Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 121650

Area Forecast Discussion
National Weather Service Indianapolis IN
1150 AM EST Tue Dec 12 2017

The AVIATION Section has been updated below.


Issued at 354 AM EST Tue Dec 12 2017

Snow showers and flurries will impact parts of central Indiana today
as cyclonic flow and forcing aloft impact the region. High pressure
will briefly build in tonight before another strong upper level
system tracks through the region Wednesday afternoon and
evening...bringing another round of snow showers. Upper level
systems will pass through the lower Great Lakes with additional
chances for snow late Thursday night and Friday then again by late
weekend. Chilly temperatures through much of the rest of the work
week will warm over the weekend.


.NEAR TERM /Today/...
Issued at 354 AM EST Tue Dec 12 2017

The cold front passed through central Indiana last evening and cold
advection has taken hold in its wake early this morning.
Precipitation to this point has been sparse and very light over the
forecast area with an axis of light snow that briefly impacted
northeast counties shortly after midnight. Temperatures were
tumbling as the cold advection strengthens. 08Z temps had fallen
into the 30s over the entire area.

Starting to see some patchy drizzle and freezing drizzle in areas
where surface temperatures were below 32 degrees. LAPS soundings
showing growing layer of dry air aloft with likelihood of lost ice
saturation through the column further supporting the drizzle aspect.
Will continue to highlight for a couple hours early this morning
before more substantial upper forcing arrives from the north.

Secondary wave aloft showing up nicely on the water vapor loop early
this morning...diving south through the western Great Lakes. This
feature will serve as a catalyst to jump start lake effect
convective snow bands over the next couple hours off of Lake
Michigan. The strength of northwest boundary layer flow around 35kts
through the morning supports convective bands making it well inland
from the most of the area with exception of eastern Lake Michigan
shoreline...into east central Indiana and western Ohio. CAMs
continue to capture this thinking nicely with well developed bands
extending into far northeast portions of the forecast area by 11-

Overall trajectories support impacts from primary lake effect bands
to remain largely confined to areas northeast of an Elwood-Anderson-
New Castle line through the morning with greatest potential for
light accums and heavier snow banding focused over Delaware and
Randolph Counties for a longer period of time. As boundary layer
flow backs from 320-330 degrees to closer to 300 degrees this
afternoon...overall bands should focus more outside of our forecast
area with Randolph County continuing to be clipped. Will carry
highest pops in these far northeast portions of the forecast area
and raise them to 70 this morning. Potential exists for some area of
Delaware and Randolph Counties to approach an inch of snow by late
day. Wind gusts in excess of 30 mph will exacerbate any heavier
snowfall in these areas and create locally challenging travel issues
through the day.

Strength of the wave aloft diving down this morning followed by
residual forcing aloft lingering supports scattered snow showers and
flurries impacting areas back south and west to the I-65 corridor
and potentially even west of there this morning as well. Intensity
and coverage outside of the northeast counties should fall back to
primarily just scattered flurries into the afternoon as drier air
builds into the boundary layer before ending towards sunset.

Temps...temperatures are poised to move little through the course of
the day as cold advection only further intensifies. With sunshine at
a premium over all but western counties for the bulk of the day...
only expect temperatures to bounce back 2 to 3 degrees most areas.
Low level thermals continue to support undercutting guidance for
highs largely in the mid to upper 20s from north to south.


.SHORT TERM /Tonight through Thursday Night/...
Issued at 354 AM EST Tue Dec 12 2017

Forecast challenges focus primarily on increased precipitation and
snow chances with a potent clipper poised to dive through the region
Wednesday afternoon and evening.

The recent pattern with amplified upper level flow highlighted by
troughing in the east and ridging in the west will remain through
the short term. Waves aloft will continue to ride up and over the
ridge then dive into the trough over the region at a 24-36 hour
cycle. Of note early this morning is a noticeably strong upper wave
on the water vapor loop moving into British Columbia. This feature
will bring the strongest upper wave set to impact the area in the
short term on Wednesday afternoon and evening.

Confidence is lower than would like at this point with specific
impacts...strength and timing of these features for the simple
reason that models are having a difficult time capturing of late the
strength of these upper waves. The system that tracked through the
Upper Midwest and lower Great Lakes Monday afternoon and evening is
a perfect case in point as that wave was underdone versus reality.
Have some growing concerns that the same thing is going on with
regards to the upper wave poised to dive into our area later on
Wednesday. Recent model runs seem to be playing "catchup" with the
potential strength of the upper wave and consequently the surface
low and considering the potency the system is exhibiting in the
water vapor loop early this morning...models may not be done playing
catchup with this system. 12Z RAOBs will be critical and may give us
better insight with this system on model runs later today.

Snow showers and flurries may linger into the evening over the far
northeast portions of the forecast area before surface ridging
overspreads the forecast area with dry conditions and a brief period
with mainly clear skies overnight. Initial impacts from the
aforementioned upper wave will come as early as Wednesday morning as
a surge of warm advection overspreads the region ahead of a
strengthening surface low which will dive into the area by the
afternoon. Introducing chances for snow showers mainly along and
north of I-74 by mid morning expanding further south by the
afternoon as the low arrives. Critical low level thicknesses appear
to warm enough on the southwest flank of the precip shield to
introduce a rain/snow mix for the afternoon and early evening.

The upper wave will track across the area during the second half of
the afternoon followed by yet another sharp upper wave diving out of
the western Great Lakes and swinging through the area Wednesday
evening. This potent one-two punch in the mid levels warrants
raising pops well above model guidance over the northeast half of
the forecast area for the afternoon and evening hours. The primary
precip type will be snow with mixing potential as mentioned above on
the southwest flank. As colder air rushes in behind the surface wave
and associated cold front Wednesday evening however...expect a rapid
transition to snow as the primary precip type over the entire area.

Potential exists for quick bursts of snow showers courtesy of the
two waves aloft tracking through the area. Greatest chance for light
accums up to an inch and possibly higher in spots will focus over
northeast counties where coldest air will reside and precipitation
will remain all snow. Will mention again that confidence is not
where we want it to be with regards to this system and its impacts
over the forecast area...but there are ominous signs suggesting that
these two upper waves could overperform above what model guidance is
suggesting this morning. Stay tuned over the coming 24 to 36 hours.

Surface ridging will return by Thursday morning behind this system
and provide a relatively quiet but chilly end to the short term.
Another wave diving southeast Thursday night could bring another
round of light snow showers. Confidence not high enough in track and
timing of this feature to go any higher than a low chance pops late
Thursday night at this point in time.

Temps...Undercut guidance for lows tonight with cold advection
persisting. Wednesday highs are a challenge with the fluid nature of
timing potential impacts from the upper wave arriving in the
afternoon. Expect there will be quite a temperature gradient across
the forecast area with the shot of warm advection ahead of the cold
front...possibly on the order of 15 degrees from northeast to
southwest. Low level thermals were supported by a general model
blend for temperatures Wednesday night on through the end of the
short term.


.LONG TERM /Friday through Monday/...
Issued at 222 AM EST Tue Dec 12 2017

Ensembles are in good agreement with the overall weather pattern
during this period. Long wave trough initially over the eastern
parts of the country will move off to the east early in the
period, leaving a more zonal flow by early next week.

Ensembles suggest a short wave trough may move across the Great
Lakes on Friday. Will go with chance PoPs for mainly light snow on
Friday. Some mixed precipitation is possible over the southern

Another upper disturbance may affect the area around Sunday, but
at this point most of the ensembles keep the higher QPF south of
the local area. Will go with chance PoPs for mainly light rain on
Sunday, and light mixed precipitation Sunday night.


.AVIATION /Discussion for 12/18Z TAF Issuance/...

Issued at 1133 AM EST Tue Dec 12 2017

VFR Conditions expected through TAF period.

Scattered snow showers should stay northeast of any of the TAF
sites this afternoon. Primary hazard to aviation today will be
gusty winds approaching 25 kts at times this afternoon. Winds will
weaken after sunset and will shift to the south by sunrise
tomorrow. Clouds at or around 3000 ft will dissipate by mid
afternoon today leaving scattered clouds above 15000 ft.




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