Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 070456

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1056 PM CST Tue Dec 6 2016

131 PM CST

Through Wednesday...

After several hours of dense fog this morning underneath a col
area/neutral point, west winds have increased and visibility has
improved over much of the CWA as a moderate westerly gradient sets
up between low pressure over northern MN and ridging that
stretches from the Canadian Rockies into the Central Plains.
Strong cold and dry advection into the region should help scour
out the remaining low clouds and fog resulting in mostly clear
skies moving in this evening and continuing overnight. Winds
remain propped up overnight due to the steep low level lapse
rates which should help prevent a good radiational cooling setup,
but still opted to bump down temperatures several degrees,
especially west of the Fox River Valley in north central Illinois
where temps dipping into the teens seems like a decent bet. This
is in good agreement with the GFS/ECMWF/EKDMOS but slightly
warmer than the Canadian and MOS which suggest some areas could
dip into the mid teens. With a continued west breeze, this should
drive wind chills into the single digits over north central
Illinois with wind chills for the remainder of the CWA dropping
into the low teens. Temperatures should rebound into the mid to
upper 20s northwest to low 30s southeast on Wednesday. Winds will
remain breezy throughout the day with gusts of 20 to occasionally
25 mph possible during the afternoon.



226 PM CST

Wednesday night through Tuesday...

After a punch of colder air, a strong system brings another round of
snow to the region over the weekend.  Below normal temperatures are
expected into next week.

The upper level trough over the plains will shift over the Great
Lakes. Cold air spreads through the region Wednesday night and west
winds increase. Min wind chills Wed night will be around zero away
from the lake and around +5F near the lake.

The cold air and fetch will result in lake effect snow over the lake
and mostly just east of Porter County IL.  Scattered flurries are
possible Thursday afternoon along and north of I-80. If lake effect
snow does form over far northwestern IN, up to a half inch of snow
may accumulate Thursday afternoon and night.

The coldest day will be Friday with highs around 20 degrees as high
pressure scoots across the region. The GFS continues to feature a
short wave trough that may bring a quick snow shower Saturday
morning.  The ECMWF does not have the same feature so only have a
chance of snow Saturday.

The next low forms over the plains Saturday night.  The larger area
of snow ahead of the low spreads overhead Saturday afternoon and
night. Guidance still differs on the exact path of the low. As the
midnight crew noted, the GFS still has a stronger low than the
ECMWF.  The ECMWF also brings warmer air that may result in a wintry
mix to about I-80. The GFS keeps the warmer air just south of the
outlook area. Given the discrepancy, kept precip as mainly snow, but
may see a mix of precip types south of I-80 Sunday. Precip may
linger longer than currently forecast with snow shifting east
Monday. Still too early to discuss accumulation amounts, but
accumulating snow is expected.

Below normal temps continue early next week, and another chance of
snow is possible north of I-80.



For the 06Z TAFs...

Main aviation concerns will remain breezy westerly winds through
the TAF period, along with the potential for development of MVFR
ceilings Wednesday into Wednesday night.

Surface low pressure continues to pass slowly north of the Great
Lakes late this evening. Blustery west winds gusting around 20 kts
at times will continue across the terminals through the period, as
the forecast area remains in the southern periphery of the
cyclonic flow around the slow-moving low. Cold air will continue
to spread in from the west/northwest, which will help to maintain
a well-mixed boundary layer with persistent gusts. Winds will
likely back to around 250 degrees Wednesday, before returning to
the 270-280 deg range by Wednesday evening, with gusts near 20 kts.

An area of MVFR cigs has passed just north of the terminals this
evening, though forecast model guidance suggests redevelopment
during the day Wednesday with bases in the 1500-2000 foot range.
This cigs look to linger into the evening, with some lowering of
the inversion allowing cigs to lower a few hundred feet from
earlier in the day. While an errant snow flurry is not impossible,
no substantial precipitation is expected.



226 PM CST

Headlines:  The small craft advisory will go into effect this
afternoon and persist through Wednesday morning. Another small craft
advisory will be needed early Thursday morning through Friday for
both winds and waves.  Replaced the gale watch with a gale warning
for the northern end of the lake late this evening into Wed evening.

Strong low pressure over NW MN will weaken as it crosses James Bay
and reaches Quebec Thursday.  The low`s cold front is just west of
the lake and will pass over the lake this afternoon.  Winds become
west behind the front. Gales are expected over the northern end of
the lake into Wed eve and occasional gales are possible over the
southern half this evening. Westerly winds to 30 kt continue into
Thursday and become northwest in the evening.  A large high builds
over the plains Thursday night and passes over the lake Friday
night.  The next low then forms over the plains Saturday night.
Winds become south to southeast ahead of the low Saturday night into
Sunday.  Guidance differs on the strength and path of the low, but
the low will impact the lake over the weekend.



LM...Small Craft Advisory...NEARSHORE WATERS
     UNTIL 10 AM Wednesday.




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