Area Forecast Discussion
Issued by NWS Chicago, IL

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985
FXUS63 KLOT 110545
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1145 PM CST Sun Dec 10 2017

.UPDATE...
826 PM CST

Have made some adjustments to the grids for tonight and Monday
based on current trends and some of the latest model guidance.

First, beefed up sky cover tonight, first for the lingering
stratus over the NE portions of CWA this evening, then for the
increasing mid-high cloudiness overnight. Still could be a window
of clear-ish skies and fairly light winds for temps to tumble,
especially southwest CWA, which would allow lows to reach forecast
values.

Trend in models has been slightly north and most guidance suggests
the CWA will spend a good bit of time in the warm sector of the
clipper tomorrow. Still look on track for a quick hitting shot of
light snow or flurries around sunrise, mainly NE CWA ahead of the
warm front. Behind the warm front modest south winds and warm air
advection should allow many areas to make a run at getting into
the 40s tomorrow, especially south of I-80. Have nudged high
temps up closer to that ball park for tomorrow.

By later in the afternoon the sfc low will track into northern IL
with pretty strong agreement in a mid-level f-gen circulation
developing to the north of the sfc low supporting a band of snow.
There has been very little model to model and run to run
consistency with the placement of this band, ranging from the
I-290/I-88 corridor on the south end to central WI on the north
end. Would seem like southern WI is the mostly likely placement
with our area remaining mostly dry tomorrow until late in the day
into the early evening as the system moves east, the f-gen driven
snow band looks to pivot southeast across our CWA. By this point,
the f-gen circulation is forecast to be rapidly weakening and the
snow band also dissipating. Increasingly, it is looking like our
CWA will end up with less than an inch of snow total, with many
areas just seeing a trace to a dusting. Didn`t make any big
changes to pop/wx grids, but midnight shift may need to back off
on pops some, especially if GFS and ECMWF come in as far north as
the 00z NAM has come in with the banded snow.

Biggest story with this system locally looks to be the wind driven
flurries and light snow showers in the wake of the system Monday
night. Bufkit soundings from the NAM and GFS have mean mixed layer
winds at or above 30kt most of the night, so gusts of 30+ mph look
quite likely.

- Izzi

&&

.SHORT TERM...
119 PM CST

Through Monday...

Main concern for the short term forecast period will be another
chance of snow tomorrow.

For the remainder of this afternoon, a stratus deck is quickly
pushing south out of Wisconsin with the northern tier counties
already under overcast skies and the stratus deck will continue to
spread south and east, bringing an end to the brief period of
sunshine today.  Temperatures will top out in the upper 20s to low
30s over the northern tier counties near the Wisconsin border and
middle to upper 30s elsewhere.  Winds are also shifting to
northwestward, which will usher in cooler air along with the
increasing cloud cover.  Under persistent cold advection,
temperatures overnight tonight will drop into the upper teens to
lower 20s.

Forecast concern will quickly shift to snow chances.  latest
guidance is a little slower in bringing the warm advection wing of
pcpn into the area, ahead of a weakening low dropping out of Canada.
The sfc low track is expected through central Wisconsin and central
Lake Michigan.  There is relatively decent agreement among the
models in this system being on a weakening trend as it tracks just
north of the CWA.  However, timing is still a concern with the
possibility that the snow will still impact the morning rush hours.
So, while some snow is expected with the warm advection zone and
amounts/rates/accumulations should be light, there may still be some
travel impacts for the morning rush hours.  As the sfc low continues
to track east through the day, another shortwave taking a more
southerly track will induce development of another weak low over the
middle Mississippi Valley by mid day.  As this system tracks east
through the afternoon, there is some indication in the guidance that
an f-gen band may set up across nrn IL/srn WI durg the afternoon
hours.  However, there is not a strong model consensus with this
scenario, but feel that there is enough confidence to at least
increase PoPs and QPF over the nrn tier counties tomorrow afternoon
with a chance for around an inch of new snowfall with the f-gen
band.

&&

.LONG TERM...
135 PM CST

Monday night through Sunday...

Meridional upper level flow will begin the period with
the extensive upper ridge across the west extending up into
Alaska. What goes up must come down, and on the downstream side of
the trough the flow aloft will be headed our way from northern
Canada. A surface low pressure will deepen just to the east of our
area Monday night as additional shortwave energy will dig into
amplifying upper trough over the eastern Great Lakes, and will
bring the leading edge of a cold front to southern Lake Michigan.
Model guidance paints some scattered light QPF Monday evening
with the front and maybe some lingering snow bands from the
afternoon, but soundings do not suggest much precip will be left
at this point. By the time the low levels get into the better snow
production region, low level moisture will be scant Tuesday
morning. This pattern will support a reinforcing shot of cold air
for Tuesday. Highs on Tuesday will only reach the low to mid 20s,
and it will be a blustery day with northwest winds of 10-20 mph,
making for daytime wind chills struggling to get into the teens.
Low level flow will be northwest which again favors LaPorte county
eastward for lake effect snow, but there is enough of a component
into Porter county on the GFS for a mention of snow showers as
low level forcing/thermodynamics are fairly good with an unstable
layer right in a prime snow production zone. It is not as deep of
a mixed layer as this weekend, but sufficient.

Warm air advection will only be modest Tuesday night, and with
weakening winds Tuesday night will be a cold one with teens area
wide, even some single digits.

The northwest flow wave train will continue as the ridge in the
west, while weakening a little, still holds for a bit longer. Warm
advection ahead of these waves and a lingering baroclinic zone in
the region will bring occasional chances for light snow through
Thursday night, but no big snow storms. Our next chance for light
snow would be Wednesday, but this appears to just clip the area.
There could also be some snow shower potential behind another
weaker cold front Thursday.

The upper ridge in the west shows signs of finally breaking down
late in the week and the upper level flow will flatten. This will
bring a bit of a milder pattern to the midwest this weekend. With
a ridge expected to be positioned across the south, any
moisture/forcing will be confined to additional northern stream
waves that, at least through the weekend remain to our north.

KMD

&&

.AVIATION...
For the 06Z TAFs...

Primary aviation concerns are with timing of a couple of
relatively brief periods of precipitation this morning into early
afternoon. Precip type also becomes a concern by early afternoon.
Strong cold front moves through toward evening, with very gusty
northwest winds developing, and light snow showers and flurries
likely tonight.

Weak high pressure ridge was drifting east across the terminals
approaching midnight, which will allow light NNW winds to back to
the south shortly. Chicago terminals were along the trailing edge
of an area of MVFR clouds around 2500 ft, and these should scatter
or move east over the next few hours.

Attention then turns to a vigorous mid-level disturbance digging
southeast across the Dakotas. Increasing ascent ahead of this
feature is expected to result in saturation and development of VFR
cigs during the pre-dawn hours, and a brief period of light snow
which will progress across far northern IL/northwest IN through
12-14Z. The mid-level trough and surface low pressure reflection
approach the area midday, with models in decent agreement in
developing additional scattered precipitation across NE IA/SW WI
and far northern IL. By this time however, warming of the column
in southerly winds ahead of the low make precip type questionable,
with some mix of RA/SN possible. Winds then veer northwest as the
low moves through late afternoon/sunset, and become strong and
gusty from 320-340 deg. Much colder air floods into the area at
that time, with gust to around 30 kts likely. MVFR cigs, snow
showers and flurries are expected to persist during the evening
and overnight hours.

Overall, have decent confidence in the general progression
outlined here and in TAF`s. Lowest confidence is on precip
coverage during the day, and precip type during the midday/early
afternoon hours.

Ratzer

&&

.MARINE...
135 PM CST

Next concern is for a period of northwest gale force
winds Monday evening into early Tuesday, and possibly some
freezing spray.

Another upper level disturbance will allow surface low pressure to
take aim at southern Lake Michigan by Monday afternoon and
evening. The low will undergo more significant deepening Monday
night as it moves to the eastern Great Lakes. Meanwhile, cold
arctic air will move over the lake as high pressure builds south
across the Plains. Winds will shift to the northwest by Monday
evening as the low and associated cold front move through, with
winds quickly increasing to 40 kt gales by late Monday evening.
Cold air, with temperatures in the teens to lower 20`s, will be
drawn across the lake by these strong winds, with the potential
for freezing spray increasing by Tuesday morning, and persisting
into Tuesday night before the winds gradually ease. A gale watch
has been hoisted for all of the open waters of Lake Michigan for
the Monday night into Tuesday afternoon. It is possible that a
gale may be needed for the IL nearshore waters also, but the low
track favors the open waters and the Indiana nearshore.

Winds will then diminish fairly quickly late Tuesday night into
Wednesday, as weak high pressure ridging spreads across the lake.
Northwest flow aloft may bring another low or two across the area
Wednesday night and Thursday.

KMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Watch...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779...8 PM Monday to noon Tuesday.

&&

$$

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