Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 221125

Area Forecast Discussion
National Weather Service Blacksburg VA
725 AM EDT Sat Jul 22 2017

A frontal boundary draped from the midwest-lower Great Lakes
east to the mid-Atlantic will stay situated around this area
into Sunday. Low pressure and attendant cold front finally
pushes east into the central Appalachians and across our area by
Monday. High pressure builds across the mid-Atlantic Tuesday
into Wednesday.


As of 304 AM EDT Saturday...

GFS seems the odd model out this forecast period as its the only one
blowing up organized convection this afternoon/evening across
southside VA into NC foothills/piedmont. Majority of the high-res
and synoptic models are in line with current conditions across the
Ohio Valley and trend convection across the eastern Ohio Valley into
the mid-Atlantic today into this evening. That is not to say to
organized storms will not impact our forecast area. Outflow from
upstream convection, combined with weak/no cap and SBCAPES in the
1500-2000 J/KG, and some modest 30-40kt bulk shear mainly north of a
Greenbrier to Buckingham line suggest severe convection potential
exists in an organized state. Meanwhile further south convection
should be more widely scattered in nature but could develop into a
multicellular broken line late in the day toward the piedmont.

Another question will be how much blowoff from upstream convection
impacts heating. Given what happened yesterday, and looking at the
model 300-500 mb field flow suggests that any high clouds will be
over the northern CWA, so heating should not be compromised too

For Pops today, kept them below likely with higher chance north of I-
64, southeast toward Roanoke/Lynchburg, and along the southern Blue
Ridge, where low level convergence will favor a little more
coverage. Lowest pops are going to be over the NC piedmont.

The cloud cover also plays a role in the heat advisory, and seeing
no changes to that, with mainly 1 hour of 105F heat index values
being reached in the piedmont around 4pm. Obviously convection could
have an impact on how hot this gets.

Still looking at highs from the mid 80s to near 90 mountains, to mid
to upper 90s east.

Tonight, models sans GFS continue to string along a couple of upper
waves from the midwest into the mid-Atlantic. NAM/ECWMF favor and
even the GFS favor another MCS potentially rolling into the central
Appalachians by dawn Sunday. Given time of day severe threat will be
limited but some stronger gusts are possible.

Still keeping higher chance pops tonight mainly north of a line from
Bluefield, WV to Roanoke to Lynchburg with less coverage in NC.

Expecting a little more clouds tonight and higher dewpoints so lows
will be in the upper 60s west, to lower to mid 70s east.


As of 300 AM EDT Saturday...

A couple of mesoscale convective systems will track across to the
north along a frontal boundary from West Virginia and Maryland. An
upper level trough will travel eastward over the Great Lakes Sunday
as a broad upper level ridge remains over the Southeast. The
tenacious upper ridge will keep the heat and humidity over the Mid
Atlantic on Sunday with heat indices reaching over 100 degrees in
parts of the Piedmont. With LIs of minus 4 to minus 6 and CAPE
around 2,000 J/kg Sunday afternoon. SPC day 2 convective outlook
placed most of the forecast area in slight risk for severe
thunderstorms. There is the potential for damaging winds and
possibly hail with stronger thunderstorms across the area. High
temperatures Sunday afternoon will range from the lower 80s in the
mountains to the mid 90s in the piedmont.

Expecting convection to spread east Sunday night and taper off to a
few showers overnight. Low temperatures Sunday night into Monday
morning will vary from the mid 60s in the west to the mid 70s in the
east. Depending on how well the convection can push the frontal
boundary southward over our CWA will play a key role for Mondays
forecast. Kept scattered showers and thunderstorms for Monday
afternoon. The highest chance appears to be along the southern Blue
Ridge and through the northwest North Carolina mountains. High
temperatures Monday will be a couple of degrees cooler than Sunday
with readings from around 80 degrees in the mountains to the lower
90s in the piedmont. Drier air moves in Monday night as high
pressure approaches from the north. Low temperatures Monday night
into Tuesday morning will generally be from around 60 degrees in the
northwest mountains to the lower 70s in the Piedmont.


As of 330 PM EDT Friday...

High pressure should build over the Northeast by Tuesday. The
frontal boundary will be shoved southward southward toward the North
Carolina coast by this time. Lingering moisture may still spark a
few diurnally-driven showers and thunderstorms during each afternoon
through this period. The best chances appear confined to the VA/NC
border on Tuesday, but more potential exists for Wednesday with
easterly to southeasterly flow from high pressure offshore. There
could be some wedging possible east of the Blue Ridge, so high
temperatures were pushed downward for Wednesday to allow for the
potential of upslope flow along the Blue Ridge and more cloud cover.
Temperatures should bounce back upward by Thursday as high pressure
heads further out into the Atlantic Ocean. Long range models hint at
another frontal boundary approaching our CWA by Friday.


As of 720 AM EDT Saturday...

Going to see showers and storms fire up by midday/early
afternoon, especially along the Blue Ridge northwest to the
Alleghanys. At the moment models still not showing enough
coverage to have thunderstorms as the main wx type but enough of
a threat for VCTS. Still looks like best coverage stays north of
a BLF-BCB-ROA-FVX line so took VCTS out of DAN`s taf.

As the case is in these patterns a cluster of storms could sink
further south impacting LWB-LYH, but at the moment it looks like
best coverage will be north.

VFR through the period outside storms. Severe threat exists so
turbulence will be an issue this afternoon/evening.

Another cluster of storms may arrive by 12z Sunday near BLF/LWB
but too far out to have in the tafs. With more high clouds
expected tonight have not included any fog.

Confidence is high on cigs/vsbys through this evening, but low
on vsbys late tonight as fog could form if less cloud cover

Aviation Extended Discussion...

Confidence is growing in a potential more active extended
aviation forecast period for Sunday into Sunday night with
multiple opportunities for thunderstorms. May see coverage shift
into southern sections by Monday afternoon as residual outflow
from Sunday jumps into North Carolina so appears more in the way
of overall VFR possible Monday based on latest guidance.
Otherwise most of the period will be VFR but with better
chances for storms in more locations, mainly afternoon and
evening late in the weekend, 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.


VA...Heat Advisory from 11 AM this morning to 7 PM EDT this evening
     for VAZ044>047-058-059.
NC...Heat Advisory from 11 AM this morning to 7 PM EDT this evening
     for NCZ005-006.


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