Area Forecast Discussion
Issued by NWS Northern Indiana

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918
FXUS63 KIWX 091750
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
1250 PM EST Sat Dec 9 2017

.SYNOPSIS...
Issued at 442 AM EST Sat Dec 9 2017

Light snow will continue across the area through early this
afternoon as a clipper system moves through the region. Most
locations will likely receive between 1 and 2 inches of
accumulation, though lower amounts are possible southeast of US
24. Heavy lake effect snow is then expected to impact portions of
northwest Indiana and far southwest Lower Michigan through this
evening. Snow totals are expected to exceed 6 inches in favored
lake effect snow belts. This accumulation, gusty winds, and
intense snowfall rates will lead to difficult travel conditions in
these areas. Cold and remaining active otherwise through next
week as several more clipper systems bring additional snow
chances.

&&

.UPDATE...
Issued at 918 AM EST Sat Dec 9 2017

Forecast still in good shape this morning with only minor
adjustments made to grids today. Weak sfc reflection over se Lake
MI will finally release ese later this morning/afternoon allowing
for more squally/cellular lake effect snow showers (with meso
vorts) to finally transition to a more impressive single band
this afternoon into early this evening. Latest HRRR/NAM/RAP
iterations, along with latest mosaic radar trends, are lining up
quite well with inherited grids/headlines. So, again, no major
changes to the fcst at this time.

&&

.SHORT TERM...(Today and Tonight)
Issued at 442 AM EST Sat Dec 9 2017

Nothing is easy with lake effect snow and this case is no
exception. Still looks like a dominant band will develop today but
hires models have various solutions for where it will set up.
This is mainly due to a more westward shift in surface low track
and potential for several mesovortices leading to a wavy dominant
band.

Main short wave trough dropping through the western Great Lakes
early this morning with digging 100 kt jet streak. Left front quad
has finally intersected modest low level isentropic lift and weak
theta e axis nosing into the west. Regional radar mosaic shows
expanding returns in warm air advection region but very dry lower
levels as seen on KILX and KDVN 00z soundings has slowed low level
saturation, especially south and east. Surface obs starting to
indicate low level saturation and expect light snow to continue
expanding slowly east. Main frontal boundary upstream across IL also
has a narrow axis of stronger returns so expect a burst of snow as
this boundary crosses the area as well. Scattered snow showers and
flurries then expected through the day as upper trough rotates
through with steep lapse rates and cold air advection.

Lake effect event remains complicated as higher resolution models
are now taking surface low on a more southerly track down the
eastern side of Lake Michigan which is further west than previous
model runs. Surface low sticking closer to sensible/latent heat
fluxes off of warmer waters versus following synoptic scale trough
further inland. Will likely see an elongated area of low pressure
as synoptic trough slides southeast and mesoscale low hangs back
near shoreline. This will likely aid in mesovort generation this
morning with cyclonic low level flow combined with shoreline
curvature and frictional forces. Several hires models picking up
on this and result is a slower and more transitory/wavy band this
morning before trailing single band becomes dominant per typical
type VI band evolution.

Models have also trended a bit warmer with low level thermal fields
as sfc-850mb delta Ts now only expected to reach mid or upper teens
through entire event and 0-2km thetaE lapse rates now only reach -2
k/km this afternoon and early evening, which is weaker than previous
runs but still sufficient for a dominant single band. Snowfall rates
of 1 to 2 inches per but hour remain possible, especially this
afternoon.

Little change made to overall expected amounts but last few runs of
RUC and HRRR advertise some healthy QPF amounts between a half
inch and three quarters of an inch reaching well inland across
eastern Starke and western Marshall counties. Given this inland
reach coupled with strong afternoon winds have added Starke and
Marshall counties to warning with potential for 5 to 7 inches and
a lot of blowing snow creating hazardous impacts. If these
solutions are correct, Elkhart and Kosciusko counties may miss out
on most lake effect this afternoon. However, morning snow and
possibility of band sliding back east this evening warrants
keeping advisory in place for now.

&&

.LONG TERM...(Sunday through Friday)
Issued at 442 AM EST Sat Dec 9 2017

Primary focus has been with ongoing event and impacts so much of
the long term period has been left in the hands of populated
blends. Few changes really from previous forecast with persistent
pattern of fast moving clipper systems followed by lake effect
snow across the north and northwest through the upcoming week.
Most models are keeping the surface lows north of our area with
each system and this would keep our system snow amounts on the
lighter side. Potential for moderate to heavy lake effect snow
continues in the wake of each clipper and have likely pops to
account for this. Medium range models are indicating a moderating
trend by next weekend into the following week as strong Pacific
jet energy breaks down western ridge leading to more of a zonal
flow across the central CONUS.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday Afternoon)
Issued at 1249 PM EST Sat Dec 9 2017

TAF focus on next 6 hours handling strong snow band that is
starting to leave KSBN with a mesolow upstream sending another NW
to SE oriented band back into the airport. Site has been at 1/4sm
for about an hour and expect slight improvement (which may be
brief) as aforementioned second band moves in. Radar returns not
as high at this time as current band but vsby of 1/2sm or less at
times can be expected. Have maintained temp group for now with
1/2sm, as there is no way to handle the highly variable vsbys with
these bands. Confidence in location and timing of main dominant
band of snow low given mentioned mesoscale features. Winds will
start out on the lighter side if not calm briefly as low passes
over KSBN, then will rapidly increase from the NW causing blowing
and drifting snow as well as continued snowshowers into this
evening.

At KFWA same band impacting KSBN will move towards the site and
cause some impacts, although hopefully in a somewhat weaker state.
Will monitor trends, but keep things somewhat optimistic. Once
this feature passes, airport may end up with little additional
activity. Will address that in the coming hours.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Winter Storm Warning until midnight EST /11 PM CST/ tonight for
     INZ003-004-012-014.

     Lakeshore Flood Advisory until midnight CST tonight for INZ003.

     Winter Weather Advisory until 7 PM CST this evening for INZ013.

     Winter Weather Advisory until midnight EST tonight for INZ005-
     015-016.

MI...Winter Storm Warning until midnight EST tonight for MIZ077.

     Lakeshore Flood Advisory until 1 AM EST Sunday for MIZ077.

     Winter Weather Advisory until midnight EST tonight for MIZ078.

OH...NONE.
LM...Gale Warning until 10 PM EST this evening for LMZ043-046.

&&

$$

UPDATE...Steinwedel
SYNOPSIS...MCD
SHORT TERM...Lashley
LONG TERM...Lashley
AVIATION...Fisher


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