Area Forecast Discussion
Issued by NWS Blacksburg, VA-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KRNK 211352
Area Forecast Discussion
National Weather Service Blacksburg VA
952 AM EDT Fri Apr 21 2017
Going to see a cold front move to the mountains Friday morning,
shifting southeast to the NC piedmont Friday evening. 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 into Monday.
.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --As of 952 AM EDT Friday...
Made some minor adjustments in temperatures shaping with latest
surface obs and glamp for late morning into this afternoon. With
a warm start, high temperatures this afternoon will climb into
the lower 60s in the northwest mountains to the mid 80s in the
Piedmont. Adjusted pops for this morning with latest radar
images and trends, then shaped towards HRRR. As the cold front
moves east this afternoon expected scattered showers and
thunderstorms to develop along and ahead of this boundary.
SWODY1 places forecast area in a marginal risk for severe storms
with strong winds and hail the primary threat. The freezing
level on rnk 12z sounding was around 9 kft, helping hail
potential. Weak near-surface winds with a substantial westerly
component will hinder low- level shear. More changes later this
As of 720 AM EDT Friday...
A few showers shifting east across the NC/VA foothills picked up
well by HRRR and RAP, and try to hang onto these showers all the
way out to southside VA and the piedmont of NC, however, radar
showing a diminishing trend. Otherwise, appears the morning will
be mainly dry with high-res models holding off until after 18z
for most of the showers and storms to move into the mountains
and foothills. Made adjustments to account for this, otherwise
no other major changes.
Previous forecast discussion...
Getting a few showers already into southwest VA/southern WV ahead of
a cold front stretching from Erie PA southwest to Paducah KY.
Mesoanalysis showed area of deep moisture convergence centered in
southern WV as of 07z/3am EDT. High-res models are showing this well
enough but as we head through dawn they weaken this area and move it
east-northeast toward the Alleghanys/southern Shenandoah Valley.
For now will keep scattered mention in the far west to isolated to
the I-81 corridor then nudge pops to likely by 12z as better lift
enters the area. With the front entering the mountains after 12z,
the upper flow out of the west-southwest should move it further east
toward 18z to the Blue Ridge. Low level flow stays westerly enough
that models showing less coverage lee of the mountains but think
enough convergence along the front with moisture pooling along front
will bring likely pops to at least the western half of the forecast
area with high chance in the east. The Storm Prediction Center
continues to highlight marginal risk for the entire area. Svr threat
will be isolated as cloud cover and instability is limited as is mid-
level lapse rates. A modest mid-level flow of 30-40 kts may initiate
some stronger winds if storms can get some altitude. So main threat
will be isolated strong/damaging winds. The better instability will
be in the NC piedmont while best wind energy appears to shift over
As for temperatures will be warm again with temps already starting
out in the upper 50s to lower 60s. Piedmont will see highs in the
mid 80s with lower to mid 70s west as clouds and showers move in
This evening the cold front should be extending from southeast VA to
Western NC. Models showing weak waves of low pressure riding along
the front and in decent agreement in placing axis of better shower
threat from far SW VA to southside VA. Since the front starts to
turn more toward east-west orientation think better rain chances may
stay somewhat further north but appears all of the forecast area
will see a decent coverage of showers tonight, with thunderstorm
chances subsiding with loss of heating. Lows should be in the 50s in
the mountains, with lower 60s across the foothills/piedmont and
-- End Changed Discussion --
.SHORT TERM /SATURDAY THROUGH MONDAY NIGHT/...
As of 430 AM EDT Friday...
An upper level trof will dig into the southeast US and develop into a
slow moving closed low as it drives a surface low through South
Carolina, while a wedge of high pressure surges into the region east of
the Appalachians. Dynamic forcing between these two features will be
formidable while the low transports abundant moisture from the Gulf of
Mexico and southeast Atlantic over the wedge. These features will all
combine to bring significant rainfall to the region this weekend and
We start off Saturday in the warm sector with isentropic lift from the
surface low over the lower Mississippi valley bringing widespread
showers to the region, and high pressure poised to begin wedging down
from the north. Most guidance brings the wedge in behind a backdoor
cold front early enough on Saturday afternoon to shut down heating, but
locations from the Mountain Empire of VA through the mountains and
foothills of North Carolina will see some instability pool ahead of the
wedge to fuel development of some thunderstorms, and shear along the
backdoor front will be sufficient to help organize the convection. It
currently appears that any threat for severe thunderstorms will be
mainly across North Carolina where some breaks of sun will help enhance
instability, but the axis of instability will have to be monitored
closely to see if it can push further north from Interstate 77 westward.
By Saturday night we will be on the cold side of the surface low
sliding by slowly to our south with a strong wedge firmly in place
blowing a stiff east to northeast breeze east of the Blue Ridge. The
latest guidance has generally trended wetter and slower, so we can
expect rainfall to persist from Saturday night into Sunday night.
Precipitation will then slowly taper off in the west toward daybreak
Monday, but locations east of the Ridge will see showers continue until
Monday night. Ensemble precipitation plumes are exhibiting considerable
spread, but a reasonable expectation will be for precipitation totals
from Friday through Monday to be in the 2.5 to 4 inch range, and this
amount of rainfall may be enough to push some streams, creeks, and
rivers into minor flood.
Temperatures will be trending much cooler as the wedge sets in this
weekend. Highs on Sunday will only reach the upper 50s east of the Blue
Ridge, with readings of 50 to 55 more common in the west.
.LONG TERM /TUESDAY THROUGH THURSDAY/...
As of 850 PM EDT Thursday...
Will 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 will then be advertised 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 southeastern CONUS.
Temperatures will be trending closer to normal Tuesday, and then above
normal for the second half of the week.
.AVIATION /14Z FRIDAY THROUGH TUESDAY/...
As of 720 AM EDT Friday...
A few showers in far SW VA, SE WV and NW NC this morning but
very scattered in nature.
Following a blend of the GLAMP/HRRR and RAP will not really see
any predominant showers until midday to early afternoon. Still
will keep thunder as vicinity, but think thunderstorms are
likely some point this afternoon at most sites. Should be VFR
then some MVFR to IFR with heavier showers/storms.
Front moves southeast of us this evening with another wave
moving along the front allowing for another round of showers
late. Should be VFR but expect some cigs/vsbys to sink to MVFR
or IFR especially in the BLF/BCB area late.
Extended Aviation Discussion...
Weather pattern active this weekend into Monday with periods of
rain/showers, heavy at times. Should expect mostly sub-VFR
condition when raining, but some VFR possible, especially north
of a LWB-LYH line.
Rain lingers into Monday as does lower cigs, and finally seeing
some VFR returning Tuesday.