Area Forecast Discussion
Issued by NWS Chicago, IL

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368
FXUS63 KLOT 111810
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1210 PM CST Mon Dec 11 2017

.UPDATE...955 AM CST

There is an area of snow/mixed precipitataion across southeast
Minnesota that will dig east-southeast today as warm advection
continues ahead of southeastward shifting clipper. There are some
decent snow rates occurring in a narrow area, and some of this
will get awfully close to the IL/WI border, but near term guidance
indicates the warm advective wing of the clipper will shift the
better moisture north of the area as this first band moves in in
the 11am-3pm time frame. Precip type from the bourgouin wet bulb
method would be largley snow if the precip makes it to the
ground, maybe some mix of rain in there. Impacts from a snow accum
perspective appear low, but there is still some uncertainty with
far south this stronger band will get, making the difference
between accumulation potential and nothing very tight and worth
monitoring across the Winnebago to Lake IL counties.

As the surface low will move into Indiana this evening, a cold
front will shift across the area. Models then indicate a brief
period of snow shower potential in the evening, scattered to
isolated in nature, with maybe a dusting. 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.

KMD

&&

.SHORT TERM...
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.

Ratzer

&&

.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 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.

KMD

&&

.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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