Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
101 PM EDT Tuesday may 26 2015
high pressure off the southeast coast will maintain a broad
southerly flow of deep moisture into the region through much of
this week. An upper level trough of low pressure to the northwest
will gradually slide east and interact with this moisture across
the area. This will result in periodic rounds of showers and
storms over the next few days.
Near term /through tonight/...
as of 945 am EDT Tuesday...
Moisture continues to increase across the region this morning as
evidenced via nearly an inch jump in precipitable waters off morning soundings
since yesterday along with a much deeper south/SW flow. This
deeper moisture also in conjunction with a weak trough axis just
west of the mountains where spotty showers continue to linger.
However cloud cover also quite widespread across the west where
will be slow to get good heating until early/middle afternoon given
surge in low level moisture. This should lead to to some
scattering or clusters of showers and storms per forecast 1k j/kg
convective available potential energy but mainly Blue Ridge west as less moisture in the east per
leftover weak ridging aloft for now. This similar to the latest
hrrr but appears it may be overdone with eastward expanse so
following closer to the latest NAM which has the best coverage
western third...and only isolated out to the Blue Ridge and little
Piedmont. Given rather wind winds aloft and lack of
shear/lapses...not expecting much severe threat outside of pulse
nature stronger cells...while appears enough movement to limit
any flooding threat unless some training occurs. Otherwise cut back on
western probability of precipitation to init this morning...followed by increasing chances
early/middle afternoon with likely probability of precipitation far west late. Temperatures may be
dampened a bit via early clouds and developing convection western
mountains...so trimmed back highs a few degrees there...but kept
80-85 from the Blue Ridge east where more sun likely.
Previous valid discussion as of 500 am EDT Tuesday...
Increasing cloud cover is anticipated today as southerly winds
transport moisture into the area from the Gulf of Mexico.
Dewpoints will trend upward and combine with daytime heating to
yield scattered showers and thunderstorms.
High pressure which passed overhead during the Holiday weekend
providing the near cloud free conditions is now positioned just
off the southeast Atlantic coast. Loss of strong subsidence will
now open the door for showers per increasing moist southerly flow
on the western periphery of the anticyclone. Dewpoints climbing
into the 60s along with daytime heating will result in increasing
instability and the formation of scattered showers and
thunderstorms. Shower activity today is expected to be most
pronounced over the mountains...primarily along and west of the
An upper level trough of low pressure over the middle section of the
country is slowly moving east with numerous showers/storms.
Outflow from some of this activity will likely reach our western
mountains tonight allowing for a continued threat for deep
convection during the overnight.
Temperatures today will be similar to Monday...maybe even a degree
or two lower in the mountains per increasing cloud cover.
Dewpoints however...will be on the increase...and offset any
perceived cooling...the higher humidity resulting in higher
apparent temperatures with the air having more of a muggy feel.
Minimum temperatures tonight will likely remain at or above 60...higher
dewpoints and cloud cover limiting the radiative cooling effects.
Short term /Wednesday through Friday night/...
as of 500 am EDT Tuesday...
Expect scattered showers and thunderstorms for much of the area on
Wednesday as an upper level trough passes across the appalachian
chain. The trough is expected to be close enough to our western
border such that shower activity will be ongoing at the start of the
day across the mountains. Best dynamics/strongest instability
associated with the trough will remain north of the area. In
addition...rainfall and widespread cloud cover early in the day will
limit heating and surface-based instability...and do not expect
organized severe thunderstorm activity...although a few strong
storms will still be possible during the afternoon and evening. On
the other hand...with deep Gulf moisture in place...will have to
keep an eye out for locally heavy rain...as well as the possibility
of isolated/minor flooding issues.
Upper level ridging will redevelop on Thursday across the southeast/
Middle Atlantic States. While weak upper level disturbances will still
pass across the area...shower and thunderstorm activity for Thursday
and Friday will be driven more by daytime heating of the soupy air.
Convection will increase late each morning...peak during early
evening...and will diminish after sunset...although do not think
rainfall will go away completely each night as spotty showers linger
across the area. With little upper level support...expect only
locally strong thunderstorms each day with no organized activity.
Upper level ridging will make for light winds aloft and slow storm
movement...so the threat of locally heavy rain will continue through
the end of the workweek.
Increased cloud cover during the period will limit heating...holding
afternoon highs in the middle to upper 70s across the mountains...and
the low/middle 80s east of the Blue Ridge. With no change in airmass
expected...expect muggy conditions each night...with lows holding in
the low/middle 60s areawide.
Long term /Saturday through Monday/...
as of 400 am EDT Tuesday...
In extended period the synoptic setup does not really change too
much. The ridge over the Atlantic coast continues to direct a steady
supply of warmth and moisture into the eastern Seaboard. At the same
there will be a chance for somewhat more organized activity
associated with a slow-moving boundary trailing from a surface low
tracking into the Great Lakes on Saturday. The European model (ecmwf) shows this
feature becoming quasi-stationary to our north Ohio-PA-NJ axis on late
Saturday into Monday and then possibly sliding farther south into
our area by later Monday. GFS perhaps a bit further north with this
feature and more progressive and eventually builds high pressure
over the northeast which pushes a backdoor type front toward the
area late Sunday. GFS ensembles solution through this period mostly
maintain the ridge over the southeast in a position that would hold
the boundary and associated precipitation further north. HPC
guidance closer to Euro solution and was basically followed for the
extended. This all means continued well above normal
temperatures...especially at night...along with abundant moisture
and high chances for at least afternoon/evening convection.
Aviation /18z Tuesday through Sunday/...
as of 1245 PM EDT Tuesday...
Starting to see isolated convection pop up across the far west
early this afternoon where moisture remains deeper and aided by
differential heating. Expect this area to gradually fill in and
become a bit more organized which will affect the kblf-klwb
corridor and possibly around kbcb if rain showers/thunderstorms and rain spill farther east.
Thus will carry a prevailing thunderstorms in the vicinity/vcsh and tempo in more in the
way of MVFR to IFR conditions pending if a shower or storm
directly impact one of these locations. Otherwise expecting mainly 4-6k
feet VFR ceilings this afternoon with little shower coverage at
klyh/kdan with isolated potential at kroa. Winds this afternoon
will remain out of the south 5-10kts...with gusts 14-18kts.
Expecting convection to fade with loss of heating this evening and
in the wake of a weak impulse sliding by to the NE. This may leave
most of the region in VFR under middle deck with perhaps some MVFR in
fog this evening where earlier rain occurred. Next shortwave
trough now to the SW will slide into the area later tonight with
an associated band of rain showers/isolated thunderstorms and rain spreading in from the SW
after midnight. Guidance has most of this affecting the mountains
mainly kblf/klwb late and east to kbcb/kroa around daybreak if not
a bit sooner. Thus including a period of prevailing MVFR in rain showers
early Wednesday morning with some of this perhaps sliding into
kroa/klyh Wednesday morning with possible MVFR. Low clouds ahead
of this feature may also bring MVFR to IFR ceilings along and east of
the Blue Ridge overnight but iffy given model tendency to bring in
stratus so held off going that pessimistic for now.
Extended aviation discussion...
Wednesday...the upper disturbance will continue to shift east
across the region with decreasing activity over the west by midday
likely redeveloping along and east of the Blue Ridge Wednesday
afternoon. Timing of this weak trough tricky as could see more
drying aloft over the west by afternoon per latest BUFKIT
soundings with convergence over the east. Latest guidance even
suggests a return to VFR west of the Blue Ridge Wednesday
afternoon with periods of MVFR/IFR out east pending convective
coverage. This boundary drifts into eastern Virginia Thursday before washing
out...so activity could be somewhat more focused along- 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. 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. Winds remaining SW 5-10kts
through much of the period...except variable and gusty in/near