Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
425 am EDT Wednesday Aug 5 2015
a stationary front extends from southern Virginia...west to
Missouri. This east-west oriented front will become the focus for
numerous to widespread showers and thunderstorms...especially for
Thursday as an area of low pressure ripples east along the frontal
Near term /through tonight/...
as of 400 am EDT Wednesday...
Surface front extends from the Tidewater of Virginia...westward into
Kentucky and Missouri. Front may drift a bit farther south
today...stalling vicinity of the Virginia/NC border. Some isolated
showers/storms are possible along the boundary during the afternoon
but most areas across our County Warning Area expected to remain dry today. Better
support for scattered to numerous showers/storms will exist along
the west end of the front over Missouri/Arkansas and into western Kentucky/Tennessee
where an upper level short wave will result in the development of
surface low pressure along the front. Models are in relative
agreement the activity which forms over the lower Ohio Valley/upper Tennessee
valleys will reach our far western County Warning Area tonight...with probability of precipitation
increasing across our southwestern County Warning Area...especially the NC high country
this evening and for the overnight. With nearly zonal middle level
flow...debris clouds and showers likely to overspread the Virginia/WV
Highlands after midnight...and then across the remainder of the County Warning Area
Thursday. The short wave driving the area of low pressure is
strong...so see no reason that showers will not persist and become
more numerous to widespread with time.
Temperatures today will be warmest across the Piedmont of NC...south
of the front with repeat daytime highs in the l-m90s. Along and
north of the front...debris cloudiness will likely hold readings in
the 80s...but may still test 90 in Roanoke and Lynchburg if enough
sun manages to get through the increasing cloud cover. For
tonight...thickening clouds will keep readings mild with lows mainly
in the middle-upper 60s.
Short term /Thursday through Saturday night/...
as of 400 am EDT Wednesday...
Widespread much needed rainfall event anticipated across the County Warning Area
Thursday as a strong middle-level disturbance tracks along a
baroclinic zone/frontal boundary located through the Tennessee Valley and
into NC. At 12z Thursday...the heaviest rain/best isentropic lift
should be located from middle/eastern Tennessee into southwest
Virginia/southern WV...translating eastward toward the Piedmont by
afternoon/evening. Surface low pressure will develop across
eastern SC/NC by afternoon/early evening...then translate
slowly northeastward Thursday night into Friday morning. Given
the extensive cloud cover expected to overspread the area early in
the day and precipitation to follow quickly on its heels with
little or no sunshine expected...plus the development of surface low
pressure to the southeast inducing a northeast wind into areas
east of the Alleghany front...essentially an in-situ wedge will be
in place as the day progresses. Expecting minimal heating and
minimal instability...as most of the models agree. This will
result in primarily rain/showers with little thunder. Isolated thunderstorms and rain
will be possible along the southern periphery of the County Warning Area where
some breaks in the cloud cover early in the day may allow those
areas to heat into the lower 80s before precipitation
begins...thus supporting some weak instability and isolated thunder.
Storm Prediction Center has outlooked the far S/southeast counties of the County Warning Area for a marginal
risk of severe. Models do indicate some shear riding along the
boundary...which could be enhanced with wedge development...but
again instability is minimal. For now...do not plan to mention
severe in the grids or the severe weather potential statement as feel threat is too low at this
time and such threat will most likely be further south.
Quantitative precipitation forecast should not be an issue. General 1.25 to 1.50 inch rainfall
amounts expected...with locally higher amounts in any areas that
see isolated convection or training of cells. However...much of the
County Warning Area has been quite dry recently...with only isolated locations
seeing more than 1/2 inch in the last 2-3 weeks.
Therefore...rainfall is not expected to be an issue...in fact the
rainfall will be most welcome in much of the County Warning Area as the recent
overall lack of rainfall and warm temperatures has left many yards
and fields parched.
Maximum temperatures Thursday will be a struggle. With
clouds...rainfall...and potential wedge development...really see
very little warming. Have leaned toward the mav guidance as
opposed to the warm European model (ecmwf). Highs should range from the 60s
mountains to the 70s Piedmont. The diurnal temperature swing will
be quite limited and potentially only a few degrees in many areas.
For the remainder of the short term period...the upper disturbance
and surface low pressure will slowly drift off the coast of the NC
outerbanks by Friday night/Saturday. Still expect widespread clouds
Friday...but rainfall should be confined primarily to areas east
of the Blue Ridge during the first half of the day...with possible
differential heating resulting in afternoon showers/isolated
thunderstorms western mountains. Confidence in this later scenario
By Saturday...the surface low and upper trough continue to drift
slowly east out to sea. Strong subtropical ridging across the
central/Southern Plains will begin to amplify into the Tennessee Valley
with 500mb heights beginning to slowly rise across our region.
However...unstable northwest flow aloft will persist and most
models hint at good potential for diurnal convection Sat
afternoon...albeit instability is marginal. Will continue with
chance showers/thunderstorms...mainly across the
mountains especially during the afternoon and evening.
Temperatures will remain cool Friday...but slowly warm closer to
normal levels Sat. Through most of the period...the range from
normal will be most notable with respect to maximum temperatures...with min
temperatures remaining near or slightly above normal given clouds and
relatively high dewpoints/relative humidity levels in place.
Long term /Sunday through Tuesday/...
as of 415 am EDT Wednesday...
Overall large scale pattern does not change during this time
frame. Upper trough remains across the eastern U.S. With a strong
subtropical ridge equally anchored across the Southern Plains.
Heights rise Sunday-Monday across our region...only to fall again
Tuesday...as a strong disturbance moves around the eastern
periphery of the upper ridge from the Great Lakes into the
central/southern Appalachians by Monday night/Tuesday. Models
disagree somewhat on the timing and impact of this event. If the
GFS solution is correct...there could be a significant convective
event Monday night or Tuesday across the region. The European model (ecmwf) would
suggest somewhat less potential. At any rate...Tuesday does appear to
bring the next best chance of widespread convection to the
area...albeit the extent and severity remain in question.
Troughing will deepen across the area Tuesday into
Wednesday...with continued chances for precipitation as a result.
Maximum temperatures will trend slightly above normal sun-
Monday...but return to near/below normal levels by Tue-Wed.
Aviation /08z Wednesday through Sunday/...
as of 200 am EDT Wednesday...
Weak frontal boundary remains over the area. However no
instability available for showers. Main by-product through the
morning hours will be scattered-broken cloud bases...mainly VFR...but can't
rule out shallow layer of MVFR cloud elements over higher ridges
vicinity Virginia/WV Highlands.
Weak surface front is expected to drift to along the Virginia/NC border
today before stalling. This front will then become the focus for
scattered to numerous showers/storms Wednesday night into
Thursday as wave of low pressure moves along boundary. Main focus
for deep convection today will be over MO/KY/TN where wave will
reach first...then activity spreading east across the central
Appalachians tonight and Thursday with opportunity for widespread
MVFR and occasionally IFR conditions which will linger from Thursday into
Until arrival of low pressure anticipate mainly VFR through the
valid 24hr taf period...mainly middle/high clouds with scattered afternoon
high based cumulus. Winds today will be out of the west-west-northwest 7-10kts with
low end gusts west of the Blue Ridge. After sunset...tendency will
be for wind direction to become more northerly with time...speeds
decreasing to less than 7 kts.
Medium to high confidence in ceilings and visibilities through the taf valid
period. Medium to high confidence in wind speed/direction through the taf
Extended aviation forecast...
widespread MVFR ceilings/visibilities possible Thursday and Thursday night as
a synoptic scale rain event evolves over the region. An area of
low pressure is forecast to ripple east along a stationary
frontal boundary. Northeast winds will develop as a result of
induced surface low pressure which will pass just south of the
forecast area...the middle-Atlantic region residing on the cool side
of the frontal boundary. Low ceilings may linger into the day Friday
before the surface low passes off the middle Atlantic coast.
For the weekend...drier conditions return with mostly VFR
conditions outside of the typical early morning fog in the usual
spots...E.G...lwb/bcb/lyh. Any deep convection over the weekend
will be mainly confined to the mountains...coverage isolated.
The next system to bring return of scattered to numerous showers
will arrive Monday night or Tuesday.