Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
325 PM EDT Tuesday Apr 28 2015
low pressure developing over the southeastern United States will
slowly move off the middle Atlantic coast by Friday. This will
bring a chance of precipitation to the region through the end of
the workweek. High pressure will then move in from the southwest...
bringing warmer and generally dry conditions to the region for the
Near term /through Wednesday/...
as of 320 PM EDT Tuesday...
Satellite images and surface observations verify that it is indeed
a very pleasant late afternoon outside. Fair weather cumulus has been
quite sparse this afternoon and after sunset any cumulus will be
dissipating early this evening. The only thing preventing a clear
sky will be some cirrus moving in from the southwest tonight. This
will make for another night of good radiational cooling but
believe the frost threat is not as great as previous nights due to
the expected cirrus preventing maximum cooling. Expect min temperatures
generally middle 40s east to around 40 west. Only getting a pixel or
two of min temperatures to 37f in the coldest areas from the Greenbrier
valley through the Alleghany Highlands so do not believe the
situation warrants a frost/freeze headline or an Special Weather Statement.
Model trends have been to keep the low developing over the
southeast suppressed to the south which spells much less in the
way of precipitation chances for our area. However...convergence
along the Blue Ridge will combine with increasing moisture to
generate some showers along and west of the ridge as the bulk of
the precipitation skirts by to our south. Expect the morning to be
relatively dry with the best chances for showers later in the
afternoon. Maximum temperatures east generally 65f to 70f with thicker clouds
holding readings to the upper 50s/lower 60s west.
Short term /Wednesday night through Friday night/...
as of 320 PM EDT Tuesday...
The models have once again slowed the timing of the low pressure
system expected to head from the Gulf Coast to the East Coast during
this period. The overall solution favors a track somewhat further
south than what had been advertised in the past two days. Probability of precipitation were
cut downward along and just east of the Blue Ridge for Wednesday
night...due to the flow turning northwesterly to promote more
downsloping. However...it places the western slopes in more upslope
flow...so probability of precipitation had to be budged upward a little for scattered rain
showers. By Thursday...a trailing upper level low should push
southward from the Great Lakes toward the County Warning Area...which will eject the
surface low pressure system away from the East Coast. The upper
level low will bring more scattered rain showers for the day. With
cape values up to 300 j/kg and lifted indices near -1 in the model
soundings...it appears the chance for afternoon thunderstorms is
limited at best. The slight chance for convection was kept in the
forecast...as a hint of instability exists since the low pressure
system is further offshore than previously projected.
Moisture should start to become more shallow by Thursday
night...with upslope rain showers still lingering over the western
mountains. Locations in the Piedmont should see decreasing coverage
of showers into early Friday...but that decrease will be delayed
west of the Blue Ridge due to the continuing upslope flow. The only
significant change in temperatures made during this period is for
the highs on Friday...as more cloud cover and cold air advection
warranted a downward adjustment by a few degrees. Chance probability of precipitation were
kept in place through most of the County Warning Area until Friday night...as high
pressure will build eastward from the Central Plains to start the
weekend on a drier note.
Long term /Saturday through Monday/...
as of 320 PM EDT Tuesday...
The ensemble means are fairly harmonious, depicting gently
undulating zonal flow across the contiguous United States this
weekend and early next week. This will be a change from the high
amplitude flow we have endured this month suggesting the cool air
will finally make a retreat. As heights/thicknesses rise look for
temperatures to return closer to normal this weekend and favoring
above normal readings...+4 to +8 degrees...for the first full week
Threat for rainfall next week looks minimal for the middle
Atlantic...with focus for showers/storms confined mainly to the middle
section of the country. It will not be entirely clear however...as
we still anticipate periods of debris cloudiness to move quickly
east within the fast zonal flow from the storms middle Continental U.S..
With the models in relative agreement...another feature of interest
sparked some curiosity. Both the GFS and European model (ecmwf) showed the
development of a cyclone...tropical?...just off the East Coast of
Florida by the middle of next week. Whether this comes to fruition
or not...it did cause ME to take pause and look at the calendar. T-
minus 33 days and counting to the official start of hurricane
Aviation /19z Tuesday through Sunday/...
as of 130 PM EDT Tuesday...
Expect VFR conditions with light winds to be the rule through
this taf period. Satellite images show some fair weather cumulus
starting to develop across the far north and believe scattered clouds
will will find their way across most of the area during the
afternoon. As we lose convective heating early this evening cumulus
should readily dissipate with just some cirrus moving in from the
southwest. Soundings in BUFKIT were strongly hinting at a VFR ceiling
developing overnight around 825mb but the moisture indicated at
this level did not initialize well compared to 12z observation and
satellite pics so will make an active decision to disregard this
moist layer. Conditions today and yesterday have been good for
drying so even with good rad cooling expected tonight...hindered
only by cirrus... will only include a brief tempo group for MVFR
fog and some scattered stratus at kbcb/klwb/kblf since these sites are
usually the most prone to ceiling/visible issues during radiational
Moisture starts to increase tomorrow as low pressure develops over
the southeast US. Trend in guidance is to keep this system
suppressed further south and this should push any chance of
precipitation out beyond this taf period so will keep things dry
with a VFR ceiling.
Extended aviation discussion...
ceilings fall into MVFR or worse going into Wednesday night with
threat of rain. Models have trended less with the rain but still
pattern favors flow off the Atlantic that gives much of the region
Upper trough axis remains over the area Thursday into early Friday
and not confident at this point on ceilings...but this appears to
favor the mountains keeping the showers at times...with sub VFR
ceilings. Should see gradually improving conditions though as wind
starts to pick up between high to the west of the mountains and low off
the coast...from the north Friday afternoon into Friday night.
The weekend looks drier as high moves overhead.
short term...precipitable water