Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
328 PM EDT Wednesday Oct 1 2014
back door front sinks south into the Carolinas tonight. High
pressure will again wedge south along the eastern slopes of the
Appalachians through Thursday. A strong cold front arrives later
Friday followed by much cooler air for the weekend.
Near term /through Thursday/...
as of 225 PM EDT Wednesday...
500 mb height rise tonight with region in a weak ridge in between
departing upper low well off the East Coast and the digging trough
over the central unites states. Weak surface cold front crossing
into North Carolina tonight will dissipate by Thursday morning.
High pressure will be over the forecast area on Thursday. There
may be some easterly flow along the Blue Ridge tonight but any
upslope will be minimal.
Models again bring low clouds back in overnight east of the Blue
Ridge. Good agreement in the short term guidance that clouds will
fill in between 06z/2am and 08z/4am. Based on the trends the past
two nights...may speed this up a couple hours. Will again have
dense fog in the western river valleys through early morning on
Guidance in the Piedmont has been too cold the past two nights.
Will trend a few degrees warmer for overnight lows. Will also
lower temperatures at higher elevations in the western County
Warning Area. Prevailing slow on Thursday is westerly. Locations
that have any clearing in the morning will again have above normal
Short term /Thursday night through Saturday night/...
as of 257 PM EDT Wednesday...
A cold front will approach the region Thursday night. With an
increase in mixing ahead of this front...a shallow wedge will
continue to erode with a piece hanging over the Piedmont into
Friday morning. As expected...models have slowed down the timing
of the front and prefrontal showers entering the area. The front
will enter the Ohio Valley late in the morning with prefrontal
showers tracking across extreme western portion of the County
Warning Area (sw Virginia/southeast wv). By early afternoon...the front moves
over the Ohio Valley and showers advance east to the Blue Ridge.
The front crosses over the mountains during the evening with
showers over the foothill and Piedmont counties...exiting during
the early morning hours Saturday. Precipitation timing may vary by
a couple of hours...as the closed low cycles and reorganizes over
the upper Mississippi Friday.
As the front tracks to the East Coast Saturday morning...the
closed low will move east into the Ohio Valley. A short wave with
this low looks to track farther to the south and over the
Tennessee Valley. With this track...thick clouds will remain over
the mountains...especially along western slopes through Saturday
morning. As this short wave exits the forecast area...west winds
will increase and clouds will eventually dissipate. Until the
clouds are gone...breezy/gusts winds should remain along higher
ridges then mix down Saturday afternoon. High pressure tracking
along the Gulf states will stretch north Saturday night allowing
winds to subside.
With the slower arrival of showers on Friday...warmed
temperatures a few degrees with l/M 70s west and u70s/l80s east.
Along with warmer temperatures...instabilities will
increase...therefore increase thunderstorms activity from isolated
to scattered Friday afternoon/evening. A dramatic change in
temperatures and humidity will be felt Saturday as temperatures
will run 5f-10f cooler than normal.
Long term /Sunday through Wednesday/...
as of 310 PM EDT Wednesday...
Following the passing of a cold front Friday night...the region
will remain under an upper level trough through early next week.
Cooler than normal afternoon temperatures likely through
Monday...then moderating towards normal by middle week. Overnight
lows Saturday and possibly Sunday night will be cold with
mountains temperatures dropping into the 30s...40s in the east.
Frost is possible across the west if winds could decouple early
Sunday morning. Ground temperatures will remain warm enough to
limit any frost to isolated areas.
The eastern half of the USA will remain under a broad but flat
trough through Tuesday. There could be a middle level disturbance
moving across the Ohio Valley Monday...but with limited
moisture...just seeing an increase in cloud cover for the area.
Tuesday maybe the next chance for rain...as a front is scheduled
to move over the region.
Aviation /20z Wednesday through Monday/...
as of 130 PM EDT Wednesday...
Visible satellite loop showed stratus east of the Blue Ridge
continuing to lift and become scattered early this afternoon.
Expect VFR conditions at klyh at the beginning of the 18z taf
forecast period. A weak short wave crossing through the middle
Atlantic region may trigger isolated to scattered showers late
this afternoon into this evening. The probability was too low at
this time to include in the tafs. The most likely location for
showers will be just north and east of klyh.
Tonight...wedge expected to set up and looking at MVFR/IFR
ceilings/visibilities reforming at lyh/Dan overnight. NSSL WRF and NAM BUFKIT
forecast both showed redevelopment of the low clouds around
07z/1am tonight. Fog again at bcb/lwb in the IFR/LIFR range late.
High confidence that IFR conditions will persist until 13z/9am.
Extended aviation discussion...
Expect VFR Thursday afternoon into Thursday night. Frontal boundary and
associated showers to impact the mountains by late Friday morning
and the Piedmont in the afternoon. Sub VFR expected in heavier
showers or isolated thunderstorms and rain.
Showers exiting the mountains by late Friday evening and the Piedmont
toward dawn Saturday. Timing of clearing is still tricky.
Saturday should be clearing out in the Piedmont to
Roanoke/Blacksburg but upslope MVFR ceilings at blf/lwb may occur into
Sat afternoon...with breezy northwest winds.
Winds diminish Saturday night. Then VFR Sunday.
near term...air mass