Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 110910
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
310 AM CST Mon Dec 11 2017

.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...
310 AM CST

Tuesday through Sunday...

Overview: Lake effect snow is possible in Porter County, IN Tuesday,
but the majority of lake effect snow should fall to the east of the
forecast area. Colder air also arrives Tuesday leading to single
digit wind chills Tuesday and Tuesday night.  Another clipper could
bring light snow to the region Wednesday, and there is a chance a
little freezing drizzle/rain could be mixed in at times. Warmer
temperatures psbly in the 40s may return for the weekend.

A classic post trough lake effect snow set up is expected Tuesday.
Strong cold air advection over the lake will likely lead to
impressive snow totals on the eastern side of Lake Michigan. The
initial wave of lake effect snow may clip eastern porter County, so I
kept likely precip chances in the forecast. However, winds will
become northwest pushing the snow band further east out of the
forecast area. As such, I expect minimal snowfall accumulation.

The bigger story area wide will be the much colder air mass. 925 mb
temps drop to -10C to -13C, and I lowered high temps to the upper
teens to low 20s. Northwest winds gusting 25-30 MPH will make it
feel like it`s in the single digits. Tuesday night should also be
chilly with a lack of clouds and diminishing winds under high
pressure.  Low temperatures will be in the single digits to low
teens and wind chills will be around zero.

Warm air advection returns ahead of the next low that arrives
Wednesday. Guidance differs on how much precip this low will produce
so I stuck with slight chance to chance precip chances.  I think
snow will be the most likely precipitation type, but forecast
soundings suggest we may be lacking ice crystals especially
Wednesday evening. If we are able to saturate, freezing rain/drizzle
is possible Wednesday evening. However, forcing looks minimal so if
freezing rain does occur, I`m not expecting anything more than a
glaze.

An upper level vorticity streamer rotates through Thursday, and it
may force a little snow. I have low confidence in whether or not it
will snow so I kept precip chances at slight chance.  Guidance then
locks on to warm air advection late in the week which could lead to
high temperatures in the 40s over the weekend.  Guidance members
have significant differences though in how the pattern will evolve
over the weekend, so my confidence is low in exact details Friday
onward.

JEE

&&

.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...
310 AM CST

I upgraded the gale watches to warnings as gales are expected across
the lake early this evening into Tuesday. Gales will persist longest
over the southern end of the lake into Tuesday evening.

Low pressure over northwest Minnesota will pass over the lake and
northern IL this afternoon.  The low will then rapidly deepen as it
moves over the eastern Great Lakes tonight. Northwest gales will
develop across the lake in response to the low, and gales gradually
diminish Tuesday evening.  A weak high pressure ridge scurries
across the western Great Lakes Wednesday before another low reaches
the region Wednesday afternoon/evening. Guidance has this low taking
a similar path near the southern end of Lake Michigan or across
northern IL, but the low is forecast to weaken as it continues to
the mid Atlantic coast. Therefore, at this time gales are not
expected with this low. Another weak low may form over the northern
or western Great Lakes late this week and deepen as it moves over
the eastern Great Lakes Friday.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Warning...LMZ777-LMZ779...8 PM Monday to 8 PM Tuesday.

     Gale Warning...LMZ743-LMZ744-LMZ745...8 PM Monday to 3 PM
     Tuesday.

     Gale Warning...LMZ740-LMZ741-LMZ742...8 PM Monday to 9 AM
     Tuesday.

&&

$$

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