Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
937 PM EDT sun Apr 26 2015
cool and unsettled weather will be in place across the
Appalachians and central middle Atlantic region this week... as a
cool upper low remains anchored over the northeast United States.
The best chance for precipitation will be Wednesday and
Thursday...as low pressure develops over the southeast and tracks
just to our south and east...before moving off the middle Atlantic
coast Thursday night.
Near term /through Monday/...
as of 930 PM EDT Sunday...
Only minor changes made to the near term. Satellite and surface
observation show that the northern part of the County Warning Area has cleared out nicely
while low/middle clouds are hanging tough along the Virginia/NC line.
Clouds should continue to erode through the early morning hours.
As of 330 PM EDT Sunday...
The Wedge is slowly eroding from the north as low pressure is
pulling away and allowing drier air to work in. Visible satellite
images show the clearing and other breaks in the clouds and this
is allowing the sun to give temperatures a bit of a late day boost. The
clearing trend is expected to slowly continue through early
tonight and this will allow for some chilly temperatures
overnight. There may be some patchy frost west of the Blue Ridge
and in some low spots across the Piedmont but do not believe
conditions will be widespread enough for an advisory so will
highlight this in an Special Weather Statement for areas west of the Blue Ridge.
We should be in for a bit of early Morning Sun to start off on
Monday...but with a pool of cold air aloft and a vigorous short
wave rotating around the upper low over the northeast US...expect
clouds to bloom by late morning and a chance/slight chance of showers
west of the Blue Ridge by the afternoon. Isolated thunder and some
graupel is not out of the question from the Greenbrier valley
through the Alleghany Highlands. Temperatures tomorrow will continue to
run below normal with highs generally middle 50s west of the ridge
to middle 60s east. The boundary layer will also be well mixed so
expect a good northwest breeze to add to the chill.
Short term /Monday night through Wednesday night/...
as of 330 PM EDT Sunday...
Two systems will be the primary players in our weather for the first
half of the work week. The first is a large upper low off the New
England coast...and the second is a southern stream system that
moves through the Southern Plains into Tennessee Valley by
With regard to the second system...model ensembles show there is low
confidence on the timing and strength of this system. The 12z GFS is
strongest and slower...while the NAM appears to be an outlier with
its faster solution. Generally followed the 00z European model (ecmwf) with this
system...which has shown good consistency keeping the system closed
through 00z Thursday.
A shortwave pivoting around the large New England upper low exits
our area to the south before 12z Tuesday. Lingering convective
showers under this system Monday evening should diminish with the
loss of heating. Meanwhile...at the surface...high pressure begins
to nose into western portion of the forecast area early Tuesday. If
winds can subside enough...temperatures in some of the higher elevations in
southeast WV may allow for patchy frost to form.
Tuesday appears to be the most tranquil day of this period...as a
weak shortwave ridge builds in advance of the southern stream system
and a blend of the bias corrected met/NAM MOS was used for highs.
We start to experience the effects of the southern stream system
Tuesday night and Wednesday as clouds Tuesday night and light rain
arrives as early as Wednesday morning in the northwest NC mountains. We
could potentially see a big drop off in temperatures in the northern portion
of our forecast area early before clouds arrive late Tuesday night.
With dew points in the 40s...it may take some time for the
atmosphere to saturate to allow rain to reach the ground.
Weak upslope flow should help focus the light precipitation from the
southern Blue Ridge int the northwest NC mountains and foothills Wednesday.
Kept high temperatures below guidance with expected widespread
Long term /Thursday through Sunday/...
as of 330 PM EDT Sunday...
While there are still differences in timing...the long range models
show a similar solution of lifting the deep eastern U.S. Trough to
the east and slowly building an upper ridge across the Ohio Valley
by the end of the weekend.
The GFS appears overdone and too far north with its placement of the
axis of heavy rain in our forecast area. Obviously this will need to
be monitored...but leaned toward the European model (ecmwf) and Canadian solutions of
keeping the axis of heaviest rain closer to the forecast position of
the surface low in the southern Carolinas.
Wrap around moisture will keep clouds...much cooler than normal
temperatures...and light precipitation through Thursday and Thursday night. With
the deepening coastal surface low...and passing of a weak upper vorticity
lobe on the west side of the upper trough axis...bumped up winds late
Thursday and Friday...but nothing that appears to warrant headlines
at this time.
As the deep surface low continues to move northeast...we should finally
see some clearing Friday...then temperatures closer to average for early
may by the weekend.
Aviation /02z Monday through Friday/...
as of 725 PM EDT Sunday...
Cold frontal boundary continues to drift south this evening. Dry
air behind the front beginning to advect in across the
north...scouring clouds and lifting ceilings to VFR. Clearing
skies...weak winds and saturated ground could allow cross-over
temperatures to be realized and have fog developing
overnight...especially in mountain valleys. Even though northwest
flow is weak...it should deter areas of fog from forming east of
the Blue Ridge. Models bring a couple of middle level waves across
the Ohio Valley and over the Appalachians Monday. One is
forecasted to track over the area by sunrise...which will limit
fog from hanging around into the morning. The second passes north
of the area and will bring breezy/gusty winds to the region Monday
Extended aviation discussion...
breezy northwest flow is expected through early Tuesday as high
pressure builds in from the northwest. VFR conditions are expected
through Wednesday before conditions worsen once again Wednesday
night into Thursday...as a low pressure system passes across the
southeast states. 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 same time.