Scientific Forecaster Discussion
fxus61 krnk 280124
Area forecast discussion
National Weather Service Blacksburg Virginia
924 PM EDT Tue Jun 27 2017
cool high pressure will build over the area from the west
overnight before passing to the coast on Thursday. Moisture
will quickly return as the high passes offshore by Friday with
possible daily rounds of showers and storms well in advance of a
cold front through the weekend. Our weather pattern may stay
active into early next week as the frontal boundary stalls over
or near the region.
Near term /through Wednesday/...
as of 905 PM EDT Tuesday...
Few changes in store with soundings/obs showing dry air in place
with only a few cumulus left on evening satellite pics. Also
expect winds to go basically calm overnight as the center of the
surface high to the west builds overhead allowing great radiational
cooling under clear skies through daybreak. Even some potential
to see a few upper 30s in the deeper valleys, pending river fog,
given dewpoints and drying of the surface over the past few
days. Otherwise will stay on the low end of guidance most spots
with 40s mountains to low/mid 50s east, as expect temps to
Chase the dewpoints down in this scenario overnight.
Previous discussion as of 200 PM EDT Tuesday...
The axis of an upper level shortwave trough is currently entering
the area. A jet streak on the east side of this feature is currently
bringing some mid to high level clouds across the area, all while
diurnal cumulus Blossom. So far, none or few of these cumulus have
continued to develop into the congestus stage. Our current forecast
for isolated showers across the mountains is probably leaning on the
more optimistic side of formation. However, none are currently
forecast, nor will they be forecast for, the start of the new tonight
period of the forecast.
Overnight, we expect skies to trend mostly clear to clear from west
to east. First the cumulus will dissipate around sunset, and then
the mid to high level clouds will exit with the translation of the
upper trough to the East Coast. Temperatures tonight will be on the
cool side thanks to good net radiational cooling, light or calm
surface winds, and boundary layer dew points already in the 40s
across much of the area. Two of our climate sites are expected to
experience a low temperature close to record levels, Bluefield, WV
and Lynchburg, Virginia. Others are expected to have lows about five to
seven degrees above the record. Please see the climate section of
this discussion for details. Low temperatures tonight will range
from the mid to upper 40s across the mountains with some sheltered
and elevated areas closer to the lower 40s. Across the Piedmont,
lows around 50 to the lower 50s will be more common.
For Wednesday, high pressure will control our weather pattern with
cooler and drier conditions in place across the region. Any cloud
cover should be limited to a few cumulus across the mountains with
sunny skies likely across the Piedmont. High temperatures will range
from the mid to upper 70s across the mountains, with higher and
sheltered area a little cooler in the lower 70s. Across the Piedmont
expect highs around 80 to the lower 80s.
Short term /Wednesday night through Friday/...
as of 240 PM EDT Monday...
The medium range models are in good agreement that an upper level
ridge builds over our area during this period, bringing a return to
warmer and more humid conditions.
With a large sfc ridge axis over the region Thursday morning and
light winds along with most clear skies, went a couple of degree
below guidance Thursday morning.
On Thursday, the surface high continues to migrate east resulting in
a light southerly flow by late in the day. While there is some
modest recovery in precipitable water values, forecast values are still expected
to be 75-100% of normal. With forecast raobs keeping a low level dry
profile, a large portion of our region will remain dry. However,
some low level moisture return into northwest NC by late in the afternoon
could spark isolated to widely scattered showers/storms.
Temperatures Thursday will be warmer, and with a downsloping flow
kept highs a couple of degree above guidance.
The more impressive surge of moisture arrives Thursday night and
Friday, which may contribute to some low clouds and patchy fog
Thursday night, along with warmer temperatures.
The models have been consistent with the idea of a weak disturbance
coming out of the western Gulf of Mexico and undercutting the upper
ridge, moving toward our region on Friday. This weakness combined
with increased instability should generate widely scattered to
scattered storms Friday with high temps near average for late June.
Long term /Friday night through Tuesday/...
as of 200 PM EDT Tuesday...
During this period of the forecast, our region will be between a
large upper ridge off the southeast U.S. Coast and an upper trof
that pushes east from the upper Midwest Saturday, into eastern
Canada into New England by Tuesday. This will result is series of
upper level disturbances moving over area and keeping a daily chance
of afternoon and evening showers and storms. The long range models
all show differences in timing and strength of these systems, but
the GFS appears to show slightly more run to run consistency,
especially later in the long term period.
The best chance for showers and storms during the long term period
will be Saturday as a surge of higher precipitable water air that the gefs
indicates in about 2 Standard deviations above normal move into
areas east of the Blue Ridge. The GFS and European model (ecmwf) indicate an upper
level wave entering the west late on Saturday.
By Sunday, there is modest agreement that a sfc boundary (likely not
the true cold front) moves into our area bringing slightly drier
air. Therefore kept lowest probability of precipitation on this day of the period.
Monday and Tuesday a low level south flow is re-established pushing
up dew points/precipitable water values/instability (especially monday). All the
long range models and ensembles hint that at least one upper level
wave moves through sometime during this time which may result in
slightly more organized convection Monday.
With 850mb temps climbing to 1-2 Standard deviations above normal
late in the period (monday-tuesday), went a degree or two above
guidance during this time. These temps combined with higher humidity
levels will likely push heat index values at least into the mid 90s
in the east for the first half of the week including the 4th of
Aviation /01z Wednesday through Sunday/...
as of 725 PM EDT Tuesday...
Cu field will dissipate with loss of heating this evening and
expect VFR all sites this taf period as high pressure settles
over the area. Expect very good radiational cooling tonight,
but given low dew points and dry antecedent conditions expect
fog to be confined to river valleys with no fog/stratus
affecting airfields. Good flying conditions will continue into
tomorrow with light winds and just a few fair weather cu and
Extended aviation discussion...
VFR conditions expected through at least early Friday. Small
chance of patchy late night/early morning mountain or River
Friday afternoon through Sunday, especially Saturday, showers
and storms return to the forecast as well as an increase in low
level moisture in general. Anticipate better chances of sub-VFR
ceiling and visibilities. Most of these during the day will be
temporary, associated with conditions immediately under any of
the stronger showers and storms. Overnight, chances of MVFR/IFR
late night/early morning fog will be greater.
as of 220 PM EDT Tuesday...
Record low temperatures for June 28th
Blacksburg, Virginia...40 in 1970
Bluefield, WV....48 in 1987
Danville, Virginia.....48 in 1987
Lynchburg, Virginia....51 in 1987
Roanoke, Virginia......48 in 1927