Area Forecast Discussion
Issued by NWS Northern Indiana

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922
FXUS63 KIWX 102107
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
407 PM EST Sat Dec 10 2016

.SYNOPSIS...
Issued at 356 PM EST Sat Dec 10 2016

Accumulating snow is expected across the region from tonight
through Sunday night. Light snow will overspread the area this
evening and increase in intensity late tonight into Sunday
morning. Snow will continue into Sunday evening, but may mix with
or change to rain Sunday evening before diminishing for locations
south of Route 30. Significant snow accumulations are possible
tonight through Sunday night, with the greatest accumulations
expected across far northern Indiana, southern lower Michigan and
portions of far northwest Ohio. High pressure follows for the
beginning of next week with calmer, but cool conditions.
Thereafter, even colder air will move into the Great Lakes region
behind a cold front expected to move through around the middle of
next week.

&&

.SHORT TERM...(This Evening through Sunday)
Issued at 356 PM EST Sat Dec 10 2016

Accumulating snow tonight through Sunday evening will be the primary
forecast challenge along with resolving expected north to south
gradient in snowfall amounts. A winter storm warning has been
issued for approximately northern half of forecast area, winter
weather advisory for the south.

Warm advection forced snow has expanded this afternoon across
eastern Iowa and will encounter relatively dry low levels downstream
late this afternoon early evening as sampled by morning KILX RAOB.
Thus, expecting snowfall rates to be on the light side this evening
as isentropic lift shifts eastward across the area. Next upper PV
anomaly ejecting eastward from the Pacific Northwest coast will
reach the Northern Plains early Sunday morning. Strengthening
southwest flow in advance of this wave will allow for increased
isentropic upglide, peaking sometime in the 12Z-18Z timeframe.
Mass/moisture convergence at the nose of this low level jet should
allow for axis of low-mid level frontogenesis forcing to accompany
stronger warm advection forcing, allowing for potential for more
focused bands of moderate snow to develop Sunday morning. Good deal
of moisture to advect northeastward into the area with PWATS
increasing into the 0.6-0.75 inch range along with 850 mb specific
humidities of 4+ g/kg. Large scale upper support will also be
enhanced by broad right entrance region of a 140 knot jet streak
across the eastern Great Lakes on Sunday.

Given persistence and strong magnitude to warm advection forcing
and anomalous moisture...still see potential of swath of 6 to 10
inch accumulations given potential fgen influence/augmentation to
WAA/isentropic lift, although always a challenge in these
moist/stronger WAA events to fully resolve evolution. A couple of
not so favorable factors to consider include snow ratios that will
be lowering with time on Sunday as warmer low/mid level air gets
transported northward. Most efficient snow ratios will accompany
the first half of this event (approximately through 15Z).
Potential of riming will need to be considered acting to reduce
ratios Sunday morning and beyond with fairly large isothermal
layer in the -5 to -10 degree C range. South winds have the
potential to gust to around 20 mph on Sunday, although lowering
snow ratios in the afternoon may tend to mitigate blowing
concerns.

Passage of mid level trough across western Great Lakes will allow
warm advection to continue into Sunday evening, with ptype issues
likely becoming a factor across approximately southern half.
Evolution of thermal profiles and persistence of isentropic forcing
across the northern half Sunday morning may lead to a fairly sharp
gradient in snowfall totals. Have issued a winter storm warning
for approximately northern half of the area, tapering to advisory
level amounts of 2 to 4 inches across the far south. NAM has come
on board with bulk of other guidance in a southward trend over the
past few runs, but confidence in exact snowfall amounts still on
the medium side (especially across the southern half) where
thermal profiles become more marginal earlier in this event. Upon
collaboration with surrounding offices have expanded winter
weather advisory across southern half of the area for these lesser
accumulations. Precip may end as rain/snow mix or drizzle/light
freezing drizzle mix late Sunday evening/early Monday morning as
deeper moisture pushes east of the area.

&&

.LONG TERM...(Sunday Night through Saturday)
Issued at 356 PM EST Sat Dec 10 2016

Upper level wave to pull off to the east early Monday with mainly
dry conditions expected during the day. A fast moving short wave
should skirt across western Great Lakes Monday night with meager and
shallow moisture profiles initially in place. This wave will have
strong baroclincity associated with it and accompanying mid deck
which may provide some seeder-feeder mechanism, so have included
low chance snow shower PoPs across the northwest Monday night. A
trend to much colder conditions behind this wave for Tuesday, and
especially by Wednesday as reinforcing boundary drops across the
area. Not much change from previous forecast in potential for wind
chills in the 5 to 15 below range for much of the Wednesday night
through Thursday night period.

Toward latter portions of the long term period, medium range models
generally agree on larger scale longwave trough taking shape across
western CONUS with frontal wave lifting into the region bringing
increased precip chances. Strong advection during this period will
lead to moderating temperatures and possible ptype concerns toward
the end of this forecast valid period.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday Afternoon)
Issued at 1236 PM EST Sat Dec 10 2016

Wind direction fluctuating between more westerly and southerly has
allowed for SBN and FWA to get into some more lake effect snow with
MVFR conditions. With wind direction changing to more westerly soon,
only kept these conditions temporarily. Elsewhere, snow in the
Central Plains is moving into the Midwest and looks to reach SBN
around 02z tonight. Clouds moving into the region may drop CIGs to
MVFR around 01z at SBN first before moving to FWA. Expecting there
may be some early onset snowfall evaporation occurring, but then
IFR conditions take over as the snow intensifies.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Winter Weather Advisory from 9 PM EST /8 PM CST/ this evening to
     4 AM EST /3 AM CST/ Monday for INZ013-015-017-018-020-
     022>027-032>034.

     Winter Storm Warning from 9 PM EST /8 PM CST/ this evening to 4
     AM EST /3 AM CST/ Monday for INZ003>009-012-014-016.

MI...Winter Storm Warning from 9 PM this evening to 4 AM EST Monday
     for MIZ077>081.

OH...Winter Weather Advisory from 9 PM this evening to 4 AM EST
     Monday for OHZ004-005-015-016-024-025.

     Winter Storm Warning from 9 PM this evening to 4 AM EST Monday
     for OHZ001-002.

LM...Small Craft Advisory from 5 AM Sunday to 4 PM EST Monday for
     LMZ043-046.

&&

$$

SYNOPSIS...Marsili/Roller
SHORT TERM...Marsili
LONG TERM...Marsili
AVIATION...Roller


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