Area Forecast Discussion
Issued by NWS Northern Indiana

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377
FXUS63 KIWX 110248
AFDIWX

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

.SYNOPSIS...
Issued at 938 PM EST Sat Dec 10 2016

Accumulating snow will expand overnight and continue through
Sunday night. Rain may mix with the snow 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.

&&

.UPDATE...
Issued at 938 PM EST Sat Dec 10 2016

Forecast still on track as 00z guidance begins to trickle in. No
changes to current headlines this evening with event just getting
underway. Warm air advection/isentropic lift snow band has finally
saturated lower levels and is expanding eastward this evening.
Already hearing reports of a half to one inch accums where this
band has been sitting. Some pockets of moderate to locally heavy
snow possible overnight over northwest Indiana and southwest
Michigan where strongest lift and moisture will be coincident.
Grids has this handled well. Question still remains later on
Sunday with how much warm air surges north and how far north it
will get. This could impact snow amounts, especially near the
warning/advisory delineation. Overnight crew will evaluate all of
the latest hires guidance and radar trends for any headline
changes.

&&

.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 00Z TAFS through 00Z Sunday Evening)
Issued at 632 PM EST Sat Dec 10 2016

Aviation forecast remains on track with deteriorating conditions
expected overnight and continuing into Sunday night. Regional
radar and surface observations show lower levels saturating
quickly across northeast IL this evening under persistent lift and
saturation. This area expected to expand east this evening and
should see KSBN begin to go down by 02z and KFWA closer to 03z per
latest guidance and trends. This initial saturation always tricky
and subject to some change. Should then see light snow persist
through the night and gradually becoming more moderate to locally
heavy at times Sunday. Some of the latest hires guidance does
indicate a possible lull to very light snow and even high end MVFR
or VFR conditions mid morning at KFWA before snow picks up again
in afternoon. Will have to monitor trends with little confidence
in such precision this far out.


&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Winter Weather Advisory until 4 AM EST /3 AM CST/ Monday for
     INZ013-015-017-018-020-022>027-032>034.

     Winter Storm Warning until 4 AM EST /3 AM CST/ Monday for
     INZ003>009-012-014-016.

MI...Winter Storm Warning until 4 AM EST Monday for MIZ077>081.

OH...Winter Weather Advisory until 4 AM EST Monday for OHZ004-005-015-
     016-024-025.

     Winter Storm Warning until 4 AM EST Monday for OHZ001-002.

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

&&

$$

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


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