Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
409 PM EDT Tuesday Jul 22 2014
upper trough over the Gulf Coast states shifts west through
Wednesday leaving a residual weak boundary and low level moisture
over the area into middle week. A cold front will be arriving
Thursday from the northwest...then exiting by Thursday night. High
pressure builds in on Friday.
Near term /through Wednesday/...
as of 205 PM EDT Tuesday...
Weak surface ridge across the region this afternoon combining with
limited instability and lack of forcing to keep overall convective
coverage at a minimum at this time. Best low level Theta-E also showing up
just south of the region which could still interact with residual
outflow to produce a bit better coverage of mainly rain showers across the
south/southeast into this evening. Elsewhere appears any late day coverage
will be quite isolated and perhaps only over the far northwest on the
edge of the current cloud shield where more heating has occurred.
Thus maintaining the highest probability of precipitation far south with middle to low
chances elsewhere into early evening.
Upper low well to the SW will continue to shift west overnight
leaving the region in a moist environment but with lack of much
support for added rain showers. In addition the low level wedge will be
breaking down as the surface high shifts across and to the south
allowing the flow to turn light westerly after midnight. This
should limit much -dz potential with only some weak leftover
convergence western slopes late tonight. Appears may be able to go
dry after this evening but with deeper moisture still over the SW
left in a token pop there overnight. Lows again warm/muggy with
most in the 65-72 range given high dewpoints and patchy fog
Next strong 500 mb trough and associated cold front will be approaching
on Wednesday as high pressure shifts farther south allowing deepening
westerly flow and a subsequent Lee trough well east of the Blue
Ridge. Models spark most afternoon convection along the southern
Blue Ridge given the weak boundary layer SW flow and better
differential heating but still iffy given westerly flow aloft and
only faint lift. Elsewhere appears rather isolated despite lingering
moisture given dry air aloft and downsloping west winds across
the east so keeping shotgun chance probability of precipitation. However the upstream pre-
frontal band of thunderstorms and rain will likely be nearing the northwest slopes late.
Thus kept in low likelys extreme west...but mostly toward sunset
at this point. Some isolated severe possible if pulse storms
organize more early...and upstream activity moves into the west in
more of a line feature...although this may be after instability is
starting to fade. Otherwise should see clouds break quicker under the
westerly flow allowing insolation and 850 mb temperatures around +20c to aid
weak compression in pushing highs into the 80s west and 90-95
Short term /Wednesday night through Friday night/...
as of 345 PM EDT Tuesday...
Wednesday night...prefrontal mesoscale convective system likely to impact at least the
western half of the County Warning Area...activity fading with loss of daytime
heating. Remnant outflow may reach Piedmont by daybreak Thursday
and debris cloudiness will likely remain to start the day
Thursday. Models are in agreement the actual cold front will
enter our southeast West Virginia counties late morning/early
afternoon on Thursday...and continue east through the day. At this time
think abundant cloudiness early Thursday may inhibit solar
insolation...limiting instability over all but the far
southeastern County Warning Area...the greatest chance for showers/storms occurring
from our Piedmont eastward into the Tidewater of VA/NC.
For now will maintain high probability of precipitation both Wednesday night and
Thursday...then end the precipitation threat quickly from west to east
with the passage of the front...leaving mainly dry conditions
for the area by Friday.
Weak high pressure will build into the area behind the
front...and should provide a day...maybe two...of precipitation free
Temperatures Thursday will be tempered by clouds associated with
the front...then trend down a few degrees for Friday. The airmass
change is not overly dry nor cool...so when the sun probability of precipitation out
Friday...readings may feel just as warm as Thursday...although the
dewpoints will not be as robust.
Long term /Saturday through Tuesday/...
as of 400 PM EDT Tuesday...
For the weekend and into early next week...the area remains
vulnerable to northwest flow disturbances tracking around the large
western/central Continental U.S. Upper ridge. A particularly strong
disturbance...noted by all the models...promises to amplify the
eastern long wave trough...a feature that should keep
conditions unsettled with near to slightly below normal
temperatures while the extreme heat remains well to our west and
well away from our part of the country.
The best chance for showers/storms looks to be Sunday as the
surface front associated with the strong disturbance crosses the
forecast area from the northwest. Potential exists for storms to
be organized with upstream mesoscale convective system impacting the forecast area.
Generally followed HPC guidance through the period. Trended probability of precipitation
up for sundays frontal passage...then down for a day or two after sundays
frontal passage. Tendency for frontal passages is to bring some
downtime as with respect to probability of precipitation before moisture and instability
Aviation /20z Tuesday through Sunday/...
as of 1230 PM EDT Tuesday...
Heating of low level moisture has resulted in an expansive cumulus
field across the region this afternoon with bases expected to rise
to mostly VFR levels in the next couple of hours. However spotty
convection developing close to kdan may keep them more in the high
end MVFR level range as well as klyh where periods of MVFR likely.
Some added rain showers or thunderstorms and rain may pop along the Blue Ridge by middle or
late afternoon so leaving in a vcsh/thunderstorms in the vicinity mention mainly where
already ongoing which excludes kblf/klyh for now. Otherwise looking at
overall cloud bases of 3-5k feet this afternoon/early evening with
southeast winds less than 10 kts.
Most convection weakens/subsides shortly after sunset with only
isolated rain showers/thunderstorms and rain perhaps lingering mountains before midnight. Fog
formation likely late evening into overnight but given less of a
low level wedge and light west winds not expecting the overall
more widespread IFR/LIFR conditions seen the last few nights.
Think the valley locations such as kbcb and klwb have the best
chance of seeing IFR/LIFR conditions with IFR to MVFR at klyh/kdan
by early Wednesday morning.
Low level flow turns more westerly ahead of an upstream cold front
on Wednesday which should allow low clouds/fog to scour out a bit
sooner Wednesday morning. Thus expecting a return to VFR at all
locations by middle or late morning Wednesday. However heating will
again lead to added rain showers/thunderstorms and rain development espcly along the
southern Blue Ridge during the afternoon and across the southeast West Virginia
sites by early evening. This may result in increasing periodic
MVFR conditions over the mountains but mostly VFR out east.
Extended aviation discussion...
Next northern stream upper trough and its associated cold front
will approach Wednesday night and work through the region on
Thursday. This will bring the best chances of more widespread
showers to the mountains and perhaps some deeper convection east
of the Blue Ridge pending timing of the boundary Thursday
afternoon. Airmass will dry out behind the front going into Friday-
Saturday. Overall expect sub VFR at times Thursday with rain showers/thunderstorms and rain
and maybe some fog Thursday night...followed by VFR Friday- Sat outside of
any valley fog at klwb/kbcb each morning.
as of 245 PM EDT Tuesday...
Kfcx radar remains down. However technicians are currently working
on the problem and we may see a return to service later this