Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
226 PM EDT Tuesday Sep 1 2015
an upper level ridge of high pressure will strengthen over the
Appalachians and central middle Atlantic region today...keeping US
in a rather stagnant pattern with isolated afternoon and evening
showers and thunderstorms through late in the week. A weak
backdoor cold front will drop south through the area by the
Near term /through tonight/...
as of 1000 am EDT Tuesday...
Only modest adjustments to the early morning package...basically
to decrease cloud cover across the area...as well as to prolong
the patchy fog along the Greenbrier River in southeast West
Virginia through 11 am. Appears that...with the beginning of
September here...we are into the time of year where lower sun
angles will allow fog to linger longer into the morning.
Otherwise...the early morning forecast package appears to be on
As of 223 am EDT Tuesday...
Ridging both surface/aloft to strengthen across the region today in the
wake of a weak passing shortwave early this morning and ahead of the
weak upper disturbance now in the western Ohio Valley. This should
result in limited forcing with any convective coverage mainly tied to
orographics and low level convergence between weak onshore flow out
east and weak northwest trajectories over the west. Models also indicated
pockets of higher surface-850 mb Theta-E over the west/SW and in the far
northwest later on. However most solutions remain with an overall disorganized
shotgun pattern to showers despite decent instability per strong
heating and lack of much cap aloft. Therefore running with overall
slight probability of precipitation mainly mountains with ribbons of more chance nature
coverage along the higher ridges in a few spots. Will be on the hot
side to start September given a bit more sun than the last few days and
light flow. This could push highs above 90 southeast and well into the 80s
elsewhere outside the higher elevations.
Weak northwest flow may take a few rain showers/isolated thunderstorms and rain just east of the
mountains into early this evening before fading but given lack of
support think most about gone around sunset. Otherwise opening upper
wave slips southeast toward the region later tonight and helps lower 500 mb
heights under strong surface ridging. This system could spark a band or
two of showers mainly northwest late but given lack of much lift after loss of
heating cut back to mostly slight chance probability of precipitation overnight mainly northwest.
However this feature may bring enough clouds to keep low temperatures up a bit
more so appears mostly uniform middle to upper 60s overnight.
Short term /Wednesday through Friday night/...
as of 345 am EDT Tuesday...
Very diffuse upper-level pattern in place at the beginning of this
period...with weak...small changes through the period that will
dictate the weather. We begin with a building upper ridge
encompassing much of the eastern 2/3rd of the nation south of the
Canadian border. Underlying this broad upper ridge are several
weak systems...a weak upper trough traversing the northern
periphery of the upper ridge through the Ohio Valley and the
remnants of Erika off the NC coast. The aforementioned upper
trough will slide southeast through the day allowing for a brief
lowering of upper heights. This feature will also track into the
region during peak heating...which should be sufficient to spark
isolated to scattered afternoon convection...especially across the
mountains. Warm temperatures aloft will inhibit any significant coverage
with this feature. Convection will be largely diurnal and should
dissipate quickly after sunset. Most favored areas for
showers/thunderstorms Wednesday afternoon/evening should be near the
I-64 corridor. However...with limited dynamics...20-30 probability of precipitation
should be sufficient.
For Thursday...the weak upper trough continues to slide southward
into the Carolinas or eastern Tennessee Valley. Meanwhile...the upper
ridge becomes very high amplitude into eastern Canada with 588dm
heights even north of the Great Lakes...all in response to
deepening low pressure along the West Coast. With the upper
trough weakening and ridging aloft...convection will be isolated
to scattered at best and almost entirely driven by
diurnal/differential heating. Best focus should be across the
western mountains. Little support for more than 30 probability of precipitation
anywhere...mostly 20 probability of precipitation.
By Friday...a strong 1030mb high begins to take shape across
southeast Canada and New England underneath the strong/high
amplitude upper ridge. 590dm heights are noted even north of the
Canadian border. As the ridge builds into New England...the middle-
week weak upper trough sinks south into the southeast u...
could become a focus for precipitation in the extended depending
on its northward extent. More importantly for our region at this
point is a healthy backdoor front beginning to take shape across
the middle-Atlantic that will advect southeast into the County Warning Area by
Saturday. A marked increase in instability is noted in advance of
this feature...which will represent slight cooling and modest
drying. We should see an increase in convection along and in
advance of the boundary from the north and northeast by Friday
afternoon...and this has been reflected in the probability of precipitation.
Temperatures will remain above normal through this
period...generally on the order of 5 degrees for daytime maximum temperatures
and nearly 10 degrees above normal for min temperatures. Most models were
in general agreement on maximum/min temperatures through the period.
Long term /Saturday through Monday/...
as of 400 am EDT Tuesday...
The backdoor cold front will move southward through the region
Saturday...followed by drier/slightly cooler air...especially
sun-Monday as surface high pressure slides southward along the middle-
Atlantic coast. The middle-week weak upper low will become cut off
in the Alabama/GA/SC region. Meanwhile...a strong upper ridge will
remain firmly entrenched across the region with 500mb heights in
the 593dm range through the weekend. Showers/thunderstorm chances
Saturday will be best in the west/southwest part of the County Warning Area near
the southwestward moving backdoor front. Convection chances sun-
Monday will be confined to the far south central/southwest parts of
the County Warning Area near the cutoff upper low and stalled backdoor front.
Temperatures will be slightly cooler through much of the County Warning Area this
period thanks to the backdoor front and increasing northeast-east
maritime flow. Dewpoints/relative humidity values will also be less...especially
across the northeast part of the County Warning Area. The exception may be the
southwest part of the County Warning Area...which will likely remain warmer and
more humid given the synoptic pattern in place. The cooling will
be most notable in maximum temperatures...which will be near normal...while
min temperatures are likely to remain well above normal.
Aviation /18z Tuesday through Sunday/...
as of 200 PM EDT Tuesday...
VFR conditions reported areawide this afternoon under a cumulus field...
scattered across the Piedmont and scattered to broken across the mountains...with
bases in the 4kft to 6kft range. Radar indicates a few showers
having developed along a klwb to khsp line...which are slow moving
and producing light to locally moderate rainfall.
Spotty showers will continue to develop across the mountains this
afternoon...with best chances along the ridges of southeast West
Virginia. Individual showers will remain small and have limited
impacts on visibility beneath...but will move slowly...which may
cause them to linger for an extended period of time. Instability
is expected to remain limited...and therefore any thunderstorm
activity should remain isolated and short lived. Convection will
fade toward sunset.
Overnight...scattered middle level clouds will move into the area as
an upper level disturbance approaches from the Ohio Valley.
Regardless...expect patchy fog to redevelop...with visibilities
across much of the forecast region falling into the 3sm to 5sm
range...and as low as 1/2sm in some of the deeper river valleys.
Ceilings most locations are expected to remain above 3kft.
Fog will dissipate quickly after 02/14z...and any low clouds will
gradually lift. Scattered showers will enter the mountains during
late morning as the upper level disturbance drifts slowly across
the northern half of the area. With greater instability on
Wednesday...expect a few showers to intensify into thunderstorms
during middle afternoon and the evening. The disturbance will be slow
moving...so convection will linger across the Piedmont after
Extended aviation discussion...
The forecast area will remain in a weakly forced synoptic
environment through midweek...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.
For Friday into Saturday 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.
as of 400 am EDT Monday...
The dew point reading at kjfz...Richlands Virginia AWOS...is incorrect.
At this time we do not know when this sensor will be repaired.