Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
712 PM EST Wednesday Jan 28 2015
high pressure over the Ohio Valley this evening will move
southeast...reaching the Carolinas on Thursday morning. Low
pressure will track from Nebraska into Ontario by Thursday
evening. This system will push a strong cold front through the middle
Atlantic region on Thursday night.
Near term /through Thursday/...
as of 715 PM EST Wednesday...
Will make minor changes to grids to incorporate most recent observation
but no significant changes needed. Infrared satellite shows thin cirrus
spilling over the top of the upper ridge to our west. Models do
show the cirrus advancing but indications are that it will remain
thin and should not impede radiational cooling with otherwise
clear skies and light winds...so low temperature forecast still looks to
be on track.
Upper ridge passes over the region tonight into Thursday morning.
850 mb temperatures moderate tonight and will be just above zero
by Thursday afternoon. Surface high pressure will pass over the
region tonight so winds will decouple. With such low surface dew
points and with only a few high clouds coming over the ridge
tonight... temperatures will plummet this evening. Will stay
closer to colder mav guidance for minimum
temperatures...especially in the valleys.
Short term /Thursday night through Saturday/...
as of 400 PM EST Wednesday...
Series of short waves sharpens upper trough with a 500mb low closing
off over New York by late Friday. Forecast area will be in favorable
left exit region of upper jet on Thursday night. First surge of much
colder 850 mb air comes in Thursday night and is re-enforced on
Friday. Along with the cold air advection...low level jet comes
around to the northwest on Thursday night with speeds of 45 to 55
knots into Friday night. Strong winds will be along the southern
Appalachians Thursday night and Friday then will shift to the
northern Appalachians on Friday night as the surface low deepens
off the coast. NAM/GFS showed pressure rises close to 6mb/6hr for
Thursday night through Friday. BUFKIT and guidance continue to
support advisory level winds behind the front Thursday into Friday
evening. Threat will remain highlighted in hazardous weather
Moisture increases on Thursday afternoon from the west. With the
deep northwest winds...will start to see clearing east of the Blue
Ridge on Friday morning. The remaining low level moisture on the
western slopes on Friday will erode late in the day. Saturday
surface and low level winds will diminish and back more to the
west. High clouds will spread over the southern County Warning
Area ahead of the Southern Plains system.
Models were in similar agreement with the timing of the surface cold
front on Thursday night. Will be trimming back probability of
precipitation before 18z/1pm Thursday. A bulk of the precipitation
associated with the system will be along and west of the Blue Ridge
but even that will not be very high amounts.
Long term /Saturday night through Wednesday/...
as of 330 PM EST Wednesday...
Temperatures below normal through the extended.
Upper level pattern for the extended will see a change as split flow
with a somewhat shallow eastern trough becomes unified as the closed
low over the Baja California peninsula opens up and ejects eastward late in the
period...leaving US with a well amplified ridge and a broad flat
eastern trough extending from a closed low way up near Hudson Bay
in Canada. This should be a fairly progressive pattern with surges
of short wave energy heading our way on a fairly regular basis.
We will start the period under high pressure Saturday night...but as
the high pushes to our east and waves ride along an approaching cold
front we will see our chances for precipitation increase for Sunday
and Sunday night. Ptype will be wintry mix at the start then expect
a mostly rain event...then back to some upslope snow showers west of
the Blue Ridge as we get on the back side of the low Monday.
Cold high pressure then builds as upslope snow showers wind down
Monday night and...after a very cold morning...quiet weather should
be the rule for Tuesday. Heading into Wednesday...the remnants of
the closed upper low will be ejecting toward US as the northern and southern
streams phase. Long range models are indicating the potential for a
/Miller a/ type low to develop over the Gulf Coast Wednesday and
track through the southeast US and off the middle Atlantic coast
Wednesday night. Temperatures are currently expected to be
borderline at best across our area for wintry weather. Nothing to
get excited about at this point...but we will be watching to see how
later model runs handle the evolution of this system.
Aviation /00z Thursday through Monday/...
as of 650 PM EST Wednesday...
High pressure will drift over the region tonight. High clouds will
increase from the west...mainly across northern portions of the
County Warning Area...in advance of an Alberta clipper...the main dynamics of
which will remain north of our County Warning Area. Scattered-bkn250 is expected at most
taf sites after midnight...keeping ceilings VFR. Not expecting any
problems with visibility overnight either as the surface air mass is
very dry. During the day Thursday...the clipper will race across the
Great Lakes into western New York/Quebec. A narrow band of
precipitation accompanies the associated front...mainly north of
U.S. 460 and west of I-81. Moisture is expected to be too limited
for much more than clouds to the south. Precipitation will be
falling from middle-based clouds...so visibility will not be an
issue...expect possibly at blf/lwb where dynamics are better and
cloud ceilings could lower to low end VFR or high end MVFR during this
activity...potentially reducing visibility then to MVFR category in
-shra. Surface/850mb temperatures should remain warm enough for all liquid
through this taf valid period.
Winds will become the main concern just after this taf period as
the clipper moves east of the region and very strong northwest
winds impact areas west of the Blue Ridge. Through the taf valid
period...however...expect calm or light/variable winds
overnight...becoming southeast 5-8 kts around daybreak...then
south-southeast-south-southwest through the day Thursday increasing to 8-12kts with low end
gusts to around 20kts west of the Blue Ridge.
Medium to high confidence in ceilings through the taf valid period.
High confidence in visibilities through 12z...then medium to high
confidence after 12z.
Medium to high confidence in wind direction/speed through the taf valid
Behind the cold front...MVFR ceilings likely to linger over the
mountains into Friday with strong northwest winds also persisting into
Friday evening. Gusts up to 40 knots are possible in the higher
elevations. Clouds will begin to clear out east of the Blue Ridge
on Friday morning...so klyh and kdan will improve back up to VFR.
Another area of high pressure shifts east into the area Saturday
with VFR expected for both Saturday and Saturday night. The next
low pressure system heading out of the SW states will bring a
return of clouds and some precipitation on Sunday and Monday with
widespread MVFR to IFR likely.
short term...air mass