Scientific Forecaster Discussion
fxus61 krnk 270746
Area forecast discussion
National Weather Service Blacksburg Virginia
346 am EDT Thu Jul 27 2017
a cold front will approach from the northwest today into Friday
sliding across our area Friday afternoon into Friday night.
Upper system should keep it cooler with a few showers and storms
around Saturday before it dries out Sunday into early next week.
Near term /through tonight/...
as of 250 am EDT Thursday...
Stratus not as prevalent as last night but is out there across the
NC mountains into portions of southwest Virginia. Mostly have high
clouds streaming in from the northeast in association with
convection moving across Illinois into Indiana.
Latest hi-res models are painting a smattering of showers this
morning mainly east of the Blue Ridge. However, area radars show
little development and will limit any isolated shower threat toward
the NC mountains where better low level convergence resides.
For today, will be watching upstream convection and how it evolves
to the east/southeast along with outflow interactions to generate
storms over our area. Models are in agreement that the mid-Atlantic
into the central Appalachians will see a decent coverage of storms,
but where it lines up for likely to categorical pops vs low chance
pops is iffy. Confidence is high based on upstream shortwave and
model initialization of it over Indiana, that best lift and dynamics
should push into WV by 18z, then move east into the Virginia
Piedmont by early evening. Leaning pops this morning toward a blend
of the hi-res arw, GFS and ecm, then blend of the GFS/ecm thereafter
which paints the mountains with high chance to likely pops, with 40
pops later in the east, but would not be surprised if this gets
bumped up with later model runs/forecasts.
The issue today will be severe wx threat from said storms. Storm Prediction Center keeps
marginal risk over the entire mid-Atlantic and central/southern
Appalachians. Per model solutions, the better upper forcing and
shear will move along and north of our northern forecast area
(lewisburg-buckingham), while the southern area gets more heating
and steeper low level lapse rates. Given we are already getting some
high clouds, issue remains cloud cover and destabilization. We have
the higher dewpoints to juice the low levels. At the moment it looks
like a day where storms could fire ahead of the main shortwave along
outflow with more organized convection moving into the Shenandoah
Valley to northern WV area this afternoon.
Should be a lull in convection this evening behind the main cluster
of storms, while more forms upstream across the Ohio Valley. There
is a potential per GFS for some clustering of storms in the evening
moving out of southeast Kentucky into the NC mountains/mountain Empire,
but will not try to get too specific beyond today. After midnight,
expect 1-2 areas of concentration, mainly NC south and southeast of
our forecast area, and the other moving from southern Ohio into
northern WV/VA/MD. Will have to see how the mid-upper flow evolves
as more complex area of storms can create density and shift
southward toward better instability. Needless to say will maintain
high chance to likely pops especially after midnight-toward morning.
Severe threat overnight should be limited with loss of heating,
though cannot rule out some rogue strong to severe thunderstorm
Another potential issue will be Hydro related, but given dry
antecedent conditions will have low confidence at the moment for
flooding, as will have to see how storms line up. The Hydro issue
may come more into Friday if the front slows down.
With more clouds around today, highs will be close to normal with
upper 70s to lower 80s mountains, to mid to upper 80s
foothills/Piedmont. You will be feeling the humidity though with
dewpoints in the upper 60s to lower 70s.
Lows tonight stay elevated due to storms/clouds and high dewpoints,
ranging from the mid to upper 60s west, to lower 70s east.
Short term /Friday through Saturday/...
as of 330 am EDT Thursday...
A significant closed low will develop over the mid Atlantic region and
linger through the weekend fueled by vigorous short wave energy moving
out of the Great Lakes. This will drive development of a surface low
over the mid Mississippi Valley that will slide along a stalled frontal
boundary draped across the region before reaching the coast as a well
developed storm system. Indications are that the energetic dynamic
environment will combine with ample instability to drive deep
convection, and may be sufficient with marginal shear profiles to
produce some strong to severe storms especially heading further south
and east. However, there is much uncertainty in exact timing/placement
of features so the severe threat for Friday remains a bit ambiguous. So
while confidence is decent for high pops, will have to wait for later
model runs to converge on a solution before the nature of our severe
threat can be better defined. Antecedent dry conditions will help
mitigate potential Hydro issues from heavy rainfall but will fine tune
this thinking based on extent of activity Thursday/Thursday
With the stacked low spinning over the mid Atlantic, wrap around
moisture will keep a chance of showers/possibly some thunder in the
forecast into Saturday night before the system starts to lose influence
over our region by Sunday. Northwesterly flow on the backside of the
low this weekend will make for breezy and cool conditions with
temperatures 5 to 10 degrees below normal.
Long term /Saturday night through Wednesday/...
as of 330 am EDT Thursday...
An upper level trough will sit over a dry surface high through the
middle of next week with no rain is in the forecast except for a slight
chance of a shower/thunderstorm from the Grayson Highlands through the
mountains of North Carolina. Expect a good amount of diurnally driven
cumulus clouds forming in the afternoon, then fading in the evening.
Temperatures will moderate back to normal by midweek. Dew points will
remain low (mid 50s to lower 60s) through Tuesday, then edge up towards
being muggy again on Wednesday.
Aviation /08z Thursday through Monday/...
as of 131 am EDT Thursday...
Stratus/stratocu sticking around along and either side of the
Blue Ridge into the alleghanys. However, most cigs are VFR and
should stay that way except tempo MVFR with fog at bcb/lwb this
morning before 13z.
Confidence is low on how long cigs last, but think they should
be VFR after 12z, with mainly mid and high clouds streaming in
ahead convection upstream over the Ohio Valley.
Which is the main issue later today. Models are all in agreement
that most of the area will see clusters of storms, question is
who gets hit and who doesn't. Confidence moderate enough to
have tempo thunder at all sites this afternoon, earlier at
blf/lwb and later across the Piedmont. Mainly looking at VFR
with tempo MVFR/IFR with heavy rain and thunder. Not going to
put gusty winds in the tafs at this point, but some of the
storms will be bringing some stronger wind gusts.
Low confidence after Thursday afternoon, as model consensus
takes heavier convection east, but another batch upstream may
arrive by 06z. For now will keep thunderstorms in the vicinity at most sites and VFR.
Aviation extended discussion...
Should start to see a front approaching with better threat of
showers/thunderstorms late Thursday night into Friday. Stronger
storms and heavy rain possible in and around the mid-
Atlantic/Ohio Valley region into the Carolinas this time frame
so be prepared for sub-VFR at times. Outside of storms expect
Saturday looks drier with northwest flow, but could see
lingering low end VFR/high end MVFR ceilings in southeast WV along with
a few added shra/tsra mainly southern/eastern sections Saturday
afternoon. Expect widespread VFR to return Sunday/Monday under
high pressure following the front.