Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
1135 PM EDT Friday Aug 1 2014
a stationary front extended from off the North Carolina coast to
southern Georgia this afternoon with high pressure wedged from
New Jersey into the western Carolinas. Little change in the
weather pattern is expected overnight. Saturday The Wedge erodes
and the coastal front remains to the southeast.
Near term /through Saturday/...
as of 1135 PM EDT Friday...
Very minor update for probability of precipitation given most of rain has moved to the
northeast...and have continued to trend down much of the rest of
the night given latest hrrr even drier overnight. Some
temperatures...especially in the Piedmont have dropped a couple more
degrees so needed to pull mins down in some locations another
notch. Spread some of the fog/drizzle back a little farther west
in spots and made sure entire Blue Ridge was included.
Previous discussion as of 930 PM EDT Friday...
Coverage of showers continues to wane as the upper wave is now
shifting east of the forecast area. Steadier area of light rain will
be east of our area before midnight...leaving only very light
scattered showers or drizzle along the Blue Ridge and east.
Evening soundings still show above normal precipitation water values and
there is still a good low level easterly component seen in the
soundings and vwp from the radar...so cannot rule out very
shallow upslope drizzle or showers although they may be hard to
detect on radar. We begin to lose the easterly component at the ridge
top level late tonight according to sounding forecasts...but think low
level wedge will be holding strong and so Worth keeping mention
of drizzle and areas of fog all night. But given these trends and
the latest hrrr and rap model runs which are handling things
pretty well now and show only very little in the way of scattered showers
here and there across northern part of forecast area...have generally
lowered probability of precipitation even more for rest of overnight to low to middle chance
Blue Ridge and east...and nothing or just slight chance west.
Other change for overnight was to lower min temperatures in parts of the
Piedmont given fact that current temperatures in a few spots were already
as low as previously forecast mins...although don't think in most
cases current temperatures will drop more than a few more degrees from
what they are now...if anything.
Preliminary look at tomorrow suggests wedge will try to break during
the day...but may hold on as long as possible over nrv and parts
of Piedmont...while breaking early across southwest and far west.
Temperatures will be tricky given the breaking wedge...and think best
chance for showers and storms will be these far western fringes
were sun is out the longest. Warmer at blf likely again compared
to lyh. No changes in that period yet though as we await more
guidance from overnight.
Previous discussion as of 614 PM EDT Friday...
Current radar suggests that main lift with upper wave that is
moving northeast out of western North Carolina is shifting into the
Piedmont areas and what upslope along the Blue Ridge exists this
evening is producing relatively small areas of light rain and
drizzle. Most of the near term guidance that is similar to the
radar now suggests much of this will gradually weaken by late
evening while the more moderate rainfall in the east quickly
shifts off to the northeast. Hard to see any reasoning for much of
any shower activity developing on the west edge of the County Warning Area and The
Wedge given that daytime instability will be lost soon. Thus am
lowering probability of precipitation for most of the west to slight chance with some low chance
remaining up across Greenbrier in case any shallow convection
farther north backbuilds...and gradually lower the likely probability of precipitation
along the Blue Ridge the chance of showers and drizzle by late
evening into the overnight. Categorical far east for just another
couple hours then dropping off to middle chance...and will determine
late if we need to drop that even more for much of the overnight
when this upper wave has moved off...leaving weakening upslope
along Blue Ridge as 850 flow turns more SW behind it. Don't think
this SW flow will be able to erode The Wedge overnight however...so
am keeping clouds and areas of drizzle and fog in all night. Made
some minor adjustments to sky cover on western fringe where some
partial clearing has taken place on the edge of The Wedge...but
this may fill back in after dark.
Also pulled mention of heavy rainfall from the severe weather potential statement as there are
not any expectations of rainfall rates high enough to cause even
minor issues except for some ponding in low-lying streets in the
Previous discussion as of 340 PM EDT Friday...
Two distinct short waves seen on the water vapor loop the first
moving across West Virginia and Virginia helped generate the large
shield of rain from this morning. The second vorticity maximum was
approaching South Carolina and was forecast to move into Virginia
BUFKIT forecast soundings showed northeast winds at the surface
east of the Blue Ridge...20-25 knot east to southeast 850 mb winds
tonight and southwest winds aloft. This transitions to southwest
winds at 850 mb as The Wedge and moisture becomes shallow on
The combination of lift from the Carolina wave and upslope flow
will result in rain along and east of the Blue Ridge tonight. Most
of the short term guidance had the more widespread and steadier
light to moderate rain moving northeast out of the forecast area
after midnight. Then overnight...precipitation will be more spotty
light rain and drizzle. Also added areas of fog in the typical
areas where The Wedge is in place.
Temperatures on Saturday will be dependent on when wedge erodes.
Expecting typical southwest to northeast progression...with
Lynchburg and the northeast counties of the forecast area this
last to break out.
Short term /Saturday night through Monday night/...
as of 250 PM EDT Friday...
Throughout most of this portion of the forecast...the axis of the
established upper level trough will remain positioned just west of
the forecast area. This will continue to allow for a good moisture
fetch off the Atlantic to progress into the region. The interplay of
upslope conditions along and near the crest of the Blue Ridge and
weak disturbances rounding the base of the main trough axis...will
keep the chance of showers and afternoon/early evening isolated
thunderstorms across our area during this period.
While the main upper trough will remain to our west...there will be
a change in just how this feature is materialized. Through roughly
Friday morning...the feature will be the base of a closed low
system near Hudson Bay Canada. This low however will shift east by
the start of the weekend...allowing this main upper trough to fade
in intensity only briefly. By Sunday...the remnant of this first
upper trough will be strengthened by a shortwave trough
progressing eastward into the Tennessee and Ohio valleys from the
west. Despite this fluctuation...the overall forecast will be as
The time period when precipitation chances will be at their lowest
is Monday night when the latest guidance offers a solution of
broadening the width of the trough...and shifting the axis a little
farther east. This is expected to have the result of likewise
shifting the best chances of precipitation east of the forecast area.
Long term /Tuesday through Friday/...
as of 1230 PM EDT Friday...
At the surface Tuesday...high pressure sits over West Virginia while
a slow moving cold front is across the Ohio Valley. Ts Bertha or
what is remaining of it is forecast to remain well off shore and
will not be a factor in our region next week. An upper level trough
remains situated across the eastern US. The upper level flow will
become more zonal by the end of the period. The front across the
Ohio Valley will slowly drift and partially stall out across
northern portions of our County Warning Area by Thursday into Friday. While precipitable waters
are not impressive...generally 0.75-1.25 inches depending on the
location...expect isolated to scattered thunderstorms each afternoon
beginning during peak heating and tapering off around sunset. Focus
of the low chance probability of precipitation are across the mountains and generally north
of Route 460.
Surface temperatures will be right around normal for the period. 850mb
temperatures of +15-17c will correspond to surface highs of middle to upper
80s east of the Blue Ridge and lower 80s across the mountains.
Dewpoints in the upper 50s to lower 60s will keep night time temperatures up a
Aviation /04z Saturday through Wednesday/...
as of 745 PM EDT Friday...
Expect surface wedge to hold east of the Blue Ridge...and any areas
west to the Alleghany front that have seen weak erosion of The
Wedge late this afternoon...will likely see it fill in during the
next few hours. Meanwhile...the main disturbance that brought the
widespread light rain to the eastern 2/3rd of the region this
morning...has shifted east of the region. The last remnants of the
associated rainfall was pulling out of the Piedmont at this time.
Some weak waves may traverse the areas east of the Blue Ridge
overnight...enough to support continued to mention of -ra at times
for a few more hours at kdan/klyh/kroa...but mostly from this
point through the overnight hours...The Wedge combined with the
lack of dynamics and moist/relatively cool ground in most areas
will yield to areas of -dz and br/fog overnight. The exception will
be klwb...where rain did not fall earlier and late day clouds
have cleared out. This should set up klwb for another night of
locally dense ground fog. Given moist lower atmosphere in place
and presence of wedge...potential for klwb to drop to 0sm tonight.
Elsewhere...expect any VFR ceilings this evening to trend quickly to
MVFR and then likely IFR after 06z in most areas east of the
Alleghany front. Visibilities will range from VFR at 00z downward to
mostly MVFR in -dz or -ra overnight/early Sat. IFR ceilings will
improve to MVFR ceilings by middle or late morning...with potential for
VFR ceilings west o the Blue Ridge by afternoon. Visibilities should improve
to VFR after 15z Sat. Winds...mostly NE-southeast through the taf valid
period at speeds of 5-8kts...except west of the Alleghany
front...I.E. Klwb and kblf...where winds will be light NE in the
morning likely becoming light southeast to SW during the afternoon. With
respect to thunderstorms and rain potential Sat afternoon...that should be little to
none at this point because of very stable air mass east of the
Alleghany front...and limited dynamics to the west. If any thunderstorms and rain
developed...they would likely only impact blf and/or lwb.
Medium confidence in ceilings and vsyb through the taf valid period.
Medium confidence in wind direction...medium to high confidence in wind
speed through the taf valid period.
Extended aviation discussion...
the broad upper trough of low pressure will linger across the
eastern United States through Sunday. Latest model indications are
that the trough will begin to shift back to the east by
Sunday/Monday...moving the best potential for any rain showers/thunderstorms and rain mainly
east of the region with time. However...an increasingly warm/humid
air mass across the region should lead to at least slightly better
potential for scattered afternoon/evening rain showers/thunderstorms and rain across the region by
sun-Mon...favoring either the mountains or areas east of the Blue
For Monday and Tuesday...the air mass will slowly become warmer
and more unstable. However...dynamics and forcing are weak at
best. Mainly isolated rain showers/thunderstorms and rain are expected during the
afternoon...with brief/localized MVFR-IFR visibilities/ceilings in
shra/tsra...mainly afternoon/early evening. Late night/early
morning fog will remain a problem each day...especially at the
usual sites...such as lwb...bcb...and lyh.
By Wednesday...a weak cold front moves slowly south from the Ohio
Valley across the middle Atlantic region bringing scattered rain showers/thunderstorms and rain to
the region...most numerous west of the Alleghany front.