Area Forecast Discussion
Issued by NWS Indianapolis, IN

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000
FXUS63 KIND 091454
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
954 AM EST Sat Dec 9 2017

.UPDATE...
The NEAR TERM and AVIATION sections have been updated below.

&&

.SYNOPSIS...
Issued at 334 AM EST Sat Dec 9 2017

The passage of a cold front through the area will bring light snow
this morning and scattered snow showers this afternoon with minor
snow accumulations. Additional chances for precipitation mainly in
the form of snow will come at times through next weekend as upper
waves of low pressure track through the area within a persistent
northwest flow. Temperatures will largely remain below normal
through much of the upcoming week.

&&

.NEAR TERM /Rest of Today and This Evening/...
Issued at 954 AM EST Sat Dec 9 2017

Dry low levels of the atmosphere kept snow at bay for a while early
this morning, but now the atmosphere is moistening up allowing more
snow to reach the ground. In addition, heavier snow showers have
developed across the middle and eastern thirds of the forecast area.

Looking at webcams across the area, it looks like many areas have
just seen a dusting. Given this, have cut back snow amounts a bit
across the area. Didn`t cut too much yet though because of the
development of the heavier snow showers and the threat for more snow
showers this afternoon. Will likely have to adjust totals more when
the afternoon snow shower situation becomes clearer.

Based on upstream conditions, raised high temperatures a bit as well.

Previous discussion follows...
Issued at 334 AM EST Sat Dec 9 2017

Light snow is slowly but steadily approaching the northern Wabash
Valley early this morning with low pressure over eastern Wisconsin.
Skies remained clear over the southeast half of the forecast area
but clouds were rapidly expanding across the region ahead of the
snow. 08Z temperatures were generally in the 20s.

The story of the day will be the potential for light snow which will
impact central Indiana in two phases: the first phase will encompass
this morning as the synoptic light snow tracks through the region
ahead of a cold front. The second phase will transition to a period
of scattered snow showers and squalls focused across the eastern
half of the forecast area this afternoon into the early evening as
cold advection intensifies behind the front and a favorable Lake
Michigan fetch likely stretches all the way down into our northeast
counties. We will discuss each phase in tandem below.

The synoptic snow will gradually expand across the northwest half of
the forecast area through daybreak then shift east through the
course of the morning as low pressure tracks east through the Great
Lakes and drags the cold front through the region. While radar
echoes are already expanding into the forecast area...residual dry
air through the boundary layer will need a bit of time to saturate
before light snow commences. The first flakes should arrive in the
Indy metro by 11-12Z then expand further south and east through
central Indiana during the morning. Light accums are possible
through midday with the morning snow...less than an inch for most
with perhaps just a dusting in some spots over the southern half of
the forecast area. Best potential to see around an inch this morning
will be over northern counties in closest proximity to the better
forcing aloft.

The more widespread light snow accompanying the frontal boundary
will largely be east of the forecast area and into Ohio by early
afternoon. After a brief respite...phase two of the system will
develop as lake effect snow bands organize off Lake Michigan in a
N/NW-S/SE orientation. CAMs continue to highlight a primary snow
band organizing by mid afternoon coming off the lake near Michigan
City and spreading well inland on the strength of boundary layer
flow that will approach 40kts for a few hours late this afternoon.
The NNW to NW low level flow through the northern half of the state
supports the band arcing generally from Kokomo to near Richmond...
focusing the greatest impacts from the lake band across parts of
Howard...Tipton...Madison and Delaware Cos. Heaviest snow showers
will be found within this band with the potential for total snow
amounts to locally approach 2-3 inches by the time snow ends later
this evening.

The arrival of cold advection aloft within the cyclonic flow supports
the development of scattered snow showers and bursts or squalls to
impact areas especially near and east of I-65 this afternoon. Model
soundings continue to show favorable thermodynamic profiles through
the boundary layer with a couple hours of subtle instability and
shallow yet steep lapse rates up through the 900 to 850mb layer. The
snow squall parameter remains impressive this afternoon and argues
for the scattered squall potential in addition to the thermodynamic
profiles present. While additional snowfall amounts will generally
be less than a half inch from any of these snow showers or
squalls...the significant impacts to motorists will be from rapid
reductions in visibility over short distances due to heavier bursts
of snow and winds gusting potentially as high as 25 to 30mph. Snow
showers will decrease in coverage this evening with the primary Lake
Michigan induced band diminishing closer to midnight in northeast
counties as weakening low level flow backs and completely shuts off
the lake fetch.

Considering that this will be the first higher impact snow event of
the season for central Indiana...feel it would be prudent to
introduce an SPS to highlight the expected impacts through tonight
with a special emphasis on the scattered squall potential this
afternoon. Most of central Indiana will see an inch or less once
snow ends...with the possibility of 1 to 2 inches northeast of a
Lafayette-Noblesville-Rushville line. As mentioned above...locally
higher amounts of 2 to 3 inches are possible wherever the primary
lake enhanced band sets up over far northeast portions of central
Indiana.

Temps...the combination of clouds...snow and developing cold
advection behind the front warrants highs closer to cool end of
guidance. Low level thermals support this as well with highs holding
in the low to mid 30s.

&&

.SHORT TERM /Tonight through Monday Night/...
Issued at 334 AM EST Sat Dec 9 2017

Forecast challenges focus on precip chances Monday and Monday night
with another clipper type system rotating through the progressive
northwest flow aloft.

Skies will clear up rapidly tonight as surface ridging expands into
the region. This will set the stage for a cold night as any thin
snow cover will have an influence on temperatures even with winds
staying up all night. Ridging will shift east on Sunday with yet
another fast moving wave aloft tracking across the forecast area
during the late afternoon and evening likely spreading more clouds
back into the region. Model guidance hinting at the presence of a
surface trough accompanying the wave aloft as it amplifies just east
of the area Sunday night. While low level moisture is largely absent
with this feature...presence of the forcing aloft may be just enough
to generate a short period of flurries by early Sunday evening...
especially over the eastern half of the forecast area. Confidence is
not high enough to introduce any flurries at this point...but will
need to monitor going forward.

After a return to a brief period of ridging overnight Sunday into
early Monday...focus will shift to yet another wave dropping through
the northwest flow. This feature is stronger than the Sunday
afternoon wave...with a surface low accompanying the upper wave into
the Great Lakes Monday afternoon and night. A warm front will shift
north of the region Monday morning and may produce a brief period of
light snow over northeast counties. Much of the rest of the day
should be dry with warmer weather as a quick shot of warm advection
bumps 850mb temperatures above freezing over the entire forecast
area. The passage of an associated cold front late Monday afternoon
will immediately cool the column with any light precipitation in the
form of a rain/snow mix or light rain transitioning back to all snow
Monday night. Could see a few tenths of an inch by Tuesday
morning...especially across the northeast half of the forecast area.

Temps...generally leaned towards cooler end of guidance for highs
Sunday and Monday as increased cloud cover at times both days will
likely stunt temp rises. Monday will see a brief break from the
colder weather as the aforementioned warm advection will enable
temperatures to range from the upper 30s to mid 40s from north to
south. Undercut guidance for lows tonight as impacts from even a
thin snow cover with clearing skies should allow temps to tumble
into the teens. Took a model blend for lows in the 20s Sunday and
Monday nights.

&&

.LONG TERM /Tuesday through Friday/...
Issued at 222 AM EST Sat Dec 9 2017

Ensembles in good agreement on the overall pattern during this
period. Deep long wave trough is expected to hold over the eastern
parts of the country, with a tendency for heights to rise towards
the end of next week as the flow becomes more zonal.

A couple of weak disturbances may affect the area around Tuesday
and in the Wednesday to Thursday time frame, but at this point
neither of these disturbances look particularly impressive. Will
go with chance PoPs for mainly light snow from Tuesday through
Thursday night, especially over the northern zones where
ensembles hit the precipitation threat harder.

&&

.AVIATION /Discussion for the 091500Z TAF Update/...
Issued at 954 AM EST Sat Dec 9 2017

Band of heavier snow allowed IFR conditions to develop at KIND. This
band is moving off to the east, so expect MVFR conditions to
dominate shortly. Some uncertainty remains whether ceilings will
remain above or below 2000ft, so for now went pessimistic and kept
them below 2000ft. Will continue to monitor.

Previous discussion follows...

Short term model guidance suggests a fairly narrow band of lift
will be moving across the terminals later this morning, exiting
off to the east around 091600Z-091700Z. As a result, expecting a
period of visibility restricitions in light snow at the terminals
through the midday hours. Brief periods of IFR visibilities in
the heavier precipitation areas.

Additional scattered snow shower activity is possible this
afternoon as steeper low level lapse rates associated with cold
advection moves in.

Ceilings today generally 020-050, with briefly lower ceilings
possible in heavier precipitation areas.

Surface winds 200-200 degrees at 8-12 kts this morning will
gradually veer around to 290-310 degrees later this afternoon.
Frequent surface gusts to around 25 kts expected this afternoon.

&&

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

$$

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



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