Scientific Forecaster Discussion
fxus61 krnk 300116
Area forecast discussion
National Weather Service Blacksburg Virginia
916 PM EDT Mon may 29 2017
a cold front will continue to push southeast of the area
overnight, followed by another front entering from the
northwest Tuesday afternoon. Yet another front passes across
Wednesday, before high pressure builds overhead Thursday.
Near term /through Tuesday/...
as of 855 PM EDT Monday...
Shallow convection continues to clip far southeast sections
while northward advance has been limited by mid level dry air as
seen off evening soundings and lower surface dewpoints per msas.
Instability has also been rather weak over southern sections so
appears given loss of heating that most added deeper convection
will remain to the south ahead of the weak front now across
southeast sections. Latest hrrr a bit more aggressive in
holding showers in well after midnight espcly far southern
third as the sheared wave aloft seen off the vapor loop crosses
the region overnight. This so far has been overdone so leaning a
bit more toward the drier rap solution in keeping in some low
pops southeast a couple hours after midnight as showers may tend
to string out along the slow boundary. Otherwise not a lot of
change with mainly cloudy skies per extensive mid deck, and
lows 50s mountains to low/mid 60s east.
Previous valid discussion as of 300 PM EDT Monday...
Pops will remain on the low end early this afternoon, with
meager convergence, however, a few showers are popping up and/or
moving in from far SW VA, with some cu development over the Blue
Ridge north of the James River.
High-res models still content on keeping focus over NC and south
today with storms becoming more robust by late afternoon
along/south of I-40, but some should sneak into our southeast
County Warning Area by 23-01z, so higher pops to 40-60 look good from
Yadkinville to South Boston. No big chances in the severe threat
with best instability and shear over the Piedmont of NC and east
of Danville in VA, to points east and south.
Drier air should filter into the low levels overnight west of the
Blue Ridge, but expect slower removal of higher dewpoints to the
east, as frontal boundary stalls from the smokys/north Georgia to
the coastal plains of NC. Seems we will keep some mid/high clouds
across the Piedmont overnight with a little clearing to the west.
Lows tonight expected to be similar to this mornings, with mid to
upper 50s west, to lower to mid 60s.
Tuesday, not much change in the airmass, though slightly lower
dewpoints are possible in the higher ridges. Southwest flow aloft
and some weak vorts tracking along it may spark isolated convection
but overall threat expected to be 20/30 pops at most in the
afternoon. Highs will range from the mid to upper 70s mountains, to
mid 80s east.
Short term /Tuesday night through Thursday night/...
as of 145 PM EDT Monday...
On Tuesday evening, the passage of a weak surface trough out of
the forecast area should allow for a brief period of dry
weather for the overnight hours. With westerly low-level flow,
the mid- Atlantic/central Appalachians region should remain
removed from higher dewpoints over the central/eastern
Carolinas. Temperatures should be fairly seasonable with
lowering humidity levels into the 50s to near 60.
Wednesday looks to be the most active day in the mid-week period.
Low-level winds back slightly toward the southwest/south-southwest
ahead of a potent mid-level shortwave and associated surface cold
front over the Ohio Valley. These two features approach the central
Appalachians late Wednesday into Wednesday evening. Should start
Wednesday with mostly sunny skies but with increasing cumulus clouds
due to steepening low-level lapse rates and modest moist advection
as winds become more southwesterly. Frontal timing is a little
slower than noted yesterday, moving across the region later
Wednesday afternoon into the mid-evening hours. Relatively strong
500 mb jet of 60 kts contributes to unidirectional though strong
speed shear with effective shear magnitudes of 50 kts. GFS and NAM
are at odds on the degree of instability due mainly to potential
daytime mixing of drier dewpoints. CAPES range from 1500-1700 j/kg
as modeled by 12z NAM using the most unstable parcel while the drier
GFS comes in with cape values about half that much. Potential would
exist for an organized scattered strong to severe thunderstorm risk
if the nam's shear/instability parameter space proves correct, less
so with the GFS. It is something that will need to carefully
monitored. Current day-3 severe convective outlook depicts a 5%
severe/marginal risk from the Blue Ridge foothills westward, with
general thunder further east. This seems appropriate given
uncertainties as described, though the threat may be slightly higher
if greater instability is realized.
Behind the front, a strong shot of cooler and drier air then advects
into the region late Wednesday night into Thursday. Surface ridge
builds in from the Midwest allowing for the first real fully dry day
experienced recently. This should knock temperatures down to highs
in the mid 70s to lower 80s except cooler in the mountains but with
notably drier conditions, and lows Thursday night in the 50s to near
Long term /Friday through Monday/...
as of 100 PM EDT Monday...
Forecast area then becomes embedded in a zonal, confluent mid-level
flow for a large part of late week into the weekend. This is
modulated by two primary 500 mb features: a persistent mid/upper-
level low and cyclonic flow over the northeast/Great Lakes, and
increasing troughing over The Four Corners leading to greater
southwest flow and warmer/more humid conditions from the plains
into the Tennessee Valley.
Warmer and increasing humidity levels can be expected through the
upcoming weekend, along with a return to a wet weather pattern in a
general sense. Timing weak shortwave troughs/vorticity maxima in the
confluent flow is difficult; and as such, i've kept chances for pops
lingering through the upcoming weekend into early next week.
Aviation /01z Tuesday through Saturday/...
as of 645 PM EDT Monday...
Weak front over the area along with the arrival of a wave aloft
from the southwest will bring periods of mid deck along with
sct/bkn cumulus across southern sections into late this evening.
This should make for mainly VFR cigs with potential MVFR around
kdan if any convection to the south lifts farther north. Will
keep in a thunderstorms in the vicinity/tempo mix for shra/tsra at kdan through at least
midnight as this point. Otherwise rest of terminals should stay
rainfree, though a light shower/sprinkle cannot be ruled out,
but nothing to obscure vsbys or lower cigs below VFR.
Latest guidance shifts most convection to the southeast well
after midnight with lingering mid deck over the east and some
high clouds over the west. Fog may become an issue mainly over
the east/southeast where dewpoints remain higher and espcly
around kdan if some rainfall occurs. Also some late night valley
fog possible over the west. Thus appears a period of MVFR/IFR in
fog/stratus will be possible espcly at klwb/kbcb/kdan, but
confidence remains low to medium of seeing much coverage.
Any fog lifts around 13z/9am to VFR and some sct/bkn mid clouds
Tuesday morning followed by sct/bkn cumulus in the afternoon.
Another front expected by Tuesday afternoon appears to bring
scattered to isolated threat of showers/storms to the area, but
will be VFR overall.
Extended aviation discussion...
Overall VFR expected for mid and late week before showers and
storms increase later Friday into Saturday. Exceptions will be
any late night river fog, or shower/wet ground induced fog.
Next threat of shower/storms Wednesday with yet another front
and decent upper support, so will have to watch for stronger
convection but appears any sub-VFR will be brief and mainly over
Thursday looks dry then more potential for scattered afternoon
and evening storms Friday through the weekend with the best
coverage on Saturday when more in the way of sub-VFR will be