a continuation of the inherited PROB30 groups. Snow
showers should begin to taper to flurries Thursday afternoon and
especially evening.

Winds should remain out of the southwest at 8 to 12 kt and cigs
should hover between 800-1200 feet through the entire TAF period.

Borchardt

&&

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

$$

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