Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 061936

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
136 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.



400 AM CST

Thursday through Monday...

Very cold and somewhat cloudy conditions will greet us on Thursday
as the cold upper trough migrates across the area. Some flurries
are certainly possible as energy will rotate southeast through the
trough axis. Highs peak in the lower 20s. While the focus of the
lake effect machine will mostly be on the east side of Lake
Michigan, the low level flow turns a bit more northwesterly such
that lake effect snow may enter Porter and possibly Lake County
Indiana later Thursday and Thursday night. More sunshine is
possible Friday, but the core of the cold air will have arrived,
making for another very cold day. Highs may not even reach 20 in
north central IL. The coldest night area wide looks to be Friday
night as a high pressure ridge will move overhead. Some of the
drop off could be tempered as warm advective high clouds arrive,
but still looking at teens. Warm air advection will continue
Saturday ahead of strengthening upper level low pressure in the

Of more significant attention in the extended period is the
potential for another round of snow Saturday night into Sunday.
The 0z GFS is amped up with all the classic signs of a powerful
winter storm (deep upper trough becoming negatively tilted,
strengthening upper jet and associated frontogenetical
circulations, strong moisture transport on a low level jet,
deepening surface low) on Sunday while other guidance is a bit
more muted, certainly reflective on how the upper trough ejects
from the west and interacts with moist low level flow. All models
show ascent developing ahead of western CONUS upper level energy
with some lighter snow getting into the Mississippi valley by late
Saturday afternoon then spreading into at least portions of the
area Saturday night, then their evolutions diverge considerably
with the EC some 18-24 hours later with bringing stronger upper
level energy capable of developing a strong surface low. The low
it develops is weaker as well but does still bring a snow/snow-
rain mix in later Sunday/Sunday night. The Canadian model is sort
of in between the two with a single system but more muted than the
GFS. GFS Ensembles support a less amplified pattern not like the
operational GFS, but predictability is somewhat low as expected.
Regardless there are signs for another chance for snow during this



For the 18Z TAFs...

Fog, locally dense, and LIFR cigs persist late this morning across
portions of northern Illinois, though have generally seen an
improving trend over the last couple hours which is expected to
continue into the afternoon. Terminals should improve to VFR
sometime this afternoon or evening with increasing west winds as
surface ridge axis departs east. VFR should then prevail through
the overnight hours with MVFR cigs returning mid to late tomorrow



409 AM CST

Main forecast concerns are with strong westerly winds and
building waves for the nearshore waters later this afternoon and
tonight, and then with possible gales over the north half of the

In the near term, southeast winds in the 15 to 25 KT range are
occurring over much of the lake. However, surface high will swing
through the southern end of the lake and allow for some
diminishing winds in this location this morning but while winds
stay elevated over the north half. This will be short lived though
as a surface trough and frontal boundary are expected to move
across the lake this afternoon into this evening. This will allow
for a transition to a southwest and west wind, with generally west
winds then expected later tonight into Wednesday. As this occurs,
expect winds to quickly increase with hazardous conditions
expected over the nearshore which will last into Wednesday morning
and possibly into that afternoon. Gales will quickly be possible
over the north half in the evening with the passage of the front,
but then once again later tonight into Wednesday. Confidence with
the duration of gales is low and so have only issued a watch at
this time, despite this possibility within 24 hours. Will need to
monitor later trends and guidance to see if the gales will prevail
or if they will be shorter in duration. Also could see a brief
window of gales over the south half this evening but have higher
confidence that this would be short lived.



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




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