Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
945 PM EST Wednesday Dec 24 2014
a wedge of high pressure along the eastern slopes of the Blue
Ridge will slowly give way to an approaching cold front overnight.
This front will move through most of the region by early Thursday
..bringing colder temperatures and gusty winds as it passes.
High pressure will then build over the eastern United States into
the weekend with generally fair weather expected into Saturday.
Near term /through Thursday/...
as of 925 PM EST Wednesday...
Cold front now pushing across the far western counties should
reach the Blue Ridge by midnight and out east during the early
morning hours. Most showers have all but dissipated along the
boundary upon encountering the lingering wedge but still appears a
few lighter bands of showers possible over the west during the
next couple of hours. However not enough to keep any thunder going
northwest with a gradual reduction in probability of precipitation west until seeing the brief
upslope kick in later on. Other concern is out east where the
front will likely slow under the SW flow aloft allowing perhaps
another speed maximum to zip up from the south into the early morning
hours. Latest hrrr continues to suggest a few added hours of rain
espcly Piedmont and supported by the 00z NAM so added in more
chance probability of precipitation east overnight espcly given filling back in of precipitation
on regional radars to the south at this time. Otherwise seeing a temperature drop of
almost 10 degrees in an hour during the frontal passage with gusts to 30-35
kts but still below advisory given leveling off of speeds once the
boundary crosses. Expect fog to finally fade enough after midnight
to drop mention for the most part overnight as well. Bumped up low
temperatures in the east where once The Wedge breaks expect a brief temperature
surge for a couple hours before gradual cooling redevelops behind
the front. However given mixing expect lows to be a few degrees
warmer than earlier thought.
Previous update as of 520 PM EST Wednesday...
Sent an update to boost dense fog coverage along/east of the Blue
Ridge this evening before the cold front finally arrives and helps
mix things out before midnight. Also issued a Special Weather Statement for the
foothills and Piedmont given the quick dropoff in visibilities
over the past hour. May need a brief advisory if conditions worsen
but holding off for now given the gradual approach of the front
from the west. Also tweaked probability of precipitation down and added more drizzle Blue
Ridge east while delaying convection over the west. Most latest
short term solutions quickly weaken this shallow convection upon
crossing the western slopes in the next couple of hours but
something to watch. Temperatures remain stuck in the 40s across the
central and east so adjusted down for steady readings there this
evening before seeing a quick jump when the front arrives after
Previous discussion as of 340 PM EST Wednesday...
At 3 PM...The Wedge of cool shallow air remained along and east
of the Blue Ridge. This morning...a short wave passed over this
wedge and brought a fair amount of rain (0.50 to 1.50 inches) to
the region. This wave has passed with only light rain/drizzle left
in its wake. Gray cloudy conditions will persist into this evening
while a lull in the rain will last for a few hours before a cold
front and associated showers enter the west late this
afternoon...exiting the Piedmont late this evening. The Wedge has
left most of the area in a stable environment but can not rule out
a rumble of thunder from elevated convection as the cold front
passes this evening.
Showers with the front may have some gusty winds with them...but
it will be the sharp pressure rises behind the front that will
bring a few hours of breezy and gusty conditions to the region.
The stronger longer lasting breezy and gusty winds will occur
early Christmas morning with a tight pressure gradient...modest
cold air advection and a 50 knots low level jet. High pressure builds
in quickly Thursday afternoon to shut down the winds.
As The Wedge erodes...temperatures are warming into the middle to
upper 50s. Area along and east of the Blue Ridge...particularly in
the foothills and Piedmont counties...may not see high
temperatures today until this evening (5p-7p). Once the front
moves east...temperatures will slowly drop into the 30s and 40s.
As the upper level trough passes overhead Friday morning...a few
showers are possible. Temperatures and boundary layer are to warm
for snow except for higher elevations and western slopes. After a
breezy cool morning...afternoon highs will warm into the 40s west
to 50s east.
Short term /Thursday night through Saturday night/...
as of 250 PM EST Wednesday...
Tranquil and mild conditions are in store for the initial Post-
Christmas period over the central/southern appalachian region. A
surface and upper ridge of high pressure centered over the
southeastern U.S. Will dominate the pattern in our region for
several days as upper troughing and storminess is confined to the
western Continental U.S.. very mild temperatures on Friday with widespread 50s
likely for highs under a pleasant late December sun. Winter
weather lovers prepare to be less than pleased.
The high will shift off the East Coast by late Saturday as height
falls and a cold front over the middle-Mississippi Valley slowly push
east. Skies will be clear Friday night and most of Saturday with
temperatures again running 10-15f above late December
normals...lows in low to middle-30s and highs low 50s to near 60.
High clouds will increase late Saturday ahead of the front with
another mild night in store as temperatures likely to remain above
freezing in nearly all areas. Probability of precipitation slowly increase as well to chance
by early Sunday in the far west with rain as the p-type in all but
very coldest locations.
Long term /Sunday through Wednesday/...
as of 300 PM EST Wednesday...
Late weekend into middle next week looks to be potentially unsettled
with large scale trough over the western two thirds of the country
kicking several short waves out into southern appalachian region.
Models offer significant differences in the strength and timing of
these waves...and amount of Gulf moisture they are able to tap.
Also run-to-run inconsistencies are making this overall a rather
low confidence forecast. Am concerned that the European model (ecmwf) in particular
has had a tendency to dig the short waves a little too much and
then ultimately drive a deepening surface wave too far north up the
Appalachians or to the west of US. The flatter more progressive
GFS has been better as of late. Latest 12z European model (ecmwf) has trended
toward the more subdued GFS for the first system headed up our way
Sunday into Monday...while Canadian now has a much stronger system
tracking just to our south on Monday and Monday night. Will lean
more toward the weaker/flatter idea advertised in the latest GFS
and now European model (ecmwf)...at least through Tuesday. Ecwmf is a little stronger
with another wave on Wednesday...while GFS is
faster...colder...and generally keeps it south of US.
While confidence in the timing and strength of these systems is
quite low right now...there is more clarity that this will at the
very least be a cloudy period...and that low level temperatures and
thicknesses will generally be warm enough not to have any real
precipitation type concerns...with the possible except of highest ridges
across the far north Sunday and Monday...with any system on
Wednesday there could be enough damming setting up that
overrunning could be some sort of wintry mix...but not even sure
we will have precipitation on Wednesday. European model (ecmwf) would suggest freezing rain.
A little more confidence at the beginning of this period...so
have some likely and high chance probability of precipitation in for Sunday...but all
rain...and most likely light. As we get into the week...the large
surface high noses in more and more and each day looks a little cooler
than the previous one which is reflected in the temperature
forecast. While I have lowering probability of precipitation each day...largely due to
increasing uncertainty...am keeping a low chance to slight chance in
through the period with more indication of some wintry precipitation as
we get into Tuesday night and Wednesday. Overall...forecast really reflects
a very broad brush rather than going exactly with any one
solution...and expect there will be quite a few changes to the
early week forecast as we get into the weekend. Not much chance
for a major storm during this period...and after Sunday its
possible that dry but cool high pressure manages to dominate...but
best guess for now is something in between. As always...stay
Aviation /03z Thursday through Monday/...
as of 610 PM EST Wednesday...
Poor flying conditions continue across most of the region this
evening with low clouds...fog and drizzle causing periodic
lowering to IFR/LIFR from kroa east...with overall MVFR ceilings to
the west. Cold front now just west of the mountains will be
preceded by a broken band of showers and possible isolated thunderstorms and rain
with strong winds between 00z/7pm and 02z/9pm at kblf/klwb and
perhaps kbcb shortly thereafter. Mixing ahead of this front should
finally help to mix out the low conditions out east with
improvement to MVFR possible by midnight east of the Blue Ridge
and then VFR overnight. Some potential to see a few western
showers sneak out east while latest short term guidance indicates
perhaps another round of showers developing out east but low
confidence at this point so wont add. However will again continue
to mention low level wind shear for the strong SW jet just above the surface out
east while mixing to the ground prompts addition of tempo stronger
wind gusts to 40 kts out west with the front.
Expect a return to VFR out east of the mountains late tonight with
prolonged MVFR lingering mountains with kblf likely dropping back
into IFR/LIFR behind the front per developing upslope. West winds
should also crank up in the wake of the front with gusts to 30 kts
possible mountains after midnight.
Thursday...MVFR ceilings and upslope rain/snow showers should
persist in the mountains...mainly kblf/klwb into the afternoon with
VFR conditions returning in the Piedmont. In between...some
lingering MVFR strato-cumulus possible around kbcb/kroa Thursday
morning before things mix out under gusty westerly winds by
midday. Winds again may gust to 25-30 kts Thursday morning before
quickly diminishing during Thursday afternoon as high pressure
Thursday night and Friday high pressure should build over the
southeast...which will provide VFR weather with lighter winds.
Another cold front with associated low pressure will approach the
region Saturday and be over or just west of the area Sunday. Given
the slower trends...appears weather may stay VFR for Saturday with
sub-VFR conditions returning with clouds/precipitation on Sunday for now.
Boundary may linger over or just south of the region early next
week with possible return to VFR Monday afternoon at this point.
as of 335 PM EST Wednesday...
With the bulk of the rainfall having shifted northeast this
afternoon storm total rainfall from the rain gage network through
18z has ranged generally from 0.50 to 1.50 inches with the highest
amounts over northwest North Carolina in parts of the upper new and
Tennessee basins river basin and across the upper Dan basin.
Rivers have seen only minor rises of a few feet and do not expect
any gaged locations to even reach action stage. In fact longer-
term precipitation deficits extant over parts of the Piedmont will
remain after this event.