Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Blacksburg Virginia
333 PM EDT Sat Jul 22 2017
a cold front will cross overhead during tonight into Sunday,
which should spark showers and thunderstorms. This front should
pass south of our region by Monday. High pressure will arrive
to bring slightly cooler conditions by the middle of the week.
Near term /through Sunday/...
as of 330 PM EDT Saturday...
Scattered showers and thunderstorms have developed along the
southern Blue Ridge, while the vast majority of the convection
along the cold front to our north has remained well north of
the County Warning Area. Storm Prediction Center has a marginal risk of severe weather south of I-64
and a slight risk to the north. The latest high resolution
models only clip areas north of I-64 later this evening. The
heat and humidity has been impressive so far in the Piedmont
with heat indices crossing into the 100-105 degree range at this
time. Highs were bumped upward, especially in the Roanoke
valley as roa is going for it in terms of a 100-degree high.
Shower and thunderstorm coverage should remain scattered through
the night as the cold front pushes southward over the County Warning Area. High
resolution models do hint at another round of convection
arriving west of I-77 by early Sunday morning. Therefore,
chance pops were kept throughout most of the County Warning Area tonight. Cloud
cover from upstream convection could limit high temperatures
from climbing as high by Sunday afternoon compared to Saturday
afternoon. Still, it will be quite hot and humid, and the cold
front may reach the Virginia/NC border by the afternoon. This
situation should allow enough instability and lift for more
scattered showers and thunderstorms to develop in this area
toward the evening.
Short term /Sunday night through Monday night/...
as of 230 PM EDT Saturday...
For Sunday evening, weather forecast models are hinting at a line of
thunderstorms entering our far western Virginia/northern North
Carolina counties during late evening as an upper level disturbance
passes overhead. Believe any storms would diminish in intensity
quickly as they cross the ridges and move into downslope flow. Have
introduced a period of higher rain chances across the west for the
possibility of this system, however admit that confidence of the
occurrence/timing/location of any organized line of storms moving in
from the west is marginal. The coming overnight forecast shift will
have the benefit of an updated set of models to have a look at and
will make adjustments to these rain chances accordingly. Outside of
these storms, convection is expected to diminish after sunset with
the loss of daytime heating.
Widely scattered showers and thunderstorms can be expected for
Monday despite a cold front approaching from the northwest. Slightly
drier air and stronger northwest wind flow will help to inhibit
convective development, though those storms which do develop may
pulse up to strong/severe intensity for brief periods of time,
capable of producing localized damaging winds. The cold front will
enter the southeast West Virginia counties during late Monday
evening, and gradually loose momentum as it continue south across
Long term /Tuesday through Saturday/...
as of 1230 PM EDT Saturday...
Looking to Tuesday, long-range weather models are indicating that
the cold front may not pass completely across our forecast region,
possibly stalling near the Virginia-North Carolina border as high
pressure shifts from the Great Lakes toward New England. Will hold
close to the previous forecast of a higher chance of spotty
afternoon showers/storms south of Highway 460...closer to the front.
Passage of high pressure off the coast on Wednesday will bring
cooler easterly windflow to the mid-Atlantic, resulting in likely
the coolest afternoon temperatures of the week. Upslope wind flow
will make for the best chances of shower/thunderstorm activity
across the mountains.
Winds will shift more southerly by Thursday, resulting in a return
of warmer temperatures and higher humidity, and greater coverage of
afternoon showers/storms, more so across the mountains. A cold front
will slowly make its way across the mid Atlantic on Friday and
Saturday, again resulting in scattered showers and thunderstorms
both days, a few of which may become severe for brief periods of
Aviation /19z Saturday through Thursday/...
as of 210 PM EDT Saturday...
Scattered showers and thunderstorms will continue to develop
over the remainder of the afternoon. The most organized
convection should stay to the north, but the chances are good
enough to keep thunderstorms in the vicinity for all taf sites except Dan. Confidence is
not high enough yet to mention thunder more strongly, but
amendments will be made if needed through the afternoon.
Otherwise, VFR conditions should persist outside of the showers
and thunderstorms. There is a threat for severe weather, so
turbulence will be an issue this afternoon.
Another cluster of showers and thunderstorms could arrive
overnight toward early Sunday morning near blf and lwb, but
confidence is too low to mention in the tafs at this point. High
clouds from convective debris upstream and a breeze through the
night in the mountains will not permit fog to form. Scattered
low clouds were kept for blf and lwb due to the potential for
MVFR ceilings. Ceilings and visibilities should be VFR during
the day with winds starting to gust from the west just past 18z
on Sunday afternoon.
Aviation extended discussion...
Confidence is growing for the potential of a rather active
period for Sunday afternoon and Sunday night. A cold front will
be passing overhead, which should spark clusters of showers and
thunderstorms. The models shift the front toward North Carolina
by Monday with the potential for convection developing in the
afternoon along the Virginia/NC border. However, most locations should
remain VFR outside of the convection. Another cold front could
push some of the moisture eastward by Tuesday, but high pressure
may possibly wedge southward on Wednesday to create the
possibility for MVFR/IFR ceilings. Conditions should return to
VFR by Thursday as the weak wedge erodes quickly.
Virginia...heat advisory until 7 PM EDT this evening for vaz044>047-058-
NC...heat advisory until 7 PM EDT this evening for ncz005-006.