Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 160513
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
1213 AM EST Tue Jan 16 2018

.SYNOPSIS...
Issued at 409 PM EST Mon Jan 15 2018

Light snow will linger overnight across far northern Indiana and
Lower Michigan with minor additional accumulations possible. Dry
otherwise with lows in the single digits. Otherwise, heavy lake
effect snow is possible for snowbelts impacted by north to
northwest winds tonight into Wednesday morning. Accumulations
greater than 6 inches are possible, especially in Berrien and La
Porte counties. See the latest winter weather message for details.
Otherwise, temperatures will fall to the single digits tonight.
Areas south of US 30 could see wind chills as low as 10 to 15
below zero.

&&

.SHORT TERM...(This Evening through Tuesday)
Issued at 409 PM EST Mon Jan 15 2018

...Significant Lake Effect Snow Possible Late Tonight into Wednesday
Morning...

Expect a diminishing trend to the snow ahead of the clipper system
as the best isentropic ascent and upper level forcing weaken
extensively, and shift eastward out of the area overnight. The dry
slot has already advanced across much of the CWA and expect this
will continue as the low lifts NE. As far as the lingering trough
axis behind our clipper, things look on track as far as the larger
scale pattern. Aloft, we`ll see the upper level low/potent PV
anomaly move over southern Lake Michigan, which gives us ample
support for snow development. Accordingly we see models develop
precipitation along the WSW-ENE oriented trough axis that develops
at the surface, with winds becoming more WSW and N-NW by early
Tuesday morning. With this convergence over the lake, expect a
mesolow to develop to our north. This will drift southward on the
long-axis of Lake MI with the surface trough through the evening. As
winds become north-northwesterly behind the low, expect our single
band to develop and move into the CWA. Though I`m confident in the
development of lake effect snow tonight as describe above, the
details on location/amounts with lake effect can be very difficult
to narrow down with any real confidence. At this point, the question
seems to be if, when, and where the mesolow will come onshore, and
what condition it will be in once it gets there.

Agree with previous shift in that it will likely be more over
Berrien county late tonight into early Tuesday morning (9-12Z) as
most of the high res model guidance seems to agree on this, and that
is when we see the trough axis move overhead. Michigan counties to
the east of Berrien may see light snow at this time, but don`t think
it will be anything of significance at this point.

From there, however, the guidance falls into disagreement. Some of
the models keep the mesolow and the trailing single band further
west across the lake, bringing it onshore in LOT`s CWA (Porter, Lake
& eastern Cook Counties)Tuesday morning and taking it eastward into
our area Tuesday afternoon into Wednesday morning. Others have the
band developing over LaPorte County, and have it drift eastward
before transitioning to more of a multiband, NNW Wind set up. Models
furthest west (including GEM), have the band not even reaching our
CWA until Tuesday evening, which could really mess up the forecasted
higher snow totals. The GEM has had a westward bias with lake effect
set ups in the past, but with the trough and potent PV anomaly in
the upper levels not sure this is a more traditional set up, and it
could be onto something.

Given these concerns, and collaboration with neighboring offices,
opted to stay with current winter weather headlines (Advisory
through 6z for Berrien, then winter storm watch for Starke,
Marshall, St. Joseph, IN, and La Porte from 12Z Tuesday into Wed
AM). Storm totals, assuming both the mesolow and the trailing
snowband come onshore in Berrien/LaPorte and drift eastward, could
be up to 6-8" with highest totals in southwestern Berrien County and
in La Porte County. Convective depths are 6-9K ft at times using the
NAM, and lift is centered within or just slightly below the DGZ-
which would yield us higher ratios. Additionally, there is a lake
superior connection, and abundant lingering moisture associated with
the clipper.

HOWEVER...If the band sets up further westward and the mesolow never
makes it far onshore in Berrien, we`ll be seeing much less.
Additionally, with the current advisory in effect, did not want to
issue a double advisory headline. Will let the evening and midnight
shifts monitor the short term trends and update accordingly.

Outside of lake effect concerns tonight into Tuesday, expect quiet
weather for inland areas as surface ridging builds in. Low
temperatures will drop into the single digits, with some areas
dropping slightly below zero. Wind chills will range from -5 to -
15F, with areas south of US 30 seeing the lower wind chills of -10
to -15. This is sub-wind chill advisory criteria, but will be
something we need to monitor closely overnight for those southern
counties.

&&

.LONG TERM...(Tuesday Night through Monday)
Issued at 409 PM EST Mon Jan 15 2018

Diminishing Lake Effect: Wednesday into Friday Night

High pressure builds overhead, and winds gradually shift to the WSW,
forcing the lake effect band eastward into Lower MI/northern IN
before it finally dissipates late Wednesday morning. Conditions will
be dry, but still relatively cold. High temperatures Wednesday and
Thursday will be in the 20s. Overnight lows will be in the teens. On
Friday, we`ll warm up some with highs nearing 30 degrees.

Warm and Wet Weekend Ahead: Saturday into Monday

WAA really ramps up late Fri into Sat as next major Pacific trough
moves into the southwest CONUS and downstream ridge-building/SW flow
envelope the Ohio Valley. Expect highs to climb into the 40s and
snowmelt to accelerate. Increasing isentropic ascent could also
bring some light precip on Sat. A very brief mix may be possible at
the onset but precip type looks to be primarily rain through the
weekend. Best chance for precip will be late Sun as primary trough
axis/cold front passes. Models in reasonable agreement for this time
range but still too early to nail down exact details. Some concern
for additional river rises with rain and melted snow but current low
track would suggest a relatively short time window of moderate rain.
Brief return to cooler temps and snow showers on the backside but
will be short-lived given prevailing W/SW flow aloft.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday Night)
Issued at 1213 AM EST Mon Jan 15 2018

Moist/cyclonic low level southwest flow just south of an elongated
west to east sfc trough will continue to support a predominately
MVFR cloud deck at the terminals through the TAF cycle. Some
guidance does hint at a trend into IFR cigs today, though opted
to hold onto a more optimistic fcst at this point. Snow
showers/flurries will also be possible mainly at KSBN as trough
axis nears. Will need to monitor this evening at KSBN as a
dominate LE single band could edge in from the west with significant
impacts/restrictions (Tuesday evening/night).

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Winter Storm Watch from 7 AM EST /6 AM CST/ Tuesday through
     Wednesday morning for INZ003-004-012-014.

MI...Winter Weather Advisory until 1 AM EST Tuesday for MIZ077.

     Winter Storm Watch through Wednesday morning for MIZ077.

OH...NONE.
LM...NONE.
&&

$$

SYNOPSIS...MCD
SHORT TERM...MCD
LONG TERM...AGD/MCD
AVIATION...Steinwedel


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