Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
313 am EDT Friday Sep 4 2015
a backdoor cold front will approach the region from the north
today before sliding through the area later tonight into
Saturday. Increasing moisture ahead of the boundary will lead to
more widespread coverage of showers and storms later today into
this evening. High pressure will wedge in from the northeast
Saturday into Sunday...and remain in place through early next
week...resulting in somewhat cooler weather over the Holiday
Near term /through tonight/...
as of 220 am EDT Friday...
Weak high pressure over the area early this morning will give way to
increasing moisture advection this afternoon ahead of a backdoor cold
front that will drift south into northern Virginia by this evening. This along
with a piece of middle level shortwave energy in the Ohio Valley that should
scoot southeast just ahead of the boundary and interact with moist
precipitable waters /strong instability to result in areas of deep convection
Most models suggest initial development along the Blue Ridge aided by
developing low level NE flow ahead of the shortwave and along an axis
of prognosticated higher 850 mb Theta-E. However a few solutions including ensembles
suggest a secondary area of better coverage out across the Piedmont
where approach of the shortwave and folding of the front east of the
mountains may induce a weak wave over the southeast. Thus trended
likely probability of precipitation a bit more toward the eastern slopes of the Blue Ridge
by middle afternoon...and then shifted south toward northwest NC/Southside
Virginia where guidance shows stronger pre-frontal convergence late per
prognosticated higher precipitable waters coming in line with the wave. Otherwise
running with decent chance probability of precipitation elsewhere although severe potential
limited by weak shear and moist soundings overall. However given
the degree of heating with highs again likely pushing 90 Blue
Ridge and points southeast...some downburst potential with any organized
slow moving cluster of storms.
Axis of rain showers/thunderstorms and rain may again organize more over the Piedmont and southern
sections before sliding SW this evening well ahead of the front and
prior to the exodus of the wave aloft. This should allow enough coverage
for continued likely probability of precipitation southeast...and chance elsewhere before
loss of heating and exiting lift cause west bound convection to become
more shallow overnight. Appears best coverage after midnight to end
up over the southern/SW sections along the nose of the developing wedge
where will be getting more of a east/southeast trajectory including low level
focus along the front. May even see a bit of drying far NE sections
late tonight and over the far west with downslope...otherwise looking
at range from low chances north to higher chances SW late. Expect overall
cloudy skies and lingering moist dewpoints to hold temperatures up despite
low level cool advection so stayed with mostly 60s for lows.
Short term /Saturday through Monday/...
as of 300 am EDT Friday...
A hybrid wedge will remain along the East Coast over the weekend.
This wedge will be centered off the New England coast and will
stretch south into Georgia. With weak isentropic lift...easterly
flow and Atlantic moisture...mostly cloudy skies are expected.
Rain/drizzle is also possible along the Blue Ridge each period and
over the Piedmont during the afternoon and evening hours. The parent
surface ridge drifts east on Monday...leaving a baggy high over the
middle Atlantic region. This will keep a chance for light rain/drizzle
along and east of the Blue Ridge into Monday night. Some dry air may
be able to advect into the region as a possible closed low forms
over northern Florida.
Temperatures will be tricky with a wedge in play over the weekend
and into early next week. Easterly flow will keep afternoon
temperatures a few degrees cooler than normal but help warm the west
with downslope flow. Isentropic lift will keep the area cloudy but
there could be a few breaks in the clouds during the
afternoon...only to fill back in during the evening. These clouds
and any rain falling could cool afternoon temperatures 5f-8f cooler
than normal. As of this forecast package...confidence is not high
for an all day rain event and any breaks in the rain or clouds will
allow temperatures to warm. With continued cloudiness...temperatures
will run muggy and warmer than normal each night.
Long term /Monday night through Thursday/...
as of 100 PM EDT Thursday...
Starting with a broad upper low over the southeast United States and
a strong ridge in the Southern Plains by Monday night. 500 mb
heights fall over the northeast United States Monday night through
Wednesday. By Thursday the northern stream becomes more dominant
with more troughing across all of the east.
At the surface and low levels forecast area will be in a deep wedge
covering the east side of the Appalachians into Monday night with
east to southeast winds. The Wedge will break down on Tuesday.
Then southwest flow will bring a return of moisture and warmer 850
mb temperatures Tuesday night and Wednesday.
A well defined cold front and the leading edge of cooler air will
reach the forecast area on Thursday. Waves of low pressure along the
front in the central United States may slow down the timing and the
eastward movement of the front for Wednesday and Thursday.
Temperatures will again be above normal in the warm sector ahead of
the cold front on Wednesday and Thursday.
Aviation /07z Friday through Tuesday/...
as of 100 am EDT Friday...
Fog/stratus the main aviation concern again overnight with
IFR/LIFR likely to develop in the valleys and in spots where
afternoon rainfall occurred namely at kbcb/klwb. Otherwise looking at
spotty MVFR in fog with only some middle deck around where stratus
does not develop.
A backdoor cold front will sink south toward the area on Friday
preceded by onshore flow and strong instability. This combined
with weak energy aloft to the northwest should lead to more widespread
convection from midday into Friday evening. Appears best coverage
early on will be along the Blue Ridge with a gradual shift east
once the boundary slides south. Think enough potential to include
a prevailing rain showers group with embedded thunderstorms in the vicinity at most locations along
and east of the Blue Ridge and thunderstorms in the vicinity in southeast West Virginia for now. Showers
may linger espcly south into the evening so kept mention in past
sunset. Otherwise looking at mainly VFR cumulus fields by late
morning...with 4-6k feet cumulus ceilings into the afternoon with MVFR/IFR
where bands/clusters of storms develop. Strong winds will also be
possible in the vicinity of storms but will hold off including
within the tafs for now given uncertainty in strength/coverage
Extended aviation discussion...
Will see a weak flow pattern with an upper trough extending from
east-west over our area Saturday into Sunday. This along with
high pressure to the northeast will cause the low level flow to
turn more easterly in time. As of now...the terminals will be VFR
during the day...with possible MVFR or worse with fog at night
into Friday night. The easterly flow expected by Saturday will
bring lower clouds in and potential for MVFR ceilings at least for the
mountains...and low end VFR at kroa/klyh/kdan. With this upper
trough there will be a daily threat of showers...so sub VFR in
these will be possible.
By Sunday-Monday the weak trough aloft moves over the southeast
coast while upper ridging elongates over the middle Atlantic. We will
see some drying take place so VFR should occur Sunday-Tuesday with
the still a threat of fog at night at kbcb/klwb.
as of 745 PM EDT Thursday...
The weekly United States drought monitor issued on September 3rd
shows the effect of a drier than normal August across much of the
area. Abnormally dry conditions...d0 on the drought scale have
been expanded into a large portion of Southside Virginia covering
much of the Dan and lower Roanoke River basins. Parts of the upper
Yadkin River basin in North Carolina are already in d1 or moderate
drought. August rainfall was as low as 50 percent of normal or
less in parts of the aforementioned areas. Several National Weather Service cooperative
climate stations in southern Virginia recorded well under 2 inches
of rain the entire month of August including Huddleston in Bedford
County at 1.17 inches and Rocky Mount in Franklin County at 1.36
long term...air mass