Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
958 am EDT Monday Aug 31 2015
an upper level disturbance passing across the Carolinas will
maintain shower activity across the area through late morning...
with additional showers then developing with daytime heating.
Upper level high pressure will then strengthen and keep US in a
rather stagnant pattern...maintaining a chance for afternoon and
evening showers and thunderstorms through the middle of the week.
Near term /through tonight/...
as of 100 am EDT Monday...
The current forecast is on track so only some minor adjustments
were made to reflect the most recent radar trends as the bulk of
precipitation associated with the short wave pulling through the
region is headed off to our east...leaving spotty showers and a
chance for some diurnal convection this afternoon.
The disturbance triggering our spotty rainfall will progress east
through late morning...with shower activity becoming isolated to
widely scattered by lunchtime. However...we can expect spotty
showers to redevelop during the afternoon as we approach maximum
daytime heating. Better chances for shower activity will be found
across the mountains...where convergence along the ridgetops will
provide added lift. Rainfall amounts will remain light...perhaps
one to two tenths of an inch in spots. Skies will remain cloudy to
mostly cloudy...which will limit afternoon highs to the upper 70s
for the mountains...and the low 80s for the Piedmont.
With the upper level disturbance out of our area by Monday evening...
expect showers to end and cloud cover to decrease. We will hang on
to the muggy airmass however...which will make for another warm
night where temperatures hold in the low to middle 60s areawide. The
decrease in clouds...coupled by light winds...will also support
another round of patchy fog.
Short term /Tuesday through Thursday night/...
as of 400 am EDT Monday...
Several weak synoptic features to note across the region
Tuesday...all underlying a broad subtropical upper ridge extending
from the south central U.S. Into the northeast U.S. Remnants of
former T.S. Erika will be located off the NC/Virginia coast and tracking
southeast around the eastern periphery of the upper ridge.
Meanwhile...a very weak area of low pressure aloft will persist from
eastern Texas into the Ohio Valley. Overall...there is little in the way
of a focusing mechanism for precipitation and any convection should
be the result of differential heating and lingering tropical
moisture across the mountains...mainly across the alleghanys of
eastern WV/western Virginia...and the Blue Ridge of northwest NC.
For Wednesday...the pattern becomes somewhat more interesting. A
weak short wave...originally emanating from low pressure in the
Pacific northwest...will track around the northern periphery of the
upper low...resulting in a temporary suppressing of the upper ridge
to the south. The weak upper trough will slide southeast-south-southeast into northern
Virginia during peak diurnal heating. With residual tropical moisture in
place...there should be an increase in afternoon/evening convection
as compared to Tuesday. Certainly...organization and coverage
should be better than Tuesday...but with weak dynamics...still
prudent to keep probability of precipitation in the low-middle chance range at this point.
The weak upper trough will sag further south to south-southwest
Thursday as a very high amplitude ridge builds toward Hudson Bay.
The upper ridge will be the underlying synoptic feature for the
entire eastern U.S. The best threat for convection will follow the
southward shift of the weak upper trough...mainly across southwest
Virginia/northwest NC...Southside Virginia...and the NC Piedmont by afternoon.
With very weak dynamics...convection will again be the result of
daytime instability and differential heating. Probability of precipitation have once again
been held mostly in the 20-30 percent range.
Under the upper ridge...850mb temperatures will hover in the +18c to +20c
range much of the period. The hottest air will remain to our
north...however. Dewpoints will hover around 60f mountains to the
upper 60s Piedmont. This will result in a continuation of above
normal temperatures through the period...especially min temperatures.
Overall expect min temperatures to average 10 degrees above normal...with
maximum temperatures 5 to 7 degrees above normal through the period. Used a
model blend for maximum/min temperatures through the period as the GFS
continues to be too warm...as it has most of the Summer. This will
result in maximum temperatures mostly in the 80s...with 70s in the mountains
and a few spots near 90 across the Piedmont.
Long term /Friday through Sunday/...
as of 415 am EDT Monday...
A late Summer...high amplitude upper ridge remains over the
northeast U.S. Into southeast Canada at the beginning of the
period with a fairly deep trough in the northwest U.S. This pattern
will remain in place through the weekend...then still appears
destined to break down early next week as the aforementioned
northwest U.S. Upper trough begins to track eastward.
Consequently...upper heights and 850mb temperatures remain well above
normal for early September as Summer hangs on. However...models
continue to indicate development of a backdoor cold front during
the later half of the week as a large 1030mb surface high develops
underneath the northeast U.S. Upper ridge. This surface high builds
southwest into the weekend...pushing the associated backdoor cold
front into the region Thursday...before it stalls across the south-
southwest part of the County Warning Area. This feature...combined with residual
antecedent tropical moisture...should serve to result in an
enhancement of diurnal convection. However...with extremely weak
dynamics aloft underneath the Mammoth upper ridge...convection will
largely be driven by low-level convergence attempting to overcome
large scale subsidence. It may end up being more of a light shower
situation with abundant cloud cover banked up along the eastern
slopes of the alleghanys. The best coverage of precipitation should
be across the northern parts of the County Warning Area Friday...shifting to the
southwest part of the County Warning Area Sat-sun.
Despite the upper ridge...the cloud cover and increasing northeast
surface winds will result in a trend toward cooler temperatures during
this period...especially maximum temperatures and most notably across the north
and east parts of the County Warning Area. Maximum temperatures will drop back from the middle
and upper 80s Thursday to the 70s by the weekend. Min temperatures will be
harder pressed to drop however with lows still mostly in the
Aviation /14z Monday through Friday/...
as of 745 am EDT Monday...
Radar indicates that the majority of heavier rain associated with
an upper level disturbance passing across the Carolinas is now
east of the taf sites this morning...leaving only spotty light
rain/sprinkles across the forecast area. Ceilings across the area
are mostly middle level...with bases ranging from 5kft to 8kft.
Visibilities across the area are ranging from 3sm to 5sm...but
expect these visibilities to improve as heating increases...after
Expect VFR conditions to persist through the day...with generally
light and variable winds. Light showers/sprinkles will persist
through the afternoon...with coverage increasing slightly across
the mountains during around maximum daytime heating. Cannot rule
out an isolated thunderstorm...however instability appears to be
strictly limited. Shower activity will diminish and skies will
scatter toward sunset...and expect to enter the night rain-free.
With partly cloudy skies and light winds overnight...expect patchy
fog to develop across most of the forecast region. Have entered
MVFR/IFR visibilities for most of the taf sites...beginning as
early as 01/06z.
Extended aviation discussion...
The forecast area will remain in a weakly forced synoptic
environment through midweek...with chances for diurnal convection
mainly from the Blue Ridge west...and some MVFR/IFR fog/stratus
development at night...especially at kbcb and klwb.
as of 400 am EDT Monday...
The dew point reading at kjfz...Richlands Virginia AWOS...is incorrect.
At this time we do not know when this sensor will be repaired.