Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 140847
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
247 AM CST Thu Dec 14 2017

.SHORT TERM...
246 AM CST

Through tonight...

Only real forecast concern in the near term is the lake effect
snow showers which will continue to impact the IL/IN state line
area and portions of Lake and Porter counties this morning. Winter
weather advisory remains in effect through 11 am, with 1-4" snow
amounts, reduced visibility and some blowing snow this morning.

Low pressure which passed across the area Wednesday continue to
move off well to the east of the region early this morning.
Blustery north winds, still gusting 25-30 mph at 330 am, were
helping to focus a north-south band of lake effect snow showers
along/just east of the IL/IN border. Fairly strong low level
convergence is expected to persist in this location early this
morning, before winds begin to diminish more quickly and begin to
show some directional shear in the boundary layer by mid-late
morning. Lake surface to 850 mb delta T`s approach 20C, though
subsidence in version around 6000 ft agl will limit convective
depth somewhat. With high-res guidance depicting the band hanging
in the same general area over the next few hours, still should see
some accumulation potential, though have decreased amounts just a
tad from earlier forecast. Highest amounts should be in Lake
county, with 2-4" totals. Far western Porter county may pick up
1-3" before the snow band weakens and becomes less organized by
late morning. Will maintain advisory headline, as travel may be
impacted across northwest IN this morning, and gusty winds will
produce some blowing and drifting. Far southeast Cook and eastern
Will/Kankakee counties may see a few light snow showers or
flurries early this morning with little or no accumulation.

Elsewhere, skies were clearing skies will allow for sunshine
today, with temps expected in the mid-upper 20`s. Another clipper
digs into the Great Lakes region tonight, though with a farther
north track than the past few. Clouds will increase, with the
potential for a few flurries mainly after midnight as the weak
surface trough approaches overnight.

Ratzer

&&

.LONG TERM...
144 PM CST

Thursday night through Wednesday...

For the early portion of the long term forecast period,
Thursday Night through Saturday Night, model guidance is in
relatively good agreement on the longwave pattern, with high
amplitude ridging over the wrn CONUS and broad troughing east of the
Rockies.  A series of nrn stream shortwaves dropping through the
fast nwly flow aloft will bring some pcpn chances for much of the
CWA.  Thermal profiles will be cold enough to support snow as the
prevailing p-type, with the main concern being location and amount
of any possible accumulation.  The general shortwave track will be
through srn WI/Lake Michigan/Lower Michigan with the strongest
forcing north and east of the CWA.  However, there should be enough
upper forcing dipping into nrn IL/IN to support some light
accumulating snow, mainly for the nrn and ern portions of the CWA,
flurries more likely for the swrn portions of the CWA.  With weak
forcing and low deep layer moisture in place, any accumulation would
likely be nothing more than a dusting.

For the latter portion of the long term forecast period, from Sunday
through Wednesday, the various longer range models quickly diverge
in how they handle southern stream systems.  The various model begin
to significantly diverge as early as Friday night and Saturday for
the wrn CONUS, but these issues will likely not impact forecast
confidence and concerns until Sunday or Sunday night.  So, given the
model differences, have opted not to make any significant changes to
the going forecast, with a general trend of near to slightly above
normal temperatures.  While confidence in finer scale details is
low, the source of systems impacting the region are of pacific-
origin, thus the trend for generally higher than normal
temperatures. P-type and coverage will be most problematic on
Sunday. While the GFS and ECMWF both indicate srn stream shortwave
energy lifting out of the southwest, which would support mentionable
pcpn, but given the timing differences, will limit PoPs to slight
chance to low chance levels.  There is a chance that much or all of
the pcpn could be rain or a rain/snow mix with short periods of all
snow at more diurnally favored times around time of min temp.

&&

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

Lake effect snow showers will affect northeast IL and far
northwest IN terminals overnight/this morning, with MVFR cigs and
MVFR/high end IFR vis at ORD/MDW overnight, and likely more solid
IFR/LIFR at GYY into Thursday morning. Gusty north winds up to 25
kts (a little higher at GYY) will diminish and back northwest this
morning.

0530Z/1130 PM CST regional radar mosaic depicts flurries and light
snow showers along the IL/IN shores of Lake Michigan, with a more
organized plume of lake effect snow north of the terminals from
UGN-ENW-RAC. This plume of heavier snow showers will move south
down the IL shore over the next 1-2 hours, with a period of light
snow expected at ORD/MDW. Based on current surface obs and high
res model forecasts, would expect ORD to remain primarily MVFR
with snow slipping south of the area by 07-08Z. MDW, a little
further southeast, may see a brief period of IFR vis. The lake
effect plume will then likely become oriented more N-S along/just
off the IL shore into GYY, and is expected to linger for a time
past sunrise. Given MDW`s closer proximity to the shore and the
expected location of the snow band, have maintained a VCSH through
sunrise, with SHSN about 10 nm east of the field. Tougher to
determine an end time at GYY, though the band should weaken and
drift east of there by mid-morning or so.

Otherwise, winds continue to diminish and back northwest this
morning, and eventually west-southwest by Thursday evening.

Ratzer

&&

.MARINE...
144 PM CST

The wind pattern over Lake Michigan will be very complicated for
the remainder of this afternoon and evening as strong low pressure
tracks across sern Wisconsin and srn Lake Michigan and then
tracks south of Lake Erie this evening. A small Craft Advisory
remains in effect for the IL/IN nearshore waters with swly winds
to 30 kt early this afternoon then shifting to nwly by late
afternoon. Winds should remain strong and nwly through the night.
While prevailing gales are not expected, there is a chance for a
short period of Gale Force gusts early this evening when cold
advection and pressure rises behind the exiting low are strongest
and would most likely be confined to the far southern end of the
Lake. Winds should diminish through the morning on Thursday as
high pressure builds across the Middle Mississippi Valley and into
the Ohio Valley. Conditions through the weekend should be
relatively quiet. The next chance for impactful weather is not
expected until early to the middle of next week as a strong cold
front pushes across the Great Lakes region. However, the longer
range model guidance is in very poor agreement on how the pattern
evolves next week, so confidence in timing of this next system is
low.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Winter Weather Advisory...INZ001-INZ002 until 11 AM Thursday.

LM...Small Craft Advisory...IL nearshore waters until noon Thursday.

     Small Craft Advisory...IN nearshore waters until 3 PM Thursday.

&&

$$

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