Heat Advisory 2, NWS Blacksburg, VA

Area Forecast Discussion
National Weather Service Blacksburg VA
309 PM EDT Thu Jul 18 2019

.SYNOPSIS...
A weak disturbance will pass east of the area this evening
bringing an end to showers and storms tonight. High pressure
will build across the region Friday into the weekend resulting
in a 3 day heat wave that will feature temperatures about 10
degrees above normal. A cold front should cross the area Monday
bringing cooler weather for next week.
&&

.NEAR TERM /THROUGH FRIDAY/...
As of 230 PM EDT Thursday...

Clusters of showers/storms continue to develop across the mountains
under the trough aloft while propagating slowly southeast toward a
residual surface lee trough in the piedmont. This scenario looks to
continue with best coverage over the southwest and then out east before
convection fades just after sunset with loss of heating. This upper
feature should finally exit/weaken overnight as mid level ridging
starts to build in from the west as heights slowly rise. This should
lead to clearing overnight outside of lingering mid deck with some fog
around espcly mountain valleys and where heavier downpours occurred
from earlier. Again quite muggy with lows in the low/mid 70s east and
60s west where a bit cooler from more widespread showers.

Heat becomes the main issue on Friday with the mid level ridge sliding
east across the region by late in the day. This along with rising 850
mb temperatures to above 20-22 deg/C and more sunshine under weak
downslope should push highs into mid/upper 90s east and 85-90 outside
of higher elevations mountains. However appears downsloping wont be
enough to help drop dewpoints too much which will hold humidity levels
up making for heat indices at above threshold advisory levels in some
eastern counties. Thus plan to hoist an advisory from the Amherst/LYH
area to Danville including points east for Friday afternoon. Otherwise
forecast soundings suggest the onset of enough capping/drying aloft to
curtail convective coverage somewhat espcly given ridging aloft.
However likely not enough per strong instability to completely keep the
lid on with orographics and differential heating likely leading to at
least widely scattered coverage southern Blue Ridge and isolated
elsewhere mainly mountains. Thus continued chance pops southwest and
20ish or less elsewhere.
&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/...
As of 305 PM EDT Thursday...

Main focus for the Friday night through Sunday night period will
continue to be heat and humidity contributing to an extended stretch
of elevated heat indices - likely to be the warmest stretch of
weather seen to this point in summer in the Blacksburg area of
responsibility. Generally speaking, model forecast guidance remains
little substantively changed regarding the potential for elevated
heat indices across the foothills, Piedmont of central VA and
northern NC, and Southside VA. With 850 mb temperatures commonly
into the low to mid 20s Celsius, this will lead to daytime ambient
high temperatures into the mid/upper 80s to the mid/upper 90s on
Saturday, while increasing cloud cover on Sunday may help knock
ambient high temperatures down a couple degrees compared to
Saturday. Little relief is expected at night, with lows in the upper
60s to the mid to upper 70s Friday and Saturday nights, and lows in
the mid-upper 60s to low-mid 70s Sunday night.

Regarding headlines, excessive heat criteria for our forecast area
involves maximum heat indices of 110F or greater with intervening
lows of 77 or greater. The afternoon forecast only brings Buckingham
County to levels which are close to that criteria, while falling
short elsewhere. Will therefore not hoist any excessive heat watches
or heat advisories for this period, as either criteria aren't met
(in the case of the a potential need for a heat watch) or are fourth
forecast period or longer (in the case of the advisory). Each
afternoon, there should be some mixing that in the afternoon helps
locally reduce dewpoints as temperatures rise that could affect
(e.g. reduce) heat index values. Will let later shifts/guidance
re- evaluate. It is looking likely that some heat-related
headline may become necessary for parts of Southside, the
foothills, the Roanoke Valley and central Piedmont in VA, as
well as the northern NC Piedmont in this period. Will continue
to message in the HWO.

Under the subtropical ridge, convective chances through the period
are generally on the small side through Saturday night. If any
storms were to develop, these would tend to be confined to the
terrain. Into Sunday, the subtropical heat ridge fractures, leaving
us in a weakness in the ridge. This should foster generally greater
cloud cover and greater thunderstorm coverage toward something
closer to scattered. Possible that one or two may become strong and
produce gusty winds and brief heavy rainfall, but it remains low-
predictability which area(s) may see this threat relative to
others.
&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
As of 115 PM EDT Thursday...

On Monday, we expect to see a full transition from the hot upper
ridge that has been over the region, to one of a longer wave trough
positioned over the East Coast through at least mid-week. At the
heart of this transition will be a strong cold front which is
expected to cross the region on Monday. Showers and storms are
forecast to be in progress by the mid to late morning across western
sections of the area with all areas of the forecast area having
greater than a 50 percent chance of showers and storms during the
afternoon. Temperatures on Monday will be notably cooler as compared
to the several days before thanks to the passage of the cold front,
abundant cloud cover, and precipitation across the area. Highs will
range from the upper 70s to around 80 across the mountains with mid
80s to near 90 across the Piedmont.

As we progress into mid-week, the upper trough builds even more
across the east with surface high pressure building into the mid-
Atlantic. Precipitation chances will linger Monday night into
Tuesday across mainly eastern and southern sections of the region,
nearest the cold front which is expected to stall near the NC/SC
border. The general trend across the entire area will be for drier
and cooler weather. By Wednesday temperatures are expected to
average around five degrees below normal for this time of year.
&&

.AVIATION /19Z THURSDAY THROUGH TUESDAY/...
As of 1235 PM EDT Thursday...

Showers along with isolated storms popping up across the south
and west will likely continue to dot the region through mid to
late afternoon with some clustering possible mainly over
south/east later on. Thus will include either a VCSH or VCTS
mention in a few locations pending release coverage and then
amend as necessary given the small nature to some of the
rainfall. Otherwise looking at VFR most sections outside of
showers this afternoon, with periods of MVFR still possible over
the mountains espcly KBLF vicinity.

Showers should push into eastern sections while fading out this
evening with loss of heating. Models continue to suggest that
low clouds and fog may fill in over the mountains again tonight,
and continued this trend based on this and current lingering
moisture. Appears the best shot at fog will be along the
periphery of the low clouds, mainly KLWB to perhaps KBCB. Fog
also possible at KBLF and even out east espcly if a specific
site sees a heavier downpour.

High pressure will start to build into the area aloft on Friday
allowing for less convective development once any stratus/fog
fade by mid to late morning mountains. Looks like best chances
of a shower or storm will be along the southern Blue Ridge and
isolated elsewhere so wont include mention and run with more VFR
at this point.

.Extended Aviation Discussion...

For Friday night into Saturday, high pressure will strengthen overhead,
with subsidence curtailing, but not completely eliminating, the
potential for diurnally driven showers and storms Saturday
afternoon. However coverage looks even less than Friday with
any storms likely confined to the higher peaks along the Blue
Ridge at this point. Otrw VFR outside of some late night fog in
the mountain valleys, with potential to restrict visibility for
a few hours prior to daybreak at terminals such as KLWB and
KBCB. A bit better coverage of showers/storms is expected late
Sunday and Monday, associated with the approach and passage of a
cold front. The front will also bring a wind shift later
Monday.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...Heat Advisory from noon to 8 PM EDT Friday for VAZ034-035-
     044>047-058-059.
NC...Heat Advisory from noon to 8 PM EDT Friday for NCZ006.
WV...None.

&&

SYNOPSIS...JH/WP
NEAR TERM...JH
SHORT TERM...AL
LONG TERM...DS
AVIATION...JH/WP