Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 262336

Area Forecast Discussion
National Weather Service Blacksburg VA
736 PM EDT Fri May 26 2017

High pressure will remain over the area this evening. A cold front
will approach the region later tonight, and stall over the area
during the day Saturday. This front will act as the focus for yet
another disturbance that will pass overhead Sunday into Sunday


As of 736 PM EDT Friday...

Made some minor adjustments in temperatures and cloud cover
for tonight into Saturday.

As of 315 PM EDT Friday...

An upper level shortwave ridge of high pressure was centered over
the region this afternoon. An upper level low was centered over
south central Canada. A little farther south in the Wabash Valley of
IL/IN an upper level disturbance was progressing eastward. A cold
front extended southwest from this feature into central Oklahoma.

As we progress through the overnight, the IL/IN feature will
continue heading eastward. It will have the impact of flatting the
shortwave ridge overhead, and place the region in a steering flow
that will be almost due west-east from the mid-Mississippi Valley
into our region. This will open the door for mid-West disturbances
that form along the associated cold front to make headway toward and
into our area late tonight into Saturday.

The bulk of the energy associated with the IL/IN feature will remain
to our north overnight, skirting the far northwestern sections after
midnight. By sunrise Saturday, convection is progged to have fired
along the approaching cold front and will be moving eastward through
the Ohio Valley and Tennessee Valleys.

SPC Day 2 Convective Outlook still has a Slight risk of severe
weather across the region tomorrow into tomorrow night in
anticipation of convection forming and racing ahead of the
approaching cold front. Around noontime onward into the evening
hours looks to be the greatest potential for the forecast area to be
impacted by this activity. Large hail and damaging winds are
expected to be the primary hazard.

After today`s brief dry period, humidity levels will be on the
increase again on Saturday, and temperatures will also inch up a few
more degrees. Low temperatures tonight will be a mix of low to mid
50s. High temperatures on Saturday will range from the upper 70s to
lower 80s across the mountains with mid to upper 80s across the


As of 230 PM EDT Friday...

Potential continues to exist for strong to severe thunderstorms and
an accompanying potential for heavy rainfall/isolated flash flooding
for the Memorial Day weekend. Identifying intensity, timing and
geographic areas in a more specific risk are very difficult to pin
down, with guidance varying widely on each of these aspects. This
appears to be a convectively active pattern across a broad area from
the mid-Mississippi, OH/TN Valleys eastward to the mid-
Atlantic/Carolinas. These details probably will not avail themselves
any sooner than each day, so the forecast really is not all that
specific. With the stated uncertainty, and as many likely have plans
outdoors, we encourage those to keep a close ear to the latest
forecast for the holiday weekend.

On Saturday night, likely capped atmosphere will keep a lid on what
is expected to be warm, increasingly humid and conditionally
unstable air mass conditions. While most areas stay dry, latest
convective-allowing guidance shows isolated to widely scattered
storms developing early evening across northern and eastern sections
of the forecast area, perhaps along any old convective outflow that
sags southward from northern VA. This area roughly bounded from
Greenbrier and northern Monroe Counties in WV eastward across the
southern Shenandoah Valley in VA and into the northern foothills of
the Blue Ridge and the VA Piedmont. Any storm that were to form may
become strong given MLCAPEs on the order of 1500 J/kg and LI values
of -6, but may prove isolated. It`s not until you look westward into
the faster westerly flow across the Mississippi and OH/TN valleys
where guidance develops more organized MCS clusters, some of which
may need to be watched for the overnight period as they near our
western border.

Dependent on how Saturday night evolves upstream, Sunday probably
features at least early-day convective debris cloudiness and
potential showers/embedded thunder. Strengthening mid-level height
gradient supports a greater increase in mid-level winds and
unidirectional, deep-layer shear values supporting organized storms.
Questions for Sunday include degree of instability/breaks in cloud
cover, any potential outflow boundaries left behind from
overnight/morning and timing any weak shortwave/vort max that may
help trigger storm development. SPC presently has a 5%
severe/Marginal Risk for Sunday for much of the forecast area.
Pending more clarity on these details, not out of the realm of
possibility that this may change.

Region becomes dominated by broadly cyclonic mid-level flow on the
southern end of a large upper low over the Great Lakes. A cold front
should be working its way eastward across the forecast area, though
it may spark showers and thunderstorms into Memorial Day. Think this
would be mainly east of the Blue Ridge according to the 12z NAM and
GFS guidance. As deep layer shear is still on the high side, could
still have a few stronger storms in that corridor during the
afternoon if frontal timing is favorable. Should see clearing take
place with cold/dry advection taking place behind the front Monday
evening, putting an end to warm and humid conditions.


As of 230 PM EDT Friday...

A brief reprieve from the unsettled conditions appears to be in the
offing for Tuesday as region becomes more influenced by broad
cyclonic flow in mid-levels. 850 mb temperatures about 3-5 degrees
cooler than the Memorial Day Weekend. Appears that Wednesday into
Wednesday afternoon may offer another period of showers/storms
associated with a weak shortwave trough rounding the broader
longwave feature, though the better chance appears to be along and
north of I-64 at this point.

We remain in the stronger belt of zonal westerlies into the late
week and the early weekend, separating warm and humid air to the
south and cooler cP air to our north. Kept a generic slight/Chance
PoP for the weekend.


As of 736 PM EDT Friday...

Good Flying conditions expected tonight, then deteriorating
conditions for Saturday.

A ridge of high pressure over the region will hold fast over
the region through tonight. Precipitation in advance of an
approaching cold front will start to enter the western section
of the area late Saturday morning. The result will be period of
VFR conditions through roughly 15Z/11AM Saturday. After that
time, MVFR ceilings will start to increase across the area from
northwest to southeast, reaching at KMKJ-KLWB line by 18Z/2PM.
Scattered showers with mainly VFR visibilities will accompany
this cloud cover. Saturday afternoon into  saturday night, MVFR
shower and thunderstorm chances increase across more of the
area, with a greater abundance of the western sections with the
arriving convection. Heavier bands of convection with potential
IFR conditions during the evening into Saturday night.

Winds will diminish this evening into tonight. Then, overnight
light winds are expected, and the direction will start to slowly
back. On Saturday, winds will increase from the southwest in
advance of the approaching system.

Confidence levels are high for ceilings, visibilities and winds
during the taf period.

Extended Aviation Discussion...

Additional convection and sub-VFR conditions probable Sunday
into Sunday night as another disturbance crosses the area.

Convection becomes less organized Monday into Tuesday with hit
and miss sub-VFR conditions. Perhaps late night and early
morning sub-VFR river and mountain valley fog.

Better organization of convection Wednesday into Thursday with
the passage of a cold front.

Confidence concerning the general weather pattern during this
portion of the aviation forecast period is moderate to high, but
confidence on specific timing of any sub-VFR condition is low.


As of 220 PM EDT Friday...

We continue to have river flood warnings in effect for the Dan River
at South Boston and on the Roanoke River at Randolph. Flood wave
from upstream gages has moved into the lower reaches of these river
basins, with crests expected at each gage Friday afternoon but will
remain above flood stage until Saturday See specific point flood
warnings in effect for these gages. Elsewhere, hydrographs at many
small creek and larger main stem rivers continue to show a general
receding trend. While the receding trend is expected to continue, a
reminder that rivers still are elevated and there are still fast
flows that can be dangerous.

Looking ahead into the Memorial Day weekend...there is the potential
for multiple rounds of showers and thunderstorms. Heavy rainfall is
possible from any thunderstorms, especially in storms that can be
slow moving, affect urban catchments and/or anchor to the terrain.
Flash flood guidance values are low (1.25-2.5" in 3 hours lowest in
the mountains and highest in the southern Piedmont), and would
precondition the region for flash flooding and rises on smaller
creeks if rainfall amounts and rates prove high enough. Confidence
is low on expected rain amounts and specific periods of time of
greater risk.




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