Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
947 am EDT Tuesday Sep 16 2014
a weak cold front will pass across the region from the northwest
this morning before stalling over the Carolinas later today and
tonight. Weak low pressure along the front may bring a few showers
into tonight before a cool wedge of high pressure settles in from
the north early Wednesday. The high will strengthen to the north
of the region through the rest of the week keeping rather cool
but overall dry weather in place.
Near term /through tonight/...
as of 930 am EDT Tuesday...
For the morning update...main adjustment to the early morning
forecast package was to increase rain chances across the mountain
Empire and southeast West Virginia as Showers Pass across the
area. Radar indicates that these showers are light and are moving
quickly...so do anticipate more than just sprinkles with this
activity. Otherwise...early morning fog has burned off for most
locations and temperatures continue to warm quickly across the
Piedmont in the westerly downslope wind flow. Low and middle level
clouds continue to limit heating across the mountains...but expect
some breaks in the cloud cover as we head into the afternoon to
allow increased heating in the higher terrain as well.
As of 250 am EDT Tuesday...
Weak cold front to the northwest will slide into the mountains this morning
before crossing the Piedmont by afternoon. Moisture remains limited
with the boundary given most support heading off to the NE but still
expect a few rain showers bands across the western slopes early on per decent
west/northwest trajectory and seen via the latest hrrr. Thus keeping a low end
pop far far west this morning and mainly clouds elsewhere with some patchy
fog around early.
Models then show a weak wave developing along the front over the
Carolinas this afternoon in response to passing shortwave energy aloft.
Some solutions a bit more robust in developing showers mainly northwest NC
ridges early this afternoon...and then spreading east across southern Virginia and
the rest of northwest NC later. This provided enough heating to develop some
instability while keeping moisture deep enough under deep westerly flow
aloft. Since appears rain showers rather widely scattered in this
regime...lowered probability of precipitation some this afternoon but kept in low chances
mainly northwest NC and isolated farther north along the Blue Ridge where
guidance shows some weak convergence along 850 mb Theta-E ridging late.
Kept in a thunderstorms and rain mention as well under the passing wave aloft over the SW
and where the best forecast convective available potential energy of around 1k j/kg could occur. Otherwise
becoming partly sunny except in the western upslope areas where clouds
likely to hang in for most of the day. This will likely make for a
range in highs from 65-70 southeast West Virginia...to the middle/upper 70s Blue
Ridge...to low 80s southeast given downslope and more sun.
Weak high pressure to slide east from the Ohio Valley to the middle
Atlantic coast overnight allowing the low level flow to become more
easterly from NE to SW after midnight. This will tend to lock in any
residual moisture and gradually increase low level moisture within the
developing wedge along the Blue Ridge in Virginia. Latest NAM looks
overdone with showers lingering behind the exiting wave aloft with more
of a transition from isolated -shra to more drizzle likely late.
Therefore left in low probability of precipitation south/east during the evening and then
included pockets of mostly -ra Blue Ridge late with drizzle/fog
included. Otherwise more clouds overnight with lows rather chilly under the
developing onshore flow putting most in the 50s except closer to 60
Short term /Wednesday through Friday night/...
as of 400 am EDT Tuesday
A series of weak disturbances will traverse the region in a
prevailing northwest flow aloft. The first will have passed through
the region by early Wednesday...with another slated for the Thursday-Friday time
frame. Toward the end of the period into the beginning of the long
term period...an upper ridge is forecast to build northward from the
Gulf Coast states. The air mass will be quite stable through the
period...especially in light of an overwhelming dominance most
of the period of a maritime easterly flow will yield extensive
periods of low clouds...especially in upslope areas along the Blue
Ridge and the alleghanys. At times...primarily during the late
night/early morning hours...the lower atmosphere should become
saturated enough to support -dz in these same areas...roughly from
tnb-hlx-roa and vicinity. Moisture is shallow and as noted above
instability is near zero throughout the period...so have only
mentioned -dz at this point and no mention of -rw and especially no
mention of any -tsra. Have kept probability of precipitation at or below 15 percent
throughout the short term. Models depict very little variation in
moisture fields and resultant temperatures from day-to-day through
the period...but there are subtle differences in the low-level wind
field that will dictate the cloud cover and resultant temperatures.
At this point...Wednesday and Friday appear to be the coolest...but The Wedge
is weakest Wednesday afternoon between short waves and a small potential
for a brief period of light northwest flow Wednesday afternoon. By Friday...a strong
wedge will be developing across the region as a large 1032mb surface
high sets up across New England and the northern middle-Atlantic coast.
Have generally used a blend of model temperatures for maximum/mins through the
period...but did lean toward the cooler met MOS for Wednesday based on
recent similar events over the past week or so. Temperatures will be
slightly below to well below normal through the period...especially
Long term /Saturday through Monday/...
as of 400 am EDT Tuesday...
A strong wedge will be in place for Saturday and it appears it could
be strong enough to allow some drier air to filter into the region
from the northeast and push the low clouds toward the mrx/rnk County Warning Area
border...namely toward Smyth/Tazewell counties and the northwest NC
mountain counties. By sun...a strong short wave is approaching from
northwest with a continued deepening of the upper trough beyond
this. The 00z European model (ecmwf) has come into better agreement with the earlier
runs of the GFS. Showers appear to be a good bed for sun into Sun
night..but confidence in thunderstorms and rain at this point is low...especially
east of the Blue Ridge. With the event being a predominate northwest flow
event...the best chance should be west of the Alleghany front. A
progressive trend toward cooler weather should follow thereafter
into early next week with an overall winterlike pattern in
place...namely a deepening upper low centered of Hudson Bay and a
series of northwest flow short waves keeping a cold cyclonic flow in
place across the region. The earlier run of the GFS flirted with
sub zero 850mb temperatures early next week...but has now come into
better agreement with the 00z GFS holding them more in the +4c to
Aviation /14z Tuesday through Saturday/...
as of 700 am EDT Tuesday...
Cold front currently crossing the far western areas will move
southeast across the Blue Ridge this morning and exit the east
this afternoon. Most isolated light showers remain confined to the
upslope areas around kblf/klwb with a separation in low clouds
near the Blue Ridge per MVFR/IFR ceilings west of kbcb and VFR to
MVFR in fog/middle deck over the east. Given deep westerly flow
expect this trend to prevail early this morning with a few -shra
lingering far west while fog/stratus slowly diminish with mixing
Fog/stratus should fade this morning around 14z/10am...with broken
VFR ceilings taking shape. However lower MVFR ceilings likely to persist
kblf vicinity with ceilings perhaps remaining MVFR until early/middle
afternoon. Another round of showers also possible this afternoon
with heating but mainly south of the taf sites excluding kdan.
Thus will leave out mention although cant totally rule out
isolated coverage developing and moving east of the Blue Ridge
late this afternoon. Gusty west winds just off the surface this
morning will diminish by this afternoon while slowly veering more
northwest with surface speeds of 5-15 knots.
Wedge will start to redevelop behind the front later tonight with
low level moisture likely leading to widespread lower ceilings
including stratus by early Wednesday morning. Also more showers
possible espcly during the evening across southern sections as a
wave passes to the south. Otherwise should init as VFR early tonight
before deteriorating after midnight with low clouds and fog
spilling back west toward the Blue Ridge by daybreak Wednesday.
This could bring at least IFR conditions to klyh/kroa/kdan and
possibly kbcb late with MVFR/IFR as well in fog at klwb/kblf.
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 build south from the Great Lakes for middle week before wedging in
from off the middle Atlantic coast by 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 espcly Wednesday 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.