Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 062354

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
554 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 00Z TAFs...

Breezy west winds are expected to persist through the forecast
period, with MVFR ceiling potential particularly on Wednesday.

Surface low pressure over northern Minnesota will continue to move
slowly to the east-northeast and pass north of the Great Lakes
through Wednesday evening. Westerly winds gusting around or a bit
above 20 kts at times will help colder air to spread into the
region, which will help keep low level lapse rates steep and the
boundary layer fairly well mixed for the duration. Lower clouds
had cleared out of the terminals late this afternoon, though an
area of 2000-2500 ft MVFR cigs over northwest IL/southwest WI are
expected to move eastward this evening, with RFD most likely to
see a brief period of MVFR cigs. Northern Chicago area terminals
including DPA/ORD may see brief cig as well, though satellite
imagery suggests the better potential for a prevailing cig would
be further north (PWK-UGN). Forecast models are in generally good
agreement in developing a more expansive MVFR cloud layer across
the area during the day Wednesday, continuing into Wednesday
evening, as another disturbance approaches the region. Wind wise,
direction will remain pretty steady in the 260-270 deg range
tonight, backing slightly to around 250 deg Wednesday before
shifting more 280-ish Wednesday evening. Gusts around 20 kts



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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