Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 01/31/22

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 519 PM CST Sun Jan 30 2022

.SHORT TERM...
Issued at 200 PM CST Sun Jan 30 2022

Through Monday night...

Overall weather concerns through the short term are minor, and
consist of the potential for very light lake effect snow across
parts of northwest Indiana this afternoon and evening, along with
some threat for patchy (freezing) fog mainly across portions of
interior northern Illinois overnight into Monday morning.

Regional satellite loops reveal an area of lake effect cloud cover
pushing inland across northeast Illinois and northwest Indiana
early this afternoon. Infrared satellite indicates cloud top
temperatures are generally warmer than about -13 C, and this has
been confirmed by a couple recent AMDAR soundings, which is on the
verge of supporting ice nucleation in the shallow cloud layer.
Webcams from Chicago suggest all snow, however, and expect this
activity will continue for another hour or two as the lake band
pivots southeastward. No accumulations are expected. Farther to
the south, this lake effect precipitation will make more of a
concerted push into Lake and Porter Counties in northwest Indiana
through the rest of the afternoon and evening, and embedded 25 dBZ
returns from the TORD/TMDW terminal dopplers suggest snow falling
at enough of a clip to knock visibilities down to 2-4 miles at
times. Even with the warm(ish) cloud tops, thinking is that
precipitation will be mainly in the form of snow given the
presence of deeper/convective appearance to the cloud layer
upstream. Lake parameters are marginal, but given the radar
returns and upstream cloud mass, could envision some patchy
coatings on elevated/less traveled surfaces through late- evening
before winds turn more southwesterly overnight.

Surface high pressure crests overhead tonight, yielding very light
winds across the region. This will set the stage for at least some
potential for fog development overnight, dependent on the
expansiveness of any lake effect cloud cover or incoming cirrus.
Latest guidance suggests we may be able to punt the higher cloud
cover east of the region late this evening/early overnight, and
some guidance is aggressively responding to this with cooling over
the snowpack leading to the development of fog. For now, have
introduced some patchy fog wording mainly for interior portions of
northern Illinois (near and north of I-88).

Otherwise, looking for the development of southerly breezes
through Monday and increasing cloud cover late in the afternoon as
a leading warm advection wing pushes overhead. A stout low-level
jet then develops through Monday night as a lead wave pivots into
the Minnesota Arrowhead Region which should result in gusty south
winds across the region and probably rising surface temperatures
after midnight.

Carlaw

&&

.LONG TERM...
Issued at 313 PM CST Sun Jan 30 2022

Tuesday through Thursday Night...

[Overview]

A complex one-two punch of winter weather Tuesday evening and
night into or through Thursday is starting to show some signs of
coming into better clarity. At this time, there is still lingering
uncertainty, but increasing confidence in the main idea of the
Tuesday eve/night through Wednesday period, for potentially
significant snow accumulations from moderate to heavy
frontogenesis enhanced snow bands in an over-running set-up. As
things stand, the highest potential snow accumulations of several
inches and thus higher travel impacts favor near and southeast of
I-55, and a sharp cut-off to the accumulating snow not too far
northwest of the increasingly likely heavier swath of snow.

The next round, Wednesday night into or through Thursday, has been
much lower confidence due to large differences in the model and
ensemble data. However, while confidence is still on the lower
side, one of the major guidance suites started to offer more
support for this next potentially high impact round of snow and
wind. Note that there may not be a full break in the snow for some
locations depending on how everything ultimately involves.
Relatively speaking, the *currently more favored* area for these
potentially significant additional accumulations and associated
higher-end travel impacts is again near and southeast of I-55.
However, with this being a fluid situation dealing with multiple
moving parts, do not be surprised if there are substantial changes
in this portion of the forecast. Finally, improved lake induced
thermodynamics with time will make synoptic lake enhancement of
snowfall a distinct possibility into northeast Illinois and far
northwest Indiana.

[Full Meteorological Details of the Set-up and Forecast Notes]

*Tuesday Evening through Wednesday*

The precursor mid and upper level pattern of the impending
regional two-part winter storm has been well advertised at this
point: a deep negative height anomaly (trough) digging into the
interior west and southwest CONUS; a strong lobe of the polar
vortex approaching Hudson Bay; and moderate to strong positive
height anomalies (ridging) over the eastern US into eastern
Canada.

At the surface, a cold front from an initial wave in the
western trough will move across the area Tuesday afternoon and
evening and then slow or stall as an elongated trough of low
pressure extends northeast from the southern Plains to
Indiana/Ohio border by early Wednesday. Due to the eastern
ridging, there will be open Gulf moisture trajectories and a warm
and unseasonably moist air mass advecting northward. Meanwhile,
due to the encroaching PV lobe from central Canada, strong
1045-1050 mb high pressure will be spilling southeastward from the
Canadian Prairies into the northern Plains.

The contrasting air masses will form a sharpening thermal
gradient (baroclinic zone) favorable for the development of a
lower to mid tropospheric frontogenetic circulation. Short-wave
energy in broad southwest flow aloft will interact with the sharp
baroclinic zone to cause a large area of banded snow to develop
(likely starting as rain in spots late day/early evening Tuesday),
with sharp cut-off expected due to very dry air moving in from
the northwest and also inherent to f-gen banded events.

Uncertain Elements Seen in the Guidance Clusters:
- Amount of northern stream (PV) and Arctic high pressure driven
  influence to the north/northwestern extent of the precip shield
- Exactly where the heavier banding sets-up

These elements are starting to come into better agreement, though
there`s still certainly time for adjustments. Note that higher QPF
in an expected banded set-up is where a model is outputting the
response to frontogenesis. Models do a good job in indicating the
likelihood of strong f-gen, but at this lead time, the actual
location of heaviest mesoscale banding can vary by 25-50 miles or
more north or south. This being said, overall operational and
ensemble support points to the earlier mentioned corridor near and
southeast of I-55 for at least 4" and possibly in excess of 6",
which we highlighted in one of our afternoon weather story
graphics. Also in support of at times heavy snowfall rates would
be the presence of steep lapse rates in the DGZ.

PoPs in the grids through Wednesday were adjusted slightly from
NBM output to reflect the above thinking and in line with high
ensemble probabilities of measurable snow. If the low level arctic
air can outpace the sufficiently cold thermal profiles aloft for
snow, that would introduce some freezing rain concern, primarily
in southern CWA.

*Wednesday Night through Thursday*

This period has been the biggest wild card, with the potential for
more substantial travel impacts due to the addition of strong
winds along with the snow due to tightening pressure gradient
between strong Arctic high to northwest and low pressure to our
south and east.

Primary Uncertain Element Seen in the Guidance Clusters:
- Whether full ejection of a strong short-wave rounding the
  western trough will occur and result in expanding,
  reinvigorated precip shield north of low pressure trough
  tracking across the Ohio Valley
- If any phasing will occur with the northern stream trough

On item one, to put it simply, the GFS/GEFS suite, has been by far
the most aggressive in the trough ejection and not "burying" the
wave in the southwest. Meanwhile, the ECMWF/EPS suite finally gave
much stronger support to the GFS idea. This is still very much in
flux given that the short-wave that will top a west coast ridge
and round the west/southwest trough is still rather subtle and
located southwest of the Aleutians. Much more advanced satellite
data and any Pacific recon will help improved sampling until RAOB
data a few days away, though still possible to have large errors
with this feature.

The general GFS/ECMWF idea is that the short-wave will eject
across the southern Plains and lower MS Valley and have some
interaction, but not fully phase with the northern stream. Did
note increased downstream positive height anomalies today and with
contribution of latent heat release, wouldn`t be surprised (based
on past experience/pattern recognition) in the PV anomaly
adjusting northward, *assuming it is strong and fully ejects*. We
still can`t discount the other guidance cluster idea that the
main wave is more sheared and northern stream races ahead of it,
keeping reinvigorated precip mainly in our southern CWA and south.
This part of the forecast will be the difference in the one-two
punch being one that produces significant to even excessive
snowfall totals in parts of the CWA, to one that is mainly front-
end driven featuring moderate to locally significant totals.

Finally, regarding synoptic lake enhancement, this item will be
dependent upon the strength of the large scale forcing that
remains uncertain. Not seeing particularly impressive inversion
heights, though they do gradually improve along with instability
as the cold air mass deepens. In these set-ups, often see speed
convergence of the brisk north-northeast flow from the lake onto
land play a role, so would be surprised if there wasn`t at least a
modest boost to totals closer to the lake.

PoPs were again tweaked a bit from NBM output in line with the
latest thoughts, particularly Wednesday night into Thursday, where
confidence nudged upward due to much stronger support from ECMWF
suite of guidance.

As we`ve been saying for the past several days, stay tuned for
updates and plan for potential/likely travel impacts, and steer
clear of verbatim model snowfall outputs being shared on social
media.

Castro

&&

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

Only real aviation concern is small threat for some ground fog
early Sunday morning. If high cloudiness clears out sufficiently
later tonight, then some ground fog could develop, particularly at
DPA/RFD/GYY. Otherwise expect VFR conditions through the TAF cycle
with south-southeast winds developing Sunday morning and slowly
increasing some through the day.

- Izzi

&&

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

$$

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