Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 101151

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
551 AM CST Sat Dec 10 2016

331 AM CST

Through Sunday night...

Challenging forecast tonight through Sunday evening with potential
for a long duration snow event with significant totals, especially
northern half of the CWA. Questions remain about how much precip
will fall and potential for mixed precip and much lower snow:liquid
ratios (SLRs) Sunday into Sunday night. This event absolutely does
not fit the conceptual model of a warning worthy snowfall event
locally, so given the expected very long duration of the event and
primarily light to moderate snowfall accumulation rates, we`re
opting to upgrade the winter storm watch to a winter weather
advisory. Since we are teetering along the edge of warning
criteria over northern most CWA, it is possible that some areas
may need to be upgraded to a warning, but after collaboration
internally and with DVN felt this was the best course of action.

Meteorologically, a fairly strong Pacific jet stream will
translate eastward into the the Great Lakes region later today
and through Sunday. Height falls in advance of a fairly low
amplitude short wave will result in back low/mid level flow across
the area today, tightening up the thermal gradient with
strengthening isentropic ascent/warm air advection likely to
result in a swath of snow developing east into northern IL late
this afternoon and into northwest Indiana by early evening.

Guidance is in pretty good agreement with QPF through 12z Sunday,
with maybe up to an inch or so of snow possible between 2-6pm
western CWA. Tonight (00z-12z Sunday) guidance paints a swath of
QPF over northern half of the CWA in the neighborhood of 0.25",
which given fairly efficient ratios should fluff up to 2 to
perhaps 5 inches of snow. Time-height cross sections suggest that
while the dendritic growth zone will be deep, especially the first
half of the night, ascent within that layer is not expected to be
particularly strong and progged RH wrt to ice is not super
saturated, so extremely efficient dendrites appear unlikely to be
the primary/dominant crystal type, none the less it should be a
dry snow that should fluff up to 15 to perhaps 20:1 ratio.

GFS and NAM both show transient waves of modest f-gen, but
neither model suggests that a deep sustained frontogenetic
circulation will become established and park overhead like last
weekend. In addition, atmosphere looks to be largely pretty stable
tonight with only a shallow layer of weakly negative EPV
developing for a short time late tonight into early Sunday
morning, which could correspond to timing-wise to some stronger
transient snow bands. Given the largely stable conditions tonight,
the potential for narrow intense banded snowfall looks low.
Again support the notion that this event tonight has the look of
a steady light to occasionally moderate snowfall event.

QPF from the various models varies more substantially heading
into Sunday. Warm air advection is expected to begin to result in
steady warming of the low to mid levels of the column Sunday. At a
minimum, this should result in a gradually wetter and less
efficient snowfall with time Sunday. In addition, the persistent
WAA should begin to push temps above freezing in the afternoon,
especially southeast of I-55 corridor and into downtown Chicago.
While forecast soundings generally keep the column cold enough for
all snow, the surface low track into southern WI conceptually
would support the potential for some mixed of precip as far north
as Chicago. Some guidance also suggests mid level drying, which
could result in the loss of ice nuclei and a transition to drizzle
Sunday, this is mainly over southern CWA. Have gone with fairly
low SLRs over southern half of the CWA in the afternoon Sunday
due to the potential for mixed precip and above freezing temps,
which does lead to a sharp southern cut off to the heavier snow
accumulations, which is pretty typical in these type of events and
being depicted pretty nicely by some of the higher resolution

Generally looking for snowfall totals in the 6 to locally 10"
range along and north of the I-88/I-290 corridor, but the snowfall
rates look to generally remain below the 6"/12 hour and 8" in 24
hour warning criteria. Also, hitting on a weekend and likely
peaking in the middle of the night tonight could lessen the
impacts some, which given the borderline warning criteria supports
starting with an advisory and upgrading when/if it becomes clear
heavy snow will materialize. In the heavier accumulating snow
swath (generally along north of I-80) expecting up to an inch of
so through 6pm this evening, 2-5" tonight, an additional 1-4"
Sunday. Snow or mix should end from west to east by late
afternoon west and during the evening Sunday over NW IN.



314 AM CST

Long Term...

Monday Through Saturday...

Main concern for the long term forecast period will be temperature
trends with the coldest air of the season expected.

Monday will be a transitional day as the system bringing the pcpn
lifts through the eastern Great Lakes and continental polar area of
high pressure builds across the middle Mississippi Valley and into
the Ohio Valley.  This will not be a particularly cold air mass and
max temperatures for Monday will be in the 20s, though this will be
8 to 10 degrees lower than the warm sector airmass ahead of the
passing cold front.  With the cold front passing across the region
Sunday night and modest cold advection setting up following the
fropa, temperature trends will likely be very flat through the day.

The surge of arctic air will begin to push into the region
following the passage of a secondary cold front Monday night.
This will be a true arctic airmass and the cold advection behind
this front will be quite strong. Max temps for the calender day
for Tuesday will likely occur during the late night hours and then
steadily drop through the day and Tuesday night. By sunset
Tuesday afternoon, temps should drop into the single digits over
the nwrn portions of the CWA, including the rockford area. There
is a chance for some light snow or flurries with the fropa, but
the system will generally be moisture starved and little
additional snow accumulation is likely. Strong high pressure will
build through the cntrl plains, and across the middle Mississippi
Valley, and become centered over nrn IL Tuesday night. By
Wednesday morning, temps will drop into the lower single digits
above zero west of the fox Valley with lows arnd 10 F east of the
I-55 corridor. A weak, sheared out shortwave crossing the midwest
will bring another chance for some light snow and the associated
cloud cover will allow for much in the way of diurnal warming. The
mid level system will move across the area relatively quickly
and bring another inch or 2 of snow on Wednesday. The coldest day
of the period will be Thursday. Temps thursday morning are
expected to range from around -5 F over ncntrl IL to +5 F over
ecntrl IL/wcntrl IN. There is some concern that even these low
temps may be a bit conservative, with some of the longer range
guidance, in particular the raw model output of the GFS and ECMWF
suggesting that sub-zero temperatures could overspread the entire
CWA, with MOS influenced guidance bringing a little too much
climatology to the mix, which would suggest the slightly higher
temperatures. Given some uncertainty, particularly in how much sky
clearing can occur overnight and into Thursday morning, have opted
not to go too extreme with min temps, but there is a chance that
temps could be a bit lower than currently forecast. The same logic
applies to Thursday and Thursday night into Friday morning. max
temps across the area should remain in the single digits on
Thursday with lows ranging from arnd -5 F to +5 F again Friday
morning. Wind Chill readings will also be a factor for Wednesday
night and Thursday night. The longer term models are indicating
enough breeziness these nights to generate wind chill readings of
-10 to -20 F. Temps should remain well below normal for Friday and
into next weekend, with a slight warming trend bringing temps back
into the upper teens to lower 20s by next Saturday.


For the 12Z TAFs...

Accumulating snowfall is expected to arrive by late this afternoon
or early evening and will continue through Sunday evening
producing several inches of accumulation over a 36 hour period. In
advance of the snow this morning and early afternoon, expect
fairly benign weather with light westerly winds and primarily VFR
conditions. There are a few flurries in the area once again this
morning, though nothing of any impact. More significant snow is
expected to develop eastward across Nebraska and Iowa through the
day today in advance of a developing low pressure system over the
Central Plains. Expect snow will spread across RFD in the 19-21Z
timeframe then across the Chicago terminals starting around the
22-00Z timeframe. Once snow commences, expect vsby to deteriorate
to IFR fairly quickly, within an hour or so and for cigs to drop
to low MVFR. Cannot rule out cigs dropping below 010 at times with
heavier snow. Expect fairly steady light snow to continue through
the overnight hours and through the day Sunday, though there may
be a couple windows of more moderate intensity snow. Have low-
medium confidence in a window of lower conditions with higher snow
rates this evening, and medium confidence in another window that
begins in the predawn hours Sunday and persists through the
morning Sunday. Have gone with prevailing 3/4SM vsby during these
time frames and may have to add tempo 1/2SM at some point if
confidence increases. By the end of this long duration snowfall
Sunday evening, the terminals could see anywhere from 4 to 9
inches of snowfall with higher amounts favored at the more
northerly locations.



354 AM CST

Moderate west winds of 15 to 25 kt will continue to very gradually
subside today to 10 to 20 kt this afternoon as high pressure
builds towards the region. The ridge axis will move across the
lake tonight with southerly flow increasing from the south ahead
of a deepening low over the Plains. The low is expected to lift
across Lake Michigan late Sunday evening and overnight with south
winds peaking at around 30 kt ahead on Sunday and then turning
west to 30 kt behind Sunday night into Monday. A small craft
advisory will likely be needed for the Indiana and Illinois
nearshore waters on Sunday. Meanwhile, another low over Hudson
Bay will drive a powerful cold front across the region Monday
night into Tuesday. As very cold air overspreads the Great Lakes
behind the front, could see gales or near gale winds at times
Tuesday night into early Thursday. Air temperatures are expected
to fall into the teens or even single digits which along with the
strong winds and high waves will contribute to some risk for
freezing spray to occur, though water temperatures remain
relatively warm and may help to mitigate the risk to some degree.



IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ019-ILZ020-ILZ021...3 PM Saturday TO 6 PM

     Winter Weather Advisory...ILZ006-ILZ013-ILZ014-ILZ022...6 PM
     Saturday TO 10 PM Sunday.

IN...Winter Weather Advisory...INZ001-INZ002...6 PM Saturday TO 10 PM




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