Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 110934

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

332 AM CST

Through tonight...

Short term concerns focus on a digging clipper system which will
impact the area today. Some minor snow accums far north are the
main issues this morning, with milder air leading to p-type issues
this afternoon before windy and colder conditions develop from
the northwest by evening.

Early morning GOES water vapor imagery shows well-developed
digging short wave moving southeast across the Dakotas. Surface
low pressure associated with this wave was located over far
northwest Minnesota at 09Z, with a warm frontal trough extending
southward into Iowa. This short wave and it`s surface low pressure
reflection are progged to continue to propagate southeastward
across northern IL today, and into northern IN by this evening.
Overall, this system appears to be a bit more wound-up than
guidance had been depicting yesterday, as well as somewhat
moisture-starved. Regional radar mosaic does show an area of light
snow across western and central WI, in a region of warm advection
and isentropic ascent associated with the elevated warm frontal
baroclinic zone. Guidance is in pretty good agreement in keeping
most of this precip north/northeast of the bulk of the cwa this
morning, grazing far northern IL mainly north of the I-90 corridor
briefly early this morning. Forecast soundings then show lack of
deep saturation across the forecast area through the remainder of
the morning hours, suggesting little/no significant precip.
Moisture then deepens midday/afternoon, as the main mid level vort
approaches/moves across the area from the northwest. Somewhat
convective-looking radar returns are evident this AM back across
northwest MN where the main mid-level vort is, and model forecast
soundings do depict steepening mid-level lapse rates in excess of
7 C/km this afternoon as the vort approaches. High-res model
simulated reflectivity develops this precip into the IL/WI border
region this afternoon. Questions remain with coverage/location and
precipitation type during the afternoon, as warmer air wraps into
the system and would likely support a rain/snow mix south of the
IL/WI border, with temps rising into above freezing (mid 30`s to
low 40`s north to south) across the cwa today. Given these trends
have lowered/limited snow accums to mainly the far northern
suburbs of Chicago near the WI border - and perhaps an inch at
best there.

As the low moves through the area late this afternoon, winds
shift northwest and increase substantially heading into the
evening. Strong cold advection will rapidly cool temps below
freezing, with steep low level lapse rates perhaps supporting
scattered snow showers and flurries tonight. No significant
accumulation is anticipated, and the main story will be the
falling temps and strong winds sustained at 20-25 mph and gusting
30+ mph tonight. Overnight lows will dip into the teens, with wind
chills around zero or in the low single digits by Tuesday
morning. Lake effect snow showers will develop down wind of the
lake into north central IN and southwestern MI, with some minor
accumulation expected over mainly Porter county. The main focus
for heavy accumulating snows should be farther east int IWX and
GRR`s cwas however.



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 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.



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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