Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Nashville Tennessee
1126 am CST Thursday Dec 5 2013
18z tafs...a strong cold front interacting with moist southwest
flow aloft will generate widespread rain through this taf period.
Ceilings will be mostly IFR with some lfir at times. Visibilities mainly
MVFR. Just after the period...06/18z...fzra will begin to develop at
ckv with significant ice possible there Friday evening.
Previous discussion... /issued 1033 am CST Thursday Dec 5 2013/
Update...new model data continues to support our forecast for
winter weather Friday into Friday night. So...we will go ahead with an
Ice Storm Warning for our far northwest counties...with main
impacts west of a line from Clarksville to Camden. These areas
will hover around freezing through midday...then gradually slip
into the upper 20s afternoon...then middle 20s evening. Main
concerns will be ice collecting on elevated surfaces...like power
lines...tree limbs and bridges. Roads may be mostly fine through
midday...but travel conditions will deteriorate into Friday
evening as temperatures drop.
For the next couple of tiers of counties...including Davidson
(nashville)...we will go with a Winter Weather Advisory for Friday
evening for travel impacts as rain turns to a mix and temperatures fall
into the 20s. Right now it looks like Nashville metropolitan rush hour
will be OK...but late evening travel and events will likely be
In the meantime...the cold front was along a Portland to
Nashville...Centerville and Linden line. The front was accompanied
by a band of showers with enhanced wind gusts to 25 miles per hour. Look for
increasing showers and falling temperatures over the northwest half of
our area through the day.
Regarding hydrology...rain just getting started...with 2 to 4
inches of rain possible through Saturday morning. We do not plan a
Flood Watch at this time. Depending on how much materializes in
the next 2 days...we may have greater flooding concerns for the
second event Saturday night into Sunday.
Previous discussion... /issued 523 am CST Thursday Dec 5 2013/
Update...added aviation section for 12z taf's.
Aviation...bna/ckv/csv...a strong cold front will move slowly
across middle Tennessee today before stalling along the Cumberland
Plateau. Look for mainly scattered showers early this morning
along and just ahead of the front. Showers will increase
throughout the day with ceilings remaining below 3 kft throughout the
taf period. Winds should remain just strong enough to prevent any
fog, although ceilings will worsen after nightfall.
Previous discussion... /issued 422 am CST Thursday Dec 5 2013/
Short term...surface cold front finally edging into middle
Tennessee this morning with mainly scattered cells at the moment
occurring along the front. Model soundings show modest low- and
middle-level instability in the pre-frontal air mass, so will leave
isolated storms in the forecast today for locations ahead of the
front. However, most of the activity will be just showers.
Heaviest precipitation will occur behind the surface front this
afternoon and overnight as the upper trough remains to our west
and moisture transport increases due to isentropic lift. Thus,
moderate rainfall will persist even as colder Arctic air plunges
into the middle state.
Still looks like a wintry mix will develop on Friday over the
northwest and overspread much of the remainder of the middle state
Friday night. Ice accumulations of up to one half inch is
anticipated in the Winter Storm Watch area, with ice accumulations
generally a tenth of an inch or less further east toward the
Interstate 65 corridor. Surface temperatures are expected to reach
freezing along the Highland Rim, north of Nashville, by late
Friday and dip to freezing in the Nashville metropolitan area early
As Arctic air pushes in on Saturday, expect highs to stay in the
30s over the biggest majority of the middle state. Temperatures are not
even expected to get above freezing along the Highland Rim and
over the northwest. This will only worsen the situation for the
upcoming icing event on Saturday night and Sunday morning, when
ice accumulations between .25 and .40 inch could occur over the
northwest, with accumulations of .10 to .20 inch as far east as
Dale Hollow Lake, Lebanon, Columbia and Waynesboro. Latest model
runs show cold temperatures pushing a little further east and
south than previous runs and have expanded the potential freezing
rain area over a broader swath of the middle state for Saturday night.
Still expect warming to occur on Sunday, as surface trough pushes
westward. However, it is doubtful that this feature will only work
as far west as the I-65 corridor. Therefore, temperatures will
probably just manage to barely creep above freezing in the
northwest. The high temperature forecast at Clarksville on Sunday
is 35 degrees. Meanwhile, a much milder 49 degrees is forecast for
Crossville. Quantitative precipitation forecast is running pretty high Sunday and Sunday night, with
most areas along and east of I-65 likely to see between one and a
half and two and a half inches of rain.
Long term...the remainder of the extended forecast looks pretty
much the same as it did earlier: perhaps a little freezing drizzle
in the northwest as the conditions turn colder again Sunday night
and early Monday, then maybe a few flurries Monday night over all
of the middle state as the really cold air starts to finally push
across the Tennessee Valley. Lows by sunrise Tuesday are expected
to be in the teens to middle 20s. Highs on Tuesday are only
forecast to be in the upper 20s to middle 30s, with lows Tuesday
night in the teens area-wide (brrrr!). Canadian high pressure
keeps US cold...but dry...next Wednesday.
Winter Weather Advisory from 4 PM Friday to midnight CST Friday
night for tnz007-008-025>027-056>060-093-094.
Ice Storm Warning from 9 am Friday to midnight CST Friday night