Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
157 am EDT Sat Aug 23 2014
over the weekend a front is expected to move south along the East
Coast...which will bring cooler air to areas east of the
Appalachians...while very warm sultry weather continues across the
middle section of the United States.
Near term /through today/...
as of 1146 PM EDT Friday...
Based on latest radar data and model data...the main axis of
heavier rain showers/thunderstorms and rain has shift just west of the mountain Empire of SW
Virginia...and not really expecting the showers upstream to be too heavy
to cause flash flooding...therefore...dropped the Flash Flood
Watch for the rest of the SW County Warning Area early. Cannot rule out the
typical ponding of water on roads...and flooded ditches.
Otherwise...updated the overnight probability of precipitation to account for the 00z NAM
which is not too far off course from what is currently going on.
May see more development in the Piedmont through dawn.
Previous valid discussion...
A backdoor cold front will begin to drop south across our area as
high pressure builds in from the Great Lakes region.
Regardless...light winds... moist soils and breaks in the cloud
cover will allow patchy dense fog to develop once again. Overnight
lows will range from the middle 60s west to around 70 east.
The cold front will continue to push south on Saturday. Will see
northeasterly windflow across the Piedmont as high pressure
continues to wedge south...while northwesterly winds will persist
across the mountains. As such...should see a redevelopment of
showers along the Blue Ridge due to the increased low level
convergence. Will still have moist atmospheric conditions to start
the day...however moisture will gradually decrease during the
afternoon as drier air moves in from the northeast.
Short term /tonight through Monday/...
as of 245 PM EDT Friday...
Surface wedge becomes firmly established Saturday night and Sunday.
Low level moisture...especially along and just east of the Blue
Ridge diminished by Sunday night. Enough dry air to cut back on
probability of precipitation on Sunday night and cloud cover for
Will keep maximum temperatures below guidance for Sunday. Monday
will be a balance between cooler airmass in The Wedge and August
Long term /Monday night through Friday/...
as of 1145 am EDT Friday...
Deep upper trough crossing through the central United States
completely flattens the Southeast Ridge on Wednesday and Thursday.
Spread of solutions on models becomes quite large by Wednesday.
Wpc was leaning toward the less amplified...more progressive
European model (ecmwf).
The surface high that wedges down the Appalachians starts out
over New England...then by Wednesday the parent high moves
offshore. By that time the amount of easterly flow and ridging
will be influenced by whatever tropical system will be father east
in the Atlantic.
A cold front over the Great Lakes on Wednesday will approach the
forecast area late on Thursday. Models suggest not a lot of
southward push of this front. Boundary may still be in the area on
Friday. As the surface wedge break down and the front
approaches...850 mb temperatures will rise.
Forecast area will be dry at middle and upper levels Monday night
through Wednesday night. Then by Thursday deeper moisture comes
back into the area along the front. European model (ecmwf) even showed drying well
below 850 mb on Tuesday.
Aviation /06z Saturday through Wednesday/...
as of 145 am EDT Saturday...
Fairly poor aviation conditions expected through the taf valid
period as baroclinic zone...oriented from NW-se...slowly drifts
westward through the weekend as high pressure moves toward the
northern middle-Atlantic region setting up a fairly classic wedge
pattern underneath a building upper ridge. This pattern will
continue to result in variable cloud conditions...along with scattered
rain showers and some thunderstorms and rain western areas during the afternoon. Until the
baroclinic zone drifts further west...cannot rule out -shra from
time to time...so have retained mention of vcsh most hours at most
sites. Ceilings will vary from VFR at times...mainly late morning
through late afternoon...to mostly MVFR at night...locally IFR-
LIFR in fog/low clouds. Visibilities mostly VFR outside rain showers/thunderstorms and rain areas
and late night/early morning fog. The usual dense radiation fog at
lwb/bcb...not a definite occurrence because of thick middle
clouds...but there is potential given the high precipitable water and saturated
ground from recent heavy and repeated heavy rainfall. As the
backdoor front/wedge evolves through the daytime...expect winds to
transition to the NE-east-northeast east of the Blue Ridge...with west-northwest-northwest
winds continuing to the west...becoming variable west of the Blue
Ridge after 00z. With wedge developing and dry air still well
upstream through the taf valid period...expect MVFR ceilings/visibilities in
br/-dz after 00z east of the Blue Ridge...with potential ground fog
developing west of the Blue Ridge because of saturated ground and
late day -shra.
Low to medium confidence in ceilings/visibilities through the taf valid
Medium to high confidence in wind/speed through the taf valid
Medium confidence in thunderstorms and rain potential west of the Blue Ridge after
Expect flying conditions to slowly improve Sunday as high pressure
continues to build in from the northeast. Will see a few showers
develop Sunday afternoon...mainly west of the Blue Ridge...however
the main concern will be early morning fog and stratus. Stratus
will be slow to burn off through the day...although breaks will
develop during the afternoon. A better punch of dry air with high
pressure building in from the northeast may be enough with
mixing/heating to allow for increasing VFR conditions Monday into
Tuesday under mainly dry conditions.
the ASOS at Lynchburg (lyh) Virginia remains partially inop due to a
board failure. Parts have been ordered with the return of
complete service unknown at this time.
The National oceanic and atmospheric administration weather radio at Wytheville Virginia...wz2500...broadcasting
on a frequency of 162.450 mhz is off the air. Verizon has been
short term...air mass
long term...air mass