Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
324 PM EDT sun Aug 2 2015
high pressure will remain across the area tonight into Monday
keeping mainly dry weather in place. A cold front drops southeast
into the region by Tuesday before stalling out during midweek.
Low pressure will ride along the front bringing a better chance
for more widespread showers and storms Thursday into Friday.
Near term /through Monday/...
as of 310 PM EDT Sunday...
High pressure over the region will quiet weather to the region once
again tonight. Expected light winds...scant cloud cover...and low dew
points will make for good radiational cooling and a repeat of chilly
temperatures by daybreak Monday. Most guidance from the 12z run seems
too warm based on the lows that occurred under similar conditions lats
night...so will be using MOS guidance for tonight which is notably
cooler than other solutions. Expect upper 50s/lower 60s east of the
Blue Ridge with low/middle 50s west and some cooler readings in the
coldest valleys. Also expect some valley fog to develop west of the
Blue Ridge and become locally dense near the rivers by daybreak.
The region will be getting squeezed between systems on Monday as a cold
front approaches from the northwest and a trough/frontal boundary to our
southeast gets a push back in our direction as low pressure develops
over the Carolinas. Best chance for organized storms will be with
better dynamics outside of our area to the north and timing of front is
late so slight/low chance probability of precipitation for a late day thunderstorm across
southeast WV should be sufficient. Southwesterly flow east of the Blue Ridge
will allow for higher dewpoints to move in and help generate some
instability during the afternoon. While the overall environment features
weak dynamics...orographic forcing just east of the ridge combined
with convergence associated with the trough/frontal boundary will be
enough to justify more slight/low chance probability of precipitation for a thunderstorm to the
east. The end result will be dry early with the chance for a late
day thunderstorm mainly east of the Blue Ridge and across
southeast WV. It will also be warmer as temperatures rebound into
the lower 90s east with generally middle 80s west.
Short term /Monday night through Wednesday night/...
as of 230 PM EDT Sunday...
Lingering upper trough well to the north will swing another cold front
toward the region from the northwest Monday night with this boundary basically
jumping to the eastern Lee trough on Tuesday. Front likely to be
preceded by a broken band of convection just northwest of the County Warning Area early Monday
evening before likely splitting/weakening upon crossing the western
ridges into drier air during loss of heating. However with guidance
showing some leftover instability northwest counties and potential for momentum
to carry convection a bit farther to the southeast pending organization...including
some low chance probability of precipitation northwest third Tuesday night but dry elsewhere.
Guidance showing much of the area in a subsidence region Tuesday into
Tuesday night along/behind the front aided by deep westerly flow and
little focus. Cant totally rule out isolated rain showers/thunderstorms and rain far west/SW with
the low level convergent flow and higher Theta-E...as well as the far
eastern perimeter where will be closer to the better instability near
the Lee trough. This also where may be far enough removed from downsloping
and better mixing in the evening to pop a shower or two so running
with 20ish probability of precipitation in these locations. Otherwise partly/mostly sunny but quite
warm/hot via compression Tuesday with highs middle/upper 80s west and
low/middle 90s east.
Next rather strong shortwave will approach from the Midwest Wednesday
and attempt to lift the boundary back north through the region Wednesday
night as warm front as the wave passes to the northwest. Best instability
and deeper moisture will be over the far west later Wednesday with
more overrunning of the front possible Wednesday night pending
just how far west this wave ends up tracking. Latest models suggest
could be too far removed for much more than Middle Range chance probability of precipitation
far north-west from later Wednesday into Wednesday night with lack
of deep convection per only weak lift/instability. However for now
will leave in some chances across the region and see how later
runs evolve given such uncertainty. Should again be quite warm/muggy
Wednesday with the 850 mb boundary sliding back north allowing
more of a SW trajectory which should at least hold highs a category
or so below values seen on Tuesday.
Long term /Thursday through Sunday/...
as of 1245 PM EDT Sunday...
Transition to more zonal flow aloft will occur early in the period as
the region gets caught under the surface-850 mb baroclinic zone and
within a nearby frontal zone stretched back into the Midwest. A couple
of shortwaves riding along these features to aid convective chances...espcly
north/west Thursday into Thursday night as decent low pressure passes
just north...supportive of high chance/low likely probability of precipitation. Stronger
energy aloft dropping over the central states upper ridge will then
start to dig out another rather strong 500 mb trough to the northwest Friday...with
an associated surface wave helping to drive the next cold front
through the region by Friday evening. Earlier models agreed on quite
a bit of deep moisture being pulled north ahead of the front/wave
on Friday with good dynamics and jet features aloft. This would
support more widespread showers/storms Friday unless the front is
faster similar to the latest GFS which would trend the best
coverage Thursday into Thursday night. Thus have trended the
highest chance to likely probability of precipitation late Thursday into Thursday night
while keeping high chances Friday for consistency and line with
the current CMC and earlier European model (ecmwf). Will be a bit cooler but still
humid per more clouds/rain showers with highs mostly 80s Thursday...
lowering to 70s west to middle 80s east Friday.
Similar to the last week or two...appears another surge of much drier/cooler
air likely behind the front next weekend as yet another 500 mb trough
carves itself out across the east. Guidance even cooler aloft
with this system given prognosticated 850 mb temperatures as cool as +12c mountains
but may be too cool that far out. However still enough to drop
highs back in the 70s mountains and only low/middle 80s out east as
high pressure builds/wedges in from the north. Residual probability of precipitation a bit
tricky toward days 6/7 but should be drier with perhaps some
leftover upslope showers west Saturday and only isolated far
southwest on Sunday.
Aviation /19z Sunday through Friday/...
as of 150 PM EDT Sunday...
Good confidence in light winds and mostly VFR conditions through
this taf period.
High pressure will continue to bring VFR conditions to taf sites
through the first portion of the valid period...then expect some
radiational fog/stratus to develop after dark. Fog/stratus was not
widespread last night and believe the most sites will get by with
a period of MVFR toward daybreak...except at klwb where conditions
look to favor typical LIFR development by about 08z. There are
also some indications in the model soundings of an increase in
low level moisture from the southeast late tonight so will
introduce some scattered MVFR clouds with the tempo for fog.
Any fog/stratus should burn off after daybreak tomorrow followed
by scattered VFR cumulus. The area will then be getting squeezed by an
approaching front front the north and a low moving up through the
Carolinas and pushing a frontal boundary in our direction from the
southeast. Believe any precipitation development associated with
these features will hold off until after this valid period so will
keep tafs dry with no mention of a ceiling...through a VFR broken ceiling is
possible very late in the period.
Extended aviation forecast...
as of 1230 PM EDT Saturday...
Mostly VFR outside of any convection through the period. A front
will set up in the middle Atlantic which could bring a few rain showers/thunderstorms and rain
by Tuesday...with perhaps a bit more coverage later Wednesday at
this point. Thus the threat of sub-VFR remains low until around
midweek...except for the typical fog potential at klwb/kbcb each
morning between 09-13z.