Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
939 PM EST Wednesday Nov 25 2015
a ridge of high pressure will remain along the East Coast through
the rest of the week. A cold front will approach the region on
Saturday and stall overhead into the start of next week. Low
pressure will move through the Great Lakes region by the middle of
next week...potentially bringing another cold front across the
Near term /through Thursday/...
as of 935 PM EST Wednesday...
Temperatures are falling a little faster than previously
expected...so lows have been bumped downward a few degrees...but
not much further due to the expected increase of clouds later
tonight. There is quite a bit of variability with temperatures
across the County Warning Area right now...with some locations either being
decoupled under calm winds...or having a southeast wind gusting
up to 30 miles per hour as is the case at the kblf ASOS. This variability
may likely continue through the night. No other adjustments were
needed in this update.
Previous discussion as of 215 PM EST Wednesday...
Through the overnight hours...additional waves of middle to high
level cloud cover will stream west to east across the area. The
greatest concentration will be across the south. The southeast-
south low level winds will just start to tap moisture from off the
Atlantic by an initial westward Route through South Carolina and
northeast Georgia. Expect the most low cloud coverage across the
northern mountains and foothills of North Carolina
overnight...with the bulk of them developing in the early morning
hours of Thursday. Low temperatures will trend milder has higher
surface dew points advect into the region. Look for most areas to
be fall to the low to middle 30s.
Thanksgiving day will be the first day in the next several to come
where moderating temperatures are expected. Heights will start to
build in advance of an approaching cold front...and warm air
advection will start to increase. Expect highs about two or three
degrees milder than those of today. A mix of low to middle 50s are
expected in the mountains with upper 50s to near 60 across the
For a number of forecast cycles...we have been pondering the
potential for some drizzle or light rain formation along and near
the crest of the Blue Ridge tomorrow. The guidance from last night
showed that potential to be less and the precipitation was removed
from the forecast. The latest guidance...other than more moisture
laden NAM...continue to forego this potential. This latest forecast
will do likewise...but continue to offer a greater amount of cloud
cover tomorrow in areas along and near the crest of the Blue Ridge.
Winds will continue to be light in the eastern parts of the area
with sustained around 10 miles per hour expected in the west. Gusts in the west
will be a little weaker than those of today with the flow not as
orthogonal to the ridge lines.
Short term /Thursday night through Saturday night/...
as of 200 PM EST Wednesday...
Initially cool and dry weather pattern will gradually transition to
more of a cloudy and dreary period Thursday night into Saturday
night as main axis of coastal surface ridge weakens and drifts
offshore...allowing low level winds to gradually veer into a
southeasterly upslope component against the east flanks of the
Appalachians on Thursday night.
At the present time...the depth of the moisture in the lower
troposphere over upslope areas appears to be shallow enough on
Thursday night to guarantee a return to abundant low level cloud
cover...but not necessarily enough to wring out measurable
precipitation...although some patchy light drizzle in preferred
upslope areas...especially southwest of Roanoke into the northwest NC
mountains appears reasonable. Elsewhere...a gradual increase in
cloud cover is expected...but not much else.
Approach of northern stream short wave moving east through the upper
and lower Great Lakes Friday into Friday night will flatten and
suppress upper ridge axis over middle Atlantic region...allowing low
level winds to continue to slowly veer into more of a S-SW component
on Friday. This should end any upsloping patchy drizzle across the
eastern flanks of the Appalachians as upslope component is lost...
but will continue to advect low level moisture north into the
area...maintaining mostly cloudy skies.
Passage of short wave trough to the north will support building
surface high pressure over the upper plains into the Great Lake
region...driving a cold front and associated showers southeast into
the forecast area Saturday into Saturday night. Greatest threat for
measurable rain during this period should occur from the Alleghany
Highlands of Virginia SW into southeast WV...where deeper tropospheric moisture
and better upward vertical motion fields will reside.
Abundant cloud cover through the period should ensure that above
freezing temperatures are maintained...but will also limit diurnal
fluctuation of temperature...with readings warmer than normal during
overnight periods...and close to seasonal normals during daytime
Long term /Sunday through Wednesday/...
as of 200 PM EST Wednesday...
Unsettled and mainly cloudy and moist period appears to be in the
offing for Sunday into Wednesday as cool front stalls over or just
south of the forecast area as upper flow backs in advance short wave
trough being ejected NE from the base of a large closed upper low
pressure system that will be centered over the SW/central portion of
Meanwhile...surface ridge of high pressure will once again become
established over New England...placing the Blacksburg forecast
area in a cool moist wedge with onshore increasingly upsloping
flow and deep weak upward vertical motion from isentropic lift and
approach of right entrance region of upper jet maximum.
Result will be an ongoing threat for additional mainly light rain
and drizzle...perhaps into Wednesday morning...until main western
system can finally open/fill and lift NE into the upper and lower
Great Lakes...allowing another cold front to sweep southeast through the
area and clear things out for most areas by Wednesday afternoon.
Exception will likely be across upsloping western flanks of the
Appalachians...where cloud cover and a continued low threat for Post-
frontal rain/snow showers may occur into Wednesday night.
Once again...with abundant cloud cover and a dirty cool high
pressure wedge reestablishing itself over most of the area...expect
limited diurnal temperature swings...with relatively cool daytime
readings...and above normal overnight readings that will remain
above freezing until after frontal passage on Wednesday.
Aviation /03z Thursday through Monday/...
as of 645 PM EST Wednesday...
Aviation conditions begin to go downhill this period...but timing
and degree of such remain rather questionable at this point.
1043mb surface high pressure anchored off the northern middle-
Atlantic coast will remain in place through Friday...drifting
slowly into the North Atlantic. Meanwhile...a very slow moving
frontal boundary will approach from the north by the weekend. High
pressure aloft will build over the region through Friday. This
leaves our area with increasingly moist east-southeast flow and as
a result increasing clouds from moist upslope along the eastern
flanks of the Blue Ridge and alleghanys.
The first vestiges of these clouds already evident from central NC
northward into the Virginia Piedmont...with Danville on the verge at
this hour of seeing a bkn035 ceiling. All indications are that
these clouds will spread northeast overnight/early Thanksgiving
day...with ceilings lowering into the 015-035 depending on elevation.
Although this is general scenario expected...considerable
differences are noted between the NAM/GFS in terms of projected
cigs/vsbys. Ceilings seem to be the main issue as the immediate ground
layer remains dry and radiational cooling will not be ideal
tonight. While the NAM advertises IFR-LIFR ceilings across the
Piedmont...the GFS generally does not...but advertises more MVFR
br. For now...have leaned toward more a ceiling issue than a
visibility issue...but have not gone as low as the NAM would
suggest...namely stopping in the low end of the MVFR range. Have
only advertised a fusibility restriction to MVFR at lwb where
low- level moisture in the form of low clouds/SC may be harder to
come by. It also remains to be seen how much the dense cirrus deck
overhead comes into play...that will be in/out overnight. Expect
MVFR ceilings to remain in place much of the day Thanksgiving as the
southeast flow persists.
Winds mostly east-northeast-east-southeast overnight at speeds of 4-7kts...becoming southeast
5-8kts Thanksgiving with low end gusts expected once again at
Low to medium confidence in ceilings through the taf valid period.
Medium to high confidence in visibilities through the taf valid period.
Medium to high confidence in wind speed/direction through the taf valid
Extended aviation discussion...
Friday into Sunday...look for a break down of the Lee side wedge
as southwest flow increases in advance of an approaching cold
front. Precipitation will become more showery in nature...and
increase in coverage from west to east with the approach and
arrival of the front. Ceilings in the MVFR range will be
commonplace during this part of the forecast...with some spots in
the IFR range. Visibilities will mainly be VFR...but MVFR
conditions will be possible under heavier showers and also during
the overnight with light fog formation.
On Monday...the front washes out over the area...but scattered
showers and areas of sub-VFR conditions are still possible.