Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 070041

Area Forecast Discussion
National Weather Service Northern Indiana
741 PM EST Wed Dec 6 2017

Issued at 311 PM EST Wed Dec 6 2017

Lake effect snow will ramp up late tonight and continue through
Thursday. 1 to 3 inches of accumulation are possible along and north
of the toll road with little to no accumulation south of there. Dry
conditions are expected Thursday night and Friday but another system
could bring widespread light snow to the area on Saturday. Cold
conditions continue through next week with highs around 30 and lows
around 20.


Issued at 730 PM EST Wed Dec 6 2017

Not anticipating major changes to previous forecast this evening.
Low level flow will begin to veer with the approach of next sfc
trough across southeast Wisconsin that will bring reinforcing shot
of colder low level air overnight. This cold advection will allow
inversion heights to rise from about 4k feet to 6k feet
overnight, along with increasing lake induced instability depth,
especially by the 09Z-12Z timeframe. As mentioned in previous
discussion, veering of winds with this trough will be somewhat
subtle and will restrict this to a multiband setup late tonight
into early Thursday afternoon. At this time, it appears
parameters will be most favorable in the 11Z-16Z timeframe when
moderate instability magnitudes should be realized and when fetch
is maximized. West- northwest fetch should favor the corridor
from central/northern Berrien counties into Cass county for the
greatest event total accumulations in the 1 to 3 inch range, with
localized 4 a possibility in better multiband setup early Thursday
morning, and when snow ratios should be maximized. May just make
a minor tweak to slightly delay lake effect PoPs this evening, but
nature of forecast through the overnight into Thursday morning
will be maintained with little change. Will continue to hold off
on advisory at this time given fetch limitation and relatively
short duration of more supportive profiles.


.SHORT TERM...(This Evening through Thursday)
Issued at 311 PM EST Wed Dec 6 2017

Main forecast concern for this period will be lake effect snow
tonight into tomorrow. Overall not a lot of changes to the going
forecast based on newest guidance. Main change was a slight delay in
start times and a slight nudge toward lower amounts. Based on
plethora of latest forecast soundings (including hi-res) lake effect
may struggle to get going in our area until almost 06Z tonight. More
favorable lake effect conditions are tied to arrival of shortwave
trough axis still over the Northern Plains per latest water vapor
imagery. 850mb thermal differentials are already near 20C but
exceptionally dry air and fast westerly flow are limiting even cloud
cover across southern Lake Michigan. Passage of shortwave trough
axis will be required to veer the low level flow, raise inversion
heights, and provide some synoptic moisture convergence. This does
not happen until ~06Z and suspect lake response will be very muted
until then.

Even after shortwave passes, lake effect setup is not ideal for
significant snow accumulation. Main limiting factors are time and
fetch. 12Z models are a little less veered with low level winds and
max wind direction now looks to peak around 310 and only for a brief
time late Thu morning before flow backs again late in the day.
Little if any lake-induced convergence signal in these regimes,
leading to a transient, multi-band setup. Instability profiles could
briefly be classified as moderately supportive. Inversion heights
climb to 6 kft with decent moisture depths/quality. Fetch does limit
moisture/heat flux from the lake though and 0-2km theta-e lapse
rates from the 12km NAM now peak around -1 C/km and only in GRR`s
area where topography of the lake favors longer fetch/residence
time. One positive is that snow ratios will likely be high with bulk
of ascent focused in DGZ. Putting everything together, expect a more
cellular, multi-band lake response with some decent rates possible
in isolated locations for a brief time 12-18Z Thu. Given transient/
disorganized bands and brief nature of best conditions...still think
most places will top out in the 1-2" range. Isolated spots may see
up to 3-4" if a more sustained band develops but confidence in that
is far too low to support an advisory...even though it is the first
event of the season.

A few flurries possible in our middle and southern counties tomorrow
morning as wave passes but not expecting much accumulation south of
the toll road given largely backed wind profiles. Temps will remain
in the 20s through tomorrow with wind chill values stuck in the


.LONG TERM...(Thursday Night through Wednesday)
Issued at 311 PM EST Wed Dec 6 2017

Lake effect will dissipate by Thu night and expect primarily dry
conditions through early Fri night with another period of dry WSW
flow. Next, rather potent shortwave dives into the region early Sat.
Models are in significantly better agreement on track and evolution,
suggesting a good chance of some light snow for our entire CWA.
Moisture obviously remains limited but dynamics are fairly
impressive in left exit region of 120+ kt upper jet streak and 1.5
PVU surface extending down to 700mb per latest GFS20. Associated low
level convergence/fgen will likely squeeze out a few inches. Also
looks like a brief period of very good lake effect potential late
Sat with northerly flow, Superior connection, good convergence, and
ample moisture/instability. Winds back by Sun morning but could
still see some significant accumulations if model trends remain
consistent. Still plenty of time for adjustments though.

Brief and minor warmup late Sun into Monday but cold NW flow quickly
returns, along with continued lake effect potential heading into the
middle of next week. Even colder conditions possible by the middle
of next week and adjusted concensus blends closer to raw guidance
given climo bias at these time ranges.


.AVIATION...(For the 00Z TAFS through 00Z Thursday Evening)
Issued at 730 PM EST Wed Dec 6 2017

The primary concern for aviation weather will be evolution of lake
effect snow showers later tonight into Thursday. Low level flow
has remained more backed in west-southwest direction through the
early evening hours, but flow should start to veer more due
westerly over the next few hours in advance of approaching sfc
trough tied to next mid level wave. Passage of this trough will
result in period of stronger cold advection and more veered flow
overnight that will support southward expansion of lake effect
snow showers across west central Lower Michigan. West-northwest
fetch will be limiting factor to more substantial lake effect
event and will promote more of a multiband setup late tonight into
Thursday morning. The 12Z-16Z timeframe appears to be the most
supportive of lake effect snow showers at KSBN given fetch
considerations and lake induced instability depths, and have
mentioned tempo IFR conditions during this period. Duration of
lake effect snow showers somewhat uncertain at KSBN as flow should
begin to back later Thursday afternoon, so have included VCSH
mention post- 18Z due to the uncertainty. Along with the lake
effect clouds have also included MVFR cigs from 2500-3000 ft late
tonight into Thursday. Steep low level lapse rates will also
promote westerly gusts to around 20 knots on Thursday.


LM...Small Craft Advisory until 5 AM EST Friday for LMZ043-046.




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