Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 261859

Area Forecast Discussion
National Weather Service Blacksburg VA
259 PM EDT Wed Jul 26 2017

High pressure to the north will continue to wedge south into the
area tonight before passing offshore the Delmarva by early
Thursday. A cold front will approach from the northwest
Thursday, sliding across our area Friday afternoon into Friday
night. Cooler and much drier air under high pressure will follow
the front over the weekend into early next week.

As of 230 PM EDT Wednesday...

Wedged up low clouds remain slow to exit espcly along the mountains
where persistent low level easterly flow continues beneath the
inversion. Expect most of the low clouds to gradually mix out to
cumulus through the rest of the afternoon so trending with less
clouds into early this evening. Any shower threat appears quite
limited as well due to lingering clouds so only keeping a mention
across the far south at most into early evening.

Otherwise high pressure to the north to shift slowly east and
offshore overnight allowing the flow to gradually veer more
southerly by morning. However potential for the wedge to flop back
in after sunset, brings the possibility of seeing some low clouds
redevelop mainly Blue Ridge. Therefore keeping things partly cloudy
overnight but dry. Given some increase in dewpoints and clouds
around supports lows mainly 60s, although perhaps only around
70 in spots out east.

Surface high slides farther offshore early Thursday in advance of
fast moving lead shortwave energy that will slide east under a
passing 5h trough well to the north. Guidance suggests could see a
late night MCS type feature with this wave that should push
east/southeast and weaken on approach in the region mid/late morning
Thursday. However timing and eastward extent remains iffy with some
solutions slower which would allow a better convective threat per
heating into the afternoon. For now given southward trends of latest
models including ensembles, will bring in chance pops for mainly
showers mountains by midday Thursday before pushing the lead
outflow/trough east during the afternoon. With a quick return of
deep moisture/PWATS under decent diffluence aloft, think enough
coverage to warrant likely pops west by afternoon, with possible
stronger convection east. Severe parameters look rather marginal
although given deep moisture could see an isolated threat from
strong winds espcly if forcing arrives later and some multicellular
organization takes shape. High temps remain tricky given potential
early clouds followed by a few breaks which could again hold the
mountains in the 75-80 range while allowing the east to bump up into
the mid to upper 80s.

As of 205 PM EDT Wednesday...

Models are suggesting convection will move over the mountains
Thursday evening from a decaying Mesoscale Complex (MCS) that will
track over the Ohio Valley during the afternoon. The GFS has evening
convection jumping from the Blue Ridge to the piedmont where a lee
trough resides. Convection continues overnight across the area but
will primary be along the VA coast. The NAM has the MCS convection
stalling over the mountains Thursday evening, with weaker storms
remaining in the area overnight. The ECM is similar to the NAM
however convection stalls further north, north of hwy 460. To cover
all scenarios, we will keep high PoPs across the mountains, lower in
the east. We will also keep PoPs through the overnight hours which
is a change from yesterday thinking of diurnal showers fading after

The forecast for Friday remains a challenge with an upper level
closed low coming out of the Great Lakes generating one or two low
pressure systems over the Mid Atlantic region. One low may develop
closer to the upper low and track across Pennsylvania Friday into
Friday night. Another low will track along the front from the
Midwest across the southern Ohio Valley. Which low becomes dominate
will determine the severity and areal coverage of storms Friday
afternoon and evening. The GFS merges both lows over eastern
Virginia with the strongest convection remaining with the northern
and upper level low. The NAM has the southern low dominating, coming
across the forecast area Friday morning with the front exiting the
piedmont by sunset. The NAM has the strongest storms tracking over
the area during the morning which will keep the chances for severe
storms low. The ECM is similar to yesterday with the strongest
convection remaining north and just a few strong to severe storms
with the frontal passage east of the Blue Ridge during the
afternoon/early evening hours. Confidence is high to increase PoPs
Friday however, the level and areal coverage of strong to severe
storms remains a question.

All models are in agreement with convection moving east by midnight
with a sharp reduction in PoPs from west to east. A chance for
showers will appear again Saturday afternoon as the closed upper low
and cold pool track over the region. Majority of the heavier showers
will be along the front which will be well east and south of the
area. Therefore, we will only carry isolated to scattered showers
Saturday afternoon.

Temperatures will also be a challenge Friday, depending on the
amount of sun. However, with high chance for showers, temperatures
will be a few degrees cooler than normal. Under the cold pool and
with northerly winds, Saturday`s temperatures will be cooler than
normal with highs ranging from the 70s across the mountains to lower
80s east.

As of 240 PM EDT Wednesday...

An upper level trough will sit over a dry surface high through the
middle of next week. No rain is in the forecast and everyone should
see fair weather cumulus clouds forming in the afternoon, then
fading in the evening.

Temperatures will be a few degrees below normal on Sunday,
moderating back to normal on Tuesday. Dew points will remain low
(mid 50s to lower 60s) through Tuesday, then edge up towards being
muggy again on Wednesday.

As of 1245 PM EDT Wednesday...

Low clouds including sub-VFR cigs and spotty MVFR vsbys remain
slow to exit espcly along the Blue Ridge where still locked
within the low level wedge by weak easterly flow. However seeing
clouds scatter out in areas east of the mountains and along the
far northwest slopes, so expecting gradual improvement to
continue as heating increases. Thus appears most should reach
VFR soon with perhaps KBCB holding onto to periods of MVFR bases
until mid afternoon.

Models suggest that low clouds may again fill back in espcly
over the mountains overnight as despite south/southwest flow,
speeds remain light which may allow moisture to pool again as
the wedge will be slow to go after dark. Still not confident
enough to lower back down to widespread MVFR but pockets of
lower cigs possible espcly near the Blue Ridge so added in a
scattered lower layer for now. Fog again remains uncertain as
more clouds could inhibit development while more clearing
valleys may promote a few hours of MVFR so left in brief MVFR at

Lead wave ahead of an upstream cold front will arrive from the
west Thursday with possible MVFR conditions arriving during the
morning mountains, and out east with renewed convective
development Thursday afternoon. However appears cigs may remain
overall VFR with more of a VFR 3-5K ft layer beneath mid deck
canopy as residual upstream showers arrive by midday Thursday.
Therefore keeping things mostly VFR into early Thursday
afternoon without any mention of showers at the end of the valid
taf period.

Aviation Extended Discussion...

Should start to see a front approaching with better threat of
showers/thunderstorms Thursday night into Friday. Stronger
storms and heavy rain possible in and around the mid-
Atlantic/Ohio Valley region into the Carolinas this time frame
so be prepared for sub-VFR at times. Outside of storms expect
VFR conditions, with possible late night fog by Friday morning.

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/eastern sections Saturday
afternoon. Expect widespread VFR to return Sunday/Monday under
high pressure following the front.




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