Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 210726
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
326 AM EDT Fri Jul 21 2017

.SYNOPSIS...
Very warm to hot and humid conditions will persist over the
area into this weekend under high pressure. A frontal boundary
across the mid-Atlantic to midwest will stay situated in this
area into Saturday, and will finally see a cold front tracking
across our area by early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 240 AM EDT Friday...

Belt of westerlies will continue to stay north of us today,  though
some hint of a southern shift appears in the models and will have to
see how upstream mesoscale convective systems over Iowa/Illinois
this morning shift east and allow for cooling of upper levels and
lowering heights later today. Have backed off somewhat on
temperatures as models are all favoring some cloud cover arriving
thanks to cu development and possibly some convective blowoff by
early afternoon. Still hot and humid, but not enough to send heat
indices to 105F. A few locations around Buckingham/Keysville/South
Boston/Danville could touch 100F for a heat index, but no advisories
planned.

SPC has us in a general thunderstorm outlook. We have plenty of
instability today and model soundings showing lack of a cap as
mid/upper ridge stay and warmer temps aloft stay south and southwest
of us. Appears best forcing and jet dynamics will favor stronger
storms across the Ohio Valley. Still any storms that fire up could
become strong to severe but isolated threat.

Following a blend of the HiRes-ARW/GFS/NAM should see widely
scattered showers and storms either arrive or develop over the
Alleghanys of WV/VA late this morning with potential outflow and
weak upper vort allowing for convection to fire downstream over the
Blue Ridge by 1-2pm. Models are favoring better coverage north of
Lewisburg WV to Lynchburg, VA then isolated/widely scattered south
along and either side of the Blue Ridge, with lesser threat further
southeast in the NC Piedmont.

Should be enough sunshine to get temperatures into the mid 80s to
around 90 mountains, with lower to mid 90s east, possibly upper 90s
if clouds are fewer.

Tonight, should see a decreasing trend of coverage of storms with
potential small area moving into the piedmont after sunset. Will
have to watch upstream once again as models are favoring
convection/MCS over the midwest, with upper vorts moving along the
northern periphery of the 5h high. Potential for another area of
showers/storms toward dawn Saturday moving over the WV mountains
toward the Alleghanys. Clearing late Friday evening followed by
increasing clouds in the west overnight. Low temperatures running
from the upper 60s mountains to lower to mid 70s foothills/piedmont.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
As of 315 AM EDT Friday...

The heat is on for the weekend with a tenacious upper ridge fighting
shortwave trofs and a frontal zone pushing southward. On Saturday, a
shortwave will travel eastward out of the Great Lakes Region to
southern New England region Saturday afternoon into Saturday
evening. Capes of 2000-3000 j/kg CAPE and LIs around -5C to -7C are
expected with this wave. Convective complexes on the northern
periphery of the upper ridge could fold southeast into portions of
the forecast area. Impressive low and mid level lapse rates should
yield sufficient inverted-V soundings and DCAPE to support
potential for damaging winds. The SPC Day 2 convective outlook
highlighted a marginal potential for severe thunderstorms Saturday
for our forecast area, with slight risk to our north. There is a
potential for damaging winds and hail with stronger storms or storm
clusters moving quickly east-southeast across the region. High
temperatures on Saturday will range from around 80 degrees in the
mountains to the mid 90s in the Piedmont. The combination of heat
and humidity will produce heat indices around the century mark in
the Piedmont. These values are not high enough to trip heat- related
advisories of 105 degrees for several hours or threaten any thermal
records. Elected to keep the highest pops in the northwest for
Saturday night, with lower threat further south and east. Low
temperatures Saturday night will remain mild with readings from the
mid 60s in the west to the mid 70s in the east.

Waves of low pressure will roll east along the frontal boundary
Sunday and try to move it south into our region. Scattered showers
and thunderstorms will develop on Sunday with the potential for
strong to severe storms. High temperatures Sunday will range from
the upper 70s in the northwest mountains to the mid 90s in the
piedmont. Heat indices will be once again top 100 in portions of
the piedmont. A healthy shortwave Sunday night into Monday will
support amplification of developing upper trof over the lower Great
Lakes. Low temperatures Sunday night into Monday morning will
generally be from the mid 60s in the mountains to the mid 70s in the
piedmont.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
As of 315 PM EDT Thursday...

At least some ongoing threat for additional shower/thunderstorm
activity, dependent on earlier convective influences, will continue
until cool frontal passage later Monday or Monday evening.

Thereafter, deepening upper trof over New England will support good
push of cooler and drier air back into the mid-Appalachian and mid-
Appalachian region Tuesday through Thursday as surface ridge builds
southeast into the area.

Little to no post-frontal precipitation threat will exist
during this period - along with a return to near normal
temperatures. Only exception may be across the extreme southern
flanks of the forecast area into midweek - where an ongoing
threat of mainly diurnally- driven showers/storms may persist
if less robust of a penetration of drier/cooler air occurs
behind the front.

&&

.AVIATION /08Z FRIDAY THROUGH TUESDAY/...
As of 110 AM EDT Friday...

Have maintained a generally VFR forecast this period with high
pressure aloft entrenched from the TN valley into the southern
Appalachians. Since cloud cover will be lagging, have moved down
a category for fog at LWB with IFR tempo LIFR in the 10-12z time
frame. Some mid clouds may slide in toward 12z to cut off fog
sooner. Elsewhere fog will be limited if at all, so none added
to other taf sites, with mainly cirrus overhead in a thin veil.

For Friday, confidence is no better than medium on timing and
evolution of potential thunderstorms. Atmosphere will be a
little more unstable compared to last couple of days under hot
and humid conditions but the triggering mechanism is in
question. Latest models runs are favoring upstream outflow to
induce showers/storms in the mountains by late morning, then
across the Blue Ridge after midday. Will have VCTS at
LWB/BCB/ROA/LYH although the coverage is questionable but looks
more concentrated north of a BKW-LWB-LYH line. 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 westerly.

Aviation Extended Discussion...

Confidence is growing in a potential more active extended
aviation forecast period for the weekend into Monday with
multiple opportunities for thunderstorms. Most of the period
will be VFR but with better chances for storms in more
locations, mainly afternoon and evening, and with better
coverage of rainfall the chances for IFR fog in the early
morning hours increases again especially for KLWB and KBCB. A
weak cold front is likely to push some of this moisture off to
the southeast by Tues or Wed for at least a couple of relatively
dry days. Overall, confidence of all weather parameters is
moderate during the extended portion of the forecast.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...None.
NC...None.
WV...None.

&&

$$
SYNOPSIS...AL/WP
NEAR TERM...WP
SHORT TERM...KK
LONG TERM...WERT
AVIATION...SK/WP



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