Area Forecast Discussion
Issued by NWS Chicago, IL

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940
FXUS63 KLOT 092047
AFDLOT

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

.SHORT TERM...
246 PM CST

Through Saturday Night...

The primary short term forecast message continues to be snow
spreading west-to-east across northern Illinois mid-late Saturday
afternoon and northwest Indiana by early evening and persisting
through Saturday night. No changes were made this afternoon to
the current Winter Storm Watch. Overall, this snow event is now
being approached and messaged as one single event, even though
meteorologically there are somewhat different characteristics
Saturday night versus Sunday afternoon and night, but for
simplicity, the break in precipitation -- or at least in
intensity -- should be limited along/north of I-80.

Back to the near term for a minute...pockets of cold advection
flurries should wane by sunset and the lake effect snow showers
into Porter County are likely to orient more east by mid-
evening. High clouds will be on the increase this evening but
beforehand there could be a fairly stout early evening temperature
drop, even into the lower teens in north central Illinois. As the
mid-level clouds increase along the 700mb thermal gradient, it is
possible some very light snow does make it down late tonight into
Saturday morning, but should not be of any impact.

Warm air advection (WAA) aloft will increase Saturday afternoon
as the first low amplitude shortwave moves into the Upper Midwest,
with overall good parameters for lift. Snow should develop east-
southeast following the mid-level baroclinic zone. With profiles
fairly cold, this spread could be a fairly fast and have bumped
up the start time of snow some, especially over north central
Illinois (Watch accounts for that well at this time). The snow
should broaden and intensify Saturday evening within the WAA
pattern, with moderate snow becoming dominant. Profiles are
favorable, especially in the evening for omega intersecting a
deeply saturated dendritic growth zone in guidance. Ratios of 14:1
to 19:1 may support a quick 2-4 inches by midnight, especially in
any evident bands that setup.

The challenges for Saturday night are still in the details, and
those are how far south will snow spread, and what axis or axes
may be favored for moderate to briefly heavy QPF/snow since the
regime favors some modest frontogenetic banding (even though
guidance is particularly strong on its f-gen fields over the local
area). Have snow spreading to the far southern CWA but do not
have much for accumulation there, with amounts ramping up
northward especially near the I-80 corridor for a gradient.
Along/north of I-80 is where semi-persistent banding would be
favored, and that will likely result in some streaks of higher
snowfall Saturday night in that area. With moisture of 2 to 2.5
g/kg Saturday night on key lifted layer, have forecast snowfall
totals roughly two to six inches along and north of I-80.

Southerly winds will be on the increase late Saturday night with
10 to 15 mph winds expected. There could be some patchy blowing
snow in outlying areas, but not anticipating that to be a major
issue.

MTF

&&

.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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