Special Weather Statement, NWS Blacksburg, VA

Area Forecast Discussion
National Weather Service Blacksburg VA
155 AM EDT Sat Sep 8 2018

.SYNOPSIS...
Frontal boundary across the region will drift slowly south into
North Carolina by Saturday. A shallow wedge of high pressure will
builds into the area behind the front then retreat north by Tuesday
as low pressure tracks from the Mid Mississippi Valley into
southeast Canada.

&&

.NEAR TERM /THROUGH TODAY/...
As of 700 PM EDT Friday...

Main update is to adjust PoPs based on latest radar trends.

Most models are keeping convection going through the night with
HiRes models focusing on the Lynchburg area. Convection this
evening, both areal and severity, is on a decline, however there
remains copious amounts of instabilities over areas that did
not see rain this afternoon and evening. If any of the outflow
boundaries from today's storms interacts with these lingering
instabilities, there will likely be ongoing convection despite
loss of heating tonight. Debris clouds may also play a factor,
helping to reduce the chances for storms overnight. For this
update, have the highest PoPs (40 percent) over the mountains
this evening, then east of the Blue Ridge overnight.

As of 339 PM EDT Friday...

The upper ridge continues to flatten and weaken as it moves east
tonight into Saturday.  A weak surface front draped across our
region will slowly sag southward tonight into Saturday. The
combination of low level convergence, boundaries, orographic lift
and heating will result in scattered showers and thunderstorms this
afternoon into tonight. The SPC Mesoscale analysis at 18z showed
from 2000 to 3500 J/KG and surface based LIs were minus 2 to minus 4.
DCAPEs were relatively high with values from 700 to 900 across much
of forecast area. The Day 1 Convective outlook has brought a
marginal risk into northern portions of our forecast area. Isolated
strong to severe thunderstorm may be possible this afternoon into
tonight with wind as the main threat. Storm movements will be slow
because of the weak mean flow, and with precipitable water values
around 1 to 1.5 inches, locally heavy rains may be possible.
The areal coverage looks to limited for flood watch. HiResW-Arw-
East, HRRR, HREF and NamNest indicated the best chances and coverage
this afternoon into this evening across the northwest portion of the
forecast area. Then, storms will propagate eastward into tonight.
Areas of fog and low clouds are possible overnight into Saturday
morning especially where it rains this evening. Low temperatures
tonight will range from the upper 50s in the mountains to around 70
degrees in the piedmont.

Use a blend of the NAM and Conshort to shape the placement of the
front on Saturday. A number of competing factors will actually
determine the southward movement of the front. These factors include
the subtropical ridge that steps to the east, 1030mb surface high
pressure sliding across northern New England, an upper low moving
into the Great Lakes, within which the remnants of Gordon are
embedded, and an easterly wave moving onto the Southeast. In any
case, scattered to numerous showers and thunderstorms will develop
Saturday. The Day Two Convective outlook keeps us in general
thunderstorms. However, some of the storms could contain heavy
rains. There is the potential for flooding issues depending on the
location of the storms and their movement. High temperatures
Saturday will vary from around 70 degrees in the north to the mid
80s in the South.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY NIGHT/...
As of 339 PM Friday...

Backdoor cold front continues to push southward through the CWA
overnight with increasing NE winds across much of the area. Blended
in some of the 12Z NAM given it is likely handling the timing of the
boundary and strength of winds behind it. Lingering evening
convection should be near and south of this boundary and so may
still be fairly active near the VA/NC border by 8pm but this should
push fairly quickly south of the CWA by midnight and possibly
sooner. Thus included thunder generally in the vicinity of where the
boundary is likely to be through about midnight and showers to the
northeast of it, and then after midnight no mention of thunder but
still some likely PoPs for showers or light rain especially close to
Blue Ridge where NE to E flow and still plenty of moisture likely to
squeeze out continuing light precip in spots, perhaps even drizzle.

By Sunday morning the back door boundary should be entirely through
CWA with possible exception of Mountain Empire region of SW VA, and
then challenging forecast in terms of where during the day if
anywhere will it try to erode or lift back north. SW winds at 850mb
to increase over us ahead of next cold front moving into OH Valley,
and sfc high over northern New England begins to shift east a bit
late in the day, so I think there is a chance that the wedge could
begin to erode late in the day along the SW and SE edges. Elsewhere
feel reasonably confident it will remain locked in for much cooler
day time highs compared to Saturday, and many locations across the
NRV, Roanoke Valley and northeast towards LYH may struggle to see 70
degrees. Went with the coolest guidance (MET). In the far SW and
also in far SE Piedmont I hedged the temps a little between the MET
and warmer MAV just in case it does begin to erode more there later
in the day. Precip-wise it may end up being a cloudy but relatively
generally dry day with only a few light showers, probably most
likely along Blue Ridge, but some models suggesting with the good
overrunning and some limited elevated instability there could be more
scattered showers that develop over the wedge late in the day and
maybe even into the evening, especially near the wedge boundary in
the Piedmont where it may continue to slowly erode overnight. So
for now until we get better hi res model guidance for the area I did
not lower the PoPs as much as I was originally inclined to, and
still have high chance to even likely near the Blue Ridge for the
day Sunday.

Monday the cold front to the west moves to near the western slopes
of the Appalachians and sfc high shifts off the coast of New England
so all indications are that by midday the wedge will quickly erode
over the entire area and allow better instability along the
weakening boundary to teh west to fill back in. Thus typical
scattered shower and storms, more numerous in west closer to
weakening frontal boundary. Do not see especially high instability,
given wedge in morning and probably fair amount of cloud cover, and
forcing along boundary is weak and also not much upper level support,
thus severe chances look low. Unsure how far east convection may
develop or translate from the mountains but with 700mb winds from
the SW I think there is a good chance that some storms do make it
east of Blue Ridge. With weak forcing as mentioned before, see a
good diurnal signal to any convection so by late evening expect most
precip to have diminished.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
As of 339 PM Friday...

All eyes on the Atlantic as Florence, likely a strong hurricane
again by then, appears to be posing an increasing threat to the
southeastern US coast, but there is still lots of time for that to
change. Strong upper ridge dominates just off the mid-Atlantic coast
with assoc sfc high pressure over much of the eastern US, which may
make it heard for the storm to turn north for awhile. In any case,
warmer conditions each day under the ridge and enough moisture to
result in diurnal and mainly mountain afternoon and evening
convection, and still maybe aided at least through Tuesday by the
ghost of the frontal boundary that will basically die along the west
slopes of the Appalachians but there will still be weak convergence
in this area. If Florence does indeed head in the direction of the
Carolina coast and even inland into Virginia, that would result in
maybe a day of sinking motion and lack of precip perhaps by Wed or
Thursday, but if it either heads more westerly into Georgia, or
somehow turns back north before reaching the southeast coast, which
will be good news for our area at least, that will allow daily
afternoon and evening convection to continue each day. If however
Florence ends up heading in our direction we could experience
impacts as early as Thursday, but more likely Friday, from
increasing north to northeast winds and then eventually heavy rain.

The major point to make at this time regarding Florence is that
while it is way too soon to have much confidence in whether or not
it will impact the RNK county warning area, everyone needs to keep a
close eye on forecasts of Florence from the National Hurricane
Center and the NWS office like ours over the weekend. By early next
week we should have a much better handle on where along the coast
and inland, if anywhere, will see the biggest impacts. Begin
considering what preparations need to be made should Florence
directly impact our area.

As far as what went into the deterministic forecast for mid to late
next week, the consensus guidance which is consistent with an
extrapolation of the official NHC forecast suggests the storm would
move into eastern VA by late Thursday into Friday, so the wind and
PoP forecast grids reflect that idea for now, but with wind speeds
toned down considerably from what they would be should Florence
actually take that track. Kept at least some chance pops for
scattered showers and storms through the week, not trying to try and
reflect a dry day ahead of the storm, since it may not even
approach.

&&

.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
As of 145 AM EDT Saturday...

Stationary front was over southern Virginia. SPC mesoanalysis
was showing enough instability to maintain isolated to scattered
thunderstorms over southern Virginia and northern North Carolina
through 12Z/8AM.

Ceilings north of the front were lowering to MVFR and will
remain MVFR for much of Saturday. Ceilings will be slower to
drop at KDAN. The farther south the front progresses and the
wedge develops the more likely that the MVFR ceilings will reach
KDAN and into northern North Carolina. Possible that this may
not be until late in the day.

Numerous showers and thunderstorms will develop by afternoon.
Rain will be heavy enough for MVFR to IFR visibilities.

Average to below average confidence on ceilings and visibilities
today and tonight. Average confidence on coverage of
precipitation.

Extended Discussion...

A wedge will build into the area Saturday night into Sunday.
The wedge erodes Monday with scattered MVFR showers and
thunderstorms. MVFR and IFR thunderstorms is likely on Tuesday
especially in the west.

Early morning fog and stratus will continue to be common each
night in the mountainous river valleys.

Confidence average for all elements.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...None.
NC...None.
WV...None.

&&

SYNOPSIS...KK
NEAR TERM...KK/RCS
SHORT TERM...SK
LONG TERM...SK
AVIATION...AMS/RCS

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Special Weather Statement
National Weather Service BLACKSBURG VA
1250 AM EDT SAT SEP 8 2018

VAZ015-016-032-080545-
Grayson-Patrick-Carroll-
1250 AM EDT SAT SEP 8 2018

...A STRONG THUNDERSTORM WILL AFFECT SOUTHERN CARROLL...EASTERN
GRAYSON AND WESTERN PATRICK COUNTIES INCLUDING THE CITY OF GALAX...

Until 145 AM EDT.

At 1248 AM EDT, a strong thunderstormS were rapidly developing
between Hillsville, Ararat, and Independence, moving northeast at 10
mph.

Pea size hail is possible with this storm.

Locations impacted include...
Galax...
Hillsville...
Fancy Gap...
Cana...
and Laurel Fork.

Torrential rainfall is also occurring with this storm, and may cause
localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

LAT...LON 3656 8089 3662 8096 3684 8056 3671 8046
      3673 8045 3665 8037 3656 8068
TIME...MOT...LOC 0448Z 233DEG 10KT 3477 8363

RAB

Special Weather Statement
National Weather Service BLACKSBURG VA
125 AM EDT SAT SEP 8 2018

NCZ002>006-VAZ015-016-032-043-044-080630-
Stokes-Rockingham-Surry-Caswell-Alleghany-Henry-Grayson-Patrick-
Carroll-Pittsylvania-
125 AM EDT SAT SEP 8 2018

...A LINE OF STRONG THUNDERSTORMS WILL AFFECT
STOKES COUNTY IN NORTH CENTRAL NORTH CAROLINA...EASTERN ALLEGHANY AND
SURRY COUNTIES IN NORTHWESTERN NORTH CAROLINA...ROCKINGHAM AND
CASWELL COUNTIES IN NORTH CENTRAL NORTH CAROLINA...CARROLL COUNTY IN
SOUTHWESTERN VIRGINIA...HENRY AND PITTSYLVANIA COUNTIES IN SOUTH
CENTRAL VIRGINIA...SOUTHEASTERN GRAYSON AND PATRICK COUNTIES IN
SOUTHWESTERN VIRGINIA...THE CITY OF DANVILLE...THE CITY OF
MARTINSVILLE AND THE CITY OF GALAX...

Until 230 AM EDT.

At 122 AM EDT, radar indicated an area of strong thunderstorms
located from near Hillsville to just east of Danville, then
southward to just east of Sparta to Mt. Airy to Yanceyville.
Movement was north at 10 mph.

Pea size hail is possible with these storms.

Locations impacted include...
Danville...
Eden...
Reidsville...
Martinsville...
and Mount Airy.

This includes The following Locations The Martinsville Speedway,
Martinsville Airport, and Danville Airport.

Torrential rainfall is also occurring with these storms, and may
cause localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with these storms.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

These storms may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

LAT...LON 3640 8101 3684 8089 3690 8068 3693 8063
      3671 8046 3675 8045 3675 8040 3679 8039
      3683 8034 3687 8024 3685 8008 3678 8003
      3683 8000 3679 7976 3684 7966 3691 7963
      3684 7915 3654 7922 3654 7914 3627 7916
TIME...MOT...LOC 0522Z 184DEG 8KT 3679 8068 3658 7918

RAB

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