Area Forecast Discussion
Issued by NWS Blacksburg, VA-- Remove Highlighting --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KRNK 202346
Area Forecast Discussion
National Weather Service Blacksburg VA
746 PM EDT Thu Apr 20 2017
High pressure over the Southeast States will move east and weaken
this evening into tonight. A cold front will approach from the west
tonight and enter the mountains Friday morning. This front will
stall near the NC/VA border Saturday, with a secondary low moving
across the area Sunday. This will keep us in a wetter pattern
through the weekend.
.NEAR TERM /THROUGH FRIDAY/...
-- Changed Discussion --As of 745 PM EDT Thursday...Surface lee trough, which in
conjunction with moderate instability facilitated development of
scattered to numerous showers and thunderstorms producing
periodic instances of sub-severe hail/wind gusts up to 40 mph
in stronger cores, has now shifted east of the central Virginia
Piedmont. Surface pressure rises/weak ridging are now taking
place across much of the forecast area and will continue to
prevail through the early overnight. Cooling temperatures should
allow for modest remnant instability to be reduced as well. Per
upstream observations and recent HRRR/18z NAM runs, it`s not
until a pre- frontal trough now sparking thunderstorms in
southeast Ohio through parts of east- central Kentucky nears our
area well after midnight when the chances for showers re-
Generally indicated diminishing shower coverage in the next
hour or two, blending in the most recent HRRR in to the prior
forecast. This keeps any remnant/rogue showers to western parts
of Mercer, Summers and along/west of the Greenbrier Valley
through midnight. As the Piedmont and Southside saw at least
some showers and with dewpoints in the upper 50s to mid 60s
common in these areas, developing shallow inversion may lead to
areas of patchy mist or ground fog. Not thinking dense fog at
this point as there is enough of a southwest flow to keep
visibilities from falling too far.
I`ve made no changes to lows at this point which look on track,
but did try to better account for current temperature/dewpoint
obs and near-term trends through midnight.
Previous near term discussion issued at 400 PM follows...
Scattered showers and thunderstorms have develop this afternoon in
the unstable warm airmass. Surface boundary with associated surface
trough acting as a trigger for storms with best coverage in
Southeast West Virginia. The combination of orographic lift and
instability creating convection along the Blue ridge mountains. The
Day One convective outlook places the threat for severe weather to
our northwest with the better instability closer to the cold front.
An isolated strong storm may be possible this afternoon into this
evening. Shaped pops this afternoon into tonight towards a blend of
the HRRR and NAM.
With the loss of solar heating this evening, expect showers and
thunderstorms to dissipate. A cold front in the Ohio Valley will
move east tonight and reach the Central Appalachians and Mid-MS
Valley by Friday morning. Ahead and along the cold front, scattered
showers and isolated thunderstorms will move into the mountains
Friday morning. Low temperature tonight will range from the mid 50s
in the mountains to the lower 60s in the Piedmont.
A wet Friday is expected for our region as a cold front pushes east
with scattered showers and thunderstorms. The Day 2 Convective
Outlook has place the entire forecast area in a marginal threat for
severe weather Friday. There is a severe weather potential with
possible thunderstorm activity along/ahead of the southward
advancing surface cold front. The main threat is for damaging winds
and hail. High temperatures Friday will vary from the mid 60s in the
northwest mountains to the mid 80s in the Piedmont.
-- End Changed Discussion --
.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/...
As of 400 PM EDT Thursday...
The need for an umbrella or a raincoat may be an understatement
this weekend. Potential for 1 to 1.5 inches of rain is high,
with moderate confidence for 2 to 2.5 inches of rain from Friday
night through Sunday night inclusive.
A stalled surface front is forecast to bisect the Mid-Atlantic
Region through the weekend, extending from the North Carolina
Coast Saturday into an area of low Pressure over western
Tennessee. This area of low pressure is forecast to slowly move
east along the stalled front providing a focus for occasional
showers and thunderstorms which may morph into an all out period
of rain or periods of rain at times through the weekend.
Our forecast area is expected to be on the cooler, north side of
the front, thus resulting in cooler than normal temperatures,
winds primarily out of the north or northeast. The only
exception may be Saturday when strong southwest wind flow aloft
across the mountains battles with the impinging northeast low
level flow coming in from northern WV/VA resulting in
potentially warmer conditions for the higher elevations and into
far southwestern VA and NC. The more dense cooler air will
eventually win out by Saturday night with the entire CWA
succumbing to the cooler temperatures by Sunday.
The severe weather threat at the present time looks marginal
per abundant cloud cover which is forecast to limit
instability. Of greater concern will be the potential for
runoff issues per increasing confidence for repetitive rainfall
and long duration which could linger into Monday. Model Median
forecasts for some of the Rivers, such as the Dan River, are
indicating at least 50 percent chance for area rivers to
experience a rise of half bank to three quarters bank full, and
a 30 percent chance for near bankfull or minor flooding. This
introduces the potential need for a Flood threat headline at
some point for the weekend, and will continue to highlight this
in the Hazardous Weather Outlook. Folks that have plans that
involve area streams and rivers this weekend should pay close
attention to the forecast and potential changes which may occur
with respect to the river stage forecasts.
.LONG TERM /MONDAY THROUGH THURSDAY/...
As of 420 PM EDT Thursday...
Low pressure will linger along the Mid Atlantic coast Monday,
the models trending slower with its departure. This suggests
Monday may turn out to be a wetter day than currently forecast
with potential for clouds to linger into Tuesday before any sort
of drying trend becomes established. For now will trend pops
upward for Monday and hang onto some cloud cover for Tuesday.
With an east or northeast wind, temperatures will be slow to
recover until the sun returns for mid Week.
A dry forecast is currently being entertained for Wednesday and
and Thursday. A surface front is forecast to move into the Ohio
Valley from the Great Lakes, but think this front will stall
north of the area as High pressure amplifies over the
Temperatures Monday will be cooler than normal, trending closer
to normal Tuesday, and then above normal for the second half of
.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
-- Changed Discussion --As of 745 PM EDT Thursday...
Earlier scattered to numerous showers and storms have dissipated
and/or shifted eastward into eastern Virginia. This leaves the
TAFs in VFR conditions that should continue through much of the
overnight. Will be watching for development of IFR visibility
fog at Lynchburg and Danville given recent rain and that
dewpoint/moisture values are currently highest. Winds generally
southwest 4-6 kts, lightest with eastern extent.
A pre-frontal trough, and later a cold front, will focus additional
showers into southeast West Virginia by 08-10z. Expect a
deterioration in ceilings toward VFR/MVFR from west to east with
increasing coverage of showers. Development of scattered
thunderstorms mainly during the afternoon, though a rough timing
would be toward the mid to late Friday afternoon hours per the
3-km NAM. Kept either VCSH and/or unrestricted showers due to
low confidence at this time range. Winds southwest around 6-10
kts, veering westerly slightly during the afternoon.
Extended Aviation Discussion...
VFR to MVFR cigs into Friday night with this front. As front
moves south Saturday morning, will see more coverage and this is
especially true Sunday. During this time, expect cigs/vsbys to
be highly variable but most of the time should be sub-VFR.
Front moves east Sunday night, with some lingering sub-VFR
possible over the mountains, but mainly VFR Monday with high
pressure nosing in from the northeast.
-- End Changed Discussion --