Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 111957

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

156 PM CST

Through Tuesday...

A compact vort max across eastern Iowa will continue to dive
southeast into central Illinois late this afternoon, then east
into Indiana tonight. The associated surface low is pulling ahead
of the upper vort, and will quickly allow cooler air to filter
back in to the area. Warm advection ahead of these systems has
allowed temperatures to climb into the mid 30s north to the mid
40s across central Illinois. There has been a persistent band of
mixed precipitation across Iowa ahead of this feature, either as
some sleet or rain, but more predominantly snow. In our neck of
the woods, the precip/virga has been coming out of a higher cloud
deck and thus some occasional non accumulating sleet has occurred.

As the surface low will move into Indiana late this afternoon and
evening, a cold front will shift across the area. We are starting
to see a band form out ahead of the main forcing which is
beginning as virga but could quickly transition to snow and this
is happening out near Freeport. As the better forcing approaches
north central Illinois there could be a brief burst of snow which
could put down a quick few tenths of an inch of snow, maybe up to
an inch across the northern tier of counties. There could even be
a mix of sleet/snow along the leading edge of the higher
reflectivity values east from Winnebago to Lake County IL. The
NARRE/RAP have been fairly consistent in suggesting a narrow
window for several hours from 3-6pm near RFD and between roughly
5-8 pm across the Chicago metro area where snow will likely be
falling. Better accumulation potential is near RFD as guidance
also suggests a weakening of the band as it gets closer to
Chicago. Surface temperatures have certainly warmed such that when
precip arrives it may struggle initially to accumulate,
especially closer to the metro, but it is certainly possible for a
coating to a few tenths.

Afterwards, strong northwest winds will bring the core of the
cold air in. The mixed layer eventually gets into a better snow
production region where there could be some flurries overnight.

Tuesday will just be blustery and cold. Northwest winds of 15-25
mph will hold through the day, and highs will remain in the low
to mid 20s. At least there will be sunshine. More clouds will
remain in place across northwest Indiana and over the lake.
Current wind forecasts would suggest lake effect potential is
higher just to our east.



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

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



For the 18Z TAFs...

Sort of complicated forecast this afternoon and evening as low
pressure tracks through the area. The first issue is wind
directions, followed by precipitation type, timing, and duration,
and then focus shifts back to gusty nw winds.

The core of low pressure will pass just to the south of the
terminals, and then spread east of the area tonight. This will
cause SE winds to hold in the 8-11 kt period for a few hours in
the better pressure falls ahead of the low, then as winds shift
further to the E, expect some decrease in speeds. Warm advective
precipitation is attempting to occur farther upstream of the upper
level low across NE IA/SE MN. Expect during the afternoon some
attempt to precipitate over the coming hours, but with drier low
levels expect this would be some light snow or sleet, non
accumulating. As the better forcing approaches RFD around 21-22z,
there could be a brief burst of snow which could put down a quick
coating of snow, possibly even a few tenths of an inch in a short
period. Cigs will fall to MVFR and vsby too, possibly briefly
IFR. As this snow axis shifts closer to the Chicago terminals,
there appears to be some weakening to the stronger forcing, but
there is still a small window for ORD/MDW in the 0z-02z time
frame for a quick coating of snow, medium confidence on timing.

Later this evening there could be a few lingering snow showers,
more so wind driven flurries. CIGS will eventually head back to
VFR. Expect NW winds to exceeds 20-25 kt from a 330-340 direction
after 3z or so at least for a few hours if not longer.



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.



LM...Gale Warning...LMZ777-LMZ779...8 PM Monday to 8 PM Tuesday.

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

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




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