Area Forecast Discussion
Issued by NWS Indianapolis, IN

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000
FXUS63 KIND 111038
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
538 AM EST Mon Dec 11 2017

.UPDATE...
The AVIATION Section has been updated below.

&&

.SYNOPSIS...
Issued at 348 AM EST Mon Dec 11 2017

After a milder day today...the passage of a cold front tonight will
bring another shot of much colder air for Tuesday into the middle of
the week. Snow showers and flurries are possible late tonight into
Tuesday with even a short period of patchy freezing drizzle in spots
prior to daybreak Tuesday. Another quick moving system will bring a
few snow showers Wednesday night and Thursday before high pressure
builds in with dry weather and a warming trend for the weekend.

&&

.NEAR TERM /Today/...
Issued at 348 AM EST Mon Dec 11 2017

Skies were mainly clear early this morning with the exception of far
northeast portions of the forecast area as the back edge of stratus
associated with a departing wave aloft linger. 08Z temperatures were
in the 20s.

Another relatively quiet day for central Indiana as focus shifts to
vigorous upper wave diving into the upper Midwest early this
morning. This feature will track southeast through the day and into
the region by early this evening. Snow had developed out ahead of
the wave over Minnesota and northern Wisconsin and should move into
the Great Lakes after daybreak with CAMs indicating potential for
the southern fringe of the the lighter snow showers to make it into
northern Indiana. This activity should remain north of the forecast
area...but expect mid level clouds associated with the snow to
expand into much of the northern half of the forecast area later
this morning.

The moisture will shift east by early afternoon with some thinning
of the cloud cover as a result. Southerly flow will develop and
strengthen as warm advection develops ahead of an attendant surface
wave and cold front which will dive into the Wabash Valley by early
evening. Could see gusts approach 20mph by late day as the front
approaches.

Temps...a brief but noticeable warmup coming today with warm
advection ahead of the cold front. Preferred the cooler METMOS for
highs with a 10-12 degree gradient from northeast to southwest across
the region. Still will be the warmest day in the last 5...and for
the rest of the work week.

&&

.SHORT TERM /Tonight through Wednesday Night/...
Issued at 348 AM EST Mon Dec 11 2017

Forecast challenges focus on precip chances and types as the surface
low and cold front sweep across the area tonight...followed by
another round of lake effect snows Tuesday that will clip northeast
portions of the forecast area.

The upper wave with surface low and cold front will move across
central Indiana this evening. Vigorous vort lobe aloft should
provide the necessary upper forcing but moisture is lacking and
largely confined to the boundary layer. Low level thermals will
crash with the passage of the front...and any light precipitation
that may begin as a mix of rain and snow or all rain should
transition to light snow showers over most of the area by midnight
or shortly thereafter. Any light snow accumulation prior to 12Z will
be a dusting at best over eastern portions of the forecast area.
Something that cannot be ignored any longer is the presence of much
drier air flooding the column above the boundary layer and creating
a short window where ice saturation is lost. With the near surface
layer falling below freezing during the overnight across the
region...a few hours with heightened potential for patchy freezing
drizzle to mix in with any light snow showers/flurries is a real
possibility. Model guidance has hinted at this potential for the
last few runs and while confidence is not high in any substantial
impact...feel it would be prudent to include a mention for a 3-4
hour period predawn Tuesday. Certainly could create some slick spots
on roads/overpasses ahead of the Tuesday morning commute.

The lake effect potential will once again crank up early Tuesday and
take center stage through Tuesday night. While the focus for
greatest lake effect snow impacts will be felt mainly north and
northeast of the forecast area...northeast counties will again have
some light accumulation potential from lake enhanced bands. Activity
will begin to develop downwind of Lake Michigan after 06Z Tuesday as
a second vigorous wave aloft dives south through the Great Lakes and
serves to amplify the upper trough over the region. Boundary layer
flow intensifies early Tuesday which should ensure that lake
enhanced bands travel well inland as they dive southeast from Lake
Michigan. Will introduce likely pops over northeast counties near
daybreak for the expected arrival of the banding.

Boundary layer flow should gradually back to a 300-310 degree
direction by the afternoon which should focus the core of the
stronger bands northeast of the forecast area. Will maintain likely
pops all day in Randolph County which would be the most susceptible
location for being impacted by lake bands even at a 300-310 degree
wind direction. Outside of the lake enhanced banding impacts over
northeast counties...models appear to be grossly underestimating
light snow shower/flurry potential over much of the rest of the
forecast area with strong forcing aloft present for much of the day.
Moisture is limited but lapse rates of 8-9 C/km are present up to
about 800mb into the afternoon hours. That combined with strong cold
advection should be more than sufficient for flurries and perhaps
even a few snow showers over much of the eastern 2/3 of the forecast
area. Winds will be gusty throughout the day as well.

Low level wind flow will back further Tuesday evening and then
diminish substantially as surface ridging builds in from the west.
This will enable any lingering lake bands to shift further away from
northeast counties Tuesday night. With the continued amplified flow
aloft over the country...the ridging will be shortlived as yet
another upper wave and associated surface low track into the Great
lakes by late Wednesday. Moisture is once again a limiting factor as
the system passes through the region Wednesday night...but enough
forcing aloft is present to generate a chance for snow showers over
northern counties.

Temps...the airmass following in the wake of the frontal passage
this evening looks arguably colder than the one that graced us on
Saturday. Undercut guidance for highs on Tuesday as most of the
forecast area remains in the mid and upper 20s. Generally utilized a
model blend for temperatures through the rest of the period.

&&

.LONG TERM /Thursday through Sunday/...
Issued at 235 AM EST Mon Dec 11 2017

Ensembles in good agreement with the overall pattern during this
period. Long wave troughing over the eastern parts of the country
is expected to move off to the east, with the flow becoming more
zonal by the end of the week.

A short wave trough is expected to pass over the Great Lakes
around Thursday. Will go with small chance PoPs for mixed
precipitation on Thursday, mainly over the northern zones.

The rest of the extended looks dry at this time as heights rise
with the lifting out of the long wave trough. Ensembles suggest
the next system probably won`t affect the area until after this
period.

&&

.AVIATION /Discussion for the 111200Z TAFS/...
Issued at 538 AM EST Mon Dec 11 2017

Scattered to broken layered cloud above 050 and unrestricted
visibility expected at the terminals today. Some lower cloud
around 040-050 may approach the KLAF terminal from the northwest
towards sunset.

Light surface winds early this morning will become 180-200 degrees
at 9-13 kts by midday.

&&

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

$$

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



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