Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
639 am EST Thursday Dec 12 2013
cold high pressure builds in from the west today before sliding
south into the Carolinas tonight. A complex area of low pressure
will approach from the southwest by Saturday and then push across
the region Saturday night. The system exits the coast on Sunday
followed by high pressure for early next week.
Near term /through tonight/...
as of 225 am EST Thursday...
Cold front now moving east of the area resulting in the onset
of increasing cold advection that will continue on gusty northwest winds
into this afternoon. Seeing a Post frontal upslope enhanced band
of low clouds that may bleed southeast into the northwest NC mountains as well
as the New River valley before fading all areas by middle/late
morning as high pressure starts to nose in from the west. However
still some snow shower threat southeast wva given arrival of the coldest
air aloft by daybreak and another passing shortwave but with the
moisture so shallow appears mostly flurries at this point. Otherwise
cold air and blustery northwest winds the main concerns today with wind
chills likely to remain in the single digits and teens mountains
all day. May be close to Wind Chill Advisory criteria early this
morning but mostly highest elevations so not planning any
headlines at this time. Temperatures look to actually fall some into the morning
with the axis of coldest 850 mb air sliding across through early
afternoon. This supports highs staying in the 20s/around 30 west
to mostly 30s out east with increasing sun after early western low
clouds dissipate and any wave action high clouds exit.
Strong high pressure builds east into the area this evening and
then shifts to the south overnight as a weak impulse aloft passes
well north. Very dry air should support mainly clear skies
although warming aloft and faint channeled shortwaves may bring a
period of middle deck in from the northwest late. Expect temperatures to level off
some after quick evening falls as mixing starts to pick up espcly
ridges as a more westerly trajectory squeezes a bit with the high
slipping to the south. Thus held temperatures up some in spots along the
Blue Ridge while keeping mostly middle/upper teens mountains to
around 20 east.
Short term /Friday through Sunday night/...
as of 400 am EST Thursday...
On Friday...high pressure will shift east from the southern
Appalachians...allowing winds across our area to shift from the
southwest. While high clouds will increase during the day ahead of
our next storm system...rising 500mb heights will allow afternoon
temperatures to warm around ten degrees over those on Thursday...
with highs ranging from the upper 30s to the middle 40s across the
mountains...while approaching 50 across the piedmonts.
Will start Friday night off with a deep upper trough approaching
from the plains...drawing Gulf moisture northward along the
appalachian chain. Meanwhile...cool dry air will be in place
across our region...as high pressure across eastern Canada tries
to wedge southward along the eastern face of the Appalachians.
Models agree that this high will not be nearly as strong as what
contributed to our widespread freezing rain event this past
weekend...but will still be significant in determining the type of
precipitation we can expect going into the night.
So...as Gulf moisture begins to overrun the cooler air at the
surface late Friday evening...expect precipitation to start as a
light snow and sleet mixture across the mountains...with a cold
rain across the Southside. Warmer air will continue to build in
through the night as a surface low pressure system associated with
the upper trough approaches from the southwest...and expect the
sleet and snow mixture to change over to rain from southwest to
northeast after sunrise...with a mix of rain and wintry
precipitation confined north of Interstate 81 by early afternoon.
For Saturday afternoon and evening...one wave of low pressure is
expected to pass east of our area along the coastal plain. For
the forecast...took into account the possibility that convection
developing across the southeast may rob US of moisture flowing
into our area...and therefore decreased the amount of
precipitation called for from model guidance.
As the low proceeds northeast...it will draw colder air into the
Greenbrier through the Shenandoah valleys...which may result in
periods of freezing rain through late Saturday evening. Ice
accumulations are expected to be less than a tenth of an inch.
Precipitation will diminish significantly Saturday night as the
low continues northeast away from our area. A reinforcing cold
front will then pass across early Sunday...which will trigger snow
showers across the mountains...in addition to producing gusty
northwest winds. Wind speeds and snow shower activity will
diminish through the day Sunday...with clearing skies...as dry
high pressure builds in from the west.
Long term /Monday through Wednesday/...
as of 318 PM EST Wednesday...
The cold air does not hanging around for long. By Monday
afternoon...a warm front is already pushing across the Ohio Valley
and will bring moderating temperatures to the region on Tuesday. A
weak cold front may move over the Appalachians on Wednesday. This
front will knock temperatures back below normal. With a dry frontal
passage...windy/gusty conditions may be a concern.
An area of high pressure will remain along the Gulf Coast for most
of next week. Any fronts crossing the region next week may have only
enough moisture to produce clouds. The chance for measure
precipitation is less than 10 percent.
Aviation /11z Thursday through Monday/...
as of 605 am EST Thursday...
Early morning MVFR ceilings at both klwb and kblf should gradually
improve to VFR by middle or late morning as high pressure builds in
and low clouds mix out. May be a brief period of MVFR at kbcb as
well early this morning...otherwise looking at VFR across the region
into tonight under clear skies once the low clouds fade. Northwest winds
will remain an issue espcly over the mountains this morning where
gusts to 25-30 kts will continue. Winds should gradually subside
this afternoon as high pressure builds overhead. West winds may
again increase across the ridges overnight as high pressure shifts
to the south late.
Extended aviation discussion...high pressure will be centered
just to our southeast Friday morning...then drift off the southeast
coast Friday night. An area of low pressure will move across Texas
Friday...then across the MS valley Friday night...approaching the
middle Atlantic region Saturday. Some light frozen precipitation may arrive
across the mountains of the central Appalachians by daybreak
Saturday. Temperatures are expected to warm above freezing
Saturday...so any wintry mix Saturday morning is expected to
change to rain Saturday afternoon. Regardless of precipitation type...
there is a high probability for MVFR ceilings and a chance for a period
of IFR ceilings/visible by Saturday afternoon. MVFR ceilings may linger both
Saturday night and Sunday. A more tranquil pattern returns by
Monday with high pressure stretching from the Tennessee Valley into the