Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
858 PM EDT Monday Aug 3 2015
high pressure will remain across the region this evening before
weakening ahead of a cold front that will approach from the west
late tonight. This front should cross the area Tuesday before
stalling over the Carolinas Tuesday night into Wednesday. Low
pressure will ride along the front bringing a better chance for
more widespread showers and storms Thursday into Friday.
Near term /through Tuesday/...
as of 857 PM EDT Monday...
Focus for the near term forecast is on the evolution of
thunderstorm cluster across NE Kentucky/SW Ohio this evening. There are
several factors that suggest this cluster will weaken by the time
it reaches our forecast area. First...this cluster is moving into
a region of lower Theta-E air. Second...it is moving into a region
of weaker low level inflow/winds. Finally...as we lose the heating
from the sun...instability will continue to diminish. All of these
factors lead US to lean toward many of mesoscale model solutions
of weakening this cluster as it enters our southeast WV counties after
03z/11 PM EDT. Storms may propogate south into Richer Theta-E
air/higher instability axis in eastern Kentucky. Will continue to keep chance
probability of precipitation in our northwest counties after 03z.
With debris cloudiness moving over the area tonight...especially
in the west...pushed up temperatures a degree or two.
As a cold front slowly moves through our area Tuesday
expect showers and storms to develop Tuesday afternoon. Models
appear more bullish on convective development with slightly better
dynamics but instability is limited so severe threat will be
marginal at best and confined to eastern sections as per the
current Storm Prediction Center day 2 outlook. Will keep probability of precipitation in the chance category
and wait to see how the situation unfolds tonight to make any
Highs tomorrow will be aided by warm air advection and increasing
downslope winds...so highs Tuesday will be in the low/middle 90s east
to middle/upper 80s west.
Short term /Tuesday night through Thursday night/...
as of 215 PM EDT Monday...
Cold front will have jumped into the Piedmont Lee trough by early
Tuesday evening before pushing east allowing high pressure to build in
from the northwest through Wednesday. With strong downsloping in place...probability of precipitation
appear quite iffy Tuesday evening except perhaps over the far west
where closer to a deeper moisture axis to the west...and along the
eastern perimeter given weaker downslope espcly when mixing weakens.
However instability not overly impressive so running with only isolated
evening probability of precipitation in these spots before dry advection wins out overnight.
Area will remain in between features and under a bubble high Wednesday
aided by west/northwest flow through the column and lower precipitable waters . Most guidance
including the latest ensembles have little precipitation under the subsidence
Wednesday so trimmed back to only isolated probability of precipitation across the SW and far
eastern counties late in the day. High temperatures Wednesday don't appear
quite as hot as earlier given decent 850 mb cooling despite downslope
aloft and more sun. Still think some low/middle 90s likely east and 80s
Moisture will begin to increase Wednesday night as an initial wave
heads east out of the Southern Plains and begins to lift the front back
north with weak overrunning in place. Guidance remains slow to moisten
things up Wednesday night with the best flux of deep moisture not
returning until Thursday per the latest GFS/Euro. Since appears best
moisture/lift will be mainly over the far west/north with the
front...will keep low chance probability of precipitation mountains Wednesday night and slights
Plume of deeper moisture advects in with the lead shortwave Thursday
into Thursday night with models showing decent instability Thursday
afternoon as well as lift with the surface wave Thursday night.
However still some uncertainty in exactly where the core of this
system will track with some solutions farther west which would
limit focus until Thursday night after loss of heating. Therefore
keeping likely probability of precipitation confined to the west and chance out east for
now. Left temperatures closer to a more blended solution pending degree
of shower coverage which could make things cooler than the 80s to
around 90 southeast on Thursday.
Long term /Friday through Monday/...
as of 100 PM EDT Monday...
Strong shortwave for early August will pass across or just north of the
region to start the period allowing a trailing cold front to slide
southeast into the Carolinas by later Friday into Friday night. This
associated with another digging 500 mb trough that will bring the onset of
the next round of slightly cooler temperatures for next weekend.
However new guidance continues to flip flop as some solutions now not
as deep or prolonged with the eastern upper trough in having a passing
500 mb cool pool early in the weekend before returning to more zonal flow
and some return in moisture by day7. This ahead of added upstream
clipper type energy that should give the initial system a bit of a
boot to start. This could also keep more clouds around at times despite
weak surface ridging in place...along with possible isolated showers
SW Sunday and mainly over the west Monday ahead of the next wave.
Thus after higher chance to low likely probability of precipitation espcly north/west with the
frontal passage Friday...will hang onto some isolated to low chance
probability of precipitation Saturday and over the far SW Sunday despite the surface front well
to the south. Otherwise thinking that much of the weekend into Monday will
remain dry pending later trends with the timing of the frontal
progression from late in the week. Temperatures to cool back to just
below seasonal levels but not quite as cool as seen yesterday for
Aviation /01z Tuesday through Saturday/...
as of 715 PM EDT Monday...
VFR conditions prevailed across the National Weather Service Blacksburg forecast area
at 23z/7 PM EDT as our region is currently sandwiched between
systems. A cold front was noted across the Ohio Valley...with a weak
warm front noted across southeast Virginia/eastern NC.
There is some discrepancy in the mesoscale models as to when showers
and storms across Ohio/north Kentucky will arrive from the northwest.
Among the mesoscale models...the hrrr is handling this complex of storm
the best and shows decaying precipitation arriving after 03z. As this
cluster of storms approaches...it will be encountering drier air
and weakening inflow which should result in this activity
diminishing...but will bring potentially MVFR ceilings to the
mountain taf sites after 05z/01 am EDT.
The anticipated clouds/precipitation will complicate the fog
forecast tonight. The best chance for fog tonight...may be in the
far east where best radiational cooling conditions are forecast.
Cloud cover will inhibit radiational cooling west of the Blue Ridge
but the exact amount of clouds and precipitation will play a big
role in fog potential. Currently feel conditions will not favor
fog and will continue to omit fog from tafs this package.
The frontal boundary will be moving through the region tomorrow
and expect some shower/thunderstorm activity to develop late in
the taf period....there is still uncertainty with the timing and
location...but signals in the models suggest best chance for storm
development will be in the eastern portion of the forecast
area...and as a result...added thunderstorms in the vicinity to kdan.
Expect winds to be westerly with some low level gusts possible...
especially tomorrow as winds become more northwesterly with the frontal
Extended aviation forecast...
mostly VFR outside of any deep convection through the period. A
front will stall across the middle Atlantic by late week which could
bring a bit more coverage of showers/thunderstorms Thursday and
especially Friday. The threat of sub- VFR remains low until
Thursday...except for the typical fog potential at klwb/kbcb each
morning between 09-13z.