Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 250540

Area Forecast Discussion
National Weather Service Blacksburg VA
140 AM EDT Sun Sep 25 2016

A backdoor front slide south into the Carolinas today, before
stalling. High pressure following the front will wedge south along
the eastern slopes of the mountains Sunday into early Monday.
Another cold front will shift in from the west Monday night into
Tuesday bringing better chances of showers and isolated
thunderstorms for early next week.


As of 125 AM EDT Sunday...

Backdoor front located near the VA/NC border will continue
southward this morning. Still a few showers skirting the piedmont
and will see transition to patchy light rain/drizzle with low
clouds along/near the Blue Ridge mainly north of Roanoke then
shifting toward Southside by dawn. Expecting some patchy fog along
the river valleys outside the wedge where its clear.

Previous valid discussion...

Low level flow remains northeasterly which should limit chances
for measurable precipitation for the next 24 hours. However,
moisture is shallow (under 4 kft), but will remain thick and
overcast through the day Sunday. With some weak warm air
advection, isentropic lift and flow turning more easterly on
Sunday, light rain/drizzle is possible along eastern slopes of the
Blue Ridge.

As the front moves to the south, cloudy conditions will keep
temperatures on the cool side for the next 24 hours or so.
Considering the depth of the wedge is around 3000 to 4000 feet,
just about everyone will see cooler than normal temperatures
Sunday. Temperatures will only warm into the 70s with eastern
slopes likely to stay in the 60s. The Bluefield-Richlands area
will be on the western flank of the wedge and could see some
erosion in the afternoon. A warm upper level ridge over the
southern Mississippi Valley could stretch east into area, allowing
southwest Virginia to warm to near 80F.


As of 245 PM EDT Saturday...

Low level wedge looks to be at its strongest Sunday night into early
Monday with the axis of the surface high just east of the mountains
aided by continued light rain/drizzle across the west and shortwave
ridging aloft. Guidance even suggests that some drier air from the
northeast could work into eastern sections during the evening given the
strength of the high, allowing for brief clearing before the flow
aloft starts to turn more southwest overnight. This should once
again propel the shield of low clouds back east by early Monday
with lingering low pops for light rain/drizzle espcly Blue Ridge
and points west into midday Monday. Lows mostly 50s to lower 60s
Sunday night with coolest air over the north/northeast.

Wedge should begin to scour out Monday afternoon as ridging aloft
flattens in advance of the strong upstream upper trof that will push an
associated cold front toward the mountains by late in the day. Models
also show quite an increase in moisture depth ahead of the front with
PWATS near 2 inches under decent southwest flow aloft. This deepening
flow looks to gradually weaken the wedge from west to east by Monday
evening although far northeast sections may not see the cool pool
exit until overnight when the pre-frontal showers arrive. However
does appear to be enough instability and added focus as the warm
front lifts north and the pre-frontal lobe enters, to support some
low chance thunder mainly south/west late Monday, with more
widespread shower coverage progressing east during the overnight.
Latest ensembles as well as a mix of the GFS/EC also support a
swath of around one half to as much as one inch of qpf espcly
central/eastern sections. Therefore running with a period of high
chance to low likelys from west to east starting late Monday
mountains and out east into early Tuesday.

Boundary may slow enough during Tuesday given the stalling nature of the
strong upper low to the north, to linger some shower pops eastern
sections, with more clearing developing mountains by afternoon. Stayed
with cooler highs Monday, 70-75, except perhaps near 80 far west where
will exit the wedge sooner. Appears enough cool advection to hold much
of the mountains in the 60s to around 70 Tuesday espcly if clouds
persist longer, with the east mainly mid 70s pending how soon the
showers end.


As of 145 PM EDT Saturday...

Upper low/trof should push east across the mid-Atlantic during midweek
with its associated cold front exiting the region to start the period.
However latest model trends suggest a somewhat slower scenario with
lots of uncertainty in just how fast this will happen given the slow
evolution of this complex upper system to the north. Based on current
and still rather progressive model solutions, could still be looking at
a few evening showers southeast Tuesday night as the front perhaps
slows per added low pressure rippling northeast. Also passing of the
center of the 500 mb trof axis/cold pool may also generate a few
showers mainly northern/western sections late Wednesday into early
Thursday but iffy. Otherwise should see much drier air invade the area
as ridging both surface and aloft take control from later Thursday into
next weekend. Expect mostly seasonal temps with highs 60s west to 70s
east with coolish overnights with some 40s mountains and 50s elsewhere.


As of 130 AM EDT Sunday...

Backdoor front situated along the NC/VA border will slide south
today. Lower ceilings already working as far southwest as
ROA/MTV/SIF and points northeast. Question will be how far
southwest this canopy of MVFR to IFR ceilings makes it. Think bCB
will soon drop to MVFR as will LWB. BLF should stay out of the
wedge clouds but could see some patchy IFR ceilings or vsbys after

Models show this wedge slow to erode Sunday with MVFR or worse
conditions prevailing most of the morning, and do not see
LYH/DAN/BCB getting above 3kft at all this taf period while
ROA may briefly rise above 3kft this afternoon only to fall back
after 00z. Drizzle may impact LYH/DAN but overall should keep
vsbys VFR, while ROA could see enhancement of drizzle this morning
given some upslope component which will drop vsbys to MVFR.

BCB/LWB vsbys will vary depending on how thick the low clouds get
but do not foresee LIFR so keeping it more IFR at times.

Needless to say not good flying conditions east of the Alleghany

Extended aviation discussion...

The backdoor front will drift to just south of the area by Sunday
night and stall. Overrunning will begin in earnest Sunday night
into Monday in advance of a strong, but slow moving, cold front
that will approach the area from the west late Monday. A healthy
line of showers and a few thunderstorms can be expected in advance
of the front as it moves through the region during this time
frame. Widespread sub-VFR cigs/vsbys are expected in the wedge air
mass Monday before the front arrives, which will also be
accompanied by a period of sub-VFR cigs/vsbys. Conditions will
improve by midweek, although as an upper low sinks southward over
the Appalachians, conditions could become unsettled enough to
continue periods of sub-VFR cigs and scattered showers.




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