Area Forecast Discussion
Issued by NWS Blacksburg, VA-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KRNK 190755
Area Forecast Discussion
National Weather Service Blacksburg VA
355 AM EDT Fri Aug 19 2016
-- Changed Discussion --A weak residual boundary just south of the area will continue to
dissipate today as weak high pressure slides in from the west
overnight into Saturday. A strong cold front over the central
United States will cross through the region late Sunday into
Sunday night. Somewhat cooler and drier air follows the front for
early next week.
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.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --As of 240 AM EDT Friday...
Rather uncertain forecast continues today with impulses within the deep
westerly flow aloft riding over lingering moisture but without much
surface support. Current embedded shortwave responsible for mid deck
and associated patchy showers should move across southern sections near
the lingering surface trough early taking residual showers to the east
after sunrise. This may allow for a brief break in clouds outside of
far southern/western sections this morning, before yet another wave
aloft arrives from the west this afternoon and shears across this
evening. Models tend to develop some instability between these features
espcly mountains this afternoon while keeping some weak low level
wedging out east within weak easterly flow. Appears guidance again
likely overdone with lift espcly the Nam in showing a swath of stronger
UVV`s over western/southern zones this afternoon with deeper convection
spreading over to the Blue Ridge. However most solutions do suggest
better coverage along the I-77 corridor into northwest NC and less
north/northeast sections where more sunshine possible. Convective
potential still looks iffy unless more heating takes shape as winds
aloft remain weak with better pwats just to the south.
Thus will include some higher pops over the south-west later this
afternoon per short term model consensus, and continued chance
elsewhere although this may be generous across northeast sections. High
temps quite tricky with clouds to start and then possibly some breaks
before filling in again. Given cooler trends of late, will lean toward
the EC mos a little more. However given such a warm start will only
take brief heating to send readings up more than expected espcly Blue
Ridge west. Otherwise going with mostly mid/upper 70s mountains to
the mid 80s out east for now.
Deeper moisture slides to the south/east later this evening leaving
only weak upslope far west late. Best coverage again early on likely
southern sections closer to the residual surface trough axis and just
south of the weak wedge to the north. Forecast soundings also show some
decrease in deep moisture over the leftover low level eastern wedge, so
mainly decent chance pops espcly south/east early, decreasing to
isolated or less overnight. Lows mostly 60s west to lower 70s east.
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.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
-- Changed Discussion --As of 300 AM EDT Friday...
Heights increase Saturday following the passing of a mid level
disturbance tracking across North Carolina Friday night. A weak
bubble high will also develop over the area for Saturday. With these
two layers of ridging, the chance for rain are less for Saturday
than previously forecasted. However, models are showing convergence
in the low levels across the mountains and along with orographical
lift in a semi-tropical environment, scattered thunderstorms are
possible during the afternoon and evening hours.
A strong cold front will move across the region Sunday, bringing a
big change in airmass for much of next week. This front will move
from the Mississippi Valley Saturday afternoon, over the Ohio and
Tennessee Valleys Saturday night, then to the Mid Atlantic Coast
late Sunday afternoon. The stronger dynamics with this system will
be north of DC and into New England. Another area with weaker
dynamics will track across the Tennessee Valley south into the Gulf
States. With no really good support for organized storms and showers
entering the area in the morning, we are not expecting any
widespread outbreak of severe weather with this frontal passage.
This front will be clearing out the tropical-like air, therefore
heavy rain is possible Sunday.
Light easterly flow should produce cooler than guidance temperatures
in the east Saturday. However, with less rain expected, high sun
angle will help warm temperatures to near normal levels. With rain
coming across the mountains during the morning Sunday, high
temperatures will range below normal. If the front is slow to move
east of the mountains in the afternoon, temperatures across the
piedmont may peak near normal in the upper 80s.
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.LONG TERM /MONDAY THROUGH THURSDAY/...
As of 310 PM EDT Thursday...
Our biggest air mass change in some time is expected for next week.
A broad area of dry, and cooler high pressure will become situated
over the region by Tuesday. Its influence will continue across the
area through at least Wednesday. By Thursday, the center of the high
may shift far enough east for moisture to start wrapping north along
its western flank. With greater low level moisture and a weaker cap,
isolated showers will be possible across the mountains during the
Temperatures will trend cooler through Tuesday before inching higher
through Thursday. Readings will be within roughly two to three
degrees of normal.
.AVIATION /08Z FRIDAY THROUGH TUESDAY/...
As of 1255 AM EDT Friday...
Weak surface trough over the region will result in considerable
cloudiness espcly over southern and western sections overnight
through Friday, with multilayer cloudiness mainly above 4kft.
Sub-VFR cloud layers are possible west of the Blue Ridge favoring
the western upslope side of the Appalachian Divide pending just
how much lower levels moisten overnight. Also combination of
westerly flow and a passing weak shortwave likely to result in
patchy showers around KBLF east to just west of KBCB through at
least early Friday. Appears more convective nature shra will occur
south of the region overnight but could clip KDAN late tonight.
Low confidence on fog formation overnight. There is considerable
mid/high level cloudiness across the forecast area and models
suggest this will persist overnight. If clouds persist, then fog
will not be an issue. If clouds thin for a time, then expect some
patchy fog espcly along the KLWB to KLYH corridor where potential
for more clearing looks to occur.
Axis of deeper moisture will remain in place under westerly flow
aloft on Friday. Another mid level shortwave riding through the
moisture plume looks to work into the mountains during the
afternoon. This may act to enhance isentropic lift initially Friday
morning over the west with mostly light showers affecting parts of
the mountains and southern sections through midday. Then questions
with the degree of heating that takes shape espcly KLWB-KBLF
corridor and points south Friday afternoon. Models likely too fast
to break things open so only keeping VCTS mention at KBLF for now
and VCSH at KLWB while leaving out elsewhere. Think mainly VFR
across northeast sections espcly around KLYH that should remain
removed from much more than some cumulus and mid deck.
Extended aviation discussion...
Appears weak high pressure may linger Friday night into Saturday
with deeper moisture mainly south/west of the region ahead of a
strong upstream cold front. This may only result in isolated
shra/tsra Saturday with better coverage holding off until Saturday
night with the pre-frontal showers. Thus could see more in the way
of VFR Saturday before trending MVFR in increasing showers from
west to east overnight Saturday night. Best likelihood of more
widespread showers and storms including MVFR/IFR will take place
Sunday ahead of a strong cold front that will pass through the
region by Sunday night. Much drier air is anticipated after
Sundays frontal passage with a dry forecast for Monday and Tuesday
promoting VFR for much of the mid-Atlantic.