Area Forecast Discussion
Issued by NWS Blacksburg, VA-- Highlight Changed Discussion --
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FXUS61 KRNK 211829
Area Forecast Discussion
National Weather Service Blacksburg VA
229 PM EDT Thu Jul 21 2016
High pressure will remain over the mid Atlantic region through
Friday. Two weak cold fronts will approach the area early in the
weekend and again early next week. The first front will slide down
from the north on Saturday but may stall across northern Virginia
by Sunday. The second front will approach the Appalachians from
the northwest on Tuesday.
.NEAR TERM /THROUGH TONIGHT/...
As of 1020 AM EDT Thursday...
Latest visible satellite imagery shows that lingering fog/stratus
has burned off and some cloudiness has started to develop east of
the Blue Ridge. Water vapor imagery and morning RNK sounding
indicate that the atmospheric column is dry and expect this to be
a limiting factor in precipitation chances today. Developing
return flow around high pressure sliding off the mid Atlantic
coast will begin to advect low level moisture into the region on
SEly flow and orographic effects along the Blue Ridge combined
with some diurnal instability warrant some slight chance/chance
POPs. However as the column mixes the dry air aloft should temper
CAPE and act to erode convective towers. A lot of meso guidance
was too robust with precipitation overnight and into this morning,
but going with the qualitative overall trend and throttling back
the quantitative numbers yields reasonable grids. Will keep a
keen eye on how/if things develop and adjust if necessary.
Temperatures will be seasonably warm once again today with highs
around 90 east to mid/upper 80s west.
Hard to identify the diffuse surface frontal boundary across the
Carolinas this morning, but certainly drier air has moved in behind
it at all levels except for the surface as see in areas soundings.
Still, lingering weak surface-based instability across the
Piedmont overnight had helped to keep a couple of very small
isolated showers going near and north of LYH, but which have now
dissipated. Despite one or two models suggesting further shower or
even thunderstorm activity, do not see that redeveloping at this
point given lack of any triggering mechanism.
Same scenario today as we had on Wednesday with modest
instability by midday but mixing of drier air aloft may limit this
as afternoon wears on and any isolated convective updrafts will
entrain a lot of dry air aloft and struggle to survive. Still,
with some weak convergence along the higher terrain of NW NC and
simply better instability in parts of the Piedmont of north-
central NC and southside VA, cannot rule out an isolated shower
or storm. Have included slight chance pops for these areas along
and east of Blue Ridge but basically for southern third of
forecast area. It is most likely however that nothing will manage
to form during the afternoon and evening other than fairly shallow
scattered cumulus clouds.
With surface high pressure elongated from off the mid-Atlantic
coast to over SW VA...winds will be very light and perhaps a
tendency to be from the southeast along and east of Blue Ridge.
Skies should clear out tonight and this may once again allow some
patchy fog to develop in a few favored valley locations, but with
each day of no additional rain the coverage of fog will become
less each night.
.SHORT TERM /FRIDAY THROUGH SUNDAY NIGHT/...
As of 230 AM EDT Thursday...
During this portion of the forecast an upper ridge over the central
portion of the country will gradually shift west. This will allow
for a better opportunity over the weekend for a cold front to move
into the area and then stall over the region, or just to our south
by Sunday night.
Friday, precipitation chances will be limited to primarily diurnal
and orographical influences along and near the crest of the Blue
Ridge. Coverage of showers and storms will increase Saturday ahead
of the cold front, and continue into Sunday with its arrival. Prior
to the arrival of the front, 850mb temperatures will gradually rise
across the region, reaching 22 to 24 C by Sunday afternoon. Forecast
highs by Sunday will be in the low to mid 90s across the Piedmont
with a range of 80s across the mountains, elevation dependent.
Heat index values will top the century mark during the afternoons of
both Saturday and Sunday across the southeastern portion of the
.LONG TERM /MONDAY THROUGH WEDNESDAY/...
As of 230 PM EDT Wednesday...
Upper ridge will extend from coast to coast through Wednesday. In
the northern stream the next synoptic scale wave crosses through
the northeast United States on Tuesday. Models were similar
bringing moisture back into the region on Monday and Tuesday ahead
of the front. Therefore this will be the time frame with the
highest probability of showers and thunderstorms. Temperatures
will remain above normal ahead of the front on Monday.
.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
-- Changed Discussion --As of 220 PM EDT Thursday...
Fairly high confidence in conditions this period with little
change in the pattern since yesterday and essentially going with
a persistence forecast for this TAF package. Do not expect any
isolated showers to affect airstrips during the next 6 hours, and
any showers at all will fade this evening with loss of heating.
We should see another night of good radiational cooling with some
fog development in the typical spots. Expect LIFR at KLWB with a
tempo to MVFR primarily at KBCB and also see a possibility for
KLYH and KDAN as well. Believe elevation of KBLF will keep them
out of any fog and KROA is not a favored location for radiational
fog. Any fog/stratus will burn off Friday morning and leave
SCT/BKN cirrus but will not clutter TAFs with another line for
Any diurnal convection will likely hold off until after this TAF
period, and expect any isolated showers/thunder to primarily be
along and west of the Blue Ridge thanks to orographic forcing and
a weak front approaching late in the day.
Winds will be light through the period.
Extended aviation discussion...
Strong high pressure aloft centered across the Midwest will
attempt to build eastward into the weekend, then retreat back to
the west early next week. Our region will remain on the eastern
periphery of the upper high and thus subject to weak disturbances
in northwest flow aloft tracking around the Great Lakes and into
the Mid-Atlantic. Moisture and instability will be sufficient for
isolated pop up thunderstorms, but a slightly better coverage will
be possible by late Friday into Saturday with a weak front/short
wave moving into the Mid-Atlantic with a better threat for
convection early next week as another front moves into the area,
stalls, and lingers over the region for several days. Hot and
humid conditions east of the Blue Ridge this weekend which may
impact density altitude. Late night and early morning fog will be
possible at the usual sites almost every day.
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