NWS Forecast Discussion

(This is mainly for locations north of I-70)

.NEAR TERM /Tonight and Saturday/...
Issued at 244 PM EST Fri Dec 22 2017

The main focus tonight and Saturday will be on precipitation
coverage, type, and timing of changeover and or mix along with
temperatures as a second cold front will drop through area overnight
as the first one exits southeast early this evening. Following
that, an upper wave will move through on Saturday.

Model 295K isentropic analysis was showing low condensation pressure
deficits spreading northwest this overnight to the northwest of the
first cold front that may briefly stall or slow down, along the Ohio
River, ahead of the second one. This should bring more widespread
rain to all but areas north or Tipton and Crawfordsville. As the
cold front sweeps through overnight, areas mostly near and north of
Interstate 70 could see light snow accumulation. Forecast Builder
was indicating a potential narrow band southwest to northeast
oriented snow band of up to 1 inch near and north of Interstate 70
but with lesser to no accumulation northwest of a Kokomo to
Frankfort to Newport line. Confidence is not great in snow amounts
due to much drier air moving in, warm road and grassy surfaces and
slightly above freezing surface temperatures. Any precipitation
should being ending from northwest to southeast Saturday afternoon
in the wake of the upper wave.

With such a big airmass change, confidence is only low to moderate
on temperatures to within a couple of degrees, so accepted the blend
with overnight lows tonight in the lower to middle 30s and highs
Saturday only in the middle 30s north to around 40 south.

&&

.SHORT TERM /Saturday night through Christmas Day/...
Issued at 244 PM EST Fri Dec 22 2017

Big challenge for the short term will be snowfall amounts and
potential headlines for the northern half of the forecast area.

Models in decent agreement that a deep and cold Canadian upper
trough will pivot across central Indiana on Christmas Eve. Models,
especially the 12z GFS were showing strong QVector convergence in
the 850 to 500 millibar level pivoting over our northern counties
Christmas Eve morning and afternoon before exiting northeast early
Christmas Eve evening. With sub-freezing temperatures throughout the
column , deep moisture per model soundings, snow ratios 15 to 20 to
1 suggesting dendritic growth potential and fairly impressive
forcing, blend snow amounts of 2 to 4 inches near and north of I-70
look reasonable. Medium confidence, the Kokomo and Lafayette areas
will likely see the highest amounts. With those snow amounts and
the travel impact, feel moderately to highly confident headlines may
be needed for Sunday and Sunday evening from around Vincennes to
Marion county and north. However, after coordination and being the
fourth period of the forecast, decided to hold off for now and go
with a Special Weather Statement and will hit strongly in the
Hazardous Weather Outlook and weather story.

The snow will be ending quickly over the northeastern counties
Christmas Eve evening with modified arctic high pressure nudging in
from the northwest and resulting in cold but dry weather on
Christmas Day. There could even be some clearing. Confidence on
temperatures is low due to the expected snow pack. However, a good
bet that at least the northern counties will be at least as cold as
the blend with lows early Christmas morning in the tees and highs in
the upper 20s at best.