Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus61 krnk 191128 
afdrnk

Area forecast discussion
National Weather Service Blacksburg Virginia
728 am EDT Wed Jun 19 2019

Synopsis...
high pressure over the southeast and a front advancing slowly
from the Ohio Valley will keep US in an active and humid
airmass into Thursday. The front moves off to our east by
Thursday evening. High pressure should return to provide drier
weather for Friday.

&&

Near term /through tonight/...
as of 255 am EDT Wednesday...

Patchy areas of dense fog observed this morning across the
mountains/foothills of Virginia/NC and WV, as enough thinning of any low
or mid clouds and a rain soaked ground have allowed for this
development. This fog will be around through about 9am, but
visibilities are expected to fluctuate between 1/4sm to over 4 miles
given the nature of the cloud cover. Will keep a Special Weather
Statement going to cover this fog issue.

Showers/isolated thunderstorms continue to slowly move northeast
from south of The Triad of NC toward far southeast Halifax County,
VA, with main lift and vort tracking east, should only see some
isolated coverage this morning, with residual deep moisture
convergence remaining in place from southwest Virginia into central Virginia.

Generally followed the high-res cam models through mid afternoon as
they are in general agreement in relation to where convection will
fire up, namely along the Blue Ridge south of Floyd into NC and
across the WV mountains by midday, then shift east northeast with
more development into the rest of the forecast area. Most of the
convection will be outflow driven, but there is some upper energy
moving across in the 15-21z time frame which should enhance deeper
convection. The Storm Prediction Center has the severe threat west
of our area today, but cannot rule out some isolated wet microbursts
given the high-pwats, with sbcapes projected to be in the 1500-2500
j/kg range this afternoon. Mean flow will be out of the west-
southwest and storms should be moving at 20-30 mph. However, like
Tuesday, as storms fire along the Blue Ridge, there is a threat of
training as more storms develop upstream, and given high pwats, some
localized flash flooding is possible. The areas that have been hit
the most with rain have been the NC counties from the Tennessee border to
the Piedmont, and into southern Virginia generally south of US 460. At the
moment not seeing enough coverage/signals for a Flash Flood Watch,
but will mention this localized threat in the severe weather potential statement.

Storms should fade early this evening, with a lull into early
overnight. The stronger shortwave pushes across the Ohio Valley with
a convective induced vort moving into the Tennessee Valley by 12z Thursday.
Models close in having area of showers/storms approaching our
mountains by late tonight, but most of the activity should hold off
until after 8am Thu.

Temps will be held down today due to cloud cover and higher rh, but
humid nonetheless. Highs will range from the 70s to around 80 in the
mountains, to lower to mid 80s east of the mountains.

Again, lows tonight will be held up due to high moisture content in
the atmosphere with 60s areawide.

Confidence in the forecast is pretty good, in terms of coverage of
storms and severe/Hydro threat being marginal or low in coverage.

&&

Short term /Thursday through Saturday night/...
as of 315 am EDT Wednesday...

Showers and thunderstorms are again expected to impact the
Blacksburg forecast area on Thursday as short wave trof and
associated cool front moves east through the region. However,
extent and intensity of convective activity during the afternoon
will be predicated on impact of mesoscale vorticity complex (mcv)
generated from convective clusters currently in Oklahoma that will
precede the main trof in the morning.

Current thinking is that decaying convective cluster and associated
weaker showers/thunderstorms and cloud debris will hold down
temperatures into early afternoon before thermal recovery begins in
earnest as clouds diminish and insolation increases with the passage
of the mesoscale convective vortex.

However, despite increasing thermodynamic support as the afternoon
progresses, lower tropospheric winds will predominantly be from the
southwest or west ahead of the approaching cool front, and at least
initially, the Blacksburg forecast area will still be in an areas of
negative vorticity advection immediately following the passage of
the mesoscale convective vortex - neither of which supports widespread convective
redevelopment.

Therefore, after the morning weakening convection, it may take much
of the day to recover sufficiently to support any redevelopment of
strong to potentially severe storms, and even then, the greatest
potential for severe weather may very well be displaced to the north
of the Blacksburg forecast area where dynamics will be stronger, or
to the south and east, where thermodynamic support will be
greater in areas not impacted by the morning mesoscale convective vortex. This well
depicted in the day 2 convective outlook from the Storm
Prediction Center - with a slight risk to the north and mainly
to the south of the Blacksburg area of concern.

Considering the uncertainties mentioned above, will continue to
mention the threat of both morning and afternoon convection, but for
differing reasons as defined above. Extent of severe weather
potential still remains uncertain, and if morning convective
activity is more persistent than currently envisioned, the entire
airmass over the Blacksburg forecast area may become so stratified
that thermodynamic recovery in the afternoon becomes so problematic
that no severe weather, and perhaps only limited convective activity
will occur - with perhaps exception of western upslope areas and
across the extreme southern or eastern periphery of the area closer
to any outflow boundaries that may have been generated by decaying
morning activity.

Looking a bit further into the future, any lingering showers/storms
should come to an end Thursday evening as cooler and drier air
arrives from the northwest following the cool frontal passage - with
dry and cooler weather then expected to persist for the rest of the
night and through Friday. However, by Friday night, yet another
convective complex is expected to ride around the periphery of a
developing upper ridge over the Gulf of Mexico, finally turning east
and then southeast from out of the Central Plains toward the mid-
Atlantic region - resulting in an increasing threat for
showers/storms to move east into the area by late Friday night.

Slowly increasing heat and humidity as the Gulf of Mexico upper
ridge continues to build, and a continuation of ridge-running
convective complexes will then likely maintain a threat for
scattered showers/storms for the rest of the short-term forecast
period (saturday into Saturday night) - even during the nighttime
periods when less activity would otherwise normally be expected.

&&

Long term /Sunday through Tuesday/...
as of 315 am EDT Wednesday...

Little change in surface and upper-level patterns are expected
through the long-term period. An upper-level ridge will persist in
the Gulf of Mexico, and a series of convectively induced mesoscale
vorticity complexes from midwestern convective clusters expected to
ride east and around the periphery of the ridge into/through the mid-
appalachian and mid-Atlantic region on a daily basis. Temperatures
will be at or above seasonal levels through the period - with
cooling by day afforded only by/through convective outflows.

&&

Aviation /12z Wednesday through Sunday/...
as of 720 am EDT Wednesday...

Expect improving vsbys/cigs as we head past 14z, but until then
a period of LIFR to IFR vsby/cigs especially in the mountains.

Showers/storms appear more likely near roa/bcb/blf/lwb today
with less coverage toward lyh/Dan. Try to time some of the
storms based on high-res models. Main chances will be in the
18-22z time range. Aside from storms should be VFR, with a light
southwest wind <10kts.

Showers/storms fade quicker after 00z Thu, with VFR through
06z, then appears fog and low clouds form around bcb/lwb with
IFR to LIFR possible.

Confidence is pretty good on improving to VFR by 14-15z today,
less on storms impact taf sites, and pretty good on late night
fog.

Extended aviation discussion...

Showers and thunderstorms will impact the area again Thursday as
a front moves across. Sub-VFR in storms, and VFR outside of the
convections. MVFR or IFR conditions from fog or low clouds may
also occur during the late night hours due to increasing
moisture. Mainly VFR Friday-Sunday outside of any early morning
fog or scattered storms Sat-sun.

&&

Hydrology...
as of 330 am EDT Wednesday...

Probability of flash flooding is above average if there is a
training of storms today, especially across the NC
mountains/foothills with 1 hour ffg of 1-2 inches exists, and
given precipitable waters 1.6 to 1.8 inches, with deep convection, rainfall
rates of 3 to 4 inches may cause issues. The storm motion is
expected to be in the 20-30 mph range, but training again may
set up along the mountains. Have decided to not issue a Flash
Flood Watch at this time, but see how storms evolve. One may be
needed for the Blue Ridge south of Roanoke into the NC
mountains, and east to the foothills of NC/VA.

Another threat of flooding exists ahead of the front Thursday.

As far as the river go, mainly getting localized runoff, and
only South Boston is expected to reach action stage by Thursday.

&&

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

&&

$$
Synopsis...pw/wp
near term...wp

National Weather Service Glossary of Abbreviations