Severe Thunderstorm Warning 142, NWS Blacksburg, VA

Area Forecast Discussion
National Weather Service Blacksburg VA
723 PM EDT Thu Aug 29 2024

.SYNOPSIS...
A cold front will continue to trigger scattered to numerous
showers and thunderstorms this afternoon and evening. The cold
front is expected to stall over the area tomorrow and into the
weekend, so additional opportunity for showers and thunderstorms
can be expected. Cooler and less humid weather arrives for next
week.

&&

.NEAR TERM /THROUGH FRIDAY/...
As of 200 PM EDT Thursday...

Key Messages:

1) A frontal boundary pushing south will keep showers and
storms in the area through tomorrow. A few storms may have
damaging winds and heavy rainfall.

2) A Flood Watch is out for our eastern counties.

Mostly cloudy skies have blossomed over the mountains, and
showers and thunderstorms are increasing in number and strength
with cloud top cooling noted on satellite. This activity is the
result of an approaching backdoor front, convective temperature
achievement/cap erosion across the higher terrain, and upper
level energy rotating from the panhandle of WV towards coastal
VA. As the front sags south this afternoon and evening,
widespread thunderstorms are likely.

Instability will be highest over the piedmont and to our east,
where dew points are in the upper 60s to mid 70s. DCAPE is
approaching 1200 J/kg over much of our forecast area however,
with dry air noted in this morning's sounding which could help
accelerate downdrafts. Thus we are most concerned with damaging
downburst winds with any severe storms, in line with SPC's Day 1
outlook. Shear is low in our area, so this does not look like a
tornado threat, unless more shear is ingested from a boundary.

CAMs show multicells growing upscale and eventually attempting
to form a loose bowing line of sorts, but the lack of
kinematic support will limit its cohesiveness. Showers and
isolated thunderstorms weaken but a few continue after dark,
mainly for the piedmont and points east and south. A few showers
will even likely be around through the overnight into Friday
morning.

Besides the wind threat, the other concern is flooding. A Flood
Watch has been issued for our far eastern counties in
coordination with local offices and WPC. QPF amounts will be
bolstered by high PWATs, moderate warm cloud depths, slow
movement, and training cells along the front. In the watch area,
expect 1 to 2 inches with isolated higher amounts possible.
Outside the watch area, most locations will see less than a half
inch through tonight. The Flood Watch is not a slam dunk with
dry antecedent conditions, but the potential is there.

Lows tonight will be in the 60s to low 70s, with highs tomorrow
in the upper 70s to mid 80s for the mountains, and the mid to
upper 80s for the piedmont. Showers and thunderstorms will
develop once again tomorrow afternoon and evening along the
mountains and south, with the boundary near the VA/NC border.
Lower temperatures and cloud cover will limit SB instability,
but another short wave will support a few strong storms and
heavy rainfall once again.

Confidence in the near term is moderate, but slightly lower for
PoPs.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/...
As of 1100 AM EDT Thursday...

Key Points:

1. Temperatures remaining above normal.
2. Daily chances of showers/storms with locally heavy possible.

A look at the 29 Aug 00Z NCEP Ensemble Mean 500 mb Geopotential
heights shows a shortwave ridge over our region Friday night
with an upstream shortwave trough over the Upper Mississippi
Valley. As we progress into and through the weekend, the ridge
gets displaced east as the trough approaches and amplifies. The
axis of this trough is expected to centered over southern
Quebec, southwest into eastern Tennessee Sunday evening. At the
surface, on Friday, high pressure will be centered just
southeast of the Canadian Maritimes with a ridge axis extending
southwest, intersecting the mid-Atlantic region. A stationary
front in advance of this high will be over VA/NC. Additionally,
deep low pressure will be centered over Hudson Bay, with an
associated cold front extending into the Mid Mississippi River
Valley. As we progress through the weekend, the Canadian low
will make progress eastward. Its associated cold front will
reach our area Saturday evening. This first front will be
followed about twenty-four hours later by second front. Behind
this second front, a strong and broad area of high pressure will
build into the Northern Plains and Upper Mississippi River
Valley.

Output from the 29 Aug 00Z Ensemble Situational Awareness Table
shows 850mb temperatures around +17C to +18C Friday night
through Saturday. These values will correspond to the 90 to 97.5
percentile of the 30-year climatology. On Sunday night, values
drop to +14C to +16C. Precipitable Water (PW) values across the
region Friday night are expected to range from 1.50 to 1.75
inches. Through Saturday, these values increase to an average a
little over 1.75 inches. For mountain regions of our area, these
numbers correspond to the 90 to 97.5 percentile of climatology.
Similar values and climatological references are expected
Saturday night, but across our entire region. On Sunday, drier
air works its way into northern parts of the region, but 1.75 to
2.00 inch PW values are still expected over primarily our
Piedmont region. Sunday night, the push of drier air continues
with values by daybreak Monday ranging from just shy of 1.00
inch in the north and 1.50 inch in the south. A look at Mean
Integrated WV Transport shows the time period of late Saturday
night through Sunday afternoon a period to watch for values
exceeding 400 units. This time of year this level of transport
corresponds to the 90 to 97.5 percentile of the 30-year
climatology. The far northeast portion of the region Saturday
night, and the eastern and southeastern sections on Sunday, are
the key locations to watch for this better than average moisture
flux.

The above weather scenario points to a period with multiple
chances of showers and/or storms. On Friday night, the
stationary front over eastern sections paired with good PW
values point to a continuation of Friday daytime activity into
the evening hours, especially in the east. Next, the arrival of
a cold front late Saturday into Saturday night, again combined
with high PW values across the area, will provide for good
coverage of showers/storms during this time period. Late Friday
night into early Saturday, coverage should be minimal and
primarily over western sections. After another decrease in
coverage Saturday night, the secondary cold front will cross the
area on Sunday into Sunday night. This front also combined with
high PW values and good moisture flux should continue the
potential for shower/storms. During all these time periods,
locally heavy rain will be possible, especially in areas where a
particular front is located, and also with the regions of
higher PW and WV flux. Temperatures are expected to average five
to ten degrees above normal through the period.

Confidence in the above weather forecast is moderate to high.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
As of 1100 AM EDT Thursday...

Key Points:

1. Trending drier and cooler
2. Isolated/scattered, mainly afternoon, showers possible
southern sections of the region.

A look at the 29 Aug 00Z NCEP Ensemble Mean 500 mb Geopotential
heights shows an upper level trough heading east of the region
as a broad upper ridge builds into the central US. At the
surface, the center of high pressure will head east through Ohio
Valley on Monday and into the mid-Atlantic and New England on
Tuesday. On Wednesday, this high will be centered off the New
England coast, but its axis will extend southwest into the
Carolinas.

Output from the 29 Aug 00z Ensemble Situational Awareness Table
shows 850mb temperatures around +13C to +15C for Monday, +12C to
+14C for Tuesday, and +13C to +14C for Wednesday and Thursday.
PW values fall through the day Sunday, reaching 0.75 inch north
and 1.25 inch south. Tuesday through Thursday, PW values across
the area will range from a little under to a little over 1.00
inch.

The above weather scenario will offer drier and cooler air
arriving from north to south on Monday. However, the proximity
of the exiting front will still offer the potential for some
isolated showers, especially in areas over the southern third of
the region. The northern third may be precipitation-free or
have minimal coverage.

From Tuesday through Thursday, drier air will continue to work
its way into the region. However, the front to our south may
wobble a bit such that some isolated/scattered showers may be
possible over mainly southern sections.

Temperatures are expected to trend cooler through the period,
reaching numbers around or slightly cooler than normal by
Wednesday/Thursday.

Confidence in the above weather forecast is moderate.

&&

.AVIATION /23Z THURSDAY THROUGH TUESDAY/...
As of 200 PM EDT Thursday...

Shower and thunderstorm activity is expected to continue for the
next few hours across the region, which will bring the potential
for reduced VSBYs and CIGs as well as gusty sporadic winds to
all terminals this evening. MVFR to IFR CIGs and VSBYs look to
develop during the overnight hours for all terminals as the
front looks to stall across the region. By the early to mid
morning, most terminals look to lift to VFR levels before
additional shower and thunderstorms develop once again areawide
by the 18 UTC timeframe. SHRA/TSRA conditions will likely
persist through the remainder of the TAF period Friday night.

Forecast confidence is medium.

EXTENDED AVIATION OUTLOOK...

Front is expected to stall over the region for the weekend. This
favors a mix of flight conditions with low confidence on flight
categories due to opportunity for showers/storms and nighttime
stratus and fog. A more bona fide push of cooler air from the
north is expected Monday, suggesting improved conditions for
early next week, although still subject to overnight valley
fog.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...Flood Watch until midnight EDT tonight for VAZ020-024-035-046-
     047.
NC...None.
WV...None.

&&

SYNOPSIS...PM/SH
NEAR TERM...JCB/SH
SHORT TERM...DS
LONG TERM...DS
AVIATION...EB/SH

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