Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
912 PM EDT Sat Aug 30 2014
an upper level system moves from Mississippi to the Ohio Valley
through Sunday. The area will remain in a typical Summer airmass
into Tuesday. A front arrives from the north by Thursday.
Near term /through Sunday/...
as of 850 PM EDT Saturday...
With isolated convection now all but gone...plan to run a few
hours without pop into the overnight given loss of heating and
best lift/convergence back to the SW over TN/al. Latest evening
soundings show a decent amount of low level instability but capped
off aloft via the lingering inversion/dry air. Appears best
moisture trajectory will continue just west of the mountains
overnight before a trailing trough axis attempts to shift east
late under the passing wave aloft as seen by a couple short term
solutions. However even this looks to arrive late when things are
more stable...so also pushed probability of precipitation back west a bit...with mainly
chance coverage southeast West Virginia down along the I-77 corridor by
dawn...and only slight probability of precipitation out toward the Blue Ridge. Otherwise a
gradual increase in middle/high clouds early on...then mainly cloudy
western half during the early morning hours with more patchy fog
possible east where should stay clear longer. Nudged up low temperatures
a few degrees in spots per moisture under the cloud canopy which
supports some spots staying above 70 east and mostly 60s west.
Sunday...models in similar agreement on track of upper shortwave but
differ on eastward extent and amount of precipitation. The 12z NAM overall
looked to bullseyeish with fingers of higher quantitative precipitation forecast across the NC mountains
into SW Virginia...so leaned the forecast toward the 12z ECMWF/GFS...which
for the most part take the higher probability of precipitation across the northwest NC/SW Virginia/southern WV
area into the Shenandoah Valley tomorrow...with less threat of
showers and storms east of the Dan/lyh corridor. Temperatures will be
tempered by the cloud cover so looking at humid conditions with
highs ranging from the upper 70s-lower 80s mountains...to middle to
upper 80s east.
Short term /Sunday night through Tuesday night/...
as of 315 PM EDT Saturday...
Scattered showers and a few thunderstorms will persist through
Sunday night into Monday morning...with greatest rain chances found
west of Interstate 81...as an upper level disturbance shifts across
the Great Lakes from the Ohio River valley. With a very moist
atmosphere in place...may see a few areas of excessive rainfall...
but believe the flooding threat will be low due to a week of dry
May see a few showers redevelop Monday afternoon as we approach
maximum heating...but believe this activity will be more
disorganized due to drier conditions as high pressure moves in from
the south...as well as a lack of upper level support. Rainfall will
diminish quickly as the sun sets and should see dry...albeit warm
and muggy...conditions persist through Monday night.
Rain chances will increase again on Tuesday as another low pressure
system moving across eastern Canada will drag a cold front south of
the Great Lakes. Better chances for rain will be found across the
mountains...where stronger upper level support will be located.
Believe models are too aggressive in pushing the front into North
Carolina per the 30/12z runs...and expect the cold front will stall
further north due to the upper level ridging in place across the
southeast states...perhaps somewhere between the Interstate 64 and
Highway 460 corridors during Tuesday night.
Temperatures will remain slightly above normal for the early half of
the week...with highs ranging from the upper 70s/low 80s across the
mountains...to the low 90s across the Southside. Lows will fall into
the middle 60s to low 70s.
Long term /Wednesday through Saturday/...
as of 330 PM Saturday...
Upper level ridging will shift northward from the southeast states
over the middle Atlantic during the second half of the week...resulting
in both warmer temperatures and weaker winds aloft. This will make
for more stable conditions that will keep shower and thunderstorm
activity disorganized through the period. Any rainfall which
develops will be driven mainly by daytime heating...which will allow
this activity to fizzle after sunset.
Expect widely scattered showers and perhaps a thunderstorm or two to
develop Wednesday afternoon along the stalled frontal boundary...
wherever it is located. High pressure passing across the Great
Lakes should keep the northern half of our area dry.
High pressure will pass off the New England coast on Thursday...
resulting in a weak wedging pattern with easterly flow off the
Atlantic. This will likely be the driest day of the period.
Winds will shift southwesterly again on Friday as another cold front
sinks south from the Great Lakes. Stronger flow off the Atlantic
will result in increased moisture...and will expect spotty shower
and thunderstorm activity during the afternoon and evening.
Expect temperatures to remain slightly above normal for the period.
Aviation /01z Sunday through Thursday/...
as of 650 PM EDT Saturday...
Other than for very isolated rain showers/thunderstorms and rain along the Blue Ridge...most
of the area remains void of convection this evening with
widespread VFR in place under a scattered to broken cumulus field.
Expect this scenario to prevail through the evening with perhaps a
few more pop up rain showers/thunderstorms and rain until the boundary layer cools after
sunset. Otherwise main concerns overnight will be with bands of rain showers
heading north from the southeast states ahead of shortwave energy
ejecting by to the west late tonight...and degree of fog
coverage. Appears some of the convection to the SW could arrive
over the far west well after midnight and slide east toward the
Blue Ridge early Sunday before fading. Thus included a vcsh
mention over the southeast West Virginia locations around dawn and extended
east to kbcb early Sunday. Expect some fog again in the valleys
given overall lack of clouds early while models indicate potential
for low ceilings to redevelop over the south/southeast late as moisture
increases. Appears best shot at MVFR in fog out east at klyh/kdan
with only spotty/brief nature fog elsewhere with clouds arriving late.
Residual rain showers band likely to dissipate heading east Sunday morning
as the main area of lift takes shape off to the west with the main
shortwave axis that will be passing to the northwest Sunday evening. This
could allow any early MVFR ceilings to fade into more of a
scattered/broken VFR cumulus field with heating at all sites during
the afternoon before instability aided by western orographics
helps pop a few rain showers/thunderstorms and rain later Sunday. Dont see much of this
getting too far east of the Blue Ridge so only including a
prevailing lower visibility in rain showers at kblf/klwb with a thunderstorms in the vicinity mention.
Elsewhere think kbcb/kroa will be on the edge of the western convective
shield so added vcsh at kbcb and left kroa and points east dry for
Showers and storms likely to push east across much of the
area...at least on a scattered basis...Sunday evening with
periods of sub-VFR possible. This will be followed by more
widespread fog Sunday night into early Monday with pockets of
MVFR/IFR in fog/stratus likely.
Impulse passes by Monday leaving the region in between this
exiting system and the next upstream cold front. This should allow
for more in the way of widespread VFR on Monday after early fog
fades with only isolated convection likely during the afternoon
including spotty MVFR.
Another cold front is expected to produce similar conditions as it
approaches from the northwest Tuesday...then perhaps better coverage of
rain showers/thunderstorms and rain Wednesday with the front passing...and across the south
Thursday where the boundary looks to stall. However timing and
southward extent of the boundary quite low confidence at this
point. Thus appears mainly VFR for now Tuesday-Thursday outside of
afternoon/evening convection and typical early morning fog in the
valleys and where earlier heavy rain occurred.
as of 730 am EDT Friday ...
The visibility sensor at kdan...Danville Regional Airport...will
be out of service until further notice. A part has been back-ordered.