Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
1223 am EST sun Nov 29 2015
a backdoor cold front will drift south into the area late tonight
before spilling into North Carolina on Sunday. The boundary will
stall just south of the region into early next week allowing a
wedge of high pressure to develop into Monday night. Weak ripples
of low pressure riding along the boundary will also result in
periods of rain before a cold front crosses from the west by the
middle of next week.
Near term /through today/...
as of 945 PM EST Saturday...
Latest radar imagery and surface observations indicate that light
rain showers are making their way across the western
ridges...and pushing across the New River valley. These showers
are developing along a slow moving cold front situated along the
Ohio River valley. As the front sinks south...rain chances will
increase across the mountains tonight...then over the Virginia
foothills during the early hours of Sunday morning. Westerly flow
and lingering dry air could keep the North Carolina Piedmont and
Southside Virginia dry until Sunday evening. Rainfall amounts over
the next 24 hours will range from a quarter of an inch /0.25/ over
the Greenbrier valley...especially north of I-64...to zero /0.00/
across Southside Virginia.
Since the front will move slowly over the area...very mild
temperatures are likely tonight...with readings from the middle 40s
to lower 50s. With rain and clouds in the region on Sunday...high
temperatures will range from the upper 50s in the Greenbrier
valley to the upper 60s over Southside.
Short term /tonight through Tuesday/...
as of 215 PM EST Saturday...
Low level wedge will continue to take shape over most of the area Sunday
night as the surface backdoor cold front slips south of the North
Carolina border in response to high pressure to the north. Initially
moisture and lift should be rather limited within The Wedge Sunday
evening prior to another weak wave approaching from the SW...and the
low level flow turning more southeast espcly across the west. Guidance
shows quite a bit of overrunning developing with this feature overnight
Sunday night with isentropic lift enhancing along the actual 850 mb Theta-
east gradient across Virginia into midday Monday. Thus will be increasing
probability of precipitation to likely/categorical from SW to NE Sunday night into Monday morning.
Impulse passes Monday afternoon in advance of the next weak ripple that
is expected to ride along the front late Monday night into Tuesday as
the flow aloft veers more south then southwest. This may leave much of
the area excluding the far SW in a lull to some degree with increasing
southerly flow likely leading to more light rain/drizzle/fog scenario
by Monday night before the next round of lift arrives Tuesday. This
supports keeping high probability of precipitation going through much of Tuesday with the more
significant rainfall along the southern Blue Ridge and SW where will be
closer to the deeper moisture and within periodic rounds of better
upper diffluence. Appears overall quantitative precipitation forecast through the period not enough to
cause Hydro issues given low rainfall rates and projections of .50 to
1.50 inches with the most along the Blue Ridge.
Will be quite chilly with small diurnal ranges expected once The Wedge
locks in and the cool pool becomes enhanced due to rainfall. This
supports lows mostly in the 40s...with highs only rebounding a few
degrees in the 40s at best Monday...and perhaps a category or two
higher to near 50 within The Wedge Tuesday. Exception could be the far
west and southeast where late day erosion of the cad could help bump
values into the 50s.
Long term /Tuesday night through Saturday/...
as of 1245 PM EST Saturday...
Surface cold front associated with the large upper low passing across
the Great Lakes will approach from the west Tuesday night and cross the
region on Wednesday. Impulses passing up ahead of the boundary along
with the associated warm front lifting north should continue to provide
enough lift to bring rather widespread rainfall Tuesday night as The
Wedge erodes. Models continue to differ on timing with the frontal
passage Wednesday given the slowness of the upper low to move east and
parallel SW flow aloft. Latest GFS has shifted closer to the earlier
wet European model (ecmwf) in bringing a decent band of showers through the area
Wednesday morning followed by a quick dry punch to all except the far
northwest where weak cool advection upslope will kick in a few added afternoon
showers. Therefore keeping likely/Cat probability of precipitation going Tuesday night followed
by high chance/likelys Wednesday morning then decreasing probability of precipitation later on.
A few lingering upslope rain/snow showers then possible far northwest
Wednesday evening under the passing cold pool otherwise clearing and
cooler overnight as high pressure starts to nose in from the west.
Large temperature range possible from highs 40s behind the front far west
Wednesday to near 60 southeast pending clouds/timing.
Tail end of the front will again be slow to push south through the end
of the week with another wave developing along the boundary over the
Gulf and heading off the southeast coast by Saturday. However most solutions
including ensembles have much less amplitude to this feature today
given a weaker 500 mb wave with strong high pressure to the north able to
shunt most deep moisture to the south/se. Will again have to watch and
see if guidance flips back to a wetter solution with time given SW flow
aloft but for now removing probability of precipitation and going dry with high pressure
building in...and wedging south by the weekend. Temperatures to return to near
seasonal norms with some below freezing values at night but mostly
upper 40s to low 50s for highs which wont be too bad for early December.
Aviation /05z Sunday through Thursday/...
as of 1200 PM EST Sunday...
Aviation conditions expected to deteriorate this taf valid period
as a slow moving baroclinic zone sags south from the Ohio
Valley/northern mid-Atlantic...surface high pressure moves toward
New England from southeast Canada...and low pressure aloft slowly
approaches from the Midwest.
Initial area of light rain spreading east from Kentucky/Tennessee into far
western Virginia and southern WV. Seeing very little in the way of much
more than 20-25 dbz returns on regional radars from this activity.
Air mass across our region is still relatively dry. Expect this
light rain to spread toward klwb/kblf/kbcb overnight...with the
main effect to result in gradually lowering ceilings/moistening of the
atmosphere...but have little minimal impact on visibility. This
first round of rain will likely have considerable difficulty
spreading east of the Blue Ridge. Ceilings will lower steadily
once the rain moves in and the lower atmosphere begins to
saturate. This will primarily impact sites west of the Blue Ridge
through this taf valid period...with ceilings lowering through the
MVFR range...as has already occurred at klwb...potentially to IFR
at blf before daybreak. During the day sun...indications are that
this first wave of rain will generally dissipate and make little
further progress east into the Piedmont leaving the west with MVFR
ceilings and -dz or sprinkles. Thus...have not included any mention
of -ra into klyh or kdan at this point. Expected scenario sun is
for precipitation in the west to trend more toward -dz with MVFR
or low end VFR ceilings...awaiting the next upstream short wave to
arrive after 00z Monday before bonafide rain spreads back into these
areas and further east. Therefore...mainly just MVFR clouds west
and low to Middle- Range VFR clouds east with any precipitation
likely confined to WV and far southwest Virginia sun. Toward 00z Monday and
after...from west to east...ceilings will lower into the MVFR-IFR
Visibilities are expected to remain VFR east of the Blue Ridge
through 00z Monday...then trend toward MVFR by 04z Monday. In the west...
visibilities will become MVFR in -ra/-dz overnight/early
sun...and likely remain MVFR through much of the day sun in
-ra/-dz/br...trending toward IFR by or after 00z Monday.
Winds are expected to be very light...favoring west-southwest-SW
overnight/early sun...but becoming variable by sun evening as a
wedge begins to develop across eastern areas.
Overall aviation conditions may remain fairly good for
kroa/klyh/kdan for one more day...but not so for kblf/klwb/kbcb.
Medium confidence in ceilings through the taf valid period.
Medium confidence in visibilities through the taf valid period.
Medium confidence in wind direction through the taf valid period.
High confidence in wind speed through the taf valid period.
Extended aviation discussion...
the cold front will finally sag south into the Carolinas Sunday
night. Moisture moving from the Tennessee Valley and over a cool
wedge will bring several days of rain/drizzle/fog and low clouds
and attendant MVFR-IFR ceilings/visibilities to the region into Tuesday
night. Another cold front will clear The Wedge out of the area
with improving flight restrictions on Wednesday. Following this
front...gusty west winds are likely Wednesday and Wednesday