Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
825 PM EDT Wed Aug 31 2016
a weak trough moves across the area this evening, followed by a more
significant front Thursday, which will bring a change in airmass.
High pressure will then build in from the upper Midwest with cooler
weather conditions for Friday. This weekend may end up wetter than
expected with Tropical Storm Hermine tracking across the Carolinas.
Near term /through Thursday/...
as of 800 PM EDT Wednesday...
Thunderstorms continue, generally along the I-81 corridor in a
weak wind convergence/upper trough area, and further back to the
northwest along the approaching front. Have adjusted pops up
across this region and brought into the northwest counties sooner
to account for this trend.
In addition, latest NHC track brings some of the wind associated
with Hermine into the southeast part of the County Warning Area as the track has
shifted westward and more inland. As a result, ran the
tcmwindtool to have the winds better match akq/rah for Friday
night/Saturday morning. For now, this just brings a period of
about 6 hours of potentially 25-30kt winds into the southeast
corner of the cwa, mainly the Caswell/Halifax area.
T/TD grids were basically on track, so only minor adjustments
needed to bring into line with current observations.
As of 400 PM EDT Wednesday...
A weak prefrontal trough providing enough convergence over the
mountains to spark a few scattered showers and thunderstorm this
afternoon. Steering flow aloft is westerly and weak, therefore
showers along the Blue Ridge will slowly drift east over the
foothills then fade with sunset before reaching the Piedmont.
An upper level trough will dive to the south across the mid west and
central plain states tonight. The portion of the cold front moving
across the Ohio Valley will creep to the southeast overnight. This
front may be along western slopes of southeast WV/SW Virginia by sunrise
Thursday. The upper level trough will begin to pivot to the east
and over the Tennessee Valley Thursday morning, which will then
push the cold front across the area during the day Thursday. This
frontal passage will be slow to cross the region as a possible
surface low may develop over North Carolina Thursday morning.
Despite winds becoming westerly behind the front, we will maintain
high pops even though showers may wane some toward the Blue Ridge
with a second line developing in the vicinity of Route 29 eastward
during the afternoon. Instabilities are not very impressive except
over Southside where temperatures could warm into the lower 90s
under mostly clear skies through mid afternoon. Can not rule out a
strong thunderstorm or two late in the day, but the overall
picture for severe is very low.
Overnight lows will remain warm and muggy with lower 60s west to
near 70f over the Piedmont. Afternoon highs Thursday will greatly
depend on the timing of the front and associated showers. We have
fairly high confident with temperatures peaking in the mid to upper
80s east of the Blue Ridge. Our confidence is lower across the
mountains with rain entering the west through much of the morning.
Slower timing of the showers will obviously result in warmer
temperatures. For now, we have temperatures warming into the mid and
Short term /Thursday night through Saturday night/...
as of 400 PM EDT Wednesday...
Upper trof expected to swing southeast through the upper Great Lakes
and into New England Thursday before pushing off the New England
coast on Friday. The big change this forecast period is models are
now leaning toward a slower solution in the tropics which means
the upper trough may or may not pick up T.S. Hermine and progress
things along as fast as previously forecast. As such have trended
the forecast cloudier and wetter for Friday and Saturday,
especially east of the Blue Ridge.
Surface front is still progged to pass just south of the forecast
area Thursday night introducing cooler conditions Friday. However,
it now looks like T.S. Hermine will be slower, with models
trending the center of the system farther northwest with time as
it gains latitude. Please refer to the tropical prediction
center(www.NHC.NOAA.Gov) for the latest information.
With models now hinting the system may take more of an inland
track, this suggests, at the very least, more cloud cover across
the entire forecast area to start off the Holiday weekend in
addition to the threat for rain east of the Blue Ridge, highest
pops from Danville to the coast.
Temperatures will take a tumble with the frontal passage Thursday
night, and pending cloud cover Friday into Saturday, may remain
cool until the sun returns later in the weekend.
Long term /Sunday through Wednesday/...
as of 400 PM EDT Wednesday...
An upper level ridge of high pressure is forecast to build across
the eastern US early next week. Initially temperatures will start
off near normal Sunday and Monday, then inch upward a degree or
two each day with above normal readings anticipated by mid week.
Once the tropical low clears the region this weekend, anticipating
a dry forecast per building heights and subsidence. Forecast
confidence does remain low through Sunday however, due to model
spread with respect to speed and track of the tropical low.
Aviation /00z Thursday through Monday/...
as of 815 PM EDT Wednesday...
Two areas of ongoing showers/thunderstorms to contend with. The
first is along a weak wind convergence/pre-frontal trough
basically aligned along the I-81 corridor. The second is further
back to the northwest with the actual front. The first area should
diminish over the next few hours, but may persist long enough to
warrant thunderstorms in the vicinity at kroa/kbcb for a couple more hours. Frontal
activity will reach the northwest part of the County Warning Area toward daybreak,
likely in a weakened fashion. Showers should move into the
klwb/kblf area toward daybreak, then toward kroa/kbcb during the
late morning/early afternoon. At this point, enough instability
may be present with the system slowing its southward movement as
well to warrant inclusion of thunderstorms in the vicinity. Front will drift slowly
southeast through the afternoon warranting the mention of thunderstorms in the vicinity at
klyh/kdan as well toward 19z. Later tafs can better refine the
actual timing of thunderstorms and rain and specific inclusion via from/tempo/prob30
Fog development late tonight/early Thursday in question because of
potential for extensive mid/high clouds overnight. However,
current thinking is that this may not reach klwb/kbcb soon enough
to preclude the development of locally dense fog. Thus, it has
been added for these sites, with MVFR locally in the Piedmont.
Winds light and variable overnight, becoming northwest 5-8kts
behind the front Thursday.
Extended aviation discussion...
Much of the extended period is subject to the track and intensity
of T.S. Hermine, now in the eastern Gulf. Latest NHC track has
shifted this further west as have most 12z runs of the models
today. Hermine may also interact with the remnants of the current
front as it stalls just to our south late Thursday. This will
take the center remnant of Hermine toward krdu by Friday
night/Saturday morning, then toward korf Saturday afternoon.
Associated wind field may impact eastern parts of the County Warning Area Friday
night/Saturday with potentially gusts toward 30-35kts along with a
period of steady light to moderate rain. Hermine will move out of
the area by Sunday allowing conditions to improve to mostly VFR
during the extended periods.
It should be noted that there will be a sharp gradient between
rain/wind east of the Blue Ridge and much drier areas back to the
west with much less wind and no rain as Hermine tracks across
central/eastern NC and southeast Virginia. Further adjustments are
likely as draw closer to the time frame of land fall.