Scientific Forecaster Discussion
fxus61 krnk 181148
Area forecast discussion
National Weather Service Blacksburg Virginia
748 am EDT Wed Sep 18 2019
a weak backdoor frontal system moved southwest through the
region Sunday and early this morning was located well to our
southwest near the Georgia/North Carolina border. Low clouds
behind the front will give way to mostly sunny conditions later
today as dry high pressure builds over the area from the
Tennessee Valley. High pressure will then remain in control
through the weekend with cool nights and warm days.
Near term /through tonight/...
as of 400 am EDT Monday...
Backdoor front has moved well to our southwest early this
morning and appeared to be drifting into northeast Georgia and
central SC at this hour, or roughly from ahn-cae-chs. Drier air
was gradually working its way through northern Virginia into southwest
Virginia with dewpoints dropping into the 50s in northern VA, still
lingering in the 60s across the southwest. Light northeast flow
will persist through today/tonight as surface high pressure
drifts slowly southwest from the northeastern U.S. The core of
this air mass is very dry with essentially 10% mean rh, so
dewpoints will continue to fall, and should reach the 40s north
to 50s south by Tuesday morning.
The northeast flow has resulted in fairly extensive low/mid
clouds this morning, which will also erode from the northeast
and drift further southwest through the day and evening,
lingering across the northwest NC mountains and far southwest Virginia
the longest. The weak upslope flow is helping to develop some
very shallow precipitation in the form of light rain or drizzle along
the southern Blue Ridge. Will retain low chance pops roughly in
the tnb-mwk vicinity through the day, decreasing to none after
Temperatures today should be relatively close to normal values,
which may seem cool considering that for the most part every
single day this September so far has pretty much above to well
above normal. Lows tonight with the increasingly dry air mass
will fall into the cool 40s north to 50s south. Finally a night
that feels like late September instead of mid-August. High
temperatures today will be mostly in the 70s north to near 80
southeast (e.G., Dan, sif). Once the clouds diminish, the dry
air will heat quickly, especially in light of the dry
ground/increasing coverage of d0 drought conditions.
/Confidence in forecast parameters/
temperatures - moderate to high,
precipitation probabilities - moderate to high,
winds - high,
thunderstorm threat - none.
Short term /Thursday through Saturday night/...
as of 145 am EDT Monday...
High pressure surface and aloft will be in full control for the end
of the week into next weekend. The upper ridge centered over the
eastern Continental U.S. Is creating a near Omega block configuration slowing
the progression of east-west systems. This will result in abundant
sunshine but cooler temps than of late. Will have several days with
near normal temps for mid-September. Considering we have been
running about +5f to +8f for the month to date this may feel more
dramatic than it actually is. Thursday night appears to be the
coolest in this period with widespread 40s. Wide diurnal ranges are
expected with the very dry ground across the entire area.
Unfortunately, no rain is expected through the period.
Long term /Sunday through Tuesday/...
as of 300 PM EDT Tuesday...
Overall model guidance is in agreement for the upper level ridge of
high pressure to remain in place over the southeast Continental U.S. Through
the upcoming weekend. The actual surface high will slowly move/drift
east to off the mid-Atlantic coast by Sunday - maintaining dry
conditions with slowly moderating temperatures as winds become
southwesterly. A cold front is forecast to move across the Great
Lakes and Ohio Valley early next week, crossing our forecast area
Monday. At the present time it appears little if any moisture
associated with the front will make it east of the mountains, shower
chances mainly confined to areas west of the Blue Ridge.
Temperatures this weekend, and ahead of the front Monday, are
expected to climb back above normal, with afternoon highs well into
the 80s Saturday through Monday, and lows creeping back into the 50s
to lower 60s. Depending on how much compressional warming we get
Sunday and Monday, due to the increasing southwest-west wind aloft,
it possible some locations Lee of the Blue Ridge could test 90
Aviation /12z Wednesday through Sunday/...
as of 745 am EDT Wednesday...
The backdoor front has moved well to our southwest this morning,
now located into northeast Georgia and central SC. Another basically
dry wedge front. Easterly maritime flow well established across
much of the County Warning Area at this hour. In addition, low clouds, mostly
in the MVFR range of 015-030 noted along and behind the front
for a couple hundred miles, or roughly back toward lyh and Dan,
where the ceilings become more broken. Some very light sprinkles
noted across the northwest NC mountains and foothills, but
otherwise no precipitation detected, and not much expected.
Little change expected through the taf valid period. The MVFR
ceilings are expected to hold steady generally along/west of the
Blue Ridge through early afternoon, then begin to break up from
the east as well as erode on the far western edge by afternoon.
Model soundings are rather reluctant to break up the low cloud
ceilings until mid or late afternoon. In addition, the models
want to bring the low clouds back in after this evening, which
has a fair degree of confidence given the cooler temperatures
and better likelihood of saturation in the boundary layer
overnight. The west/southwest part of the cwa, say roughly ukf-
mwk-bcb and westward would be the most favored. Thus, blf may
remain with MVFR ceilings throughout much of the day. There
should be at least a few hours, especially across the Piedmont
of VFR conditions during the evening and early overnight hours.
Fog, other than cloud induced fog at locations with higher
elevations (e.G., Bcb and blf) is not expected, but some MVFR
visibilities in fog at these higher elevations can be expected.
Winds will be northeast to east through the period at speeds of
/Confidence in forecast parameters/
ceilings - moderate,
visibilities - moderate to high,
winds - moderate to high,
thunderstorm threat - none.
Extended aviation discussion...
High pressure to our northeast building southwest over the
region combined with a strong ridge of high pressure aloft will
result in mostly VFR conditions throughout the period with no
precipitation expected at this time. The exception will be late
night and early morning fog in the usual spots (e.G., Klwb and
Confidence is moderate for ceilings and visibility Wednesday
night through Sunday.