Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 110906
AFDLOT

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

.UPDATE...
855 PM CST

Evening Update...

No changes planned to forecast or headlines at this time, though
will be making some minimal changes to near term pops based on
radar/obs trends.

Initial region of warm-advection forced light snow overspread the
region late this afternoon as anticipated, with heavier (mainly
moderate) snowfall within a region of frontogenetically-forced
northwest-southeast bands. Recent radar trends show this banding
has weakened and lifted northeast across southeast WI, Lake
Michigan and northern IN at this time. However, 00Z DVN sounding
depicts a fairly deeply saturated column with nice veering wind
profile signature associated with strong warm-advection. Within
this region of persistent upglide, latest high-res guidance
continues to support regeneration of transient f-gen banding which
should continue to result in periods of moderate or greater
intensity snowfall. While we`re currently in a lull in radar
returns especially across the western parts of the cwa, stronger
returns are blossoming across portions of eastern IA, and would
expect an uptick in coverage and intensity later this
evening/overnight as forcing and moisture advection persist. With
the first band working to moisten initially dry low-levels, will
likely see additional bands more efficient in lowering vis and
producing heavier snow, and latest RAP soundings suggest a decent
period of deeper dendritic growth after 05-06Z.

Initial bands of snow have produced 2+" of new snow in many spots
across the warning area, which is in line with going forecast
amounts through 06Z. Thus not inclined to make any big changes at
this time, with expected redevelopment and increase in snow
coverage and intensity later tonight.

Ratzer

&&

.SHORT TERM...
309 PM CST

Through Sunday Night...

The main forecast concerns with the winter storm continue to
revolve around total snow amounts, as well as the extent of the
mixing/changing to rain in my far south later Sunday into Sunday
evening.

Not much has changed in the overall thinking in terms of snow
amounts, with only minor tweaks made from the updated snow
amounts from this morning. Generally this will continue to result
in the highest totals of generally 8 to 12 inches north of I-80.
Amounts still look to tapper off into the 1 to 4 inch range over
my far southern counties (Livingston, Ford, Iroquois and Benton
counties). As a result, no changes are planned to the current
headlines.

An impressive band of moderate snow continues to develop east-
northeastward over eastern Iowa and western Illinois in
association with an initial band of frontogenesis along the
leading edge of the increasing isentropic upglide (warm air
advection). This area of snow should begin to result in a quick
onset of moderate snow from west to east over the area within the
next few hours. This could result in up to a quick inch of snow
through early this evening, especially over north central
Illinois. Once this snow onsets, it appears that it will continue
at times through the night and into Sunday morning over northern
Illinois and northwestern Indiana as warm air advection continues.
Mesoscale banding will also continue to be possible over northern
Illinois overnight as steep mid-level lapse rates remain in place
above 650 MB. As a result, snow amounts tonight alone could be in
the 3 to 6 inch range, especially north of I-80 where higher
ratios of around 15 to 1 will be likely.

Periods of moderate snow look to continue Sunday morning, though
there could end up being a break, or a period of lighter snow
during the morning, before another period of wetter snow onsets in
the afternoon in association with the approach of the mid-level
disturbance. This could again result in periods of moderate, to
possibly at times heavy snow over the area into early Sunday
evening. Temperatures still could get warm enough in my far south
for a mix or transition to a period of rain late Sunday into the
evening, though it appears areas farther north would remain
mainly snow. Snow ratios will likely be lower (around 10 to 1)
during the day, but another 3 to 6 inches will still be possible.

The snow should gradually come to an end from west to east during
the evening on Sunday. There could also be some patchy blowing
snow Sunday night as the winds pick up a bit on the back side of
the surface low. However, given the wet nature to the snow on
Sunday, blowing snow would likely be a minor impact.

KJB

&&

.LONG TERM...
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.

&&

.AVIATION...
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

&&

.MARINE...
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.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Storm Warning...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022
     UNTIL MIDNIGHT Monday.

     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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