Scientific Forecaster Discussion
fxus61 krnk 211149
Area forecast discussion
National Weather Service Blacksburg Virginia
749 am EDT Sat Sep 21 2019
high pressure at the surface and aloft will remain over the area
into the first half of next week. A weak upper disturbance will
move across the area today, while a weak cold front approaches
the area late Monday. Warmer and more humid conditions will
return to the area through the first half of next week.
Precipitation chances will remain minimal to none.
Near term /through tonight/...
as of 430 am EDT Saturday...
A weak impulse aloft was moving through the region this morning
from the northwest. This has triggered a few showers mainly west
of the Alleghany front where moisture levels are higher.
Otherwise, low to mid clouds linger across the southern Blue
Ridge, mainly far southwest Virginia and northwest NC, with mostly
clear skies elsewhere. Patches of fog had developed primarily
in the river valleys west of the Blue Ridge. Fog was a bit more
widespread today than yesterday morning thanks to the slowly
increasing dewpoints and return of southerly flow around the
slowly southeastward moving surface high. Temperatures averaged
about 8 degrees warmer this morning than at this time yesterday
ranging from the mid 40s at Burkes Garden to the mid 60s at
Roanoke and across much of the Piedmont.
Isolated showers may pop up through the afternoon west of the
Blue Ridge as the weak impales drifts east and dissipates. No
meaningful rain is expected. Will confine pops to areas west of
the Blue Ridge and keep no higher than 25% at this point. Clouds
will linger again to some extent west of the Blue Ridge, but it
should be mostly clear to the east. Tonight will bring more of
the same with partly cloudy skies, patchy fog, again a bit more
widespread and of longer duration than the previous days, and a
continued slow warming trend.
Temperatures today will reach above normal levels again with
near 80 west to mid and upper 80s east, with 70s in the western
mountains. Lows tonight will be in the 50s west to the 60s
/Confidence in forecast parameters/
temperatures - moderate to high,
precipitation probabilities - low to moderate,
winds - moderate,
thunderstorm threat - little to none.
Short term /Sunday through Tuesday night/...
as of 330 am EDT Saturday...
On Sunday, the upper ridge rebuilds across the region after today's
passage of a weak shortwave trough. No precipitation and
temperatures around ten degrees above normal are expected Sunday
into Sunday night.
On Monday, a cold front is still progged to make progress toward the
western extent of the forecast area. However, it will be encountering
the persistent upper level ridge and relatively dry air across the
region as it arrives. With the Gulf of Mexico cut off as a moisture
source, any moisture associated with the front it will have to bring
along with it from the Great Lakes/Ohio Valley region. Showers
will be isolated to scattered in distribution with little if any
activity east of the crest of the Blue Ridge. Other than
slightly cooler temperatures in the west, although still above
normal for this time of year, no notable change in our
temperature pattern is expected in Monday with the front still
to our west.
Monday night, the front is expected to cross the area with some
isolated to scattered showers across the west. Lows temperatures
will be slightly cooler than those expected Sunday night.
On Tuesday, drier and cooler air will work its way into the region.
High temperatures will still be a few degrees above normal, but
cooler than the previous day's readings.
Confidence in the above forecast scenario is high.
Long term /Wednesday through Friday/...
as of 153 PM EDT Friday...
A broad ridge then begins to build into the southeast United
States for Wednesday and especially Thursday. There are still
some model differences in regards to rainfall. The GFS is
finally advertising some rainfall due to a weak frontal boundary
draped west to east across the region and a generally flatter
flow at 500mb. However, the European model (ecmwf) is much faster in building in
the 500mb ridge and leaves our area with little to no precip
through this entire forecast period. Other than these
differences, the models tend to agree temperatures will likely
be above normal with 850mb temps reaching anywhere from +16 to
+20. Will likely side with the persistence of a dry pattern and
go with a blend of the European model (ecmwf) and nbm for this period.
Aviation /12z Saturday through Wednesday/...
as of 745 am EDT Saturday...
Surface high pressure, which has been northeast of the region,
now located over the forecast area. This high was moving slowly
east off the North Carolina coast which will allow low level
winds to become west to southwest this morning. Despite good
radiational cooling, fog has been hard to come by this morning
and time has run out for anything significant. A very brief
window of fog is still possible through 13z at klwb and kbcb.
With the exception again of the usual spots (e.G., Klwb and
kbcb) the remainder of the taf valid period will feature VFR
conditions with a persistent inversion in the lower atmosphere
and trapped marine moisture from several days of easterly flow
across the region yielding once again a fair amount of cloud
cover during daytime heating, especially in the west. A weak
upper-level disturbance will help in this regard as well. There
could even be a few sprinkles in the western mountains, but
overall feel atmosphere is too dry for much if anything, so am
not advertising it in any tafs at this time. What little
precipitation there was a few hours ago has already dissipated.
Winds becoming west-southwest-west 5-8kts after 13z, with low end gusts
possible around kblf.
/Confidence in forecast parameters/
ceilings - moderate to high,
visibility - moderate to high, except high 14z-06z,
winds - moderate to high,
thunderstorm threat - none.
Extended aviation discussion...
High pressure will keep conditions VFR into Sunday. Only
exceptions may be late night and early morning fog in the
mountain valleys which would impact the usual terminals (e.G.,
Klwb and kbcb).
A weak cold front may bring scattered MVFR showers on Monday,
mainly in the mountains. Otherwise VFR Tuesday and Wednesday.