FXUS61 KRNK 201905

Area Forecast Discussion
National Weather Service Blacksburg VA
305 PM EDT Thu Sep 20 2018

Isolated to scattered afternoon showers or thunderstorms will give
way to high pressure building in from the east tonight. This high
pressure area will shift back eastward Friday as a cold front
progresses into the Ohio Valley and western Appalachians. Weekend
looks cloudy with chances for rain on the increase. Above-normal
temperatures to continue until late this weekend when abundant
clouds and rain will lead to near to below normal temperatures.

As of 245 PM EDT Thursday...

While upper-level ridge continues to build east, cumulus continues
to deepen within the Blue Ridge mountains in an air mass
characterized by surface-based CAPEs of 1500-2000 J/kg. Terrain-
focused convergence focused by light southeast winds have proven
enough to overcome background subsidence. Showers have also formed
along remnant frontal boundary lingering in our far eastern
counties. Indication from 12z model guidance and finer-resolution
CAM output shows isolated to scattered storm coverage initially
focused along the southern Blue Ridge, but we should see storm
coverage increase in the northern Blue Ridge/southern Shenandoah
Valley late afternoon into mid-evening. Showers and storms will be
slow to move and although PWATs are only around an inch or so, the
slow moving nature of the storms could lead to isolated spots where
downpours/locally heavy rains are possible. Storms/showers should
weaken but then give away to surge of lower-level moisture now
evident along the mid-Atlantic coastline later tonight and
overnight. While low-level flow is light southeast/east-southeast,
BUFKIT profiles from most models indicates a heavily saturated lower
atmosphere (extending to a depth from sfc to 850 mb) from Lynchburg,
Danville westward to Roanoke/Blue Ridge. Suspect that we'll see
lowering and thickening clouds overnight especially along and east
of the Blue Ridge. Periods of mist possible as well given humid
conditions and small T-Td spreads, though visibility likely lower in
the western mountains into northwest NC where skies probably will
remain thin enough to permit more radiational cooling. Coolest lows
tonight in the low-mid 60s west of the Blue Ridge/mountains of
northwest NC, while tending in the upper 60s into the Piedmont.

Morning low cloud cover in northern/northeastern sections will lead
to at least initially cooler temperatures (a slower heating rate),
but these clouds should give way to mostly clear/partly cloudy
cumulus skies by late morning. High pressure should shift eastward
and developing pressure gradient between the high and pressure falls
associated with a cold front should lead to a bit stronger
southwesterly winds around 5-10 mph. With more southwest component
to the winds and the cold front still well to the west, shower/storm
coverage Friday should prove limited and mainly to the west of the
Blue Ridge and Alleghany Highlands. Still do think coverage is less
than today, however. Little overall change to highs in the 80s and
continued elevated humidities (dewpoints in the 60s to low 70s).

Overall forecast confidence is moderate.

As of 250 PM EDT Thursday...

Upper ridge across the southeast states will gradually flatten to
start the weekend as a passing mid level trough pushes a weak
surface cold front toward the area Friday night and into northern
sections Saturday. Could be isolated showers Friday evening well in
advance of the boundary with any convection fading overnight per
loss of heating. Front looks to sink in from the north Saturday with
guidance showing quite a bit of instability in advance of the low
level boundary Saturday afternoon. However upper support remains to
the northwest with main focus along the Blue Ridge to start, with
perhaps some linear convective feature sinking south by Saturday
evening as depicted by several solutions. This could support an
isolated pulse severe threat given dry air aloft espcly Blue Ridge
east on Saturday if instability is able to overcome lack of much
support aloft. Therefore bumped up pops to high chance/low likelys
Saturday afternoon and across the southern half of the area into
Saturday night where lingering instability, deeper moisture and
theta-e will persist. Still quite warm ahead of the front until
convection fires Saturday with highs in the 80s given weak westerly

Front becomes more disjointed Sunday into Sunday night with the
surface boundary sinking to the south while the front aloft looks to
remain northwest of the area until Sunday evening. Behind the
surface front, will see a low level wedge take shape from the
northeast with this scenario driving much cooler air and low clouds
across the region by Sunday night. Degree of significant rainfall
looks uncertain during this period as well given ripples of energy
aloft riding to the northwest, while the forcing with the surface
front remains to the south. However think the main area of vorticity
aloft will drift in from the northwest by late Sunday, and despite
lack of instability should be enough upslope/overrunning of the low
level cool pool for more rain/shower coverage into Sunday night.

Temperatures should tumble within the wedge on Sunday with highs
remaining in the cool 60s over the mountains and northeast sections,
with a few 70s in the far southern and western perimeter counties.

As of 130 PM EDT Thursday...

Weak stalled front across or just south of the region to start the
week will begin to lift back north as a warm front into Tuesday in
response to increasing ridging over the southeast states ahead of a
stronger trough to the northwest. This should keep the threat for at
least scattered showers and storms going espcly north Monday near
the boundary, and more isolated nature across the area Tuesday as we
slide into the warm sector with less forcing by Tuesday afternoon.

Rather strong 500 mb trough will push the next stronger cold front
in from the northwest on Wednesday preceded by an axis of deep
moisture and lift that should bring more widespread showers/storms
for midweek. However model timing differences continue with the GFS
fastest bringing the best coverage Wednesday, while the Canadian
slowest having more showers Thursday, with the 12Z ECMWF a
compromise at this point. Latest ensembles do suggest more of a pre-
frontal band of deeper convection later Wednesday into Wednesday
evening followed by drier/cooler weather Thursday. Thus will have
the higher pops Wednesday afternoon/evening and decreasing chances
into Thursday.

Models do suggest that much of the upper trough will shear
out to the north which could result in a slower frontal passage
espcly with the core of the cooler air aloft progged to stay north
of the region by late week. Otherwise after a return to warm/muggy
conditions with highs back into the 80s by Tuesday, will see a cool
down back closer to normal including less humidity into Day7.

As of 139 PM EDT Thursday...

Generally VFR with coverage of deepest Cu largely within about
20 miles of the Blue Ridge. Scattered showers and a few
thunderstorms possible between 20-02z within the Blue Ridge,
tending to develop earlier further to the south and later
further to the northern end of the Ridge. Those ideas all hinted
at by latest short-term model guidance such as HRRR and the 3-km
NAM. Indicated VCTS at Roanoke where confidence is greater of a
few storms at least around, where VCSH offered at Blacksburg
and Lynchburg where there's less certainty of thunderstorms but
can't entirely be ruled out either. Storms should begin to
dissipate around mid-late evening. Winds light southeasterly
(3-6 kts) east of the Ridge and more variable west.

After midnight, guidance is coming around to idea of restricted
conditions due to both fog and stratus as light east/southeast
component shunts moisture against the Blue Ridge. Have shown
MVFR to IFR stratus at Roanoke, Lynchburg and Danville with
MVFR fog/mist. Southeast component is pretty light so not clear
cloud cover will extend beyond the Ridge; therefore think LIFR-
VLIFR fog is more probable at Blacksburg and Lewisburg. Winds
tend light southeast east of the Ridge to light/calm west.

Into Friday, fog and lower stratus should thin out giving way to
VFR cumulus. May see some isolated showers develop along the
Alleghany Highlands late in the TAF period but away from any
TAFs. Winds turn southwest 3-6 kts, with gusts to 16 kts at

Confidence is moderate in TAF elements.

Extended Discussion...

A few storms producing brief sub-VFR restrictions possible
Friday afternoon but coverage appears limited. A cold front
will approach the forecast area from the northwest this weekend,
increasing the potential for sub- vfr conditions associated
with scattered shra/tsra. Sub-VFR conditions may persist into
Monday as boundary lingers over the Blacksburg forecast area.


As of 741 AM Thursday...

River flood warning continues along the Dan River at South




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