Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
1145 PM EST sun Nov 29 2015
a backdoor cold front will drop south through the area this evening
and into the Carolinas overnight before stalling. The boundary
will remain just south of the region early next week allowing a
wedge of high pressure to develop into Monday night or Tuesday.
Weak ripples of low pressure riding along the boundary will also
result in periods of rain before a cold front crosses from the
west later Wednesday.
Near term /through Monday/...
as of 940 PM EST Sunday...
Probability of precipitation were increased along the Virginia/NC border...as radar continues
to show echoes traversing over this area. A few minor adjustments
to dewpoints...temperatures...and sky cover were also made based
on the latest observations. Temperatures are expected to slowly fall
into the 40s overnight. Patchy fog and periods of light rain
should be the main story for tonight...so no other tweaks are
needed at this time.
As of 230 PM EST Sunday...
A weak back door front will sink into the Carolinas tonight. In the
wake of the front...an area of cool high pressure will remain wedged
over the forecast area through Monday. Meanwhile...a wave of low
pressure will track from the Tennessee Valley to the Ohio Valley.
Warm moist air from this wave will overrun The Wedge...bringing
rain/drizzle and low clouds into the forecast area. A short wave
wave being ejected out of the Tennessee Valley will bring moderate
rainfall to the southwest corner of the area (bluefield-
Richlands...mountain Empire and the North Carolina highlands)
overnight. This wave will track northeastward and exit the Lynchburg
listening area early Monday afternoon. Another wave may enter the
forecast area Monday evening...bringing another round of rain to the
Rainfall amounts through Monday will vary from an inch or so over
the North Carolina Highlands /1.00/ with the highest amounts along
the eastern slopes of the Blue Ridge /1.50/ to a quarter to half an
inch elsewhere /0... to an inch on eastern slopes of the
southwestern Virginia Blue Ridge /1.00/.
Hope you have enjoyed the warmer than normal weather this weekend.
Once the front sinks south and rain falls into The
Wedge...temperatures will drop into the lower to middle 40s overnight
and stay there through the day Monday.
Short term /Monday night through Wednesday/...
as of 130 PM EST Sunday...
Very wet scenario will persist into the middle of the week as
overrunning of the low level wedge continues into Monday night in
advance of a surface warm front that will be lifting north overnight.
This ahead of another surface wave to the west should act to enhance
rainfall coverage again Monday night into midday Tuesday as this
boundary attempts to slide through the region while convergence
enhances. Speed maximum aloft also passing across over the weakening
wedge should support another round of decent rainfall so running
with mainly categorical/likely probability of precipitation into Tuesday morning.
May see somewhat of a break espcly over southern and eastern sections
Tuesday afternoon/evening as the boundary lifts north and the 850 mb flow
briefly turns more westerly. This before backing again ahead of the
main upper trough to the west by Tuesday night. Models continue to slow
the associated cold front with this upper system to the west...with
most now showing an axis of rain redeveloping across the mountains
Tuesday night and spreading slowly east during Wednesday. This
along an axis of much higher precipitable waters that surge up from the
southwest and in a corridor of lift with another surface low
traversing up the boundary. Although latest forecasts show limited
instability...soundings support more in the way of shallow
convection which would result in higher rainfall rates on top of
already saturated soils by then. Guidance timing and quantitative precipitation forecast amounts
still remain a bit different per the faster/weaker NAM/CMC
solutions and the much wetter 12z GFS. This likely the period
where a Flood Watch may be needed so will keep mention going in
the severe weather potential statement about possible water issues espcly along the southern Blue
Ridge/foothills by then when another couple of inches could fall
over the mountains.
Otherwise will keep high probability of precipitation for most sections Tuesday afternoon/night
and include a band of likelys or higher crossing the region from northwest to
southeast during Wednesday with the surface front. Steady temperatures mostly in the
upper 30s to middle 40s Monday night should start to slowly rebound
Tuesday as The Wedge weakens...with mostly 50s all sections and
possibly close to 60 far west per 850 mb warming to near +11c. Pending
timing of the cold front to the west Wednesday and eastward expanse of
heavier rain...could see highs zooming into the 60s east and 50s west.
Long term /Wednesday night 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. Ahead of this will still need to keep in some low
probability of precipitation mainly east for the slow to exit surface cold front Wednesday
evening...and across the northwest Wednesday night for upslope where may be a
bit of snow mixed in. 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 Thursday-Sunday and given most cold air
locked to the north expecting near or slightly above seasonal highs
after thursdays cool down with only some lows below freezing
under the surface ridging.
Aviation /06z 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 235 PM EST Sunday...
A prolonged rainfall event will occur across the region starting
tonight and lasting into much of Wednesday as moisture overruns a
wedge of cool air along the Appalachians. Initially rainfall is
expected to be rather light with much of the steadier rain focused
along the southern Blue Ridge/foothills into the mountain Empire
region where up to 2 inches could fall through Tuesday evening.
Elsewhere totals of around an inch could occur although likely
less across parts of the Piedmont and near the Interstate 64
corridor. Heavier showers are then expected along a cold front
later Tuesday night into Wednesday. Rainfall rates with this
precipitation could be much higher and fall on already saturated
soils...with another 1 or 2 inches 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 is also possible given the prolonged nature to the
runoff and totals that could approach 3 to 4 inches in spots.
Thus a Flood Watch may be required during early next week for
parts of the region pending the rainfall totals over the next
couple of days before the heavier rain arrives.