Area Forecast Discussion
Issued by NWS Northern Indiana

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049
FXUS63 KIWX 111804
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
104 PM EST Mon Dec 11 2017

.SYNOPSIS...
Issued at 100 PM EST Mon Dec 11 2017

Chances for heavy lake effect snow will increase tonight and
continue into Tuesday night as a clipper system moves over the
area. Several inches of accumulation are possible along with
blowing and drifting snow especially north of Highway 30. Please
see the latest winter weather products for the latest information.
Otherwise, temperatures will remain well below normal through
Wednesday with highs mostly in the 20s.

&&

.SHORT TERM...(Today and Tonight)
Issued at 418 AM EST Mon Dec 11 2017

...Winter Storm Watch for Heavy Lake Effect Snow...

Active pattern continues with another potentially big lake effect
event on the way but impacting more counties further east and
southeast. Areas well inland and near our southern border may see
advisory level snowfall if band becomes stationary.

First things first and approaching short wave and associated arctic
front must be dealt with. Weak isentropic lift beginning early this
morning but best focus looks to be near and north of IN/MI border.
Hires models have been backing off on qpf amounts and focusing best
pcpn chances in extreme north with this initial warm air advection
light snow. Short wave is impressive and tightly wound on water
vapor imagery diving out of southern Canada and into the Dakotas
this morning. Regional radar mosaic and sfc obs show light snow
quickly spreading southeast and expect this to clip the far north
after 12z. Have used the conshort hourly pops for timing as this
has best short term verification and looks reasonable compared to
timing and placement in hires and spectral models. With the quick
moving nature and weak lift now only expecting around an inch in
far north with possibility of up to 2 inches close to our northern
border. Have also trimmed southern edge of pcpn chances with few
if any models showing pcpn in the drier regime over the far south.
Brief break in pcpn expected late morning into afternoon with
models again shifting best chances ahead of main short wave
further north. Did keep mid to high chance pops in north which was
higher than most blends as HRRR and RUC indicate some light pcpn
clipping this area again this afternoon and evening.

Short wave to pass this evening with arctic front crossing the area
in the evening. Expect some snow showers to move in along and behind
this front with deepening cold air advection. Winds will also pick
up and become quite strong and gusty. Delta Ts to quickly increase
after 03z and become very favorable for the lake effect after 06z as
winds become northwest. Would expect some lake enhanced snow showers
immediately behind front mid evening transitioning to pure lake
effect overnight. Accumulations of 1 to 3 inches expected before
more intense bands get going toward or just after 12z Tue.

&&

.LONG TERM...(Tuesday through Sunday)
Issued at 418 AM EST Mon Dec 11 2017

Very impressive lake parameters coming together on Tuesday as
extreme instability is present for the entire day as sfc-700mb
delta Ts eclipse 30 setting stage for intense snow bands. NAM12
0-2km ThetaE lapse rates remain impressive and nearing -4 k/km.
Flow will be from a north northwest direction with tap into Lake
Superior moisture for pre-conditioning before long NW fetch over
Lake Michigan. Hires models all in general agreement with several
long and intense bands streaming well southeast and into Ohio and
beyond. The differences as expected are on where each of these
bands will set up and how they will migrate during the period.

At this time step there is enough confidence for a watch as
synoptic and mesoscale parameters will support very intense bands
with snowfall rates likely reaching 2-3 inches per hour on
Tuesday. The extreme instability, lake induced CAPE nearing 1000
j/kg on NAM12 bufkit, would also suggest rare thunder snow a
possibility. Strong flow of 30 to 40 knots expected to push these
bands well inland and they should remain quite strong through
central portions of CWA. Several models have high end advisory or
low end warning criteria amounts as far south as Kosciusko and
noble counties. With combination of snow and wind have opted to
include these inland counties in the watch with significant
impacts expected. Also thinking advisories will be needed even
further south and southeast where the most dominant band ends up,
possibly into Fort Wayne area and beyond on Tuesday.

The only possible negative for this event is the
inversion heights appear to remain between 7 and 10kft through
entire event. However, strong lift is coincident with saturated
DGZ and with upstream conditioning think convection should be
sufficient to occur in the saturated layer. Expect event to begin
waning late Tuesday night as low level flow backs and drier air
works in crashing inversion heights overnight.

Total accums right now expected in the 5 to 9 inch range with
local accums around a foot possible where one of the bands remains
stationary. Question remains on which of the multiple bands
expected will become most dominant. Also must point out that this
type of flow/event often leads to merging bands and increased
snowfall rates. A few hires models hinting at this but no way to
anticipate when or where it will actually happen.

Still looks like another clipper Wednesday into Thursday followed by
more lake effect. Only adjustment to blend was to increase NW area
pops to likely for expected next round of lake effect. Warming
trend for the weekend with highs in the 40s possible.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday Afternoon)
Issued at 100 PM EST Mon Dec 11 2017

An area of frontal lift will be in the vicinity of the terminals
this afternoon; however, expect nearly all of the snow with this
system will remain just to the northeast of the sites. Otherwise,
a clipper system will race southeast over northern Indiana this
afternoon. Once this system passes tonight, winds will increase
rapidly from the northwest with lake effect snow developing. Other
than inversion heights being a little low, optimum snow growth in
the dendritic zone should occur with delta T values topping 20C.
There should be plenty of preconditioning from upstream waters
including Lake Superior. Given sustained winds over 20 mph,
blowing snow is expected. Visibilities of a mile or greater may be
too generous at SBN after 12Z.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Winter Storm Warning from 1 AM Tuesday to 7 AM EST Wednesday for
     INZ004>006-008.

     Winter Weather Advisory from 1 AM Tuesday to 7 AM EST Wednesday
     for INZ014-016>018.

MI...Winter Storm Warning from 1 AM Tuesday to 7 AM EST Wednesday for
     MIZ077>079.

OH...NONE.
LM...Gale Warning from 10 PM this evening to 7 PM EST Tuesday for
     LMZ043-046.

&&

$$

SYNOPSIS...Skipper
SHORT TERM...Lashley
LONG TERM...Lashley
AVIATION...Skipper


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