Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 172056

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
256 PM CST Wed Jan 17 2018

256 PM CST

Through Thursday...

The main concern is the potential for patchy blowing and drifting
snow in open areas of northern Illinois north of I-88 and near
the lake in northwest Indiana through a portion of the overnight.
Southwest winds will peak this evening through the early
overnight, with gusts up to 25-30 mph, highest north of I-80.
With the dry, fluffy snow cover in place that hasn`t undergone
much if any melting and little consolidation, hence the concern
for patchy blowing and drifting snow. If winds are any stronger
than forecast, then blowing snow could become more of an issue,
while on the other hand, if gustiness is less frequent than
expected, blowing snow may not materialize. Despite winds
remaining steady to gusty tonight, the refrigerating effects of
the snow cover should allow for a gradual fall in temperatures,
particularly later in the overnight into early Thursday when
winds are expected to ease some. Have forecast lows in the lower
to mid teens, which with the wind will yield wind chills to start
Thursday within a few degrees of zero.

Strong warm advection will continue aloft on Thursday, forming a
stout inversion and limiting mixing to under 925 mb. With gusty
southwest surface flow over the expansive snow cover and the stout
inversion, suspect that high temperatures will again end up below
MOS guidance and on the lower end of the climatology of progged
925 mb temps. Forecast highs are from around 30 to the lower 30s
(28-33 range). Full or nearly full sunshine and these temperatures
should start the slow melt and consolidation process of the snow
pack. Gusts topping out in the 25-30 mph range during the
afternoon will continue to make it feel more chilly than it
otherwise would be with temperatures near or around freezing, with
wind chills in the teens to lower 20s.



305 AM CST

Saturday through Tuesday...

Focus in the longer term, is on a deep low pressure system which
is expected to lift through the region during the latter part of
the weekend and early next week. Guidance has remained consistent
with the track of the surface low moving near or just north of the
forecast area, which would support a continued warm-up and mainly
a rain event for northern IL/northwest IN, before colder air
wraps in on the back side of the slowly departing system on
Monday. Of course it`s important to note that this system is still
a late day 4 through day 6 event, and fine details will likely
change a bit as we get closer in time.

Medium-range guidance (GFS/GEM/ECMWF) are in general agreement in
deepening an upper level trough across the southwestern CONUS
Saturday, with a closed upper low evolving over the south-central
Rockies by early Sunday. This system is then progged to move to
the mid-Mississippi Valley by early Monday. The forecast surface
low track is generally from Kansas to northwest IL and then
central or northern Lake Michigan. Given this scenario, the WFO
LOT cwa would remain largely in the warm sector in advance of the
low through early Monday, before the occluding cold front moves

Surface temps are expected to remain above freezing Saturday
through Sunday night, as warm/moist advection gradually saturates
the low levels. This would likely lead to development of
drizzle/light rain Saturday night, though the slightly cooler
ECMWF would suggest the potential for a little mixed precip across
northern IL and especially near the IL/WI border during the
night. Milder temps, in the 40`s to around 50 degrees, continue to
advect in from the south on Sunday, with rain eventually becoming
heavier by Sunday night as the system approaches, and low-level
theta-E increases in advance of the approaching low. Precipitable
water values are progged to approach 1.00" Sunday night, during
the period of greatest rainfall potential. The occluding cold
front then pushes east across the cwa Monday, bringing colder air
back in which would support a mix/change to snow or snow showers
during the day as the upper trough axis moves through aloft. While
this is the scenario supported by current model guidance, some
spread in timing and details does exist. We`ll have to continue to
monitor the evolution and track of this system over the next
several days.



For the 18Z TAFs...

VFR conditions are in store through the forecast period.
Southwest winds will gradually increase through this afternoon
with gusts into the mid 20 kt range by late this afternoon through
part of the overnight, before diminishing some in the pre-dawn
hours of Thursday. May need to consider adding temporary VSBY
restrictions to RFD and possibly DPA for blowing snow tonight if
winds are as gusty as forecast. Guidance continues to indicate
50kt winds at about 020 late this evening and overnight, so if on
the other hand we lose the surface gusts late this evening, low
level wind shear may need to be added to the TAFs. Steady
southwest winds will continue on Thursday, with gusts in the 20-25
kt range, especially during the afternoon.



250 AM CST

A broad and strong area of high pressure is in place over the
central and southern Great Plains this morning while low pressure
is moving east across Hudson Bay. A moderate west to southwest
gradient will be in place between these two features over Lake
Michigan. Winds are expected to increase to gale force across
northern Lake Michigan by mid morning and spread south across the
remainder of the lake by mid to late this afternoon. Winds should
drop back below gale force late tonight as a weak ridge builds
into Ontario. Another low is expected to track across the Canadian
Prairies late in the week and will bring another window of gales
to Lake Michigan Friday and Friday night. Late in the weekend, yet
another low is expected to deepen over the central Great Plains
and lift across the western Great Lakes late Sunday night or
Monday. Gales will once again be a possibility ahead of the low
and will be likely behind the low.



LM...Gale Warning...LMZ777-LMZ779 until 3 AM Thursday.

     Small Craft Advisory...nearshore waters
     until 4 PM Thursday.




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