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NWS Area Weather Forecast Discussion
County Warning Area [CWA]: RNK
Regional NWS Weather Office: Blacksburg, VA

FXUS61 KRNK 260533

Area Forecast Discussion
National Weather Service Blacksburg VA
133 AM EDT Wed Jul 26 2017

A weak front across northern North Carolina will shift south of
the area tonight into Wednesday before dissipating. High
pressure north of the region should continue to ridge south
across the area through Wednesday night keeping overall dry
weather in place. Another cold front approaches from the
northwest Thursday into Friday exiting to our southeast by
Saturday morning.


As of 125 AM EDT Wednesday...

Made a few adjustments to pops as high-res models showing a few
showers possible later this morning across the southern Blue
Ridge. The 00z Nested NAM and HiResARW are showing more
development of showers than the HRRR and RAP as well as the GFS.
Usually the NAM comes in too fast/too wet on return flow
scenarios, so will side a little drier.

Previous discussion from Tuesday evening...

No major changes needed to the going forecast. Did make a few
minor adjustments though. First was to remove isolated/slight
chance PoPs for northern NC tonight as it appears that in
surface METARs that the inverted trough axis has shifted
southward. Visible satellite imagery trends imply that deeper
cumulus development has been few and far between in this area
where residual instability is the greatest (though is not
significant). Also increased sky cover some for the after-
midnight period east of the Blue Ridge into the foothills in
VA/NC. Recent RAP and 18Z NAM/GFS guidance continue to show
increased low-level RH/stratus as 2-m AGL to 850 mb flow becomes
light easterly with passage of backdoor front to our northeast.
Coverage may be somewhat greater in the southern Blue Ridge
versus along the southern Shenandoah Valley. Outside of that
though, forecast appears to be in good shape with counties west
of the Blue Ridge likely to see sharply falling temps with
sunset and areas of patchy fog in the New, Holston and
Greenbrier River Valleys. Lows in the upper 50s to middle 60s
appear on track, though if stratus comes in sooner than
presently anticipated, possible that lows may need to be
adjusted up some in late-evening update.

Previous near-term discussion issued at 215 PM follows...

Surface front has drifted farther south this afternoon with dry
advection having pushed into northern North Carolina per latest
dewpoints. This has basically shifted instability to the south of
the CWA with only the far southwest currently holding onto a bit
more of a cumulus field. Appears best chance at seeing any isolated
showers will be across the North Carolina mountains where a weak
inverted trough lingers and expect a little more convergence as
winds start to turn more easterly by early evening. However not more
than a 20 pop at best with clear skies elsewhere this evening.

High pressure to the north shifts east overnight allowing a
little deeper easterly trajectory to develop by morning. Some
guidance again trying to bring low level moisture back into the
Blue Ridge espcly far south where dewpoints should be a little
higher. Think too dry elsewhere as models remain too moist, so
running with mostly clear to partly cloudy overnight with patchy
fog/stratus around. Should be quite comfortable overnight with
low dewpoints and better radiational cooling under the high
allowing temps to fall well into the 60s most spots, with even
some 50s possible in the valleys.

Will start to see low level moisture start to slowly rebound on
Wednesday as the high to the north shifts farther east resulting in
a gradual veering of the flow to a more southerly component later in
the afternoon. This seen via model jump in dewpoints and northward
shift in progged PWATs from south to north during the day. However
instability remains lacking as well as upper forcing with any
convection likely due to weak southeasterly flow convergence and
orographics. This again mainly over the southwest with ridging
holding across the north and east so trimmed pops back to only
isolated/low chances mainly southern Blue Ridge. Otherwise expect
any low clouds to mix out to more sunshine by afternoon with highs
cooler, mostly upper 70s to mid 80s, within a lingering east-
southeast flow until late.


As of 205 PM EDT Tuesday...

Surface high pressure will weaken Wednesday night into Thursday as a
cold front approaches from the west. This front is stuck in zone
flow and will slowly move from the upper Midwest Wednesday night to
the Ohio Valley Thursday. This front will pick up speed, moving over
the mountains early Friday morning, then east of the Blue Ridge
Friday afternoon as an upper level low tracks across the Great Lakes.

Diurnal heating and orographical lift should generate isolated to
scattered showers and thunderstorms across the mountains early
Thursday afternoon. These storms may slowly drift over the foothills
towards evening. As the front approaches the mountains early Friday
morning, the chance for wide spread showers will increase. Despite
these showers coming in during non-diurnal heating, linger
instabilities and a 30-40 knot low level jet could allow some strong
storms to pass over the area overnight. The chances for showers and
thunderstorms will move east of the Blue Ridge Friday morning,
exiting the piedmont by sunset. The best dynamics with this front
will remain north over Pennsylvania and Maryland Thursday night then
along the New England coast Friday. For the forecast area, the
severe weather outlook for Friday is marginal with scattered strong
storms to possibly a few severe cells across the foothills and
piedmont counties. If the front is slower by 6 or more hours, the
severe threat could increase to slight as this would place the
frontal passage during peaking heating Friday afternoon. Even if the
timing is slower, the best dynamics will remain to the north.

Temperatures will be slightly warmer than normal Thursday with low
to mid 80s west of the Blue Ridge and upper 80s to lower 90s east.
Friday will be a lot cooler across the mountains with morning
showers and a frontal passage. High temperatures will range from the
mid 70s to near 80F across the mountains. Temperatures will warm to
near normal east of the Blue Ridge with afternoon temperatures in
the mid 80s.


As of 240 PM EDT Tuesday...

An upper level low will track from the Great Lakes Friday, then
south over Virginia Saturday. Heating under this cold pool will
bring scattered showers to the region Saturday afternoon.
Temperatures will actually be cooler than normal for the first time
since late June.

Surface high pressure will build over the region starting Sunday.
This ridge of high pressure will keep the area cool and dry into
early next week.


As of 127 AM EDT Wednesday...

Should be mainly a VFR forecast, but will have to watch for
stratus forming along the Blue Ridge, especially south of
Roanoke. Some models are aggressive in stratus development while
others maintain a more scattered nature. Appears best threat of
sub-VFR cigs will be south of ROA-BCB so not affecting TAF

Exception will be LWB where flow from the northeast allows for
fog over the Greenbrier to flow toward the airport. Thinking a
LIFR vsby/cigs is likely by 10z. Confidence low on BCB getting
fog as dewpoint depression at 05z was 9F.

For Wednesday, fog lifts by 13z. Again some question on timing
stratus scattering out but by 15z most areas should be VFR.
Should see renewed VFR fair weather cumulus for the afternoon;
recent guidance continues to show spotty showers later in the
TAF period but feel these are overdone with large mid-level dry
air in place. Thus will not mention in the TAF at this point.
Winds mainly light east to southeast through Wednesday.

Aviation Extended Discussion...

Should start to see a front approaching with better threat of
showers/thunderstorms Thursday into Friday. Outside of storms
expect VFR conditions, with possible late night fog by Friday

Saturday looks drier with northwest flow, but could see
lingering low end VFR/high end MVFR ceilings in SE WV along with
a few added shra/tsra mainly southern sections Saturday
afternoon. Expect widespread VFR to return Sunday under high
pressure following the front.





Forecast Discussion from: NOAA-NWS Script developed by: El Dorado Weather