Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 140451

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1051 PM CST Sat Jan 13 2018

258 PM CST

Through Sunday...

Main concern for the short term forecast period will be increasing
chances for snow tomorrow afternoon.

For the remainder of the afternoon, lake effect snow over far nwrn
Porter County will continue to push off to the east.  High pressure
centered over Iowa will build across the region tonight, allowing
northwest winds to become light and variable or even calm.  Under
clear skies and light wind, radiative cooling will help drop
temperatures will drop slightly below 0 F outside of the highly
urbanized areas of the Chicago Metro area.  A few isolated, low-
lying areas could see temps drop to -5 F, while the while the more
highly urbanized areas should see temps in the low to middle single
digits above 0 F.  Since there is no snow cover at this time,
temperatures should not radiate as low as a snow-cover would allow.
Also, since winds will be near calm, wind chill readings should not
drop quite as low as would occur if there was some winds.  The high
will move off to the east tomorrow, with increasing southerly winds,
with increasing warm/moist advection.  Have adjusted PoP trends for
tomorrow afternoon with snow chances ramping up quickly in the
afternoon with, at least, light snow overspreading the area in time
for the afternoon/evening rush hours.  While little or no
accumulation is likely through the afternoon, there may be some
visibility restrictions that could impact the afternoon commute.
Precipitation type in not a concern as thermal profiles will be well
below 0 C, with all precipitation falling as snow.


223 PM CST

Sunday night through Saturday...

Main forecast concerns/challenges are with the snow potential
Sunday night into Monday morning, and then with returning low
temps and wind chills through mid week.

Expect snow chances to quickly increase Sunday evening ahead of
digging trough/low and associated surface reflection. Initial dry
conditions in place don`t appear to be too much of a factor as a
period of WAA ahead of this system should allow saturation to
occur by early to mid evening across much of the CWA. During this
time, should see snow begin and likely over the entire CWA. Then
anticipate a period of light snow for much of Sunday night into
Monday morning, slightly increasing in intensity overnight into
the early morning hours Monday morning, with the arrival of
stronger/additional forcing tied to the approaching trough/low and
surface low. With the increasing forcing, some is located within
the snow growth zone. However, as continues to be the trend, the
bulk/strongest forcing looks to be just outside of this zone. Once
again expect a period of light, maybe briefly moderate, snow to
occur Sunday night into Monday morning, ending from west to east
by mid to late morning. Still anticipate area wide snow totals of
2-3 inches, likely a dry snow. On the backside of the system on
Monday, will see increasing winds/gusts with the arrival of the
CAA. This increasing wind could support some blowing snow on
Monday, with this new/dry snow. Will continue to monitor this,
especially for open and rural areas. Some additional scattered
snow showers may be possible later in the day Monday through
Monday night and early Tuesday. However, don`t expect much of any
additional accumulation. Keeping an eye on returning lake effect
snow chances, that do increase late Tuesday and Tuesday night.

The CAA on Monday will support a nondiurnal temp trend, with
falling temps likely through the day. Monday night and Tuesday
night will be rather cold nights, with temps around zero or below.
Elevated winds during this time will be a concern, as this could
support rather low wind chills of 20 below zero or lower. Then
anticipate a warming trend through the end of the work week and
early part of the weekend.



For the 06Z TAFs...

Quiet conditions in the near term, focus will be on
timing the arrival of snow and the magnitude of the visibility

Bottom line up front: Expect a steady snow from roughly 0z Monday
through 18z Monday, with lingering snow showers after. IFR visbys
expected. 1/2 to 3/4SM vsby is a much lower chance and not likely
the most dominant condition, more likely 1-2SM. Dry snow.
Accumulation of 2-3" or a little more a good bet. Rates around a
quarter inch per hour.

Details: High pressure just to our west will shift overhead
tonight for light winds, maybe a weak offshore wind at ORD/MDW.
The high will pull east Sunday, and winds will shift to SW then S,
ramping up slightly through the day reaching 10-12 kt in the
afternoon. Warm advection ahead of the incoming system will work
to lower cloud bases through the afternoon, with a fairly rapid
saturation commencing after about 22z from RFD southeast to IKK.
The leading edge of snow will be on the nose of the warm
advection and thus snow will commence from southwest to northeast.

While it is plausible some snow will arrive at the Chicago
terminals before 0z, higher confidence exists in it holding off
till around then. Once the snow hits it will be brief 4-5SM snow
before quickly deteriorating to IFR visibility. Snow rates do not
appear to be too high as this is not a banded snowfall event,
therefore expect steady rates generally around one quarter of an
inch per hour. Ceilings will lower to IFR eventually. At this
point this event does not look to be a 1/2 to 3/4 mile visibility
snow for any long period but we could see it briefly at times.
Will wait for later TAF issuances to hone in on TEMPO periods for
below 1SM vsby. Highest rates appear to be at onset of snow and
then again around the early portion of the morning rush (9z- 14z
Monday). The main snow period will end early afternoon Monday, but
there will likely be continued snow showers even through Monday
afternoon and even possibly some very light snow or flurries into
Monday night.

Storm total snow accumulations from Sunday night through Monday
about midday appear to be generally in the 2-3" range, with some
areas possibly to see above 3" simply due to the long duration of
the snow. Generally a dry snow is expected.



1049 PM CST

Expansive arctic high pressure across the Great lakes
and extending to the Ohio valley will shift east and become
elongate from the Ohio valley to New England on Sunday. Low
pressure will shift towards Lake Michigan Sunday night, then
slowly and pass over the southern tip of the lake Monday. Expect
a ramp up in southerly winds ahead of the low, with 30 kt winds
expected, possibly a few gale force gusts. With the low reaching
the lake anywhere from the southern third to central portion of
the lake, expect highly variable winds with winds shifting west
southwest across the southern half, and east-northeast winds for
the north half. While the main low will shift to the eastern Great
Lakes Tuesday, a portion will linger across southeast Lake
Michigan into Tuesday. Winds will shift to north and eventually to
northwest. Lake convergence may make for fairly gusty winds on
Tuesday. High pressure looks to then shift south of the lake mid
to late week and will turn winds to west- southwest for rest of
the week. With relatively low pressure across Quebec winds will
remain elevated to around 30 kt, but there is some gale potential
during this time.






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