Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
749 am EDT sun Apr 26 2015
with the departure of an upper level system from the middle Atlantic
this morning...high pressure building in from the Midwest region
will become the main weather feature for our area today. Another low
pressure system is expected to affect the southeast and middle Atlantic
region during the middle of the week.
Near term /through tonight/...
as of 345 am EDT Sunday...
Keeping an eye on an area of rain...with the occasional embedded
thunderstorm...making its way east across central Virginia this
morning as an upper level disturbance moves across the region. Rapid
update models indicate that the majority of this rain will exit our
area for the coastal plain by 7 am...leaving only isolated
showers...which will be intermixed with areas of fog and drizzle
through late morning.
With the departure of the upper level disturbance...high pressure
building in from the Midwest states will gradually take over as our
dominating weather feature...causing winds across our area to slowly
shift northwest through the day. The very slow arrival of drier air
means that low clouds will remain across our forecast area through
much of the day...keeping a firm lid on daytime heating. Although
slightly warmer than those observed on Saturday...high temperatures
this afternoon will remain well below normal for late April...
generally holding in the low to middle 50s most locations...with upper
40s possible in a few spots near the Interstate 64 corridor.
Clouds will start to break up during late afternoon/early evening
east of the Blue Ridge...with cloud cover decreasing across the
mountains after dark. The arrival of high pressure will also cause
the pressure gradient to increase for the central Appalachians...and
we can expect winds to become breezy after midnight. Looking once
again at another cool night...with Sunday night lows ranging from
the middle 40s across the foothills in North Carolina...to the low and
middle 30s along the Interstate 64 corridor. May be a few spots where
temperatures fall below freezing in the north...however the
increased wind speeds will likely prevent frost formation. Will let
the day shift take a look at another batch of model data...and
determine if any frost/freeze statements are necessary for tonight.
Short term /Monday through Wednesday night/...
as of 400 am EDT Sunday...
Convoluted upper air pattern remains in place through the period.
The main features being a deep 529dm upper low off the northeast
U.S. Coast extending well back into the Ohio Valley/mid-Atlantic
region...a closed low coming out of the Desert Southwest into
Texas...and a elongated ridge in between between the central
Canadian provinces down into the central u... the southeast
U.S. For Monday and Tuesday...our region will remain largely under the
influence of the northeast U.S./Northern middle-Atlantic upper low. A
spoke of positive vorticity advection/short wave will rotate through the region from the
north on the west side of the elongated upper low. With 850mb
temperatures continuing to hover near 0c and cold 500mb temperatures -20 to -24c...
the atmosphere will be quite unstable. Any morning sunshine will
quickly result in cumulus/SC developing and likely -shra across the
mountains by late morning through the early evening. Temperatures
will remain well below normal...and any showers will easily bring
those areas back down into the 40s during the afternoon. In
fact...850mb temperatures could support some -sn at the highest elevations
Monday morning...but have not included in the grids at this time.
Showers will be considerably less likely in the downslope Piedmont
areas. Less clouds in this area will also allow maximum temperatures to warm
back into the 60s...yet still below normal.
For Tuesday...the deep upper low finally begins to drift east further
out to sea...while the upper low across Texas drifts into the
arklatex region. A new upper low begins to drift south out of the
central Canadian provinces...while the skinny/complicated upper
ridge drifts into our region. This should be the one day with
little to no -shra activity with no forcing evident aloft and weak
ridging. Increased sunshine should allow temperatures to warm a tad
closer to normal...but 850mb temperatures only warm a few degrees to
about +4c across much of the County Warning Area.
For Wednesday...the arklatex upper low drifts into the southeast states
while low pressure aloft drifts southward into the Great Lakes.
These two system will merge across the region into Thursday as yet
another upper low forms across the middle-Atlantic by Thursday. Light
rain will begin to spread into the region Wednesday as heights aloft
lower as the upper lows converge across the region. This...along
with cloud cover and increased precipitation will ensure continued
well below normal temperatures. Quantitative precipitation forecast does not appear to be an issue
at this point as the heaviest rainfall will remain well south of
Long term /Thursday through Saturday/...
as of 430 am EDT Sunday...
The GFS and Canadian have come into better agreement with the
European model (ecmwf) on development and evolution of a coastal low/almost
Miller-a type setup off the southeast U.S. Coast Thursday. This will
spread rain into the region from the south. As the southern stream
low becomes absorbed into southward moving upper low from the
Great Lakes...a deep 540dm upper low will evolve across WV/Virginia into
early Friday. This will prolong the light rain...northerly flow...and
unseasonably cool temperatures right into Friday. With the region
largely under a cool/stable/northerly flow...precipitable waters across the
region are generally around 1/2 inch during this time frame...so
heavy rain does not appear to be a concern. That should remain
well southeast along coastal areas and then off the Virginia/Maryland coast
with time. But...850mb temperatures will continue to hover near 0c
throughout the period as the upper low dumbbells across the middle-
Atlantic...keeping maximum temperatures for sure well below normal...but min
temperatures slightly below normal because of cloud cover
Aviation /12z Sunday through Thursday/...
as of 745 am EDT Sunday...
Deep upper low off the northeast U.S. Coast will sink slowly
southward to off the middle-Atlantic coast by Monday. Associated
west-east oriented frontal boundary from tys-rdu...will sink
slowly south as well. Wrap around -shra will continue across the
County Warning Area with weak forcing from the north on back side of upper low.
Double barrel surface low...one in the eastern Tennessee Valley and the other
off the NC coast will shift southeast with time. Winds will back
from NE to northwest through the taf valid period. This will keep a moist
Atlantic flow east of the Blue Ridge until evening and a moist
upslope flow west of the Blue Ridge. -Shra will continue
throughout the day...but gradually diminish and shift southeast by
afternoon. Will drop definite -shra from from groups after 18z...but
include vcsh through the remainder of the afternoon. IFR-LIFR ceilings
through 15z will gradually improve to MVFR by early-middle afternoon
east of the Blue Ridge...but likely only to IFR to low end MVFR
west of the Blue Ridge in upslope flow. Visibilities generally now VFR
east of the Blue Ridge...but lingering MVFR-IFR west of the Blue
Ridge. Expect most areas to improve to VFR by late morning/early
afternoon...with the exception of blf which could linger with
IFR-MVFR visibilities much longer...and which may return after 00z. Will
need to watch spoke of positive vorticity advection/short wave approaching from the north
late tonight with possible renewed shower activity across the
mountains of WV at that time. Winds NE 5-7kts through middle-
day...backing to northwest 6-8kts during the afternoon/evening.
Medium confidence in ceilings and visibilities through the taf valid period.
Medium confidence in wind direction/speed through the taf valid period.
Extended aviation discussion...
breezy northwest flow is expected for Monday and early Tuesday as
high pressure builds in from the northwest. As noted above...a
short wave will rotate around the back side of the northern
Atlantic upper low...which will keep variable clouds and scattered -shra
across the mountains...especially during peak heating. MVFR ceilings
may linger in these areas...but VFR ceilings to no ceilings are expected
across the Piedmont. Winds will be northwest 5-10kts...with some
low end gusts possible west of the Blue Ridge. VFR conditions are
expected Tuesday before conditions worsen once again Wednesday-Thursday as a low
pressure system passes across the southeast states...which will
affect the middle Atlantic on Wednesday into Thursday. Confidence is
growing that this system will result in widespread rain for the
forecast area...as well as persistent MVFR/IFR conditions. An
upper low will also build southward from the Great Lakes at the