Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 230322

Area Forecast Discussion
National Weather Service Blacksburg VA
1122 PM EDT Mon Jul 22 2019

A cold front extended from a low in Pennsylvania into central
Arkansas. This front will move slowly southeast, reaching the
coast by late Tuesday. High pressure with cooler temperatures
and lower humidity will cover much of the eastern United States
Wednesday through Saturday.


As of 1119 PM EDT Monday...Focus now turns to the next round of
showers with embedded storms, in two general areas: (1) the
leading edge of the cold front which now is across south-central
WV into northeast TN and (2) a steady area of rain associated
with a weak wave low northwest of the Knoxville TN area. This
latter feature is spreading moderate to locally heavy rain
across a good part of the I-40 corridor in TN and north-central
NC, which has contributed to instances of flooding in parts of
eastern TN.

An early look at 00z guidance still continues to show these two
features working in tandem/merging to re-blossom showers and
embedded storms across the Blue Ridge south of Roanoke on
eastward into the VA/NC foothills into the overnight. Most
models have a steady shield of rain indicated for the overnight;
while less overall instability may make rain rates more gradual
than the afternoon, PWATs are also shown on SPC`s mesoanalysis as
being closer to 2" in areas where heavy rain is currently
falling across middle- eastern TN. Will hold on any changes to
flash flood headlines at this point - do think that by Tuesday
morning the greater threat for rains is mainly east of the Blue
Ridge, but both Watches still appear otherwise valid.

Previous discussion from this afternoon...

Models show development of thunderstorms over the foothills and
piedmont between 2PM-5PM this afternoon moving east to the coast by
late this evening. This area getting a lot of sun, increasing the
instability east of an Appomattox to Reidsville line. GSO 12Z
sounding had enough dry air aloft to support enhanced downdrafts in
storms this afternoon. DCAPE was already 1000-1200 J/kg in that area.
Precipitable water still at very high values, up to 2.1 inches in
the piedmont tonight. Plus precipitation potential placement
forecast was indicating the possibility of heavy rain. Will be
including this in the forecast for all of southwest Virginia,
southeast West Virginia and northwest North Carolina tonight. Not as
unstable over the mountains due to more cloud cover today but that
area will get repeated rounds of showers and thunderstorms into late
tonight. At this point not planning on expanding Flash Flood watch
in time or area, but even the faster moving storms east of the Blue
Ridge will have high rainfall rates.

Upper pattern will amplify tonight and Tuesday with a deep long wave
trough digging over the eastern United States. With trough digging,
strong upper diffluence in the right entrance region of the upper
jet will be over the Mid-Atlantic region through Tuesday morning.

Surface and 850MB front will be into southeast Virginia and central
North Carolina by Tuesday afternoon. Main axis of deep moisture
transitions from the mountains to the southeast along with the front.
Air mass above 700mb dries out behind the front Tuesday but low
level moisture will take longer to exit. Timing of the front and
drying behind the boundary is similar on the synoptic scale
guidance. Maximum temperatures on Tuesday will be below normal
behind the front, due in large part to extensive low cloud


As of 235 PM EDT Monday...

500 MB heights continue to lower over the eastern United States
Tuesday night and Wednesday. Air mass continues to dry out Tuesday
night and Wednesday. Precipitable water values drop below one inch
Tuesday night. Any rainfall behind the front will be light. Surface
and low level winds become northeast for Wednesday and Wednesday
night with high pressure centered over Pennsylvania. As low level
moisture erodes, expecting much more sun on Wednesday. Even more
noticable will be surface dew points lowering into the 50s on
Tuesday night and Wednesday. Both high and low temperatures will
remain below normal through Wednesday night. Blend of guidance looks
reasonable through the period.


As of 1025 AM EDT Monday...

The longwave trough over the eastern CONUS responsible for the mid-
week cool down will be shifting east and lifting out by Friday with
h5 heights building back into the 588 to 591 dm range by the
weekend. The negative h5 anomalies will become slightly positive by
Sunday across the most of the eastern seaboard. The rising
heights will be accompanied by slowly increasing temperatures
and humidity somewhat more typical of mid-summer, with
temperatures closer to seasonal normals. A surface ridge will
remain centered over the central Appalachians extending north
into New England through the weekend which will keep convection
to a minimum. A slight uptick in convective coverage is
possible with the slowly increasing moisture but there is no
synoptic signal to indicate anything substantial and triggering
mechanisms likely to be at the mesoscale or below.


As of 805 PM EDT Monday...

While most TAFs should stay VFR next couple hours, will be
focusing on Danville for a short-term thunderstorm threat as
storms move northward from the Triad. Overall, a relative break
in storm chances is anticipated through about 03z.

After 03z, deterioration in aviation conditions is expected as a
southward-sagging cold front links up with a disturbance moving
east-northeast from Middle TN. Most TAFs should see a renewed
potential for showers and embedded thunder, with rainfall rates
potentially supporting sub-VFR visibilities at times. See TAFs
for specific timing (indicated by period of SHRA VCTS). As this
merger occurs, expect ceilings to drop to widespread sub-VFR
(mainly MVFR, periods IFR ceilings) into the overnight. While
coverage of showers/embedded thunder should progress southeast
as Tuesday progresses, little improvement in ceilings should be
expected on Tuesday, and it may take most of the day to clear
areas to the Blue Ridge.

Though midnight, look for winds to stay predominantly
southwesterly 4-7 kts. A wind shift to northwest is expected
overnight through Tuesday to around 4-8 kts, though
northwesterly speeds may tend to increase more after the 00z
TAF period with ridging building in from the northwest.

Forecast confidence in aviation elements is moderate.

.Extended Aviation Discussion...

The front pushes its way south and east into the Carolinas by
Tuesday evening. Drier weather including widespread VFR
returning for Wednesday through Saturday.


As of 757 PM EDT Monday...

Flash Flood Watch remains in effect for the western half of the
CWA until 2 AM Tuesday, and for the southern Blue Ridge in VA/NC
and the foothills of NC until Tuesday morning.

While rainfall has proven to be quite heavy this afternoon, it
has been fairly short-lived due to faster-moving storm clusters.
This has kept rainfall from reaching excessive levels. 13z
National Water Model near-surface soil saturation analysis and
RFC flash flood guidance are both showing a highly variable soil
moisture pattern across the CWA making runoff predictions based
on variable convection that much more uncertain.

Late tonight, a re-blossoming of showers and embedded storms is
likely as an MCV-enhanced wave low moves from Middle TN into the
northern mountains of northwest NC, and as a sagging cold front
progresses into eastern WV and southwest VA. Recent 18z
guidance indicates at least moderate- intensity rain with
embedded convectively- enhanced rain rates falling over a longer
period of time overnight across the southern Blue Ridge to the
Interstate 40 area. While rainfall may not prove overly
excessive from an intensity standpoint with less expected
convective instability, the long duration of high-PWAT infused
showers/storms may offer some potential flood problems,
especially in or near hilly spots.


VA...Flash Flood Watch until 2 AM EDT Tuesday for VAZ007-009>014-
     Flash Flood Watch until 8 AM EDT Tuesday for VAZ015-016.
NC...Flash Flood Watch until 8 AM EDT Tuesday for NCZ001>003-
WV...Flash Flood Watch until 2 AM EDT Tuesday for WVZ042>044-507-


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