Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
341 PM EDT Wednesday Jul 30 2014
an upper trough of low pressure will retrograde west to the
Mississippi River valley...and the center of surface high pressure
will shift to our east. The result through the weekend and into
the early part of next week will be an increased chance of showers
and storms across our region starting on Thursday.
Currently...Friday into Saturday looks to be the time window with
the greatest potential for the entire area.
Near term /through Thursday/...
as of 320 PM EDT Wednesday...
We will continue to watch to potential areas of convection heading
into the early evening hours. The one will be centered just
southeast of the forecast area...but its influence may be extend far
enough north for some isolated showers and storms in areas
around...east and south of Danville Virginia through roughly 800 PM. The
other area will be in association with some instability that has
developed just west of the southeast Wes Virginia portion of the
forecast area. While areas around and west of Lewisburg WV may see a
stray shower between now and roughly 500 PM...chances will increase
as we approach 700 PM. A disturbance with an area of steep lapse
rate air is currently moving through Ohio...and should be
approaching the area toward the early evening. Both the hrrr and rnk
WRF-arw offer solutions that bring the activity to our doorstep and
then have it dissipate. Have allowed for the potential for some
isolated thunderstorms this evening in areas near and north of
Interstate 64 in WV and Virginia. Other isolated showers will be possible
near the crest of the Blue Ridge from the late afternoon into the
early evening. While instability is not strong in this
area...convergence within this area may be enough for a few showers
to form. The main focus will be across the far northwest and far
southeast parts of the region.
A high pressure ridge axis was situated west-east across the area
currently. Easterly flow on the south side of this feature has
helped advect higher dew points in the lower 60s back toward the
area as close as Roxboro NC and South Hill Virginia. The center of the
high is prognosticated to start shifting eastward tonight. This will help
allow for a gradual trend towards low level winds with a greater
southerly component across the region. In turn...higher dew point
air is expected to continue its progression towards and then into
the area. By daybreak Thursday...some of this moisture will become
banked against the crest of the Blue Ridge with increased cloud
cover over northwest North Carolina and neighboring sections of
southwest Virginia. Low temperatures tonight should be milder as
compared to those of this morning. Readings around 50 to the lower
50s are expected across the mountains with a mix of middle to upper 50s
across the Piedmont. A few spots across Southside Virginia and
neighboring north central North Carolina can expect lows around 60.
Of our six climate sites...Lewisburg has the potential of being
close to a record low tonight. See the climate section of this
discussion for details.
On Thursday...the trend towards east-southeast low level flow and greater
moisture will continue. Increasing clouds will be the norm
throughout the day with upslope rain showers developing and growing
in coverage. The greatest concentration will be across the northern
mountains of North Carolina...northeast along the crest of the Blue
Ridge to near Floyd Virginia. High temperatures will range from the middle
70s to around 80 in the mountains to the low to middle 80s across the
Short term /Thursday night through Saturday/...
as of 330 PM EDT Thursday...
Models consistent bringing a short wave through the southwest flow
aloft Thursday night through Friday night. Thursday night and Friday
there is also some support from the diffluent area of the upper jet.
May keep probability of thunderstorms low since guidance is showing
little cape and instability over much of the forecast area. By
Friday morning...precipitable water values will be back in the 1.0
to 1.5 inch range with surface dew points in the 60s. Wpc 24 hour
quantitative precipitation forecast from 12z Friday to 12z Saturday of 0.25 to 0.50 inches looks
reasonable. However...models do not have a particularly good
consensus of where the heaviest amounts will be. Easterly component
of low level winds favors the eastern slopes of the Blue Ridge.
Putting the highest probability of precipitation in from late
Thursday night through late morning on Friday.
Wide range in guidance for maximum temperatures on Friday. With all
the clouds and precipitation expected...have trended toward the
cooler bias corrected mav numbers.
Long term /Saturday night through Wednesday/...
as of 330 PM EDT Thursday...
Front at the surface remains along the coast Saturday night through
Monday. On Tuesday and Wednesday...a cold front will push into the
region from the northwest.
Until this front GOES through...there will be a daily threat of
showers and thunderstorms each afternoon and evening. Challenging to
narrow down which day of time frame will be more favorable for
precipitation...although models suggest a short wave coming through
the Ohio Valley on Sunday may bring a more organized area of showers
and thunderstorms to the County Warning Area.
Pocket of warmer 850 mb air with +20 temperatures comes out of the
Mississippi Valley...reaching the forecast area on Wednesday based
on the European model (ecmwf).
Aviation /20z Wednesday through Monday/...
as of 100 PM EDT Wednesday...
High confidence in light winds through the taf period.
Medium/high confidence VFR east of the Blue Ridge.
Medium confidence VFR yielding to MVFR/IFR in fog/stratus tonight
west of the Blue Ridge.
Weather this taf period should be quite similar to what we had
yesterday as energy rotates around the upper trough and combines
with instability in the cold pool of air to produce a good cumulus field
and some isolated showers...especially from the Blue Ridge westward.
Believe the cumulus will be a VFR scattered v broken type situation. Will lean
optimistic and keep scattered except for kblf/klwb where broken will be
more likely due to proximity to short wave energy. Will also keep
tafs dry since extent of any showers will be quite limited.
Expecting another round of MVFR/IFR fog and stratus overnight for
klwb/kblf/kbcb but if cumulus/SC cloud deck lingers it may delay onset
and ultimately limit extent of fog/stratus. Will choose to go with
quicker cumulus/SC dissipation this evening/tonight and more aggressive
fog/stratus overnight. Moisture will be on the increase from the south
tomorrow as more short wave energy rounds the base of the upper trough
but any significant affects look to be beyond this taf issuance.
Extended aviation discussion...
a broad trough of low pressure will be across the eastern United
States through Sunday. However...it will start centered over the
region...and then retrograde west to near the Mississippi River by
Sunday. This transition will allow for better chances of
showers...and perhaps a few thunderstorms...to occur across the
region as Atlantic moisture will be able to work its way into the
area. Appears coverage will be more isolated on Thursday with
lingering overall VFR...then increasing from Friday into the
weekend as a deeper southerly trajectory starts to develop. Along
with this moisture will come better chances for sub-VFR
conditions...especially under any heavier shower or storm that
forms...but also during the overnight hours as boundary layer
moisture gets trapped under the nocturnal inversion.
July 31st record lows:
the National oceanic and atmospheric administration weather radio at Wytheville Virginia...wz2500...broadcasting
on a frequency of 162.450 mhz...is off the air. Technicians are
waiting on delivery of parts to make repairs. The earliest the
radio will be working again is Friday.
short term...air mass
long term...air mass