Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
1043 PM EST Thursday Dec 18 2014
a weak disturbance will cross the region tonight. Building high
pressure is expected through the weekend...though clouds will
Near term /until 6 am Friday morning/...
an utterly poor snow Crystal growth habit exists over the County Warning Area this
evening...as evidenced by the pinhead sized snowflakes/grains that
have been scattered across the area in the form of "flurries." In
addition...areas just to the north and northwest of our County Warning Area have
started to report freezing drizzle. This seems likely to continue
to filter into at least the northern reaches of the County Warning Area as
moisture increases...but still little moisture exists above about
the -8c isotherm. As such...while deeper moisture may allow some
trend toward snow shower activity...it seems insufficient to
preclude freezing drizzle from continuing to develop in the
northern portions of the County Warning Area.
Additionally...with an incoming short wave trough and a bit of
elevated moisture...some seeder-feeder type of snow flurry action
does seem possible for a short window as the wave tracks
through...however model cross sections...time height
sections...and plan depictions all generally reveal a quick trend
toward zero microbars/second of layer negative Omega values between
00z and 06z this evening. As a result...with dry air between the
two moist layers...weakening lift aloft...and poor Crystal growth
below...it would seem just a general expansion in the flurries for
a few hours as the wave tracks through should be the main problem
for the night over the rest of the County Warning Area. Unfortunately...Road
temperatures have trended near freezing through the evening...so
reports of numerous accidents have come in across the I-70 and
south corridor as light snow initially melted then refroze on
roadways. Luckily enough...this wave is tracking through rather
quickly...and after its passage...even boundary layer relative humidity values
start to fall off a bit...so it would seem even the poor growth
habit may deteriorate by morning. Fries
Short term /6 am Friday morning through Sunday/...
surface high pressure is prognosticated to build in under upper trofg over the Great Lakes
Friday...ending any lingering morning snow showers. The surface high is expd to
bring dry weather through the weekend...though persistent moisture below an
inversion may keep skies mcldy. A shortwave moving across the southeastern Continental U.S.
Lt Sat/Sat night is now prognosticated to stay far enough S to remove all
precipitation from the forecast for areas S of pit. A weak shortwave is prognosticated for
Erly sun across the Great Lakes...but with a lack of deep moisture and the
surface high overhead kept the forecast dry.
Slightly below average temperatures should rtn to near seasonal levels by sun.
Long term /Sunday night through Thursday/...
while the models agree on a deepening upper level trough to close
off over the Great Lakes through the period the details of how
this exactly unfolds will be critical. By Monday southern stream
shortwave ahead of main northern stream upper trough digging into
the northern plains will produce low pressure across the Carolinas
moving off shore of the East Coast. The southeast ridges may be
brushed by light rain from this system. Next up is deepening low
pressure over the Ohio Valley ahead of sharply digging upper
trough and closed low moving into the western Great Lakes. Upper
Ohio Valley looks to be on the warm side of the system Tuesday
into Wednesday with rain...followed by strong cold front and cold
advection later Wednesday night and Thursday with snow showers.
With such a strong system aloft and fast deepening low pressure at
the surface timing of cold air and exact track will likely change
as we get closer to Christmas. All in all previous forecast is
little changed with only minor adjustments based on newest wpc
Aviation /04z Friday through Tuesday/...
MVFR ceilings will be expected at all terminals overnight along with
light snow. Think that IFR ceilings will occur across northern sites of
fkl/duj...and an isolated IFR visible cannot be ruled out in a heavier
snow shower elsewhere. Scattered snow showers could persist north
and east of Pittsburgh after daybreak...but coverage will be
minimal. Although model guidance indicates that ceilings should be
scattering out Friday afternoon...do not have high confidence in
this considering winds will remain out of the west/northwest...and
have kept tafs more pessimistic.
Outlook.../Friday night through Tuesday/...
stratus and restrictions may persist into the weekend. Next storm
system will bring restrictions Tuesday.
PA...Winter Weather Advisory until 6 am EST Friday for paz007>009-