Area Forecast Discussion
Issued by NWS Indianapolis, IN

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AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
220 PM EST Fri Dec 8 2017

.UPDATE...
The LONG TERM section has been updated below.

&&

.SYNOPSIS...
Issued at 330 AM EST Fri Dec 8 2017

Cold weather will continue through next week as the region remains
under a northwest flow aloft. Chances for snow will come tonight and
Saturday as a surface low passes through the lower Great Lakes.
Additional chances for precipitation mainly in the form of snow will
come at times next week as upper waves of low pressure track through
the area.

&&

.NEAR TERM /Rest of Today/...
Issued at 933 AM EST Fri Dec 8 2017

Forecast is on track. Plentiful sunshine and a dry atmosphere will
allow temperatures to rebound into the 30s today. Only made minor
adjustments to the hourly forecasts as needed.

Previous discussion follows...

Clear skies with the coldest temperatures of the fall so far as the
region resides under an elongated ridge of high pressure. 08Z
temperatures were largely in the teens across central Indiana.

The high pressure ridge will serve as the dominant influence today
as the Ohio Valley remains between a frontal boundary impacting the
southeast states and a fast moving surface wave poised to drop out
of Manitoba by late in the day. Model soundings and RH progs
indicating the presence of an abundance of dry air and subsidence
through the column...with any clouds associated with the approaching
surface wave and cold front approaching from the northwest holding
off until late afternoon at the earliest. Winds will remain from the
southwest most of the day with potential for sustained to approach
15mph this afternoon as the surface pressure gradient tightens.

Temps...warm advection will develop ahead of the approaching cold
front today but the bulk of it holds off until the afternoon.
Considering the frigid start this morning and a lessened impact from
warm advection...trended highs towards the cool end of guidance
ranging from the low to mid 30s.

&&

.SHORT TERM /Tonight through Sunday Night/...
Issued at 330 AM EST Fri Dec 8 2017

Forecast challenges focus on snow chances developing late tonight
and Saturday as the aforementioned surface wave and associated cold
front swing through the region.

The surface low will be located over the upper Midwest early this
evening...rotating into the Great Lakes by Saturday morning and
swinging the cold front into the region.  The system will then
transfer its energy to the developing surface wave off the New
England coast Saturday night. Decent model consensus exists on the
track and timing of the surface wave and its eventual energy
transfer but as is usually the case with these fast moving clipper-
type systems...differences remain with respect to potential snow
amounts.

Snow associated with this system will come in two parts...the more
widespread light synoptic snow that accompanies the front...and the
scattered snow showers Saturday afternoon and evening that carry
more of a lake enhancement. One noticeable trend off the 00Z data is
the system is slower dropping into the Great Lakes with the result
that the arrival of the snow ahead of the cold front will be delayed
from previous forecasts. Will not introduce any pops until midnight
or after in the northern Wabash Valley...with more substantial light
snow not becoming widespread until the predawn hours and through
daybreak as the more focused forcing aloft and an axis of mid level
deformation works into the region. The synoptic snow will continue
to shift east through the morning hours before tracking into Ohio
with the front around midday. Snow amounts through midday Saturday
will range from a dusting up to 1.5 inches from southwest to
northeast. The greatest focus for light snow accums will generally
be near and northeast of the I-74 corridor with highest amounts in
northern counties in closest proximity to the best forcing aloft.

As strong cold advection takes over Saturday afternoon...the second
part of the snow potential will develop as lake effect snow bands
organize off Lake Michigan in a N/NW-S/SE orientation. CAMs in
particular focusing the development of a dominant snow band by late
afternoon coming off the lake near Michigan City and spreading well
inland on the strength of boundary layer flow that may very well
exceed 40kts for a few hours. While there will likely still be some
fluctuation in exactly where this band aligns...a NNW to NW low
level flow supports the band setting up in a vicinity from near
Kokomo arcing into NE Tipton...northern Madison...then Delaware and
Randolph Counties. Heaviest snow rates will be found within this
band with highest potential for additional accumulations into
Saturday evening.

Outside of the main lake band...expect additional bursts of snow
showers and perhaps a few squalls focused near and east of I-65
through the afternoon as low level thermodynamic profiles briefly
align with subtle instability shallow yet steep lapse rates below
850mb and continued cold advection. Most of these snow showers
should diminish to flurries if not completely end by early Saturday
evening with the dominant band holding out through midnight or just
after in northeast counties before backing and weakening low level
flow completely shuts off the Lake Michigan fetch. By late Saturday
night...potential is there for total snow amounts locally up to 3
inches in the vicinity of the dominant snow band in northeast
counties...with 1 to 2 inches along and northeast of a Lafayette-
Noblesville/NE Indy suburbs-New Castle line. Amounts are expected to
be less than an inch southwest of this line.

The second half of the weekend will see a return to quiet weather as
high pressure reestablishes. Breezy conditions will persist on
Sunday with cu developing within the cold advection. The passage of
an upper wave through the Great Lakes Sunday night will spread more
extensive clouds cover across the northeast half of the forecast
area as it passes by.

Temps...undercut guidance for highs both Saturday and Sunday as
strong cold advection and low level thermals argue for colder
daytime temperatures. Highs will remain in the lower and mid 30s for
most of the area. Utilized a blend for lows through the period in
the teens and 20s with a lean towards the colder end of guidance
Saturday night courtesy of fresh snow cover and clearing skies.

&&


.LONG TERM /Monday Night Through Friday/...
Issued at 220 PM EST Fri Dec 8 2017

Models are close enough that the model blend initialization
was accepted for most items.

The eastern half of the country will remain under an upper trough
through the long term. A series of waves will ride through this
trough, bringing occasional chances for precipitation through the
period.

Of course there are timing and some location differences between the
models, but there is too much uncertainty that far out to try and
pick a favorite. Thus kept the intialization`s PoPs intact.

The intialization is again too warm later in the long term as it
trends toward climatology, so cut temperatures as needed there.

&&

.AVIATION /Discussion for 08/18Z TAF Issuance/...

Issued at 1200 PM EST Fri Dec 8 2017

VFR conditions through 8Z Saturday, MVFR after.

The main focus for this TAF period is the timing and intensity for
snow tonight and tomorrow morning as a low pressure systems moves
through central Indiana. Snow is expected to begin around 8Z in
LAF and spread southeast through the early morning. Temporary
reductions in visibilities will be the primary hazard to aviation
with MVFR ceilings moving in as the snow exits the area.
Additional snow is possible after 18Z, but with low confidence on
timing and exact location there will be no mention in the TAF for
this issuance.


&&

.IND WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SYNOPSIS...Ryan
NEAR TERM...Ryan/50
SHORT TERM...Ryan
LONG TERM....50
AVIATION...White



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