Area Forecast Discussion
Issued by NWS Chicago, IL

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610
FXUS63 KLOT 092131
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
331 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...
317 PM CST

Sunday through Friday...

The main concerns during the period continue to first focus on
the second half of the winter storm expected to impact the area
through Sunday night. However, very cold weather, along with
another chance for some light accumulating snow will be the focus
around midweek.

There is still a decent amount of uncertainty with the specifics
of the snow accumulations on Sunday into Sunday evening. However,
given the fact that it now appears that they may be little to no
break in the snowfall, which should start during the later
portions of Saturday. We felt it prudent to issue a winter storm
watch earlier today for the northern portion of the area in
collaboration with the neighboring offices.

In spite of the fact that model guidance continues to differ on
QPF amounts on Sunday into Sunday night with an overall weak
surface low forecast, they do all indicate signs that could
support periods of moderate snowfall. Most notable of all is the
prolonged strong isentropic upglide (warm air advection), through
the period. In fact, both the GFS and ECMWF advertise winds on the
285K through 295K surfaces of 35 to 45 KT blowing nearly
perpendicular to the isobars. This suggests that continued ascent
and warming/moistening conditions should continue to support
periods of snow through the day. Additionally, precipitable water
values up around 0.6 inches and mixing ratios of around 3g/kg are
forecast over the area during the day, which is favorable for a
heavier wet snow. The Garcia method, which can work well in these
warm air advection regimes suggests this could support up around 6
inches of snow during the day Sunday alone. As a result, I have
tended to favor the more moist 12 UTC GFS solution on Sunday.

This strong warm air advection also suggests that precipitation
type issues could arise by late Sunday into Sunday evening,
especially for areas south of I-80. Even so, a few inches of snow
could accumulate over my south during the early portion of the day
Sunday before it tries to mix with or change to rain. Given
uncertainties on how far north the rain snow line with get later
Sunday, I have continued to mention at least a small chance of
snow over my far south, though if the GFS is right, periods of
snow early Sunday could transition to rain south of interstate 80
during the afternoon. This makes snow amount forecasts for my
southern areas even more tricky as they could end up being limited
by the possibility for rain later in the day.

Overall, for snow amounts, I have continued to have the highest
amounts in the watch area, with the potential of 4 to 6 inches
during the day Sunday, and possibly another 1 to 3 inches Sunday
evening. This would result in totals (including Saturday and
Saturday night) of 6 to 12 inches in the watch area. Farther
south, amounts could tapper off quickly as you approach central
Illinois. While the current watch continues to cover the areas of
best potential to get 6+ inches, a county or two south of the
watch area could need an advisory (for 2 to 6 inches) as we get
closer to the event.

If that were not enough, more snow is possible Tuesday night and
Wednesday in association with an arctic push of very cold air.
This could again result in some snow accumulations over portions
of the area, though the very cold conditions will become the main
story over the area for Wednesday and Thursday. Highs look to only
be either in the single digits or the teens, with lows possible
dropping below zero in some areas at night. Winds with the arrival
of the arctic airmass could also result in wind chills in excess
of -20 Tuesday and Wednesday nights, so wind chill headlines could
be needed.

Beyond the current forecast period there continue to be signs of
more snow, so the larger scale weather pattern looks to remain
favorable for periods of active weather and cold for at least the
next week.

KJB

&&

.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...
325 PM CST

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. A stronger arctic cold front is
then expected to move over the lakes region late Tuesday. This
will result in a period of impactful west-northwesterly winds
over the lake Wednesday into Thursday as an arctic cold front
moves over the lake. Gales will be a good possibly, along with the
threat of freezing spray during this period.

KJB

&&

.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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