Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
936 PM EDT Friday may 29 2015
high pressure offshore will cover much of the eastern United
States Saturday and Sunday. A cold front will approach from the
northwest late in the weekend...then south of the area by
Monday...resulting in more widespread showers and thunderstorms
into Tuesday. Cooler weather with high pressure will build in the
middle of next week.
Near term /through Saturday/...
as of 935 PM EDT Friday...
Showers/thunderstorms rapidly dissipating with loss of daytime
heating. As of 935 PM EDT...the only remaining area was over
portions of the Greenbrier valley into the Alleghany
Highlands...where confluence in vicinity of decaying outflow
boundary and forced ascent has maintained sufficient upward
vertical motion to maintain storms a bit longer than elsewhere.
A second small area approaching from the south toward Wilkesboro,
NC...where boundary layer still a bit warmer and not worked over
from earlier convective activity.
Regardless...even these areas should experience dissipation
before midnight as lower levels stabilize and with no forcing
aloft to maintain upward vertical motion.
Previous forecast for overnight period still on track and
As of 245 PM EDT Friday...
upper ridge axis pushes slightly southeast tonight and Saturday
but will still be over Virginia and North Carolina. A weak surface
front extended from the Delaware-Maryland-Virginia to Ohio. Do not expect this
boundary to make any farther progress south.
If there is any lingering showers and thunderstorms after the loss
of daytime heating...models showed the most likely location will be
in the far northwest County Warning Area. The same location has the
best chance of thunderstorms again Saturday afternoon. Plan putting
in little to no chance of rain from 07/3am through noon/17z
Similar to Friday morning...locations that have rain this afternoon
are more likely to have fog overnight. Will add in more patchy fog
Guidance was too warm for the lows Friday morning. Will lean toward
somewhat cooler met guidance for overnight lows. Expect maximum
temperatures for Saturday to be similar to Friday.
Short term /Saturday night through Monday night/...
as of 230 PM EDT Friday...
High pressure will remain anchored off the coast of the Carolinas
during this portion of the forecast...all while an upper level
trough and surface cold front push east through the mid-west. The
result will be increasing chances of more than simply diurnally
based showers and storms. We will gain increased jet dynamics with
the approach of the trough/front...especially across western parts
of the region. The front itself will move into and across the area
Temperatures through the period will trend slightly cooler thanks to
increased cloud cover...and a trend toward slightly lower dew points
behind the front Monday night.
Long term /Tuesday through Friday/...
as of 245 PM EDT Friday...
The cold front will wash out over or just south of the area by
Tuesday afternoon. After this point...the region will start to
transition back a southerly flow pattern as a low/trough develops
across the lower Mississippi River valley...and high pressure
becomes centered again off the coast of the Carolinas. Daytime
heating along with pieces of energy ejecting off the lower
Mississippi Valley low will help yield increasing chance of showers
and storms again by Thursday into Friday. Temperatures will also
start to trend milder Thursday into Friday.
Aviation /02z Saturday through Wednesday/...
as of 805 PM EDT Friday...
Backedge of scattered showers/storms now mainly north of Highway
460...continuing to move north...and now showing signs of
weakening with loss of daytime heating. Have therefore removed
threat for any additional precipitation at all terminal forecast
points except for klwb...which will likely remain under a threat
until around 02z/10pm.
Otherwise...previous forecast thinking concerning the development
of MVFR or IFR fog later tonight in areas that received rain
earlier today still seems reasonable...and this supported by
latest lamp guidance. Klwb...klyh...and kbcb have the greatest
potential for development of late night visibility restrictions...less
so at kdan...kblf...and kroa.
Similar to Friday morning...expect most late night fog to
dissipate by 13z/9am...to be quickly followed by development of
cumulus with onset of surface heating. However...unlike
Friday...expect some subsidence nudging in from the southeast such
that any diurnal redevelopment of showers/storms by late
morning/early afternoon expected to be less in overall coverage
and mainly confined to southeast West Virginia. Therefore...thunderstorms in the vicinity
comment inserted into kblf and klwb terminal forecast points at
17z/1pm...but not at the remaining aviation forecast points.
On Sunday...a moist unstable air mass will remain in place across
the region with only weak forcing. This will result in mainly slow
moving diurnally driven deep convection during the afternoon and
evening hours followed by stable conditions and patchy MVFR fog at
A frontal system will approach from the north Sunday night. This
boundary should finally sink south into the region by Monday resulting
in widespread convection...with additional rounds of rain showers/thunderstorms and rain
into at least Monday night before the boundary sinks to the
south. This should make for more frequent periods of sub-VFR from
Sunday night into Tuesday morning with lower ceilings/visibility reductions
due to showers/storms during the day...and in fog overnight into
the early morning hours each day. Lower MVFR/IFR ceilings are likely
behind the front on Tuesday and Wednesday with easterly flow and
wedging in place resulting in MVFR ceilings.
May see enough drying to bring an end to the lower clouds and
precipitation on Thursday and Friday. These are the most likely
days to be VFR.