Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 102109
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
309 PM CST Sat Dec 10 2016

.SHORT TERM...
309 PM CST

Through Sunday Night...

The main forecast concerns with the winter storm continue to
revolve around total snow amounts, as well as the extent of the
mixing/changing to rain in my far south later Sunday into Sunday
evening.

Not much has changed in the overall thinking in terms of snow
amounts, with only minor tweaks made from the updated snow
amounts from this morning. Generally this will continue to result
in the highest totals of generally 8 to 12 inches north of I-80.
Amounts still look to tapper off into the 1 to 4 inch range over
my far southern counties (Livingston, Ford, Iroquois and Benton
counties). As a result, no changes are planned to the current
headlines.

An impressive band of moderate snow continues to develop east-
northeastward over eastern Iowa and western Illinois in
association with an initial band of frontogenesis along the
leading edge of the increasing isentropic upglide (warm air
advection). This area of snow should begin to result in a quick
onset of moderate snow from west to east over the area within the
next few hours. This could result in up to a quick inch of snow
through early this evening, especially over north central
Illinois. Once this snow onsets, it appears that it will continue
at times through the night and into Sunday morning over northern
Illinois and northwestern Indiana as warm air advection continues.
Mesoscale banding will also continue to be possible over northern
Illinois overnight as steep mid-level lapse rates remain in place
above 650 MB. As a result, snow amounts tonight alone could be in
the 3 to 6 inch range, especially north of I-80 where higher
ratios of around 15 to 1 will be likely.

Periods of moderate snow look to continue Sunday morning, though
there could end up being a break, or a period of lighter snow
during the morning, before another period of wetter snow onsets in
the afternoon in association with the approach of the mid-level
disturbance. This could again result in periods of moderate, to
possibly at times heavy snow over the area into early Sunday
evening. Temperatures still could get warm enough in my far south
for a mix or transition to a period of rain late Sunday into the
evening, though it appears areas farther north would remain
mainly snow. Snow ratios will likely be lower (around 10 to 1)
during the day, but another 3 to 6 inches will still be possible.

The snow should gradually come to an end from west to east during
the evening on Sunday. There could also be some patchy blowing
snow Sunday night as the winds pick up a bit on the back side of
the surface low. However, given the wet nature to the snow on
Sunday, blowing snow would likely be a minor impact.

KJB

&&

.LONG TERM...
249 PM CST

Monday through Saturday...

Concerns in the long-term portion of the forecast include the
potential for dangerous cold Tuesday night through Thursday night,
as well as a few opportunities for snow. Late next week, a more
significant storm system could take shape, with accompanying
accumulating wintry precipitation.

Anomalous mid and upper ridging across the Aleutians and
northeastern Pacific near Alaska will favor a very cold pattern
continuing through the upcoming week, as true Arctic air plunges
southward across the country. A piece of the polar vortex will
drop to near Hudson Bay, supporting 500 mb heights of near or
below 480 DM. Meanwhile, the fast progressive jet stream pattern
will remain as well, with low amplitude clipper type disturbances
possible Monday evening and again Wednesday. With the Arctic air
mass in place, any snow from these potential systems will be of
the fluffy/high-ratio variety. Thus, while the current operational
guidance would not suggest any significant events, snow will
easily accumulate on surfaces for at least some travel impacts.
Also, some previous model runs have offered potential for more
energetic disturbances and more notable accumulations, so cannot
rule this out either. Again, latest forecast suggests that period
of interest for potential fast moving snow producers are Monday
evening and Wednesday.

Returning to the potential for dangerous cold in the mid-late work
week period, behind the possible Wednesday system, shortwave
around impressive ~480 DM mid-level low near James Bay will send a
potent thermal trough our way. Models have exhibited run to run
variance in the magnitude of the cold air mass that reaches our
latitude, though there was consistency in today`s guidance in 850
mb temperatures of -20 Celsius or colder Wednesday afternoon
through early Thursday. With deep snow pack in place, this thermal
profile will easily support temperatures barely budging and then
falling Wednesday afternoon followed by widespread subzero
readings Wednesday night/early Thursday. Expansive high pressure
spreading toward area will also add a westerly wind component to
support wind chill values of -20F or colder (advisory criteria or
colder) for at least a portion of the area. Weak "warm" advection
will ensue aloft on Thursday, though frigid start and no mixing
under the high could keep temperatures in the single digits area
wide, if not colder in a few spots.

Finally, despite long lead time, concern is increasing for a
significant winter event later in the week. A very impressive
thermal gradient will exist across the Plains with Arctic high
pressure to the north. At the mid and upper levels, there is
decent guidance continuity in the idea of deep trough carving out
into the intermountain West, with a temporary surge of height
rises for the eastern 2/3 of the CONUS. Associated surface cyclone
will be working with aforementioned impressive thermal gradient,
as well as very strong WAA ahead of it, potentially supportive of
mixed precip for at least portions of the area. This will depend
on low pressure track and how far north warm surge makes it.
Cold northerly flow supplied by Arctic surface high over the
northern Plains, as well as deep snow pack locking in the cold
increase the concern. With it being 5-6 days out, however, have
maintained snow as precipitation type in the Friday-Saturday
period. There is enough confidence in precipitation occurring for
likely PoPs Friday night. Will need to watch this period closely
as it draws closer.

Castro

&&

.AVIATION...
For the 18Z TAFs...

The incoming snow producing system is the main concern through the
TAF period, with associated CIG, VSBY and runway impacts. Light
snow will overspread the area this afternoon and become steadier 1
to 2 hours after onset, bringing VSBY to IFR and then likely to
LIFR. CIGs will also deteriorate, probably to higher end IFR,
though confidence is low-medium on CIGs tonight. There is a well
developed snow band from central IA to western IL and if the heart
of this band moves across any terminal this evening, at least
occasional 1/2SM VSBY will be possible if not probable. It`s also
possible that VSBY will improve back to 1SM+ at times tonight,
though confidence is low on specific timing.

Large scale lift will further increase ahead of a weak surface
low on Sunday morning, with forecast soundings favorable for an
extended period of at least frequent 1/2SM or less VSBY. Given
extended lead time, opted to introduce temporary 1/2SM VSBY into
the TAFs. Confidence is medium-high in at least occasional 1/2SM
VSBY. CIGs will also likely deteriorate to LIFR during the day on
Sunday. The snow will taper off Sunday evening, beyond the TAF
period.

Regarding snow accumulations, the latest forecast is for 8 to
locally as much as 12 inches to fall. Given the cold temperatures
the past several days, the snow will more readily accumulate on
runways than the event last weekend. The snow will be high ratio/light
and fluffy, this afternoon through the overnight and transition
to wetter/lower ratio on Sunday. West southwest winds of 10 kt or
less will turn south-southeast (130-160 deg) tonight, with an
increase in speed by early Sunday morning when gusts of 15 to 20
kt are expected. Direction will become due southerly (160-190 deg)
Sunday afternoon.

Castro

&&

.MARINE...
354 AM CST

Moderate west winds of 15 to 25 kt will continue to very gradually
subside today to 10 to 20 kt this afternoon as high pressure
builds towards the region. The ridge axis will move across the
lake tonight with southerly flow increasing from the south ahead
of a deepening low over the Plains. The low is expected to lift
across Lake Michigan late Sunday evening and overnight with south
winds peaking at around 30 kt ahead on Sunday and then turning
west to 30 kt behind Sunday night into Monday. A small craft
advisory will likely be needed for the Indiana and Illinois
nearshore waters on Sunday. Meanwhile, another low over Hudson
Bay will drive a powerful cold front across the region Monday
night into Tuesday. As very cold air overspreads the Great Lakes
behind the front, could see gales or near gale winds at times
Tuesday night into early Thursday. Air temperatures are expected
to fall into the teens or even single digits which along with the
strong winds and high waves will contribute to some risk for
freezing spray to occur, though water temperatures remain
relatively warm and may help to mitigate the risk to some degree.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Storm Warning...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-
     ILZ012-ILZ019-ILZ020-ILZ021 UNTIL MIDNIGHT Monday.

     Winter Storm Warning...ILZ006-ILZ013-ILZ014-ILZ022 UNTIL
     MIDNIGHT Monday.

     Winter Weather Advisory...ILZ023 UNTIL MIDNIGHT Monday.

IN...Winter Storm Warning...INZ001-INZ002 UNTIL MIDNIGHT Monday.

     Winter Weather Advisory...INZ010-INZ011 UNTIL MIDNIGHT Monday.

LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742...4 AM Sunday TO 9
     AM Monday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...4 AM Sunday TO 3
     PM Monday.

&&

$$

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