Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 091746
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1146 AM CST Fri Dec 9 2016

.UPDATE...
1045 AM CST

We have opted to issue a winter storm watch for counties mainly
along and north of I-80 with the exception of La Salle County for
late Saturday afternoon through Sunday evening for an increasing
potential for 6+ inches of snowfall.

While the 12 UTC NAM continues to struggle with this event, the
global guidance has continued to show a better handle on this
evolving snow event this weekend. As such, a prolonged warm air
advection snowfall event is looking more likely across the area,
beginning as early as late Saturday afternoon. While uncertainties
still exist with the most favored time of heavier snow, continued
strong isentropic upglide should result in periods of moderate
snow supportive of good accumulation. Periods of heavier snow
will also be possible over northern Illinois in the vicinity of
mesoscale bands of frontogenesis, which should greatly aid in
forced ascent.

At the present, we have opted to keep southern sections of the
area out of the watch. While accumulating snow will still be
possible in these areas, it currently appears the heaviest snow
will fall over northern Illinois in and near the watch area. It
also is evident that the track of the lower level system could
very well bring warmer air into our southern counties by late
Sunday. This could result in a change over to a period of light
rain before the precipitation ends Sunday evening, and this could
limit accumulations farther south.

Otherwise for today, have added flurries/light snow shower
mention through mid-afternoon. For Porter County Indiana, true
lake effect showers are expected to continue and some minor
accumulation of up to a half inch or possibly one inch could
occur through this evening, mainly in the northeast part of the
county.

KJB/MTF

&&

.SHORT TERM...
410 AM CST

Through Sunday Night...

Main concern for the short term forecast period will be the
developing winter storm which is expected to bring snow to the
region as early as Saturday afternoon.

In the meantime, today and tonight will be relatively quiet with
mostly cloudy skies and unseasonably low temperatures. Highs today
will be in the lower 20s with lows tonight in the middle teens.
With winds of 5 to 10 mph overnight, wind chill readings will be
in the single digits.

As for the upcoming snow event, the longer range model guidance,
with the continued exception of the NAM, are in relatively good
agreement. Timing is still a bit tricky as the upper level pattern
is evolving into relatively zonal flow and timing of individual
shortwaves can be difficult. However, the GFS/ECMWF are in
relatively good agreement on the timing and strength of individual
waves, so confidence is increasing on the details and impacts of
this system. There should be 2 relatively different phases to the
pcpn. Initially, snow should begin to move into the wrn portions
of the CWA by early Saturday afternoon. Conditions should still be
sufficiently cold through a deep layer of the atmosphere that pcpn
type is not in question and will begin as snow. The initial wave
of pcpn will be driven by large scale warm advection in advance of
a developing sfc trough over the wrn plains. As sfc high pressure
settles off the Carolina coast, the GLFMEX will become more open,
with deep layer moisture streaming nwd ahead of the trough. Upper
level support should be somewhat weak as sheared out shortwaves
track across the upper Great Lakes. From Saturday afternoon
through early Sunday, the heaviest snow should be over the
northern portions of the CWA, generally north of the I-80
corridor. Given the cold in situ conditions, snow to liquid ratios
should be a little higher than typical, around 17:1. Current
indicators suggest that 4 to 6 inches may fall by early sunday
morning north of I-80 with amounts diminishing rapidly to the
south.

By Sunday afternoon, pcpn type becomes more of a concern. Sfc low
pressure developing over the plains should approach the middle
Mississippi Valley by mid day Sunday, further increasing the
warm/moist advection. Sfc temperatures will begin to creep upward
through the day, but with warmer air aloft overspreading a colder
sfc, where temps should still be at or slightly below freezing.
This will set up the potential for a at least a mix of fzra/ra or
fzra/sn and a narrow corridor of freezing rain. Timing of the mix
pcpn phase of the system remains the biggest uncertainty. The
latest guidance would suggest that pcpn could be all snow into
early afternoon with the changeover to mixed pcpn and then to
liquid occurring over the srn portions of the CWA through the
afternoon and into the evening hours. The sfc temperature trend
should be atypical as well, with temperatures continuing to climb
through the evening. At this point, it looks like the freezing
rain could reach as far north as the I-80 corridor through the
evening hours, while pcpn over the southern portions of the CWA
becomes all liquid. The northern portions of the CWA, especially
north of the I-88 corridor will likely remain all snow, though
with the warmer air spreading north, liquid to snow ratios should
decrease, dropping to 10-12:1. The greatest uncertainty in timing
of the evolution increases through Sunday night as the ECMWF
trends a bit faster than the GFS in lifting the sfc low across nrn
IL and to the ern Great lakes. This appears to be a result of the
GFS showing a bit more phasing of a middle stream shortwave across
nrn IL and a nrn stream shortwave tracking across nrn Wisconsin.
Given the relatively fast zonal flow aloft initially, would tend
to favor the faster solution of the ECMWF, which would ultimately
lead to pcpn tapering off a bit quicker than the slower, more
phased GFS solution. But at this time range, the differences
between these 2 solutions would ultimately only cause minor
differences in eventual snow totals. Based on the currently
favored solution, the northern portions of the CWA could see an
additional 3 to 5 inches of snow, with snow amounts, again,
rapidly dropping off to the south. For the entire event, currently
expect 7 to 10 inches of snow north of the I-80 corridor, with the
highest amounts closer to the Wisconsin border. The far southern
portions of the CWA may only see 1 to 2 inches.

&&

.LONG TERM...
306 AM CST

Monday through Friday...

Dangerously cold temperatures will be the main forecast story
next week with sub-zero conditions possible as we head through
midweek. Area of low pressure that will bring our snowy conditions
over the weekend is progged to lift across the eastern Great Lakes
to New England on Monday. GFS spits outs some light QPF early in
the day despite forecast soundings indicating quickly drying mid
levels with subsidence associated with building upper ridge
overspreading the area in the afternoon. Could be some spotty
light flurries or possibly even drizzle given the shallow depth of
the moisture which eventually tops out in the -8 to -10C range for
ice nucleation, but in general think the models may be overdoing
QPF for Monday given the shallow moisture and weak forcing.

Meanwhile on Monday, a strong jet topping an anomalously strong
upper ridge over northern Alaska into the Arctic Ocean will help
dislodge an extremely cold Arctic airmass and push near -40 850MB
temps across the Canadian Prairies on Tuesday with -20C to -30C
temps overspreading portions of the upper Midwest Wednesday and
Thursday. Highs around 30F on Monday will give way to single
digit or teens for lows Wednesday morning behind the Arctic front
with little recovery during the day Wednesday. Models continue to
show some spread in exactly how cold we get, with the latest
forecast remaining conservatively on the warmer side of solution
envelope mainly due to the influence of warmer MOS guidance. Raw
output from the latest runs of the GFS and ECMWF indicate the
possibility of sub zero highs on Thursday over some if not much of
the CWA with widespread sub-zero conditions both Wednesday night
and Thursday night. In addition, breezy conditions in advance of a
strong 1040mb high building into the northern Plains Wednesday and
Thursday will result in (again conservative) wind chill values of
10 to 20 below.

Deubelbeiss

&&

.AVIATION...
For the 18Z TAFs...

The concerns in this TAF period are ongoing flurries/light snow
showers and some occasional MVFR cigs possible. The primary main
snow time remains beyond this current TAF...more on this in the
final paragraph.

Cold advection flurries/snow showers are very slowly pushing
southeast as drier air moves in aloft. The period of MVFR
visibility is likely done at ORD and MDW, but if it was to occur
again early this afternoon it would be brief. Spotty MVFR cigs
are noted in observations, but all 2000ft or higher, so not a huge
impact.

Winds will be diminish and turn southwest by Saturday morning and
then south by late Saturday afternoon prior to arrival of the
snow.

There could be some flurries Saturday morning again, but the main
snow will arrive at RFD between 20Z and 23Z and ORD and MDW
between 22Z and 01Z. Snow should become moderate in intensity
possibly quickly after starting and persist through Saturday
night. This snow should be a lighter water content. While
intensity of snow may wane some Sunday morning, an uptick in snow
is expected during the afternoon into the evening. This snow will
have a higher water content. Total accumulations in excess of six
inches are becoming more favored for ORD, MDW, and RFD, with two
to five inches of that by daybreak Sunday.

MTF

&&

.MARINE...
306 AM CST

Moderate northwest winds gusting to near 30 kts are in place early
this morning across Lake Michigan but should very gradually ease
into the 15 to 25 kt range through the day as high pressure builds
towards the region. A small craft advisory remains in effect
through late this morning for the Illinois nearshore waters, and
through the afternoon into the early evening for the Indiana side
when waves should finally diminish below criteria. Low pressure is
expected to develop over the Central Plains Saturday night into
Sunday turning winds southerly across Lake Michigan, then the low
is expected to lift across southern Lake Michigan Sunday night
with north to northwest flow overspreading the lake behind the low
Monday. Another cold front associated with a low over Canada will
push across the lake on Tuesday with a bitterly cold airmass
moving in behind the front. West to northwest gales are possible
especially Wednesday into Wednesday night and there is also a
concern for freezing spray.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Storm Watch...ILZ003-ILZ008-ILZ010...3 PM Saturday TO 3
     AM Monday.

     Winter Storm Watch...ILZ004-ILZ005-ILZ006-ILZ011-ILZ012-ILZ013-
     ILZ014-ILZ020-ILZ022...6 PM Saturday TO 3 AM Monday.

IN...Winter Storm Watch...INZ001-INZ002...6 PM Saturday TO 3 AM
     Monday.

LM...Small Craft Advisory...INDIANA NEARSHORE WATERS UNTIL 6 PM Friday.

&&

$$

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