Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 110931

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
331 AM CST Sun Dec 11 2016

330 AM CST

Through Tonight...

Periods of light snow will continue through the remainder of the
day with a couple windows where some locations could see bursts
of moderate or even briefly heavy snowfall.

Early this morning, low pressure is centered over the OK/TX
panhandles with axis of surface pressure falls stretching
northeast into northern MO then to near DBQ giving a preview of
where the developing surface low will track through the day today.
Aloft, water vapor imagery shows an upper low digging across
eastern MT with sheared vorticity stretching southwest into
northern UT. As these features move east across the plains today,
this will result in modest surface cyclogenesis with broad strong
WAA occurring across much of the Midwest. Radar imagery shows
light to moderate snow stretching from northern IA/southeast MN
ESE into northern IN and southern MI on the nose of the strongest
mid level WAA which is expected to peak across the local area
through the morning then shift east by late morning. Latest runs
of the RAP continue to indicate F-Gen increasing through the
predawn and early morning hours across SE MN ESE across far
southern WI and far NE IL this morning. This corridor of strong
forcing superimposed with steep lapse rates above roughly 700mb
continues to indicate some potential for mesoscale banding which
could result in some locally higher snow totals, and Boone-
Mchenry-Lake IL seem to have the best chance for seeing banding
at this time.

Guidance is in decent agreement with a little bit of a lull in the
snow amounts in the mid to late morning hours as this initial
wave of WAA forcing shifts off to the east. Forcing is expected to
increase again this afternoon, though, as 500mb height falls
associated with the upper wave begin to overspread the region. RAP
again shows a corridor of strong F-Gen, this time between the I-80
and I-88 corridors though is more transient in nature and
confidence in mesoscale banding lower. In addition, warming of the
column will produce a deep isothermal layer of -3C to -5C which is
not overly favorable for large dendrite growth and will contribute
to lower snow to liquid ratios, likely in the 8-11:1 range.

Snowfall totals for the rest of today through midnight tonight are
expected to be in the 4-8 inch range for much of the Chicago
Metro area and northwest Indiana with 3-6 over north central
Illinois, and amounts quickly tapering south of I-80. Again, while
light snow is likely though most of the day, we may see bursts of
better accumulating snow early this morning and again during the
afternoon hours. Including snow that has already fallen, we are
expected to see storm Total snow of 6-10 inches for the Chicago
Metro, 5-9 over north central Illinois, and again amounts taper to
just a few inches as you head south of I-80. No changes are
planned for the current headlines.



305 AM CST

Monday Through Sunday...

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 that brought the
pcpn lifts through the eastern Great Lakes and high pressure
builds across the middle Mississippi Valley and into the Ohio
Valley. Since this will be a continental polar air mass and not
particularly cold, max temperatures for Monday will be in the 20s,
though this will be 8 to 10 degrees lower than the warm sector air
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.

A surge of arctic air will begin to push into the region following
the passage of a secondary cold front late Monday night or early
Tuesday morning. 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 shortly after
midnight and then steadily drop through the day and Tuesday night.
By sunset Tuesday, temps should drop into the lower single digits
over the nwrn portions of the CWA, including the Rockford area.
There is a chance for some light snow with the frontal passage,
but the system will generally be moisture starved and little or no
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 below zero west of the fox Valley with lows in the
lower single digits above zero across the remainder of the CWA.

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 still 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 much of the the area should
remain in the single digits on Thursday with lows ranging from
slightly above to slightly below 0 F 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. A new low is
expected to develop over the central plains late next week,
bringing some warmer air back, but it will also be the next
significant weather make, bringing the next round of slow to the
region. While the warm advection in advance of the system will
bring temperatures back into the middle 20s to lower 30s, the
strong isentropic lift in the warm sector ahead of the system
should also bring the next round of measurable snow for Saturday,
only to be followed by another arctic blast next Sunday.


For the 06Z TAFs...

Confidence in trends have increased some since the 00z tafs as a
lull in the snow has developed late this evening. Guidance shows
light to moderate snow developing back across the terminals in the
early morning hours with a period of moderate to perhaps briefly
heavy snow centered just before sunrise. Then trends continue to
suggest another lull in the precip mid morning Sunday. During
these lulls...light snow will still be falling but vis/cig will
improve to at least ifr and perhaps mvfr. Low pressure will then
move across the area Sunday afternoon and this period now looks to
be the time when the heaviest snow and lowest vis/cigs will occur.
Timing will need some adjustments/tweaks but appears snow will
once again develop in the late morning and continue through the
early evening with a period of heavy snow possible during the
afternoon. Tempo timing is centered on most likely timing for
heavy snow but its possible that once the snow begins...17z-19z...
it may quickly lower to 1/2sm or 1/4sm. Confidence only medium
from this distance so additional changes are possible with later
forecasts. As the surface low departs Sunday evening...should be
a sharp cutoff to the back edge of the snow Sunday evening.

South/southeast winds 10-15kts will continue through late Sunday
morning and turn more southerly for a time before turning more to
the south/southwest Sunday afternoon...then shifting westerly
Sunday evening. Gusts to 20 kts possible Sunday morning. cms


211 AM CST

Low pressure developing across the Plains early this morning will
lift northeast toward the western Lakes this afternoon, and will
cross northern Lake Michigan tonight. Surface pressure falls of
2-3 mb per 3 hours ahead of the low will serve to tighten the
pressure gradient across the region ahead of the low, with south-
southeasterly winds increasing into the 25-30 kt range this
morning. Winds will ease late this afternoon/early evening as the
gradient weakens in the vicinity of the surface low center,
veering to the west and increasing again after midnight as the
trailing cold front moves through. Cold arctic air will produce
steep low-level lapse rates over the relatively warm lake waters,
with winds expected to increase to 30 kts again overnight and
early Monday, easing later in the day as the low pulls further
away to the east. A strong secondary arctic cold front will move
across the lake Tuesday night, deepening the mixed layer further
and producing a windy period with westerly gale force winds
possible from early Wednesday through early Thursday, especially
over central and northern parts of the lake. High pressure ridging
eventually moves across the western Lakes late Thursday
night/Friday allowing winds to subside. Longer range model runs
suggest the potential for another deep low to lift across the
western Lakes next weekend.

For the IL/IN nearshore waters, the increasing south-southeast
winds this morning and blustery west winds tonight into Monday will
support small craft advisory conditions.



IL...Winter Storm Warning...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-

     Winter Weather Advisory...ILZ023 UNTIL MIDNIGHT Monday.

IN...Winter Storm Warning...INZ001-INZ002 UNTIL MIDNIGHT Monday.

     Winter Weather Advisory...INZ010-INZ011 UNTIL MIDNIGHT Monday.

LM...Small Craft Advisory...ILLINOIS NEARSHORE WATERS UNTIL 9 AM Monday.

     Small Craft Advisory...INDIANA NEARSHORE WATERS UNTIL 3 PM Monday.




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