Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus61 krnk 200542 
afdrnk

Area forecast discussion
National Weather Service Blacksburg Virginia
142 am EDT Thu Jun 20 2019

Synopsis...
a stationary front will remain draped over the mid Atlantic
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 today/...
as of 111 am EDT Thursday...

Trends in radar of convection moving into far SW Virginia appears to
be weakening. High-res models are somewhat in agreement that
this area of showers and isolated storms will impact southeast WV to northwest
NC early this morning before falling apart somewhat as it moves
into the New River valley. Made some adjustments to pops to
account for this and added more fog as well, especially along
and east of I-81 where cloud cover is limited, so should see a
bit of fog, though mainly patchy in nature.

Previous discussion from Wed evening...

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 /tonight 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
valleys.

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
refining.

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 /06z Thursday through Monday/...
as of 120 am EDT Thursday...

The aviation forecast will be tricky as we have a dying complex
of showers/isolated storms heading toward a lwb-blf-vji line
through 12z. Confidence is low in actual long term showers at
the blf/lwb sites, so kept vcsh for now, as instability
decreases and thunderstorm threat likewise.

Will see fog and/or low clouds at times, with fog more prevalent
in the east where skies have cleared for the most part. Still
confidence high on sub-VFR vsbys and at times cigs, but low
confidence on how long these conditions last and likely will see
fluctuations in vsbys between LIFR to MVFR through 12-13z.

For Thursday, high-resolution models vary on timing of storms,
but appears will see some where leftover current batch faded
across the mountains this morning, mainly along/east of the Blue
Ridge between 12-18z, then secondary surge of showers/storms as
the front moves across in in the afternoon. Stronger storms are
possible across the Dan/lyh and points east corridor later this
afternoon, with gusty winds. Gusty winds are also expected as
front moves across, in the 20-30kt range out of west.

In essence confidence high that storms will be around today, but
low on timing. Any storms will drop cigs/vsbys to IFR or lower
given high moisture content.

Should see drying by late in the day in the west, to evening in
the east.



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.

&&

Hydrology...
as of 125 am EDT Thursday...

Appears threat of flash flooding will not be as high given less
coverage Wednesday. Storms today will still be rain efficient,
but storm motion will be faster, so they should not linger over
the same area. Some places could see 2 to 3 episodes of storms
during the course of the day, so these areas may need to be
watched, especially the mountains.

&&

Rnk watches/warnings/advisories...
Virginia...none.
NC...none.
WV...none.

&&

$$
Synopsis...precipitable water
near term...ph/pw/wp

National Weather Service Glossary of Abbreviations