Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
436 am EST Tuesday Dec 1 2015
a cool wedge of high pressure will be in control for most of the
day again...but will slowly mix out by afternoon allowing a slow
rise in temperatures. Moist flow aloft from the southwest and a
couple more waves of energy will continue to bring more rainfall
to the region through tonight. A cold front will cross from the
west on Wednesday...bringing a final round of rain before finally
ushering in some drier air behind it for the rest of the week.
Near term /through tonight/...
as of 425 am EST Tuesday...
Latest wave of precipitation overrunning cold wedge is lifting north
across forecast area early this morning and more moderate precipitation rates
should lift north of about the U.S. 460 line around daybreak...but
some areas of drizzle or lighter showers may be left behind so
only lowering probability of precipitation to chance category south of this wave. Another
wave pushing out ahead of cold front in Mississippi Valley is in
central Tennessee and should move into western portions of the
forecast area by midday but most guidance keeps this in western
sections. Thus east of about Interstate 81 may see only very light
rain or drizzle for much of the day time hours. This will allow
runoff along the Blue Ridge to settle down a bit before next wave of
rain moves in by late afternoon and into evening from the west.
Recent hrrr runs have pretty decent handle on current precipitation and
timing of these waves today it appears.
Meanwhile...the other issue today is how long The Wedge will hang
on...and all guidance including high resolution models with good
handle on terrain and good vertical resolution suggest it erodes
by midday with temperatures moderating by afternoon. This due to fairly
good SW winds at Top of the Wedge and lack of significant precipitation
into wedge and loss of dry advection from the northeast as surface
high moves off northeast U.S. Coast. Hard to go too strongly
against these high res...but did lean more toward cooler and
slower erosion of wedge in far northeast areas such as lyh...and
keep temperatures pretty steady until middle afternoon in those areas before
only slowly rising. No sunshine expected but by afternoon the
ceilings will lift somewhat while precipitation takes a short break most
areas...until the next wave moves in late in the day west and
spreads east by late evening.
Overnight...precipitation increases in moist SW flow ahead of cold
frontal boundary which will not cross over Appalachians until
Wednesday...but looks like one more good slug of rainfall by
midnight and after...with focus along and just east of Blue
Ridge...and concern is much of this will occur over areas that
have received the most rainfall since late Sunday. The current
Flood Watch area still looks good...with another one and a half to
two inches expected in parts of northwest NC and foothill region through
overnight tonight...and at least one and half up through the New
River valley and along Blue Ridge. This could easily be enough to
result in some streams coming out of their banks by early Wednesday
morning and then continuing into Wednesday. Temperatures Tuesday night will
generally be in the 50s and will remain steady or may even slowly
rise...especially in Piedmont.
Short term /Wednesday through Friday night/...
as of 330 am EST Tuesday...
By Wednesday morning the upper level closed low moving into the Great
Lakes region will be driving a well developed surface wave along the
cold front as it passes through the area. This will enhance dynamic
forcing in an environment that is saturated thanks to several days of
deep southwesterly flow bringing abundant moisture into the region. The
end result will be a burst of precipitation with the front as it moves
through on Wednesday with additional rainfall amounts approaching one
inch from the NC mountains into Virginia from the mountain Empire through the
southern Blue Ridge. The Flood Watch for this area looks to be right on
target and currently do not see a reason to expand the area of
concern...though will continue to monitor near term rainfall trends to
see if an expansion becomes necessary.
Once we get behind the front late Wednesday and Wednesday night...
blustery northwest winds will usher in colder air and generate some
upslope rain/to/snow showers along the far western slopes. No
significant accumulations are expected though some location may see a
dusting. High pressure will then continue to build over the middle
Atlantic region and bring US fair weather through the end of the
Temperatures will surge ahead of the front on Wednesday as any remnants
of The Wedge vanish...with highs in the lower 60s east of the
ridge/middle 50s west which is 5 to 10 degrees above normal. Conditions
more typical for early December will arrive behind the front with lower
50s east/middle 40s west.
Long term /Saturday through Monday/...
as of 100 PM EST Monday...
A weakness in the 500 mb pattern moves across the southeast United States
but operational and ensemble models differ in the intensity and location
of this feature. Despite this weakness...overall 500mb heights will rise
in the east into the weekend. At the surface high pressure will remain over
the region through Saturday. Temperatures will be moderating through the
period and by Sunday and Monday will be slightly above normal. Expected dry
weather through day 6 then a southern stream system brings a probability of
precipitation back to the area on Monday.
Aviation /09z Tuesday through Saturday/...
as of 1150 PM EST Monday...
Poor flying condition expected during the taf period with
low clouds...rain and drizzle...though some break in the rainfall
may occur between 12-18z...though confidence in this small window
is medium. Nonetheless most of the period will IFR or worse.
Heavier rains will start to move toward blf/lwb by late Tuesday
night...beyond this taf period.
Extended aviation discussion...
Conditions may improve temporarily Tuesday night as The Wedge
boundary lifts north before a cold front from the west on
Wednesday brings additional rain and some relatively brief IFR
conditions. Behind the front later Wednesday conditions will
finally improve to VFR in northwest flow...although some IFR in upslope
areas at kblf and klwb may linger into early Thursday morning behind
front. Behind the front...gusty west winds are likely from late
Wednesday into Thursday. VFR conditions should dominate through
as of 430 am EST Tuesday...
A Flood Watch remains in effect from this evening through
Wednesday evening for western portions of the forecast area
Tuesday night and Wednesday...and including the NC foothills. This
area still looks on track with an additional one and a half to
two and a half inches of rainfall today through Wednesday for most
of this region...and all additional rain will go into runoff at
Small streams and creeks may rise above bankfull. Low water
crossing and poor draining areas in urban locations may be
flooded. Tributaries that feed into the Yadkin River...the
headwaters of the New River...the Roanoke...and the upper Dan
River may experience flooding. Some minor flooding is not out of
the question on the Dan and Roanoke rivers well downstream of the
Blue Ridge by Thursday.
Virginia...Flood Watch from this evening through Wednesday evening for
NC...Flood Watch from this evening through Wednesday evening for
long term...air mass