Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
1258 PM EDT Wednesday Sep 2 2015
a ridge of high pressure will remain over the region through the
end of the week. Upper level disturbances tracking through the
ridging will result in widely scattered mainly diurnal showers
and storms into Friday. A weak backdoor cold front will drop
south through the area by the end of week bringing somewhat drier
and cooler air for the weekend.
Near term /through tonight/...
as of 1255 PM EDT Wednesday...
Our latest forecast will reflect a change upward by an average of
two to three degrees of the high temperatures. Several locations
are already within a degree or two of the earlier forecast highs.
With at least four hours of additional heating at a conservative
degree per hour...an increase in the forecast highs are warranted.
Another adjustment has been to the coverage of isolated showers
across the southern half of the crest of the Blue Ridge and
adjoining foothills from this point Onward. Initiation commenced
about an hour or two earlier than previously predicted. Our
forecast will now reflect this coverage as ongoing. No other
notable changes at this time.
As of 930 am EDT Wednesday...
Early morning fog has dissipated in all but parts of the
Greenbrier and New River valley regions. However...by 1100 am...at
vast majority...if not all...of this fog will have dissipated as
well. Precipitation coverage across the region currently is zero.
There were a few isolated showers over Buckingham County around
the top the hour..but these have since faded. Latest mesoscale
models paints very little if any precipitation across the
northern parts of the area through 17z/1pm...and then start
ramping up the coverage after that time. Have tweaked the
probability of precipitation grids and the weather grids to better
reflect this trend. No other notable changes at this time.
As of 240 am EDT Wednesday...
Overall hot and humid weather for early September to persist today as
surface high pressure remains in place across the mountains. However
shortwave energy in the form of an opening upper low across the Ohio
Valley will ride southeast through the 500 mb ridge and over the northern
half of the area this afternoon. This should result in better lift
espcly along and just south of the Interstate 64 corridor early on and
across the eastern half through late in the day. Latest guidance also
showing better instability with a little more cooling aloft...but still
uncertainty with coverage given overall broad subsidence in place under
rather warm 500 mb temperatures. Current Storm Prediction Center/rnk WRF solutions along with the
latest hrrr remain the most widespread with rain showers/thunderstorms and rain north half and
along the Blue Ridge. This while most other solutions including
ensembles producing less including only widely scattered shallow
convection mainly confined from the Virginia Highlands east to around
klyh for the most part. Therefore aligned highest chance probability of precipitation across
the far north with a period of lower chances south/east during middle/late
afternoon and only isolated coverage far SW. Given weak flow aloft not
expecting much of a severe threat but could see a bit more clustering
of slow moving convection right near the core of the upper wave late.
Otherwise looking at mainly pc with middle deck early...then more cumulus this
afternoon with highs again reaching the low 90s east and well into the
80s elsewhere pending timing of convective initiation.
Impulse passes this evening leaving the region in between waves and
under surface high pressure. This should allow most convection to fade
by midnight given loss of heating and some overnight clearing per weak
northwest flow aloft in the wake of the middle level system. Thus keeping some
low end chance probability of precipitation going mainly east this evening before trending dry
with fog again overnight. Lows perhaps slightly cooler given some drop
in dewpoints and espcly where any cooling evening rainfall occurs but
still mostly low/middle 60s mountains to upper 60s east.
Short term /Thursday through Saturday night/...
as of 415 am EDT Wednesday...
Weak upper-level short wave will be dissipating across North
Carolina drifting slowly into the southeast states where it
appears it is destined to become a weak cutoff that will
retrograde/linger across the southeast/mid-south into next week as
a large high amplitude upper ridge continues to build from the
south central/central U.S. Into southeast Canada and the northeast
U.S. Over the top of the weak cutoff low. Upper support will be
minimal at best with best dynamics...what they are...having
drifted south of the area. Any convection will largely be the
result of differential heating and focused primarily across the
western mountains. Really cannot support much more than 30 probability of precipitation at
By Friday...a large surface high developing across the northeast
U.S. Underneath the Mammoth upper ridge will begin to push a
backdoor front from the middle-Atlantic southwestward. Upper dynamics
are very weak...but a marked dewpoint gradient will exist along
the southwestward advancing boundary as very dry air advects
southwest into northern Virginia. This should be sufficient to support
isolated to scattered showers and thunderstorms...with the best
coverage shifting southwest toward evening. Instability is quite
notable across the region in advance of this backdoor front...so
even with weak dynamics...with the noted instability and precipitable waters in
the 1.5 to 1.7 inch range...30-40 percent probability of precipitation seems reasonable.
For Saturday...the backdoor front continues to drift southwest and
should be located from central WV through southwest Virginia...near the
I-77 corridor...into central NC. Very dry air is noted advecting
into the region from the northeast with dewpoints dropping into
the 50s and precipitable waters falling to near 0.50 inch across the northeast
part of the County Warning Area. Any convection Saturday should be limited to the
I-77 corridor and points westward...primarily during the
Temperatures will remain well above normal Thursday and Friday
ahead of the backdoor front...with maximum temperatures well into the
80s...near 90 across the Piedmont. Min temperatures will hold mostly in
the 60s to even near 70 across the Piedmont. These readings are
around 5 and 10 degrees above normal...respectively. MOS guidance
has been trending too much toward climatology...so needed to nudge
up most areas from 2-3 degrees to better match what has been
Long term /Sunday through Tuesday/...
as of 445 am EDT Wednesday...
The focus through the period will be on the aforementioned weak
and increasing broad upper low anchored across the southeast/mid-
south part of the U.S. Our County Warning Area will be stuck in the transition
zone between this increasingly moist/unstable air mass to the
south and the persisting upper ridge and associated
subsidence/very dry and warm air mass across the middle-Atlantic
region. Shower/thunderstorm chances will gradually creep northward
through the period...but likely remain confined to areas west of
I-77 Sunday...then creep further north toward the U.S. 460
corridor by Monday and much of the County Warning Area by Tuesday. The best
chances throughout the period will remain across the western NC
mountains. The models have continually delayed the arrival of a
front toward the middle part of next week...and with the upper ridge
sandwiched in between the low pressure area to the south and the
stronger westerlies along the Canadian border...it may be some
time before it actually reaches our area. Hence...Summer appears
destined to hang on much of next week.
Maximum temperatures should be near normal at the beginning of the
period...but perhaps still slightly above normal west of I-77. Min
temperatures will remain near normal to slightly above normal far
southwest. Maximum and min temperatures will creep to slightly above normal
levels again as we move into early next week.
Aviation /18z Wednesday through Monday/...
as of 640 am EDT Wednesday...
Fog and stratus remain spotty across the region early this morning
with the typical valley locations such as klwb/kbcb seeing IFR or
worse at this time. Expect could see a brief reduction in visibility as well
around klyh/kdan as well during the next hour or two before
heating helps dissipate any residual low level moisture. Otherwise
waves of middle/high clouds ahead of the upper system to the northwest the
main aspect once any low clouds burn off with VFR expected at all
locations from middle morning Onward.
Upper wave over the Ohio Valley will shift slowly southeast into
Virginia this afternoon while slowly weakening. This will allow
more instability with steeper lapse rates and lift. Showers and
storms will be more scattered this afternoon with best forcing
across the northern foothills/Piedmont to the New River valley.
As of now will keep the going thunderstorms in the vicinity at kroa/klyh and kbcb after
19-20zz/3-4pm...but think coverage will be less further west and
over the southeast...so only going with a vcsh at klwb-kblf while
keeping mention out of kdan for now. Overall expect VFR
conditions outside any convection into this evening with ceilings of
4-6kft with towering cumulus and cumulonimbus in the afternoon.
Extended aviation discussion...
The forecast area will remain in a weakly forced synoptic
environment through Thursday...with chances for diurnal
convection mainly from the Blue Ridge west...and some MVFR/IFR
fog/stratus development at night...especially at kbcb and klwb. A
weak backdoor cool front may enhance the chances for showers and
storms later Friday before drier air arrives Friday night.
For Saturday into Sunday will see high pressure in control though
the position of the high over the middle Atlantic may slide more MVFR
ceilings off the Atlantic into our eastern taf locations at least to
start the weekend.
as of 400 PM EDT Tuesday...
The dew point reading at kjfz...Richlands Virginia AWOS...is incorrect.
At this time we do not know when this sensor will be repaired.