Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 191149

Area Forecast Discussion
National Weather Service Blacksburg VA
749 AM EDT Wed Jul 19 2017

A weak upper low over southeast Virginia will move south along
the North Carolina and South Carolina coast line through Friday.
High pressure aloft will build east from the Midwest toward the
southern Appalachians and Southeast U.S. through the weekend. A
cold front will move southeast into the region from the Great
Lakes late in the weekend.


As of 330 AM EDT Wednesday...

Showers and thunderstorms, associated with a weak upper low now
located over southeast Virginia, dissipated quickly after sunset
with no convective activity left in or near the CWA at this
hour. The upper low will drift further south along the North
Carolina coast today and tonight. The associated cold pocket
aloft will dissipate with 500mb temps warming from -10C
yesterday to around -6C by the end of the day today. Lapse rates
will be markedly weaker and overall the atmosphere much more
stable. All this will be the result of the upper low moving away
and heights building eastward from the Midwest where a strong
597dm upper ridge is located.

Confidence in convection today is low. Situation is entirely
different than yesterday and none of the models really
indicating much occurring at all. HRRR and NSSL WRF very quiet
with little to no convection indicated through the end of its
cycle. Nonetheless, not comfortable with temperatures warming
into the 90s and dewpoints well into the 60s of completely
unmentionable pops, so have included a 15% pop for all areas
east of the Alleghany front. Feel that areas along and west of
the Alleghany front will be closer to the upper high and too
stable for convection. The Piedmont and/or along the Blue Ridge
should be the most favored areas for convection today. Then
again, I could also see a scenario with no convection at all. It
is rare to have two back-to-back days like yesterday and again
it is worth noting that the synoptic situation is far from
favorable for such today compared to yesterday.

Temperatures will be on an upward trend the next couple of days
as the upper ridge builds east. 850mb temps have been hovering
near or just below 20C. These will be creeping up toward +22C to
+24C over the next couple of days. Not enough rise today to
really be reflected in the max temps, so have only raised them a
degree or two in most areas. This will yield low 90s Piedmont to
mid 80s west, except upper 70s higher elevations of the
mountains. Overnight lows Thursday morning should be
considerably warmer than this mornings temps as a consequence of
the lack of convection to lead off with a rather cool evening.
Lows Thursday should range from the lower 70s Piedmont to the
lower and mid 60s across parts of the Greenbrier and New River
Valley as well as mountain valleys throughout the west.

Finally, do expect fairly widespread areas of dense fog this
morning from late day rain Wednesday and calm winds, clear
skies, and effective radiational cooling this morning. Fog will
likely be less widespread Thursday morning with the lack of late
day rainfall and warmer overall minimum temps, hence greater
T/Td spreads.


As of 230 AM EDT Wednesday...

Quieter yet hotter weather in store Thursday, as dome of high
pressure slides east from the southern Plains and Mid-Mississippi
Valley. Still enough low level convergence where an isolate storm
could pop up along the higher terrain in the afternoon but most will
be dry. Temperatures should peak out in the mid to upper 80s across
the mountains, to the lower to mid 90s east. Dewpoints in the 60s
should keep heat indices mostly in the 90s east.

Friday, the upper center of the high remains over the TN valley,
while southern belt of westerlies stays situated from the northern
Plains, east to the mid-Atlantic. Will have to watch for convection
firing upstream and density of an MCS being able to propagate
southeast into our area. Most of the 0z models are favoring some
convection sliding toward the northern CWA in the afternoon, roughly
from the Greenbrier valley of WV eastward toward Lynchburg. Not too
confident given the flow aloft and the warmer temps aloft with the
ridge in place, so will have mainly slight chance pops in the
mountains, to low chance over the Alleghanys, while keeping most of
the piedmont/foothills south of Roanoke/Lynchburg dry.

Friday`s highs should be a few degrees warmer if we have no
convective blowoff, with upper 80s to lower 90s in the mountains, to
mid to upper 90s in the piedmont/foothills/Roanoke. Dewpoints
creeping up will make this the day where heat indices could exceed
105 degrees in some of the piedmont area.

Heading into Saturday, 5h heights fall slightly with southern belt
of westerlies dropping toward the south into Virginia. This combined
with an frontal boundary across the mid-Atlantic and upstream
convection should allow for at least outflow driven convection to
fire during the afternoon. Highs again will be dependent on clouds,
but should be a degree or two cooler than Friday but hot
nonetheless, with mid 80s to near 90 west, to mid to upper 90s east.
Again with dewpoints near 70, heat indices could threaten 105
degrees along/east of Highway 29 Lynchburg/Danville south into

Any convection Friday-Saturday should weaken/dissipate during the
nighttime hours, while lows stay elevated in the upper 60s to mid


As of 315 PM EDT Tuesday...

Very warm/hot weather expected to persist through much of the
long-term forecast period. Weak upper closed low expected to
drift west into AL over the weekend, then shear slowly northeast
back toward the southern and central Appalachians by early next
week. Meanwhile, a series of short wave trofs in the northern
stream are forecast to scoot east through the Great Lakes, with
the strongest on Sunday into Monday likely to push cool
front/baroclinic zone south toward or through the Blacksburg
forecast area later Monday or Monday night.

Whether the surface front can progress this far south while
pushing against a strong Bermuda high pressure system still
remains somewhat in question, but the frontal proximity,
combined with at least some moisture streaming back northeast
in advance of southern closed/shearing low should support a
slowly increasing threat for mainly diurnally-driven
showers/thunderstorms into portions of the Blacksburg forecast
area, especially in the mountains (Saturday) and into most
areas Sunday into early next week as the front nears or moves
southeast through the region.

Little overall change in airmass expected until the possible
frontal passage - with a continuation of relatively hot days
with warm nighttime lows, but with only a very limited threat
for any record breaking readings. Cooler air may arrive by
Tuesday, driving temperatures back down 5-7F and closer to
seasonal levels.


As of 745 AM EDT Wednesday...

Patchy dense fog is the immediate problem this morning with some
sites, such as LWB and BCB reporting 0SM. This will continue for
another hour or so as a result of rainfall late in the day Tuesday
leaving the ground wet/moist at sunset. With calm winds and
mostly clear skies now, conditions are ideal for radiational
cooling and fog. Most of the dense fog is confined to the New
and Greenbrier River Valleys. Again, this should only be an
issue for another hour or so. There is some concern that patchy
dense fog and associated low ceilings will reoccur Thursday
morning, but with less rainfall to perhaps no rainfall late
today, the fog may tend to be less in duration, intensity, and
areal coverage.

After sunrise, conditions will improve quickly to VFR and
should remain that way through the remainder of the TAF valid
period. Not expecting a repeat of the widespread showers and
thunderstorms that were observed Tuesday under the weak upper
low as the upper low drifts further east-southeast and upper
ridging moves eastward from the Plains. 500mb temps will warming
from -8C to -10C to around -5C during the next 24 hours, which
will leave a much more stable atmosphere in place. Showers and
thunderstorms will be isolated today and confined mainly to
areas east of the Blue Ridge closer to the upper low. Not enough
confidence to include in the TAFs at this time.

Winds will be light and variable through the TAF valid period
and mostly calm at night. Some tendency for winds to be
southeast across the Piedmont, but even there, speeds will be
5kts or less.

Medium confidence in visibilities and ceilings through 13Z, then
high confidence in VFR conditions.
Medium to high confidence in wind speed and direction through
the TAF valid period.

Aviation Extended Discussion...

The pesky weak upper low will drift into the southeast states
toward the weekend and then eventually retrograde westward along
the Gulf Coast. Eventually it could figure into our weather once
again early next week. However during the time frame of Thursday
through Sunday, generally VFR conditions are expected as ridging
aloft spreads eastward from the Midwest toward the Southeast
U.S. A frontal system could approach the area late in the
weekend with an increase in convection at that time. Late night
and early morning fog can be expected at the usual sites (e.g.,
LWB, BCB, LYH) most likely each day through the period with
IFR-LIFR conditions possible in the 08Z-13Z time frame each day.




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