Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Blacksburg Virginia
427 am EDT Wednesday Sep 17 2014

a cool wedge of high pressure settles in from the north today
before weakening overnight into Thursday. Another larger area of
high pressure will strengthen to the north of the region Friday
into Saturday before passing offshore on Sunday. A cold front
will cross the area from the northwest on Monday bringing a better
chance of showers to start the week.

Near term /through tonight/...
as of 245 am EDT Wednesday...

Msas showing the cold front now well to the south with surface high
pressure starting to nose in from the north allowing an initial shot of
low level dry advection per current dewpoints. As the center of the
ridge passes north this morning...and to the middle Atlantic coast this
afternoon...will see better wedging take place early this morning and
likely persist at least Blue Ridge east into this evening. However
degree of stratus and drizzle still in question this morning given the
dry air and delayed setup of The Wedge under a rather high level strato-
cumulus canopy to init. This may tend to allow more breaks this afternoon
espcly if low clouds are slower. Thus cut back on clouds/drizzle
early similar to the drier GFS and trended with more clouds during
the morning.

Weak wave aloft should pass to the south today as well with
another nearing the western slopes this evening. May see some weak
instability develop west of the Blue Ridge this afternoon so
included an isolated rain showers mention I-77 into northwest NC and over the southeast
on the tail of the passing impulse. Elsewhere...keeping some mention in
for -ra Blue Ridge pending depth of The Wedge but may end up pc
far west and NE with mostly cloudy central/south but even there
some breaks possible later.

High temperatures remain tricky given timing of The Wedge/low clouds and
current spread in MOS guidance. Since already running warmer and have
potential for more sun in spots...bumped up going values more in line
with the European model (ecmwf) 2m temperatures which gets some locations along The Wedge
perimeter above 70 with mostly 60s within the cad.

Weak wedge lingers espcly east overnight with added clouds west ahead
of another shortwave diving southeast in the northwest flow aloft. However setup not
totally ideal for good wedging given weakening of the middle
Atlantic surface high and more north/northwest flow just above the
boundary layer moist layer. This along with plenty of dry air
aloft supports little more than clouds/patchy drizzle eastern
slopes late along with some fog in spots so trimmed probability of precipitation. Lows
mainly 50s under the low level cool pool with some 40s possible
valleys espcly if more clearing develops.

Short term /Thursday through Saturday night/...
as of 400 am EDT Wednesday...

A weak short wave will move through the region early Thursday. The track
will take the best lift/moisture from eastern Kentucky/eastern Tennessee toward
far SW Virginia/northwest NC. At this point only expecting some middle clouds and
possibly a passing sprinkle...but not enough support to include any
-shra in the grids at this point. Following this trough...the broad
upper low anchored over Hudson Bay will deepen briefly. A strong
Canadian surface high will translate eastward toward New England by
early Friday and into a very favorable wedge position. As the surface high
strengthens toward 1030mb...deepening easterly flow will spread down
the east side of the Appalachians into the southeast states.
GFS/ECMWF/NAM all in good agreement on upslope winds along the Blue
Ridge/Alleghany front likely squeezing out -dz Friday morning...which
translates further west-SW by afternoon toward the Virginia/TN/NC border
region. Abundant low clouds...especially Blue Ridge westward...will
hold temperatures below normal. By Sat...the flow quickly becomes more
zonal as heights rise in response to a broad upper ridge building
across the south central/southeast states. Meanwhile...another short
wave digs southward through the Great Lakes with the associated
frontal system reaching the region by sun. Thus...with The Wedge
eroding quickly Sat and SW flow developing in advance of the next
short wave...we should see warmer temperatures and more sunshine
than will be the case Friday.

For temperatures...less impressed with the potential wedge setup for
Thursday...if any. The first surface high is very weak and dissipates as
overall northwest flow aloft prevails. Have removed mention of drizzle for
early Thursday and also warmed temperatures somewhat Thursday in anticipation
of returning SW surface flow...or at least southerly flow...and likely
some sunshine by afternoon with the dissipating weak wedge.
Adjustment to MOS guidance was needed for maximum temperatures all three days.
For maximum temperatures Thursday...have used a compromise between the warmest mav
MOS and the coldest met MOS...thus just a tad warmer than the
European model (ecmwf) MOS. For Friday...however...have undercut all model
guidance...especially the mav MOS which is way to warm and in
disagreement with the cloud field and -dz/-ra it generates.
Potentially...some areas Friday may not get out of the middle to upper 60s
west of the Blue Ridge. By Sat...southwest flow...more
sunshine...and notably higher heights/temperatures aloft...585dm at
500mb...should support low 80s across the Piedmont to the lower 70s
in the mountains. Again...guidance does not have a good handle on
this and is too cool.

Long term /Sunday through Tuesday/...
as of 400 am EDT Wednesday...

Upper trough will move through the region Sunday...accompanied by a
chance of showers. With prevailing northwest flow...the best chance will be
west of the Alleghany front across eastern WV. Latest models forecast
enough instability to consider thunder...also given current expected
timing of the convection reaching the County Warning Area late afternoon/early
evening. However...our only neighbor to mention thunder at this
point is in east will leave out at this point. At any
rate...this would appear to be one of the more typical events we
often see as the system crosses the mountains where most
precipitation wanes rather quickly after it passes east of the
Blue Ridge.

The main story behind the upper trough Saturday will be a deepening
trough across eastern Canada and thus drastically lower
heights/temperatures aloft across the eastern U.S. This will
dominate the region through the first half of next week as it slowly
translates toward the Canadian Maritimes. A reinforcing short wave
will drive what appears to be one of the coolest air masses of the
fall season so far through the region Mon-Tue. 850mb temperatures drop into
the +2c to +6c range by Tuesday with min temperatures potentially into the
30s in the mountains and 40s elsewhere by middle-week next week. The
GFS tends to be slightly colder than the European model (ecmwf). Precipitation beyond
Sat appears to be pretty scant...but likely extensive upslope clouds
across the alleghanys with northwest upslope flow and strong cold air advection. Still too
early to mention any -shra or potentially even -shsn at the highest
elevations with the cold air mass middle part of next week...especially
since the best upper support remains north of the County Warning Area with the
secondary reinforcing upper trough. Thus...have not made any
significant adjustments to the long term grids at this time.

Aviation /08z Wednesday through Sunday/...
as of 1255 am EDT Wednesday...

Mainly VFR ceilings in the wake of the passing wave/front to the southeast
will slowly lower overnight per increasing NE flow under the
developing wedge. This should bring ceilings down to MVFR across the
east in the next few hours with periodic IFR over the west where
moisture remains trapped at kblf/klwb. Low confidence in how fast
stratus/fog and/or drizzle develop overnight but appears that
locations along the Blue Ridge including kbcb/kroa will see ceilings
and perhaps visibilities drop right before daybreak...with similar trends
also possible at klyh/kdan. However think current higher cloud
bases will delay the overall fog formation with this occurring as
ceilings lower by dawn. Thus continued leaning toward more IFR/MVFR
late tonight with some fog...though think fog will be less than
the lower ceilings.

Some degree of low level moisture will remain stuck across the
region Wednesday morning and just how fast this erodes tricky
pending just how deep the low clouds get overnight. Latest
guidance remains spread from eroding much of the low ceilings per the
GFS to at least MVFR holding for much of the day espcly Blue
Ridge and points east like the NAM/CMC. For now given the
potential for ceilings to hang longer within The Wedge...trended
toward holding MVFR in longer espcly kroa and points east with
return to VFR southeast West Virginia sites by afternoon and at kbcb around
18z/2pm. Any improvement in the east likely to be short as The
Wedge flops back after dark with possible MVFR ceilings or lower
redeveloping during Wednesday evening.

Extended aviation discussion...

Broad upper trough remains in place across the eastern states for
the remainder of the week allowing a large area of high pressure
to remain wedged across the middle Atlantic through the weekend.

This should push the pesky deep moisture further south away from
the region. However bigger concern will be with trapped residual
low level moisture that may keep southern/western locations stuck
in periods of MVFR ceilings into Friday as the low level east/southeast flow
takes shape. High pressure may be strong enough to push the low
level moisture to the southwest Saturday and with heating/mixing
could see things return to overall VFR for early in the
weekend...but not until Saturday afternoon at this point. Flow
turns more SW by Sunday ahead of the next front allowing for
widespread VFR with any western showers likely not arriving until


Rnk watches/warnings/advisories...


near term...jh
short term...rab
long term...ams/rab

National Weather Service Glossary of Abbreviations