Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
138 am EDT sun Jul 13 2014
high pressure will give way to return southwest flow and
increasing moisture through Sunday. Then a strong cold front
approaches from the west on Monday and cross the region on
Tuesday. Cool high pressure follows the front for the latter
half of next week.
Near term /through today/...
as of 930 PM EDT Saturday...
Patchy fog overnight...mainly in the mountain valleys where some
isolated showers occurred late Saturday afternoon. Other than
that...anticipate a quiet overnight...with near normal
Evening rnk sounding revealed modest cape...1100 j/kg...enough to
drive shower activity vicinity of the late day wind convergence along
the mountains. Did not have any severe reports. Best coverage was
over the NC high country...and over the higher ridges just
northwest of Roanoke. Pretty descent storm occurred in eastern
Craig County Virginia...east of New Castle. This storm deposited a
healthy 1.50 inch of rain...a narrow core...most of this falling
in the National Forest. This same storm cell had the highest
concentration of cloud to ground lightning of all the storms that
occurred in the County Warning Area today. Of the official reporting stations...we
measured 0.11 in the gage here at the National Weather Service office in Blacksburg...a
10 minute thundershower. There was no cloud to ground lightning
observed with this shower...but we did hear some in-cloud
Thunderstorms associated with an upper level trough over the upper
Midwest will result in an advancing area of cirrus clouds...this
debris cloudiness moving downstream and potentially impacting our
County Warning Area late tonight. The result will be the potential for less net
radiational cooling which may reduce the amount of fog development.
Low temperatures tonight will be just slightly higher than those
realized Saturday morning thanks to the potential for more cloud
On Sunday...we expect greater coverage of showers and storms as
compared to what is occurring today. While daytime heating will help
fuel convection...an added feature will be increased jet dynamics
aloft with the approach of an upper trough. We still expect the
greatest coverage across the mountains...however...isolated
convection will be possible even across the Piedmont. High
temperatures Sunday in the west may be slightly cooler than those
realized today thanks to a greater potential for precipitation and
more cloud cover. Across the Piedmont...readings may be slightly
higher than those of today.
Short term /tonight through Tuesday night/...
as of 355 PM EDT Saturday...
Any scattered convection will dissipate in the evening...although
left a small area of slight chance probability of precipitation in northwest NC mountains area due to
some weak convergence and light precipitation suggested by NAM. While
there is likely to be a break in the far west between dying
evening showers and when chances increase again toward morning
with left-over activity trying to reach western slopes from
overnight convection far to the west...kept a low chance pop going
in far west...increasing to middle chance toward morning since
confidence is low in terms of exactly when some of this might
arrive. Most likely it will not be until after 12z Monday
morning...but NAM suggesting faster timing if enough of a cold
pool develops with an organized line well upstream overnight...the
dying remnants could sneak in earlier. Certainly not confident
enough to drop probability of precipitation entirely for the overnight back on western
slopes. Lows Sunday night will be bit milder in the west given
arrival of clouds overnight ahead of convection...and going on
warm side of guidance there...and with cooler guidance in east
where skies will be partly to mostly clear. Right now have blf
and Dan with same low temperature...and would not be surprised if blf is
Monday the front approaches from the west with increasing shear
and moisture...but big question is instability and amount of any
dry air to aid in downburst production. Much depends also on how
much lingering shower activity can get into the west in the
morning...which would reduce instability further. Some
convergence in Lee trough develops later in the day too...and with
forecast soundings suggesting better mixing and difference in temperature and
dew pt out there...wondering now if the better severe potential
will be along and east of the Blue Ridge vs. In the mountains.
With surface front still to the west though...and heating ahead of it
could still cause some trouble...so at this point dont want to try
and differentiate severe threat between west and east...but am
leaning toward a minimal threat overall...with a little better
potential in the east. Will not know much more until we get a
better handle on what could be lingering Monday morning and how
much heating will occur to help increase instability beyond what
models are suggesting now. Bumped up highs a bit in east with the
idea of more sunshine early...and guidance still going with some
middle 90s. Much bigger difference in daytime highs from west to east
Monday compared to last couple of days.
Decreased thunder potential late overnight Monday night but not
eliminating entirely...with middle chance probability of precipitation given plenty of moisture
and frontal boundary moving into west. By daytime...models
suggesting a much stronger Lee trough developing and idea that
the surface front will eventually jump into this position by late in
the day. Initially...there could be some activity along weakening
surface boundary across west...but instability is very minimal.
Again...better in east. Any severe chances should be across
east...and depending on where the boundary is by middle
afternoon...it could actually be just southeast of the forecast area.
Cooler with highs due to extensive cloud cover and scattered to
Some cooler and slightly drier air to begin pushing in from west
overnight Tuesday night...but trends are to slow this moisture
boundary down a bit...and still think there could be lingering
showers in the east much of the night with slowly moving
boundary...but left it no higher than chance probability of precipitation in far east by
evening. We have been running much cooler than guidance for
overnight lows...and this may be the case in far west...but not so
sure about east with lingering showers and uncertainty in how far
east boundary will be...so bumped up lows a bit in the east.
Long term /Wednesday through Saturday/...
as of 1230 PM EDT Saturday...
By 12z Wednesday morning the cold front will be across eastern Virginia.
High pressure builds in bringing drier and much cooler conditions.
As the high shifts to the east towards the NE states by the end of
the week our wind direction will gradually shift from northwest to an east/southeast
flow. There will be a noticeable drop in humidity behind the front
as dewpoints crash into the 40s and 50s. 850mb temperatures will range from
+11-15c which will coordinate to surface temperatures 5-10f below normal.
Higher elevations of the mountains may not make it out of the 60s on
Wednesday and Thursday...while even lower lying areas will only
range from the low 70s to near 80 east of the Blue Ridge. Night time
lows will also be 5-10f below normal. While its too soon to talk
about any potential record lows...several of them may be within
reach should we get sufficient radiational cooling.
At the end of the period...the GFS continues to hint at a shortwave
disturbance pivoting towards the region from the Southern Plains for
Friday and Saturday. The flow becomes more southerly and moisture
increases ahead of the disturbance. Placed a broad area of low
chance probability of precipitation across the area for those two days.
Aviation /06z Sunday through Thursday/...
as of 125 am EDT Sunday...
High pressure was offshore with broad southwest flow bringing deep
moisture back into the region. MVFR fog had develop at kbcb where
it rained briefly Saturday evening. Visibility may occasionally
drop to LIFR through 12z/8am at kbcb. Expecting VFR conditions
for the rest of the night at kroa/kdan and kblf. Some MVFR fog may
develop at klyh and klwb...but this will be dependent on the
amount of cloud cover. At 06z/2am satellite pictures showed a band
of middle level clouds from New York to northwest North Carolina
moving east. High confidence that more clouds from out of the Ohio
Valley and from thunderstorms over Illinois will reach the
forecast area this morning... will remain VFR.
Sunday will be comparable to Saturday with mainly VFR conditions.
Cloud build-ups expected by early afternoon...with scattered
middle/late afternoon showers...primary development vicinity of the
mountains. Kbcb and kroa are the most likely locations to be
impacted with a thunderstorm.
Extended aviation discussion...
Monday night through Tuesday night...a better chance of shower
and thunderstorm activity...along with the potential for a greater
coverage of sub-VFR conditions will exist thanks to a deep upper
level low/trough and its associated dynamics progressing eastward
through the Great Lakes region and then into New England. Its
associated surface features will offer a cold frontal passage
Tuesday afternoon or evening along with a slight chance of severe
weather as offered by the Storm Prediction Center.
High pressure will try to work its way into the area Wednesday
into Thursday. There is some question as to just how far south of
the area the cold front will make it before the region is on the
west side of the high...and thus we return to a southerly flow of
moisture into the area. Either way...low level moisture will
likely not completely dry out...and the potential for overnight
IFR/MVFR fog is promising...especially for areas that receive the
most rainfall Monday into Tuesday.
By Friday...most guidance is in agreement that a wave will move
along the stalled front and bring another substantial round of
showers and storms to the area.