Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 201738

Area Forecast Discussion
National Weather Service Blacksburg VA
138 PM EDT Thu Jul 20 2017

High pressure building eastward across the area will provide
hotter temperatures today through Saturday. At the surface, a
frontal boundary will slowly track from the lower Great Lakes
today, to the Northern Virginia area by Friday evening, before
stalling. A cold front will move into the area early next week.


As of 944 AM EDT Thursday...Overall no significant changes
needed to the going forecast. Overnight fog/stratus still
lingers in the Greenbrier River Valley and the New River in WV
but this should erode in short order as the sun continues to mix
it out. Otherwise, turning increasingly warmer today with what
should be nearly full sunshine. I say nearly because looking to
the northwest across the Ohio Valley is some scattered to broken
mid-level cloudiness from the weakening convective complex that
moved across Chicago and the upper Midwest overnight. Mid-level
trajectories offer the potential for some of this mid to high
cloudiness to filter into southeast WV and Tazewell County in
VA. Thicker coverage anticipated west of the forecast area, and
it`s not clear this will necessarily curb strong heating for
these counties either. Reviewing 12z RAOBs at RNK, ILN and GSO
reveals dry mid-level air with some weak capping. Suspect that
with warming thermal profiles today the cap will hold tough, and
if anything were to break through the cap it would likely be
limited further by the drier mid-level profile. Feel that
higher-resolution guidance that breaks out some isolated showers
or storms this afternoon today is overdone, so left as dry. No
changes to highs at this point - though depending on opacity of
mid/high-level clouds, may make some slight adjustments down.

Previous near-term discussion issued at 310 AM follows...

Upper ridge over the southern Plains and Mid-Mississippi Valley this
morning extends eastward today to the mountains, while the upper
trough along the coast of the Carolinas, weakens and actually starts
to slide back southwest toward the Florida Panhandle tonight.

8h temps reach +22C today and with a lack of cloud cover somewhat of
a westerly downslope component, though not strong, temperatures
should heat up into the mid 90s over the piedmont this afternoon as
well as urban areas further west, like Roanoke and Lexington.
Subsidence behind the upper trough and westerly flow should keep
dewpoints somewhat lower to offset heat indices, but some areas
could exceed 100 degrees for an hour in the Buckingham to South
Boston area around 3-4pm this afternoon.

Threat of convection looks very low, so have removed isolated threat
along the Blue Ridge. May see some storms nearby over the Alleghanys
west and north of Hot Springs after 4pm, but appears better coverage
will be further north over northern WV/PA.

Tonight, 8h flow turns more northwest and a couple of models have a
piece of vort energy/possible convective vort moving from
Indiana/Ohio around midnight, to the central Appalachians by 8am
Friday. Some hint in the previous forecast for this possibility so
keeping slight chance of thunderstorms late tonight northwest
Lewisburg/Hot Springs. Otherwise, muggy with lows from the mid to
upper 60s mountains, to lower to mid 70s east.


As of 400 AM EDT Thursday...

Appearing to be an active to potentially very active period for
convection, thanks largely to a warm/hot, very unstable, and
moist air mass that will be in place acted upon by an active
northwest flow on the northeast side of a strong subtropical

We begin the period with a strong 594dm upper ridge anchored
just to our southwest across the TN Valley. An active northwest
flow will be established from the Upper Midwest toward the Ohio
Valley and into the Central Appalachians and Mid-Atlantic
region. Several ripples are embedded within the flow that will
trigger MCSs across the Upper Midwest/Ohio Valley and which will
propagate east to southeast into the warm/unstable air mass
present across our region. The upper ridge will not be located
sufficient far enough northeast to suppress the convection much
if at all. As we move toward the Sunday, the upper ridge will
break down further and retrograde back into the South Central
and Southwest U.S. This will open the door for even more MCSs to
track into our region from the northwest.

Timing of individual short waves that will trigger convection is
difficult and varies among the various models. Initial thinking
is that Friday afternoon could be quite active as most models
show a short wave moving southeast from OH into WV during peak
heating. The next short wave may track further north across the
northern Mid-Atlantic, but the first short wave and associated
convection will leave an outflow boundary in the region Saturday
that will most likely be the focusing mechanism for additional
convection, especially west of the Blue Ridge.

On Sunday, a stronger short wave will push a cold front
southeast through the area that will likely stall just to our
southeast early next week, a common theme this summer so far.
Another round of strong convection is likely Sunday as a result.
SPC has already outlook roughly the northern 2/3rd of the CWA
for a marginal severe threat Saturday/Day 3, with a marginal
threat just to our north Day 2. This certainly seems reasonable
given the pattern that will be in place.

The other concern over the weekend will be the warm/hot
temperatures and accompanying high humidity levels. With the
center of the upper ridge remaining to our southwest and 850mb
temps topping out around +22C, don`t see anything dramatically worse
than what we have already experienced so far this summer. High
temperatures over the next few days will range from the mid to a
few upper 90s east of the Blue Ridge to the mid 80s in the
west, near upper 70s to 80 in the higher elevations. Past heat
waves of , e.g., late June/early July 2012, have seen 850mb
temps closer to +25C and over a larger area than even what is
expected with the +22C area this time. Dewpoints across the
Piedmont will hover in the lower 70s much of the next several
days with mid to upper 60s western areas. This will net heat
indices below 105 criteria for the most part Friday and just a
brief period in a few of the far eastern Piedmont counties
Saturday around the 105 criteria. Will continue to highlight in
the HWO, but will not be issuing any Heat Advisory headlines
with this package. This is not to say that it won`t be hot and
humid because it will, just about everywhere. So, just keep that
in mind as you go out and about outside the next few days. For
what it is worth, am still not convinced that BCB will see 90,
as we have not yet this summer. With 850mb temps topping out at
+22C or less, am reluctant to forecast 90 at BCB, so held the
max there at 89 for both Fri and Sat.


As of 315 PM EDT Wednesday...

Broad troughing will develop across the Eastern U.S. through the
first half of the next week, similar to what we saw earlier this
week. The upper ridge will relocate to the southwest U.S.
leaving us in an active northwest flow aloft, at least at times. A
frontal boundary should be pushed southeast through the CWA
Sunday into Monday. The GFS suggests that the boundary will be
pushed far enough south to remove our CWA from the convective
threat for much of the period after Monday. However, the ECMWF
is less excited about this possibility and that would be more
consistent with what we have seen repeatedly this summer. Have
retained the chance to high chance pops previously advertised
for this period.

With the upper ridge weakening and shifting way from the away,
a notable drop in 850mb temps is expected from +20C on Sunday to
around +15C or so by the mid part of next week. High
temperatures will drop back into the 80s, even in the Piedmont,
with 70s for highs in the mountains for the first half of next
week. Low temperatures will drop back to the 50s mountains and
60s elsewhere. Very pleasant temperatures for late July!


As of 138 PM EDT Thursday...

Medium to high confidence in VFR conditions through the
afternoon and overnight. Cumulus with isolated showers have
developed in the mountains but their highly sparse nature with
vertical development somewhat stunted should not pose any
restrictions for specific airports and will not include in the
TAF at this point. May see some unrestricted haze at times near
Danville and the southern Blue Ridge where air mass is more
humid. Should see cumulus diminishing in areal extent with
sunset. Toward late evening and overnight, mid to high level
cloudiness spills east- southeastward from a forecast MCS
complex expected to pass north into the mid- Atlantic region.
The high clouds lends to lowered confidence than prior days on
patchy radiation fog development in the western river valleys.
Winds west around 3-7 kts diminishing to light and variable by

For Friday...confidence is no better than medium on timing and
evolution of potential thunderstorms. It appears that near-term
guidance keys on re-development of thunderstorms, perhaps
locally strong, on an old convective outflow boundary from
aformentioned overnight MCS, confined from and north of a line
from the Greenbrier Valley/Alleghany Mtns to Lynchburg. Though
VFR should predominate, introduced VCTS with CB for Lewisburg,
Roanoke and Lynchburg at 15z, and this may be the earliest
storms get going. Confidence on thunderstorm development
elsewhere too low to include mention in the TAF. Potential
exists for storms to become strong with gusty, erratic winds
with localized turbulence, lightning and brief IFR or lower
visibility in downpours. Outside of thunderstorms, winds should
be light southwesterly.

Aviation Extended Discussion...

Though periods of VFR certainly the case, a potential active
extended aviation forecast period for late week into Monday
with multiple opportunities for thunderstorms. Confidence of all
weather parameters is moderate during the extended portion of
the forecast.

Scattered thunderstorms with brief IFR or lower visibility
restrictions Friday afternoon near/north of a line from LWB-
ROA- LYH may sink southward by evening. Clearing is anticipated
late in the evening before scattered storms re-develop again by
afternoon Saturday.

A cold front will move into the region Sunday night into Monday.
With an increase in convection, better chances of sub- VFR
conditions in association with convection along the front during
this period. Overnight fog will be more likely Sunday night
into Monday morning.




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