Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 171759

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1159 AM CST Wed Jan 17 2018

1159 AM CST

After a cold start to the day, (sub-zero lows for parts of the
area were likely the last for a while), increasing southwest winds
will help temperatures recover this afternoon to the upper teens
to low-mid 20s, highest in Chicago. This is still several degrees
below normal for mid January. Expansive snow cover over the region
may keep some locations a bit short of forecast highs. The
increasing southwest winds, with gusts up to 20-25 mph during the
mid to late afternoon, are in response to strong Arctic high
pressure of 1045 mb settling over the mid South while low pressure
moves over Hudson Bay. These winds will continue to make it feel
quite chilly despite some moderation in temperatures.

In addition, also due to the southwest winds which will further
increase tonight with gusts up to 25-30 mph, have some concern for
patchy blowing and drifting snow in open areas late this
afternoon through much of the overnight. This is especially the
case where winds will be strongest in northern Illinois and near
the lake in northwest Indiana where snow is deepest. Have added
patchy blowing and drifting snow to the grids in these areas.
Should winds be any stronger than forecast, blowing snow could
become more of an issue.



305 AM CST

Today through Friday night...

A period of dry weather, with slowly moderating temperatures, is
expected to last through the end of the week. Blustery winds will
keep it feeling chillier however, especially until temps rise
above normal Friday.

Early morning satellite imagery indicates the deep upper trough
that brought snow and colder conditions to the area over the past
few days was finally sliding east of the area. Backing of low
level winds through 850 mb has shifted the lake effect snow plume
east of Porter County overnight, allowing advisory headlines to
expire as scheduled. Strong deep-layer subsidence was evident with
120+ meter 500 mb height rises developing in guidance progs as
the trough pulls away to the east today, and upper ridging
develops across the area. At the surface, a large 1044 mb high
pressure was analyzed over the Southern Plains, and this feature
is progged to spread southeast across the lower Mississippi Valley
and Gulf coast region today and linger for the next few days. As
this occurs, our low level flow will continue to back to the west-
southwest, allowing warmer temps to spread east across the
Midwest. While warm-advection will be persistent during the
period, fresh snow cover of 3-6" in most parts of the forecast
area will tend to limit warming at the surface, keeping the mixed
layer rather shallow, and resulting in most of the warming
occurring aloft. This, combined with the relatively cold nature of
the surface air mass across the Plains to begin with, supports
undercutting MOS guidance over the next few days. In addition, our
location in the stronger pressure gradient on the northern
periphery of the strong surface high pressure to our south, will
make for modestly blustery winds during the day, and will produce
wind chills only in the single digits today, and in the teens to
lower 20`s Thursday. The good news is that the dry arctic air mass
should prevent much in the way of cloud cover, allowing a good
amount of sunshine both today and Thursday.

By Friday, the sprawling high over the Gulf Coast shifts east to
the southeastern U.S. and allows our low level winds to back more
southerly, which along with persistent warming aloft should help
produce a bit more of a bump in our temperatures to well above
freezing in most areas. Upper 30`s to lower 40`s look reachable,
higher if snow cover has less of an impact.



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...3 PM Wednesday to 3 AM Thursday.

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




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