Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Blacksburg Virginia
1251 am EDT Thu Mar 30 2017
high pressure will continue to wedge south along the eastern
slopes of the Appalachians today before passing offshore this
evening. Low pressure will pass to the northwest of the area
Thursday night into Friday with a trailing cold front crossing
the region by Friday night. High pressure follows this system
with mainly dry weather expected over the weekend.
Near term /through today/...
as of 645 PM EDT Wednesday...
No major changes are in store for the early evening update. The
one change reflects less cloud cover across the area currently,
and through the next few hours, as compared to the previous
forecast. We still anticipate a return of clouds, and some
light upslope patchy light rain, along and near the crest of the
Blue Ridge, but with an onset now a little later than
previously advertised. Have also adjusted hourly temperatures,
dew points, and winds based upon the latest observations and
expected trends through the evening hours.
As of 310 PM EDT Wednesday...
High pressure has begun building into the region this afternoon
from the north to start setting up a wedge. Easterly flow around
the high has allowed clouds to develop for areas in the
Piedmont. The mountains will also begin to see some cloud cover
in the evening to overnight hours ahead of a low pressure system
moving in from the west. Temps tonight should remain relatively
mild due to this cloud cover. Virga, eventually turning to rain
falling into the dry lower layers should cause the cloud deck
and temperatures to fall. High temps tomorrow should be much
cooler than today, much closer to normal. Trended toward the
higher end of guidance for the low temps tonight, and the lower
end for highs Thursday. Expect precip to begin mainly along the
Blue Ridge through North Carolina, and become more widespread
over the course of the day Thursday as the cold front
Short term /tonight through Saturday/...
as of 230 PM EDT Wednesday...
Onset of more significant rainfall will start Thursday night as deep
southeast flow transitions to a more south/southwest trajectory in
advance of a warm front approaching from the southwest. This aided by
initial upslope flow ahead of an axis of deeper moisture along a lead
shortwave ejecting east from the parent Midwest upper system. Models
appear a bit fast although increasing support along with strong
diffluence above a sharp 850 mb Theta-E gradient would suggest a quick
uptick in pops to likely/categorical by daybreak Friday. Guidance even
showing some weak instability late Thursday night over the southwest
near the approach of the triple point to the west so added a thunder
mention there. Appears south/southeast winds again quite gusty across
the highest elevations but below advisory levels given deep saturation
and limited mixing below. Lows mostly in the 40s to lower 50s as could
even rise some late as warmer air aloft arrives.
Main period of more widespread heavier showers likely Friday morning
into early afternoon as the upper low to the west swings a front aloft
across the region. This should allow some deeper warm air to the south
to spill north with a few hours of moderate to perhaps heavy rain
decreasing during the afternoon as a mid level dry slot arrives. Likely
to be some embedded thunder as well espcly closer to the warm
front/triple point lifting across the south and west. This also
supported by a strongly sheared environment while severe potential
possibly limited via rainfall and some degree of wedging pending just
how fast the cool pool lifts out Friday. Expect added convection Friday
afternoon mainly east where more heating possible if can get some
breaks behind the axis of deep moisture, and ahead of the actual
upstream cold front as cooling aloft takes place. Otherwise upwards of
an inch of rain or more possible espcly Blue Ridge with high pops
tapering from the southwest by Friday evening. Highs Friday all
dependent on when the heavier rainfall exits, as for now staying on the
cooler side of MOS, per the initial wedge and lingering clouds/showers
until late that should hold most in the 60s.
Wave departs offshore Friday night into Saturday leaving decent
northwest upslope flow in place as Post frontal cool advection kicks
in. This should bring an end to showers across the far east by Friday
evening with pops mainly confined to the northwest ridges espcly Friday
night into Saturday morning where high end chance to low likelys on
tap. Elsewhere downslope drying under a gusty northwest breeze to bring
clearing skies later Friday night and a cool but mainly sunny day
Saturday under slowly diminishing winds. Lows Friday night mostly in
the 40s under steady cooling aloft, then 50s/around 60 mountains
Saturday to some downslope aided low 70s southeast.
Long term /Saturday night through Wednesday/...
as of 200 PM EDT Wednesday...
Shortwave ridging will take hold to start the period as surface high
pressure builds in from the northwest making for a dry/pleasant end of
the weekend. This will basically be between systems as the surface
ridge ends up wedging south along the mid-Atlantic coast by Sunday
night. Similar to past events, expect south/southeast flow ahead of the
next vigorous southern stream upper low heading out of the Southern
Plains to start generating low clouds later Sunday night, with spotty
light rain or drizzle possible southern Blue Ridge late. Lift will then
steadily increase along with stronger overrunning late Monday into
early Tuesday as the main wave passes to the west and a warm front
pushes north into southern sections Tuesday morning. This will bring
another round of at least light/moderate rain espcly Monday afternoon
into early Tuesday before the warm front passes north per latest wetter
European model (ecmwf). Convective threat again focused along the retreating warm front
and perhaps within the warm sector Tuesday pending timing and
subsequent heating ahead of the trailing cold front Tuesday afternoon.
System exits Tuesday night into Wednesday with shortwave ridging
returning, allowing for a period of drier weather for middle of next
week. High temps to stay above normal, mainly 60s mountains to low 70
east, except cooler within The Wedge Monday when some 50s could occur.
Lows to start out on the cool side, mostly 40s, before rebounding under
clouds/warm advection to mainly 50s into midweek.
Aviation /06z Thursday through Monday/...
as of 1245 am EDT Thursday...
Cloud cover blossoming across the region espcly east of the Blue
Ridge late tonight and expect the canopy to become more
widespread through early Thursday morning. Confidence is high
that cloud bases will continue to lower as low level moisture
increases under deepening southeast flow. Areas closest to the
Blue Ridge can expect ceilings to drop into the MVFR or even IFR
range by early Friday, with low end VFR to ocnl MVFR common
farther east across the Piedmont, and farther west into parts of
West Virginia. Patchy light rain or drizzle will also develop
in the preferred upslope regions with visibilities VFR to MVFR
through the forecast period within drizzle/fog today. Any sub-
VFR ceilings will likely remain at those levels through the
remainder of the latest taf forecast period. South to southeast
winds to also increase along the western ridges with gusts
topping 25 kts possible at kblf this afternoon/evening.
Confidence in the general trend through the forecast period is
moderate to high. Confidence specific boundaries between VFR and
MVFR ceilings low to moderate.
A complex storm system approaches Thursday night with a warm
front lifting into the area by early Friday ahead of a passing
cold front Friday afternoon. Widespread showers and low cigs
likely to result in a period of sub-VFR continuing from Thursday
night into much of Friday before some improvement possibly
occurs with the cold frontal passage later Friday. Best rain
chances appear later Thursday night into Friday morning when
could see embedded thunderstorms as well as MVFR to IFR vsbys.
With the frontal passage by Friday evening, MVFR and IFR
conditions look to persist over the western mountains along with
gusty northwest winds.
Extended aviation discussion...
High pressure should bring improving conditions during Saturday
although appears sub-VFR cigs along with a few residual showers
may linger at kblf/klwb into Saturday afternoon. Otherwise will
see VFR return later Saturday through Sunday under diminishing
winds as high pressure slides across. Next similar low pressure
system to that seen at the end of this week will approach by
Monday with sub-VFR conditions likely returning Sunday night
into Monday as rain again transitions to showers and storms to
start the week.