Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 01/18/21

Area Forecast Discussion...UPDATED

National Weather Service Chicago/Romeoville, IL

 906 PM CST Sun Jan 17 2021

.UPDATE...
905 PM CST

The going forecast is in solid shape. Similar to the past 48+
hours, there is a low potential for patchy freezing drizzle
between the light snow/flurry action through Monday morning.

The radar this evening continues to show areas of flurries with
transient zones of more light snow (such as out our window here
in Romeoville as of 845 PM). In northwest Indiana, the forcing for
this light snow was still associated with today`s primary short
wave as well as some lake enhancement and lake-induced
convergence. This is slowly tapering, but still some pockets of
light snow further west, likely owing to subtle lift from a
possible weak sheared wave on the backside of the broader cyclonic
mid-level flow. Satellite indicates cloud tops of -13C or so and
most reports, automated and from mPing, indicate light
snow/flurries. Aircraft soundings and the 00Z DVN sounding do show
a wee bit of spread in the temperature and dew point traces. That
hints that pockets of drizzle remain possible, and occasionally
we see an unknown precip automated report that is likely that.
But cloud bases are generally above 1,500 ft in most of northern
Illinois and southern Wisconsin, making it difficult for any
coating drizzle to manifest itself. So not expecting much of a
noteworthy threat with any of the precipitation over the next
several hours.

Further upstream observations over southeast Minnesota and far
northern Iowa do indicate some lower cloud bases and an area of
light precipitation -- primarily light snow/flurries. This is
associated with another backside sheared short wave. Will have to
watch how this angles into north central Illinois very late
tonight and possibly northeast Illinois around daybreak. Forecast
soundings from the RAP and NAM for northern Illinois do show some
weak omega during that time. This and the saturation look to be
collocated up to the -13C height, which would point toward
flurries/light snow if enough forcing can materialize this far
east-southeast.

MTF

&&

.SHORT TERM...
329 PM CST

Through Monday night...

While some of the heavier snow rates have gradually waned over the
last few hours, light snow continues to linger over much of the
region this afternoon. This will continue to be the case into the
evening hours when snow is expected to end from west to east as the
upper low/vort max over central Illinois pinwheels its way off to
the east.

It appears unlikely that any additional snow accumulations today
will exceed more than a few tenths, but depending on how much low-
level wind convergence we can muster out over the lake late this
afternoon and evening, surface-850 mb delta Ts of around 13 degrees
Celsius, equilibrium levels potentially rising to 7000-8000 ft
AGL, and the presence of 100-200 J/kg of lake-induced CAPE could
introduce some potential for lake-enhanced precipitation over
northwest Indiana. If this were to materialize, could see another
inch or so of snow falling there (mainly Porter County) before
accumulating snow ceases later this evening. Elsewhere, we`ll lose
much of the deep saturation through the dendritic growth zone as a
ribbon of drier air aloft advects in, but with just enough
saturation within the lower portion of the residual cloud ice layer,
could see us squeezing out some flurries through the overnight hours
provided that we can get enough forcing from any of the vort lobes
gyrating around the back end of the upper low. However, considering
that we saw some reports of drizzle mixing in with the snow earlier
this morning and that none of the hi-res guidance really had a
good handle on how things evolved today, we`ll have to watch for
the potential for freezing drizzle again tonight as perhaps cloud
tops may end up warmer/lower than currently progged.

Turning our attention to Monday, a shortwave disturbance over
eastern Montana will shoot southeastward towards central/southern
Illinois overnight and into tomorrow. The combination of DPVA,
favorable jet exit region dynamics, and modest isentropic upglide
look to foster the development of precipitation just out ahead of
this disturbance tomorrow afternoon and evening. However, with
the better forcing looking to stay well to our south, most, if not
all of the CWA should remain dry tomorrow. It`s not entirely out
of the realm of possibilities that the northern fringes of the
blossoming precip shield graze our far southern counties
(Livingston, Ford, Iroquois, and Benton) resulting in flurries up
to maybe a dusting of light snow, but there isn`t enough of a
signal in either ensemble or deterministic guidance at this time
that would justify a formal mention of precip in the forecast for
this.

Cloudy skies will keep our high temperatures for Monday capped in
the lower 30s. Clearing skies and cold advection behind the
aforementioned disturbance will result in Monday night being one
of our coldest nights of the year so far with low temperatures
ranging from the teens to the low 20s. However, these lows will
still generally be at or above the normal climatological minimum
temperatures for this time of year.

Ogorek

&&

.LONG TERM...
253 PM CST

Tuesday through Sunday...

The shortened work week (for some) will be relatively quiet from a
precipitation perspective, with current only chance for snow being
on Tuesday evening. Looking ahead into next weekend and early next
week, there is a chance for a more noteworthy wintry precipitation
producer.

Tuesday will feature below normal high temperatures in the 20s for
a change, along with breezy westerly winds gusting up to 25 mph.
The parent short-wave for Tuesday evening`s clipper and light snow
accumulation potential is currently way up in northern Canada
near the Arctic Circle. The wave will zip south and then southeast
with a weak surface reflection. With today`s 12z cycle, the ECMWF
has very good support from its 51-member ensemble, along with the
addition of the GFS, GEFS, and the CMC. Increased PoPs into the
chance range I-80 and north, which is the current preferred path
of the light QPF swath.

Tomorrow night`s clipper wave passing to our south was just a few
days ago modeled to affect the CWA, so certainly can`t rule out
changes of that nature with such a low amplitude feature for
Tuesday evening`s clipper. That said, barring big changes, would
expect higher than climo snow ratios around 15:1 and up to an inch
or so of snow possible. Light snow could spread into parts of
northwest/north central Illinois later Tuesday afternoon. Will
mention the potential for minor travel impacts in the text of this
afternoon`s HWO.

A much stronger clipper/hybrid will zip across the Canadian
Prairies to north of the Great Lakes Wednesday through early
Thursday. This will bring a minor warm-up, especially on
Thursday, which could be fairly mild in the 35-40F range ahead of
the cold front trailing from the aforementioned low. Westerly
breezes will pick up again ahead of and behind the front. A shot
of colder air (in the context of the mild winter thus far) will
follow for Thursday night into the weekend as expansive strong
high pressure overspreads the region. The cool and dry surface
high will set the stage for our possible winter system to close
out the weekend.

Regarding this potential for wintry precip, the general idea has
pretty solid ensemble support out on day 7, and will be associated
with energy ejecting from a trough in the southwest and low
pressure or troughing initially over the central/southern High
Plains. However, the eventual likely outcome in terms of evolution
and sensible weather impacts will remain in flux for several
days. If a stronger system occurs, may have to contend with the
potential for mixed precipitation Sunday into Sunday night, while
on the other hand a weaker wave could create an warm advection
/over-running type snow scenario, or a miss altogether if the
energy gets suppressed. For now, have a chance of snow in the
official forecast on Sunday. Stay tuned for the latest forecast
updates on this possible system. The pattern does look to remain
active into next week beyond day 7 thanks to western troughing
(-PNA pattern) amidst continued blocking in the Arctic and North
Atlantic (-AO/-NAO) and ridging near western Alaska.

Castro

&&

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

Synoptically-driven -SN is quickly exiting to the southeast at
TAF issuance, with intermittent MVFR visibilities lingering for
another hour or two at ORD/MDW. Ceilings will also quickly improve
to high- end MVFR and possibly VFR levels at times into the early
overnight hours amid isolated flurries or brief VFR -SHSN.

A low to mid-level wave over southern Minnesota will track
southeast into central Illinois through mid-morning Monday.
Ceilings are expected decrease into low-end MVFR (to perhaps IFR
at RFD) levels with a slight increase in -SHSN coverage. A small
chance exists for FZDZ to mix with the -SN during this time,
especially if ceilings lower further than expected. After daybreak
Monday, any remaining precip should diminish with gradually
increasing MVFR ceilings throughout the day.

Winds are expected to remain W to WNW around or under 10 knots
through the period, strongest Monday afternoon.

Kluber

&&

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

$$

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