Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
1143 PM EDT Thursday Aug 21 2014
a strong upper level ridge of high pressure near the Gulf Coast
will keep northwest flow in place over the region through the end
of the week. This will result in periodic rounds of showers and
storms into Saturday before a cold front drops through from the
north Saturday night. High pressure following the front should
bring drier and cooler air into the area by early next week.
Near term /through Friday/...
as of 1115 PM EDT Thursday...
In between systems at this time...with weak surface high over the
alleghanys...though higher Theta-E resulting in a few showers
across WV into the Virginia Piedmont. Overall mainly watching upstream
mesoscale convective system over Indiana/Ohio...and models sharing that in the morning
hours after 4 am into dawn...it will be reaching our southeast WV
counties. At what intensity...we expect it to be weaker given
time of day with lack of any instability/thermodynamics. Models
also favor weakening the vorticity overnight as well. Main axis of
higher Theta-E is shifting more over western WV/eastern Kentucky so this area of
showers and storm may dive more south-southeast through the next few hours.
Still not seeing enough of a threat overall for flooding though
will have to watch some of our WV/far SW Virginia counties which
received a generous 1-2 inches earlier today in spots. Right now
isolated flash flooding is possible but looks like system will be
moving faster...given broad 800 mb flow of 30 knots.
No changes to the forecast lows which are expected to be in the
middle 60s to around 70...from west to east. Keeping fog in as
well...mainly out east where some clearing out will take place.
Previous valid discussion...
Next wave slides down the residual boundary Friday with a
possible fading mesoscale convective system on the doorstep across the northwest early on before
it weakens crossing the ridges. However confidence low on this
outcome given guidance uncertainty but does appear better coverage
in order espcly northwest early and then again in the afternoon as may
see more widespread convective bands form along residual outflow
pending timing. Very strong northwest jet aloft for late August would
support more linear bowing type storms with potential for some of
this coverage to cross the Blue Ridge despite the deep downslope
component if can get enough breaks to enhance afternoon
instability. However models remain drier with more debris clouds
around in the morning...and a mixed out scenario in the afternoon
which would again limit probability of precipitation to the north/NW. Thus trended in that
direction with likelys extreme northwest and chance elsewhere. High temperatures
remain quite iffy as warm enough aloft to zip into the 90s east
given more sun while could get stuck in the 70s far west if rain showers
arrives earlier and persists. For now used a blend of the mav/met
MOS with support from the earlier slightly cooler ec 2 meter
Short term /Friday night through Sunday/...
as of 245 PM EDT Thursday...
Synoptic scale pattern changing to a high amplitude trough in the west
and ridge in the east. By Sunday the 591 dm center of the 500 mb
ridge is over the Tennessee Valley.
Best push of the cold front and deeper moisture to the southwest
will be Saturday night and Sunday. Until then forecast area remains
in baroclinic zone with the best probability of precipitation
dependent on upstream storms and timing of short waves. Will have
highest probability of precipitation in for Friday night.
As high pressure builds in...a surface low tracks down the East
Coast. By Saturday the low will be over the Carolina coast which
will help bring surface and low level winds over the forecast area
around to the east. This low moves well offshore as The Wedge
strengthens on Saturday night and Sunday.
Since forecast area will be well into The Wedge on Sunday have kept
maximum temperatures below most guidance.
Long term /Sunday night through Thursday/...
as of 1130 am EDT Thursday...
Surface high pressure will be in a favorable location over New
England to wedge well down the east slopes of the Appalachians
Sunday night and Monday. Support for The Wedge weakens by Tuesday.
High pressure will remain over the forecast area through Wednesday.
Wednesday night and Thursday a cold front will approach the area.
Large 500 mb ridge aloft will cover much of the lower Mississippi
Valley across the eastern United States through the period.
The deepest moisture shifts southwest out of the forecast area on
Monday and Tuesday. Will have these days dry in the forecast. The
chance of precipitation returns to the area on Wednesday and
Thursday ahead of the front. Expect maximum temperatures to be
warmest on Wednesday and Thursday with The Wedge gone and more
potential to mix down warm 850 mb temperatures.
Aviation /04z Friday through Tuesday/...
as of 735 PM EDT Thursday...
Storms will be over Dan at taf time...and exiting going on 0130z.
Elsewhere will be mainly dry though isolated to scattered showers
are possible. Fog will be the main concern overnight if we get
enough clearing of the low levels. Feel confident enough to keep lwb
LIFR per previous forecast after 06z...but will see more cloud
streaming in ahead of next impulse that is over northern Indiana this
evening. This will allow fog to erode after 09z...but will keep
IFR. Will also have IFR at blf/bcb later tonight.
Models showing main instability axis and Theta-E to hang over the
WV mountains Friday...and storms should move in by 15-17z here...then
issue is location further east and coverage. Have thunderstorms in the vicinity at all
sites with -shra in blf/lwb...delaying thunder until afternoon
east of WV.
Upper level disturbances will be moving through periodically and
interacting with a stationary front over the area for the next few
days...resulting in periods of showers and thunderstorms. Due to
the high moisture content of the air...expect stratus and fog
during the overnight periods with areas of MVFR/IFR most
pronounced between midnight and 9am each day. Models are
indicating that a deeper easterly sort of wind flow will setup
for this upcoming weekend. This suggests that low ceilings may
persist even during the daytime periods...Saturday and
Sunday...with widespread MVFR conditions across the area this
weekend due to moisture banking up against the east side of the
Appalachians. Better punch of dry air with high pressure building
in from the NE may be enough with mixing/heating to allow for
increasing VFR Monday into Tuesday under mainly dry conditions.
the ASOS at Lynchburg (lyh) Virginia remains partially inop due to a
board failure. Parts will likely need to be ordered with the
return of complete service unknown at this time.
short term...air mass
long term...air mass