Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 181648
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
1248 PM EDT Fri Aug 18 2017

.SYNOPSIS...
A cold front will cross the mountains from the west late this
afternoon before sliding southeast of the region later this
evening into tonight. The front will remain over the Carolinas
Saturday into Sunday before dissipating. High pressure works in
from the Ohio Valley during the weekend, then overhead Monday
resulting in drier weather across the region.
&&

.NEAR TERM /THROUGH TONIGHT/...
As of 955 AM EDT Friday...

Surface cold front over central Kentucky still on track to reach
the western mountains later this afternoon before spilling east
of the Blue Ridge this evening. Best support aloft will again
shift just north of the region as a weak shortwave trough
crosses in from the northwest. However degree of instability
quite impressive with MLCAPE values near 3000 J/Kg espcly Blue
Ridge east this afternoon given very moist dewpoints and strong
heating. In addition, axis of 850 mb theta-e ridging forecast
to enhance off the eastern slopes of the mountains where the
increasing mid level jet likely supportive of a few stronger
bands/clusters of storms as seen via the latest HRRR. This also
where expect the overall deep westerly flow to back more
southwest this afternoon as current convergence with western
slope pre-frontal clouds/showers tends to jump toward the
residual eastern lee trough this afternoon. Thus have bumped up
pops a little mountains this morning and from the foothills east
this afternoon where expect enough coverage for a ribbon of
likely pops. Otherwise mainly chance to scattered pops as the
transition in convection from west to east takes place. Think
enough heating espcly central/eastern sections to bump up highs
as thickness supports 90-95 east, and 80s west outside of the
far western elevations where may stay in the 75-80 range.


Previous discussion as of 341 AM EDT Friday...

Quiet wx wise this morning aside from dense fog in the Greenbrier
Valley, with a few low clouds across the piedmont/foothills. Today`s
forecast centers on cold front located this morning from the Great
Lakes, southward into west central Kentucky. Progression of the
southern section of the front has slowed due to weaker flow aloft.
This will be the challenge today where storms fire up and how much
sun we get ahead of the front to allow for increased instability.
Shortwave over southwest TN at 300 AM shifts northeast into NE TN by
12z, then slows as it shifts east of the Blue Ridge this afternoon.
Further north several embedded vorts will shift east in faster flow.
Sfc front will be located from the central Appalachians to mid TN by
midday, then east of the Blue Ridge by early evening. Severe
parameters are marginal for most of the forecast area, but SBCAPEs
progged in the 2000-3000 J/KG range along/east of the Blue Ridge
this afternoon with marginal shear mainly across the northern CWA
may allow for some storms to bring gusty to damaging winds. At the
moment best support per models for severe is much further northeast
toward DC/MD/PA. SPC has marginal risk over most of the forecast
area today, with slight risk northeast of Lynchburg, and low/no
threat in the mountains of far SW VA/WV. Wind is the main threat,
but low to medium confidence on the overall threat. Again, isolated
threat.

As for rainfall, showers/storms should move along fast enough to
prevent any rainfall issues, so flood threat is low.

As for rain chances, with some upper support and instability plus
low level convergence per front and orographics, should see a
coverage of 40 to 60 percent across the area this afternoon. Models
have been too aggressive lately on coverage, but now with the front
in play, should be a little better. Still not everyone will see rain.

Staying muggy with highs a degree to several cooler than Thursday,
thinking more clouds arriving and storms firing up by midday. Highs
will range from the upper 70s to lower 80s mountains, to upper 80s
to lower 90s east of the Blue Ridge, but including
Roanoke/Lexington.

Will see the southern half of the sfc front lag over NC this
evening/overnight which should keep shower/storm threat in the NC
piedmont and southside VA at least until midnight. Will keep high
chance early evening from Farmville, VA to Danville/Reidsville, then
dry it out by dawn Saturday. Lows staying muggy, except drier air
advecting in late in the mountains allowing temps to fall into the
lower 60s, while the piedmont drops to the upper 60s to lower 70s.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
As of 300 AM EDT Friday...

Frontal boundary will stall Saturday to our south across the
Southeast States. Upper trough will rotate east across the region
Saturday afternoon into Saturday night. Drier weather will prevail
with subsidence and northwest flow. The ECMWF tried to push the
moisture associated with shortwave further south in southeast West
Virginia. We will see if this trend continues. High temperatures
Saturday will range from the mid 70s in the mountains to the lower
90s in the piedmont.

In the wake of the shortwave, better subsidence arrives Saturday
night. Low temperatures Saturday night will vary from the upper 50s
in the northwest mountains to the upper 60s in the Piedmont.

The upper flow will flatten under slowly building heights on Sunday
with surface high pressure building in from the northwest. The low
level flow turn more southeast Sunday afternoon and could return
some instability into Sunday evening. An isolated shower or
thunderstorm may be possible in the south. High Sunday in the upper
70s in the mountains to the lower 90s in piedmont.

High pressure will slide eastward Sunday evening into Sunday night
with low temperatures generally from around 60 degrees to near 70
degrees.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
As of 300 AM EDT Friday...

Flat upper ridging will remain in place to start next week as surface
high pressure shifts from the Mid Atlantic on Monday to off the
southeast coast by midweek. This along with warming aloft within a west
to southwest trajectory should keep things quite warm and increasingly
humid from later Monday into Wednesday.

Models are coming in a little wetter for Monday with isolated to
scattered showers and thunderstorms as higher PWATS begin to advect
back north. On Tuesday, scattered showers and thunderstorms are
possible with deeper moisture aided by orographics. Highs likely
around 90 east Monday/Tuesday and 80s elsewhere except possibly a
little lower Tuesday far west pending clouds/showers.

500 mb pattern will once again become more amplified as another upper
low crossing north of the Great Lakes helps dig out an even deeper
eastern trough by the end of the period. This feature will also propel
a strong cold front toward the area Wednesday with the boundary likely
just south of the region during Thursday. Lead pre-frontal trough axis
likely to spark better coverage of showers/storms Wednesday
afternoon/evening, followed by cooler/drier air during Thursday and
Friday. Therefore will include higher chance to low likely pops
Wednesday and then mostly residual showers southern sections
Thursday pending timing of the frontal exodus by then. Should see
some cooling due to clouds including more showers Wednesday, then
cool advection behind the front resulting in highs only 70s
mountains to lower to mid 80s east Thursday and Friday.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
As of 1240 PM EDT Friday...

Most showers with isolated storms continue to linger across the
far western areas, mainly just west of KBLF-KLWB early this
afternoon where seeing pockets of MVFR cigs.

Latest models continue to develop showers/storms across and
espcly east of the mountains this afternoon into early evening
ahead of the upstream cold front. Appears most of this will
initially be scattered in nature over the west before perhaps
organizing into more bands/clusters along/east of the Blue Ridge
by mid afternoon. Shra/tsra should then push into eastern
sections by early this evening prior to exiting shortly after
00z/8 PM if not sooner. Thus running with mostly VCTS/VCSH at
all locations with tempo groups for lower MVFR conditions in
showers across the west, and deeper convection east where IFR
vsbys may occur.

Appears fog will be stuck in the valleys and eastern locations
that see heavier rainfall this afternoon although uncertain
given gradual advection of dry air late. For now included sub-
VFR in fog at KLWB/KBCB/KLYH late tonight while leaving out
elsewhere. Some stratus also possible with the fog and perhaps
even at KBLF with weak upslope flow.

Outside of convection, winds will be mainly west to southwest
at 5-15 kts ahead of the front this afternoon before turning
light/variable overnight.

Weak high pressure will build into the area Saturday bringing
VFR conditions under northwest winds at 5-10 kts.


Extended Aviation Discussion...

Mainly VFR conditions for the second half of weekend under weak
high pressure. Scattered thunderstorms and periods of MVFR
conditions return next week, with late night/early morning fog
possible almost any day. Better potential for sub-VFR will come
Wednesday when more widespread showers and storms arrive with
the next cold front.

&&

.EQUIPMENT...
As of 400 AM EDT Friday Aug 18th...

KFCX doppler radar expected to be down for the rest of the
month due to a failing bull gear. Technicians are working on the
radar, through next week and the radar will most likely stay
down completely as the repairs are being made.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...None.
NC...None.
WV...None.

&&

$$
SYNOPSIS...WP
NEAR TERM...JH/WP
SHORT TERM...KK
LONG TERM...JH/KK
AVIATION...JH/RAB/WP
EQUIPMENT...RAB/WERT/WP



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