Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 090950
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
450 AM EST Sat Dec 9 2017

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

A clipper system will bring light snow to the entire area through
early this afternoon. Most locations will likely receive between
1 and 3 inches, 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.

&&

.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 06Z TAFS through 06Z Saturday Night)
Issued at 1250 AM EST Sat Dec 9 2017

Expect conditions to deteriorate through this morning and afternoon
as the clipper system currently over WI and Western Upper Michigan
drops southeastward into Northern Indiana. The worst conditions are
expected at KSBN, where Lake Michigan will provide additional
moisture/instability and enhance precipitation in the wake of the
low. At this point, have conditions dropping to MVFR by 9z at KSBN,
and to IFR/LIFR by 16z as the low is overhead. Per upstream
observations, it looks like once we overcome drier air we could see
visibilities drop to around 2 miles, then possibly dropping to less
than 1 mile in any bands that develop later this morning/afternoon.
There is some uncertainty as we seem to be struggling with dry air
in place over the area. This may prevent things from dropping as far
as I have in the TAF at the moment. Additionally, there is still
model disagreement about where the lake effect band is placed
tonight, with some shifting it westward of KSBN and others keeping
it directly overhead. Will add further detail to this scenario with
later issuances, but for now hedged on the side of caution and went
with LIFR conditions. At KFWA, expect we`ll see a drop down to MVFR
visibilities and ceilings today as the system crosses through. A
brief drop to IFR is possible, but wasn`t confident enough to
include in the TAF at this point given our dry air situation.
Otherwise, winds will shift to the northwest through the period, and
begin to gust into the 20-25 knot range by this afternoon.


&&

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

     Lakeshore Flood Advisory from noon today to midnight CST tonight
     for INZ003.

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

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

     Lakeshore Flood Advisory from 1 PM this afternoon to 1 AM EST
     Sunday for MIZ077.

     Winter Weather Advisory until midnight EST tonight for MIZ078.

OH...NONE.
LM...Small Craft Advisory until 10 AM EST this morning for LMZ043-046.

     Gale Warning from 10 AM this morning to 10 PM EST this evening
     for LMZ043-046.

&&

$$

SYNOPSIS...MCD
SHORT TERM...Lashley
LONG TERM...Lashley
AVIATION...MCD


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