Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 090439

Area Forecast Discussion
National Weather Service Indianapolis IN
1140 PM EST Thu Dec 8 2016


The Aviation section has been updated below.


Issued at 315 PM EST Thu Dec 8 2016

A pair of low pressure systems will bring chances for accumulating
snow late this weekend into Monday and again next Tuesday.
Temperatures will be below normal through the period.


.NEAR TERM.../Tonight/
Issued at 923 PM EST Thu Dec 8 2016

Pesky flurries continue across the area, and have expanded a bit
further south. Have added flurries to the grids across the entire
area. With tail end of weak vort lobe unlikely to exit the area
until late, have gone ahead and taken the flurries out til morning.
Additionally, latest hourly numerical guidance and obs trends
suggest min temps will likely get down into the upper teens many
areas, and made adjustments accordingly. Previous discussions

Issued at 602 PM EST Thu Dec 8 2016

Upped cloud cover overnight in accordance with satellite trends and
HRRR low level moisture progs. Remainder of forecast in good shape.
Previous discussion follows.

Issued at 315 PM EST Thu Dec 8 2016

Models in good agreement that cold air advection will continue
tonight as northwest winds continue ahead of strong surface high
pressure over the Plains. However, thick cloud cover associated with
cyclonic flow aloft should keep temperatures from falling too much
overnight. Consshort handles this well with lows in the lower 20s.
Would not rule out a few flurries.


.SHORT TERM.../Friday through Sunday/
Issued at 315 PM EST Thu Dec 8 2016

The main concern for the short term will be snow potential late in
the weekend associated with warm air advection and upper jet
dynamics. Models have some timing issues with subtle waves in thelow
amplitude fast upper flow, and differences in strength of
warmadvection on Sunday. With this in mind, low confidence
forecastespecially Sunday, when some areas should see the snow
change to ormix with rain or possibly some other mix. With this low
confidence,will go with a blend.

The early part of the weekend will be quiet with strong high
pressure expected to provide cold but dry weather through at least
most of Saturday. Then, things get interesting with 100 plus knot
jet stream over the area and warm air advection. Models were
agreeing on 2-4 g/kg specific humidity values on the 290 K level by
Saturday night. Critical thicknesses favor all snow Saturday
nightand a mix across roughly the southern half on Sunday
afternoon.Condensation pressure deficits suggest the snow will start
offacross our far northwestern area late Saturday afternoon and
notherntwo thirds Saturday night. Precipitation chances will
increase tolikely Sunday and occasional Sunday night as a surface
low liftsnortheast nearby. The 12 ECMWF was further south with the
track ofthis low, taking it across south central Indiana. The 12z
GFS wasstronger and took it across far northern Indiana. As it looks
now,there is a potential for several inches of snow across
ournorthernmost counties by Monday morning and lesser amounts
south.However, the exact track of the low will be the key. Any
change inthe track will have large implications on precipitation
type andsnow amounts.

MOS suggests below normal temperatures through the period. See
noreason to deviate from a blend. Did not use diurnal
temperaturecurve Sunday afternoon as warm air advection continues
ahead of aPlains low pressure center.


.LONG TERM /Sunday Night through Thursday/...
Issued at 255 PM EST Thu Dec 8 2016

The period begins with a weak warm front lifting north across the
area. An upper trough begins to amplify over the western great
plains...particularly South Dakota and Nebraska. The GFS is stronger
and more amplified with this feature...leading to a stronger surface
low passing across Illinois and Indiana Sunday night/Monday morning.
This allows for more warm air into the forecast area on the front
end and a quicker return of cold air after the low passes. It also
allows for higher QPF across the area. Mid/long range models have
not been in good agreement as it pertains to this portion of the
forecast lately. Thus...confidence not high in timing...depth and
strength of the cold air in advance of and behind the passage of the
surface low...affecting precipitation type. Model initialization
went with a blend...which seems reasonable.

Tuesday night sees the passage of a strong cold front with model
differences in the amount of precipitation Tuesday and Tuesday
night...but both the GFS and ECMWF signal the coldest air of the
season pushing in behind the front...with temps at 850mb in the
teens Wednesday afternoon/evening as an example.


.AVIATION (Discussion for the 09/06Z TAFs)...
Issued at 1140 PM EST Thu Dec 8 2016

MVFR ceilings will likely persist through the night. VFR conditions
may return Friday afternoon.

Widespread stratus in cyclonic flow aloft will persist overnight
into Friday. Scattered flurries and occasional snow showers may
briefly reduce visibility but this should be minimal.

Winds will remain northwesterly through the period, generally around





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