Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
1004 am EDT Wednesday may 27 2015
high pressure off the southeast coast will maintain a broad
southerly flow of moisture into the region through the remainder
of this week. An upper level trough of low pressure will remain
over the central part of the country...disturbances aloft
occasionally ejecting northeast out of the trough and interacting
with the moisture over our region. This weather pattern in
combination with daytime heating and instability will result in
periodic rounds of showers and storms over the next few days.
Near term /through tonight/...
as of 1000 am EDT Wednesday...
Much of the remainder of the day will be spent in between two
upper level waves of energy...with one exiting to the northeast as
of middle morning...and another over Mississippi Valley area not
arriving until overnight. Many areas had considerable cloud cover
early this morning but much of the west has cleared off with
efficient heating for much of the day. Still with weak flow and weak
convergence...dont see much in the way of any organized
convection. Sounding quite moist...but with high freezing level
and not many level of dry air...and modified sounding only showing
SBCAPE of 1200 j/kg or so...any scattered storms today will be
fairly weak and see very little if any threat for severe. Mean
flow should keep any storms moving along so also very little
threat for flooding issues...especially given drying trends over
last several days and minimal coverage of showers/storms yesterday.
With some heating in mountains and weak convergence near Blue
Ridge...think this will be most likely area for initial
development by midday...then drifting into foothills and Piedmont
later in the afternoon through early evening where best
instability this afternoon is forecast. Least coverage may be
back over western slopes where showers early this morning likely
stablized atmosphere somewhat. High res models support the idea of
minimal coverage...especially over far west...and thus have
lowered probability of precipitation a little more with slight chance far west to middle
chance from Blue Ridge east.
Previous discussion as of 530 am EDT Wednesday...
Ridge of high pressure is anchored just off the southeast Atlantic
coast with a trough of low pressure over the middle section of the
country...its associated surface cold front extending from the
western Great Lakes southwest into Kansas. Southerly flow around
the periphery of the coastal anticyclone and the Midwest trough
was resulting in an abundance of moisture flowing north from the
Gulf of Mexico into the eastern one third of the Continental U.S.. surface
dewpoints from Ontario Canada southward...all the way to the Gulf
Coast were in the 60s. This moisture rich airmass will remain in
place the next several days helping to fuel scattered showers and
thunderstorms over a big chunk of real estate.
Best chance for organized thunderstorm activity today will be
from the Ohio Valley northeast into New England associated with
short wave impulse exiting the upper level trough and passing
north of our region. Other organized deep convection will occur
farther west along the tail end of the surface front over the
central and Southern Plains. For our forecast area...the shower
activity is expected to be more disorganized and mainly diurnally
driven. Heating today should result in build-ups along the Blue
Ridge...the development of scattered showers/storms then moving
northeast into the foothills and Piedmont during the middle/late
afternoon. Deep layer flow of 20-30kts out of the south-southwest
should keep the showers moving along fairly quickly. Since the
activity is expected to be disorganized...trimmed probability of precipitation into the
40-60 percent range...emphasizing scattered as opposed to
numerous or widespread.
Temperatures will be Summer-like with highs in the 80s and lows
in the 60s...complimented by dewpoints in the 60s.
Short term /Thursday through Saturday night/...
as of 500 am EDT Wednesday...
Upper level ridging will become established on Thursday across the
southeast and Middle Atlantic States...resulting in light winds aloft
heading into the weekend. Do not expect much change in the weather
pattern for the low levels of the atmosphere...with southerly winds
maintaining the flow of moist subtropical air along the appalachian
For the Thursday through Saturday period...expect to start each
morning with patchy fog burning off...giving way to partly cloudy
skies by late morning. Spotty showers and thunderstorms will
increase in coverage across the mountains during each late morning/
early afternoon...with coverage peaking during early evening.
Convection will weaken and fade around sunset...but will likely see
a few showers linger through the overnight hours. Patchy locally
dense fog can be expected to develop each night...especially in
river valleys and in areas which received significant rainfall the
Weather forecast models do not indicate any disturbances passing
across the area which will support organized severe thunderstorms...
but cannot rule out a few storms pulsing up to strong/severe
intensity at times. Light upper level winds will make for slow storm
movement...indicating the possibility of locally heavy rain where
cells stall or train...and perhaps isolated minor flooding issues if
rainfall remains prolonged.
Temperatures across the area will be similar each day...with highs
ranging from the upper 70s and low 80s across the mountains...to the
middle 80s across the Piedmont. Overnight lows will range from the
upper 50s in spots...to the low/middle 60s for most of the area.
Long term /Sunday through Tuesday/...
as of 500 am EDT Wednesday...
Looking into Sunday...the next upper trough makes headway into the
ridge...just like its predecessor a few days prior. Anticipate this
feature to generate a greater coverage of showers and storms...
especially across the western half of the area. The surface cold
front associated with the trough is expected to enter the area
Sunday night...and stall overhead or just to our south on Monday.
The front will buckle back north early next week as southerly flow
around the west side of the high re-establishes itself. This will
keep the chance of showers and storms in the forecast...again with
the focus with the greatest concentration in the west.
Aviation /14z Wednesday through Sunday/...
as of 800 am EDT Wednesday...
Forecast area is well into a moist airmass with multi-layer cloud
cover. Most cloud bases are at or above 5kft above ground level...but there is a scattered
layer between 1500-2500 feet that may become broken attms especially when
associated with rain showers. With moist environment can't rule out
potential for rain showers but not confident on coverage. Little in the
way of showers on radar this morning...but anticipate activity to
increase with the heating of the day with greatest coverage during
the afternoon and evening.
Winds near the surface are mainly out of the south but may
briefly turn to the west over the mountains this afternoon as the
showers and thunderstorms develop over mountains and progress into the
foothills and Piedmont.
Extended aviation discussion...
Showers an thunderstorms may be somewhat more focused along and east
of the Blue Ridge Thursday.
For the remainder of the period...a moist unstable air mass
remains across the region with weak triggering mechanisms and
dynamics. A frontal system will approach from the north by the
weekend...but will likely remain well north of the County Warning Area through
Sunday. Thus...no change in air mass or the overall synoptic
pattern is expected through the period.
Early morning low clouds/fog and associated IFR-LIFR ceilings/visibilities become
increasingly more likely through the period...especially following
any late day rainfall. Otherwise...expect mostly VFR-MVFR ceilings
outside heavier showers/thunderstorms.