Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus61 krnk 200112 

Area forecast discussion
National Weather Service Blacksburg Virginia
912 PM EDT Wed Jun 19 2019

a stationary front will remain draped over the mid Atlantic
through tonight into Thursday. By Thursday afternoon, a cold
front will shove this stalled frontal boundary offshore. High
pressure should return to provide drier weather for Friday.


Near term /through Thursday/...
as of 911 PM EDT Wednesday...

A few decaying showers in the region will continue to weaken
with loss of heating. The focus then turns to the complex of
thunderstorms across central Kentucky into northern Tennessee at 01z (9 PM
edt). Most of the cams agree this complex will weaken as it
approaches our area after 06z (2 am edt) tonight, as instability
continues to drop and low level winds veer to the
southwest/west reducing the inflow into this complex. With
SBCAPE values still a couple of hundred j/kg, still would expect
this complex to produce some rain, especially west of I-77, so
kept chance probability of precipitation in place during the pre-dawn hours in this
area. Little changes were made to temps, winds, but cloud cover
was lowered early this evening before low clouds/stratus return
late tonight.

For Thursday, a cold front will push eastward from the Ohio
River valley and arrive over the mid Atlantic during the day.
Some prefrontal convection could arrive by daybreak but
eventually decay over the mountains. However, the cold front
will produce another round of showers and thunderstorms by
midday that persists until sunset. The threat of severe weather
and flash flooding seems a little greater on Thursday as
compared to this afternoon, but confidence is too low to
pinpoint exactly where these threats will materialize. For now,
all these weather threats have been mentioned in the severe weather potential statement.


Short term /Thursday night through Saturday night/...
as of 131 PM EDT Wednesday...

A broad upper trough will initially be responsible for sensible
weather conditions Thursday night into Friday, in the wake of
thursday's cold frontal passage. This trough will bring with it a
noticeable drop in humidity levels from present elevated levels,
and a period of breezy northwest winds Thursday night into part
of Friday. While this upper feature will be to our northeast
through late week/early weekend, at the same time an anomalous
upper ridge associated with summerlike heat and increasing
humidity will be building over the lower-mid Mississippi Valley.
This will set up a period of enhanced mid-level northwest flow
later Friday into Saturday from the upper Midwest across the
Ohio Valley and into the southern/central Appalachians. Majority
of guidance over the last 24 hours suggests will likely be a
convectively-active storm track on the periphery of the warm and
conditionally unstable air mass brewing over the lower Ohio/Tennessee

Decreasing pops for Thursday night will be the general trend, though
still some stratus/light showers to linger in the WV mountains into
the Blue Ridge. Also in the Post-frontal regime will be seasonally
strong 6-hourly pressure rises between 3 and 7 mb, coinciding with
850 mb jetstreak perpendicular to the Blue Ridge of around 45 kts,
most of which lie locked above a subsidence inversion. Mav-based
wind speeds are around 20-22 kts between 00z-06z Friday however,
with lighter speeds moving into the daytime hrs Friday. Will show
wind gusts overnight into early Friday in the mountains nearing 35
mph, with about 20-25 mph east of the Blue Ridge. It's possible
these winds may be strong enough to bring some weakened trees down
along the southern Blue Ridge where considerable rains have fallen
of late for overnight into early Friday. On the whole, Friday
should see decreasing wind gusts and an overall pretty pleasant
day with plentiful sun.

As mentioned, for late Friday into Saturday we'll likely be needing
to keep a close eye toward the northwest for the potential for
convective storm complex(es) moving southeastward either close to or
over the Blacksburg area of responsibility. While models have shown
or at least hinted at the convective potential for a few model
cycles, remainder of the period regarding pop chances, especially on
timing when and where they are the greatest, is pretty low-
confidence. Details on location, timing and to an extent the scope
of any severe or Hydro threat will hinge on any leftover boundaries
or leftover embedded convectively-enhanced vort maxima. GFS-based 0-
6 km bulk shear is around 35-50 kts Friday night into Saturday, and
while summerlike humidity levels boost cape values during the
daytime hours, a plume of steep mid-level lapse rates shown by the
GFS in the northwest flow may keep thunder chances going into the
evening/overnight. That's certainly a background environment that
would Foster strong to severe storms, but again is conditional on
aforementioned mesoscale details which are unresolvable at this time
range in model-world. Current thinking is areas southwest of a
Lewisburg to Roanoke to Reidsville line, where the GFS shows the
periphery of the instability axis lying. Given stated uncertainties,
will show 20-30% pop across this area Friday night, with a dry
forecast elsewhere. Generally will keep highest pops in the 40-50%
range South/West of that Roanoke-Reidsville line Saturday, with 20-
30% elsewhere. Will hold off on any severe weather potential statement mention regarding late
week/early weekend severe/Hydro at this point; but the severe/Hydro
threat is something that we'll have to be carefully monitoring and

Temperatures on the whole are near to close to late-June climatology
in the 70s to mid 80s. Will start to increase the humidity level
late Friday night and into Saturday.

Forecast confidence is moderate to high for Thursday night into
friday; uncertainty regarding placement and location of convective
complexes renders confidence at low to moderate levels for Friday
night into Saturday.


Long term /Sunday through Wednesday/...
as of 133 PM EDT Wednesday...

Amplified ridge axis initially over the Ohio/Tennessee valleys begins to
shift eastward over the Blacksburg forecast area by Sunday.
Increasing heat and humidity is likely to be the rule for Sunday
and most of Monday. GFS has advertised 850 mb temperatures in
the low +20s c by Monday, which will likely push temperatures
east of the Blue Ridge into the low 90s, with heat indices in
the mid to upper 90s. Potential for scattered showers and
thunderstorms should exist as well on Sunday in the heat of the
day but coverage may be limited by absence of larger-scale
forcing mechanisms.

Later Monday a rather strong shortwave trough aloft looks to topple
the ridge and will drag a weakening cold front across the
Appalachians and mid-Atlantic region. This should bring with it a
greater chance for thunderstorms areawide; while a few strong storms
possible, too much uncertainty to place in the severe weather potential statement or in the zone
forecast. A bit cooler, although temperatures likely still to run
close to or just above late-June climatology for Tuesday.

Forecast confidence is overall moderate for this period.


Aviation /01z Thursday through Monday/...
as of 740 PM EDT Wednesday...

Isolated to widely scattered showers and thunderstorms along a
weak surface trof may move over bcb, roa and lyh between now and
02z (10 PM edt) before waning with loss of heating. Later
tonight, lingering low level moisture and light winds will
likely produce another round for MVFR to IFR vsbys and ceilings,
especially at the mountain taf sites and lyh.

Confidence is lower regarding the timing of showers and
thunderstorms for Thursday, as a complex of storms across the
Ohio Valley this evening March east. With limited instability,
and a turn to southwest low level winds, this complex will
weaken and may completely dissipate before reaching our area.
For now, kept vcsh in during the early morning hours for the
mountain taf sites.

Re-development of scattered thunderstorms is expected Thursday
afternoon ahead of a cold front, and again included thunderstorms in the vicinity at all
taf sites to account for this. Winds may be issue, especially
for small aircraft tomorrow with gusts winds 20-25 kts expected
during the afternoon.

Extended aviation discussion...

The cold front should head offshore by Thursday night with
breezy winds expected. Dry weather and VFR conditions with
lighter winds will follow for Friday into early Saturday as
high pressure builds overhead. Scattered showers and
thunderstorms could return during late Saturday through Monday
with MVFR conditions possible in the strongest storms. MVFR may
also be possible due to any low clouds or fog during the early
morning hours, especially in the mountains.


as of 225 PM EDT Wednesday...

The probability of flash flooding is above average if there is
training of storms this afternoon, especially across the NC
mountains/foothills where 1 hour flash flood guidance of 1-2
inches exists. Given precipitable water values over 1.5 inches
with deep convection, rainfall rates of 3 to 4 inches may cause
some issues. The storm motion is expected to be in the 20-30
miles per hour range, but training may set up along the Blue Ridge. No
Flash Flood Watch has been issued at this time so that trends
can be observed on how the storms evolve first. If a watch is
needed, it may be for the Blue Ridge south of Roanoke into the
North Carolina mountains and eastward to the foothills.

Another threat of flooding exists ahead of the cold front on
Thursday. As far as the rivers go, there is just localized
runoff, and only South Boston is expected to reach action stage
by Thursday.


Rnk watches/warnings/advisories...


Synopsis...precipitable water

National Weather Service Glossary of Abbreviations