Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
431 am EST Monday Nov 30 2015
a cool wedge of high pressure will dominate through early Tuesday...
with a warm front lifting back to the north late on Tuesday. Waves
of upper level energy will lift over this wedge resulting in
continued periods of rain. 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 415 am EST Monday...
Wedge firmly in place across forecast area as temperatures continue
to steadily drop behind it. Back door frontal boundary now well to
the south...and initial main wave of moisture is shifting west
across mainly the southern two thirds of forecast area. Rainfall
totals as of 4am have been a half in to as much as an inch across
northwest NC...otherwise less than this. Behind this initial wave moving
out this morning...rainfall rates are expected to taper off
during the day to very light rain or drizzle but likely not end
completely given a southerly 850mb jet continuing most of the day.
Recent hrrr runs suggest significant reduction in precipitation
production later this morning with most of the precipitation shifting
north...until late afternoon. Area radar mosaic suggests this
trend will occur but again given the moist flow over The Wedge and
some degree of upslope along Blue Ridge...do not expect rain to
completely end for any of the day...but it may be very light. Thus
continue categorical probability of precipitation all day but with lowering rainfall
amounts and some shift toward northern part of area as day GOES
on. As wedge continues to strengthen during the day have depicted
temperatures steady or even continuing to drop in some locations through
much of the day...and maybe some slight rises again
late...especially on southwest fringes. For the most part...highs
will be from early this morning...and went slightly cooler than
most guidance for the day.
By this evening another weak short wave approaches and is likely
to spread another area of mainly light rain across southern
portion of area...followed by a stronger wave sliding up the west
side of Appalachians as 850 flow turns back to southwest. This in
advance of deepening and approaching upper trough to the west. At
this point do not think this precipitation will be quite as much as what
we have already seen across southwestern portions of the
area...and much of this will remain across the west and will be
shifting north by Tuesday morning into areas that have not seen
as much rain through this morning. Thus not expecting any
hydrology problems through tonight...but see the hydrology
section of this discussion for possible impacts from additional
rain Tuesday into Wednesday. Near steady temperatures will
continue overnight tonight.
Short term /Tuesday through Thursday night/...
as of 400 am EST Monday...
Wet scenario is set to play out Tuesday and Wednesday as deep moist
southwesterly flow is established ahead of a cold front approaching
from the west. Precipitation will come in surges as short wave energy
moves through the upper flow...but the main period to watch for
potential heavy rainfall will be later Tuesday night through Wednesday
morning. Dynamic forcing will increase as a vigorous upper closed low
moves into the Ohio Valley and generates a series of waves along the
cold front as it moves west to east across the region. This will
enhance precipitation rates while also slowing the front down...leading
to additional rainfall totals of up to another inch or so especially
from the mountains of North Carolina into Virginia along the Blue
Ridge. However there is uncertainty in where this heavy rain will fall
in relation to where heavy rain will have fallen Sunday night through
Monday. Will therefore hold off on issuing any flood watches until it
becomes more clear exactly what areas will be receiving successive
bouts of heavy rainfall. Will continue to highlight the most likely
candidates from the Blue Ridge westward in the severe weather potential statement.
Behind the cold front there will be some light upslope shower activity
along the far western slopes changing to snow showers Wednesday night
with no accumulation expected. Snow showers will wind down Thursday
morning as high pressure builds in with fair weather.
Temperatures will moderate with a good surge of warm air as The Wedge
erodes ahead of the front...allowing readings east of the Blue Ridge to
reach the lower 60s on Wednesday. Cooler air arrives behind the front
and brings readings to normal/slightly below normal levels for Thursday.
Long term /Friday through Sunday/...
as of 1230 PM EST Sunday...
Lingering upper troffiness across the middle Atlantic will gradually
weaken and be replaced by shortwave ridging by the weekend ahead of yet
another upstream closed low to the west. This should allow a large dome
of more Pacific nature high pressure to build east from the plains into
late week before drifting north and becoming wedged down the coast
Saturday into Sunday. Also still some uncertainty with the possible
development of a 500 mb upper low along the southern Appalachians
underneath the broad upper ridging by the weekend. This again could
interact more with deeper moisture along the leftover coastal
baroclinic zone but this still appears less likely now per latest drier
ensembles and latest GFS/CMC that keeps precipitation shunted to the south.
Otherwise will be going dry for Friday-Sunday and given most cold air
locked to the north expecting near or slightly above seasonal highs
with only some lows below freezing under the surface ridging.
Aviation /09z Monday through Friday/...
as of 1145 PM EST Sunday...
Surface observations indicate that a cold front continues to sink
southward across the forecast region this evening as Canadian high
pressure wedges southward across the middle Atlantic from New
England. Rain continues to overspread the area as upper level
disturbances approach from the West...Riding along the front.
Rain...light to occasionally moderate...will fall into the cool
layer of high pressure through the night...maintaining IFR
ceilings in most locations. The low clouds will also make for
reduced visibilities...especially across the mountains where these
clouds may intersect the higher terrain. Visibilities in other
locations may fluctuate between 2sm to 6sm through the night as
areas of heavier rain pass over/away from airports.
IFR conditions are expected to be the norm through the end of the
30/06z taf period. Rainfall coverage will diminish during Monday
afternoon and evening as a break develops between upper level
disturbances riding across the forecast area. As such...may see
ceilings improve to low end MVFR where rainfall diminishes for an
extended period of time...as the drier air flowing in with the
high pressure wedge causes ceilings to lift by a few hundred feet.
Occurrence and timing of these improvements will be difficult to
pinpoint. Another upper level disturbance will approach the area
late Monday evening...bringing another round of rain...as well as
a return of widespread IFR/LIFR conditions for the forecast
Extended aviation discussion...
The cold front will remain south of the region through Tuesday
night. Moisture moving from the Tennessee Valley and over the 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. A
cold front from the west on Wednesday will clear The Wedge out of
the area with improving flight restrictions by later in the day...
but first could bring some brief heavier rain. Following this
front...gusty west winds are likely late Wednesday into Thursday
with VFR conditions expected.
as of 420 am EST Monday...
A prolonged rainfall event will continue across the region
lasting into much of Wednesday as moisture overruns a wedge of
cool air along the Appalachians. Rainfall through 4am Monday has
been close to an inch acros portions of wautaga County NC...and
otherwise a half to three quarters of an inch across much of the
southwest portion of the forecast area. Additional rainfall from
additional waves of moisture today through Tuesday could total
another 1 to 2 inches in some of these areas but rates will
generally relatively light. However...heavier showers are then
expected along a cold front late Tuesday night into Wednesday.
Rainfall rates with this precipitation could be much higher and
fall on already saturated soils...with another inch or more of
rainfall possible through the end of Wednesday. Runoff from this
rainfall on top of the earlier rainfall could lead to localized
flooding issues... especially along the Blue Ridge and points
west. Some river flooding cannot be ruled out given the prolonged
nature of the runoff and final totals that could approach 3 to 4
inches in spots. Thus a Flood Watch may be needed by Tuesday or
Tuesday night for at least some western portions of the forecast area
depending on how much rain occurs through tonight and forecast trends
for what is expected with the cold frontal rainfall Tuesday night and