Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 261823

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

Low pressure will exit to the northeast of the region this
morning followed by high pressure that should build in by
afternoon bringing drier weather on gusty westerly winds. The
high weakens overnight allowing a warm front to lift north
across the region before stalling. Weak disturbances will ride
east along this boundary resulting in a return to periodic
rounds of showers and thunderstorms for the holiday weekend.


As of 115 PM EDT Friday...

No major changes expected for afternoon forecast update. Diurnal
cumulus field will continue through the late afternoon and then
rapidly decrease coverage with the approach of sunset. Forecast
high temperatures are all looking reasonable with temperatures
expected to be within plus or minus two degrees of the official

As of 955 AM EDT Friday...

Morning forecast update will reflect primary cosmetic changes on
the trends of the temperature, dew point, and sky cover based
upon the latest observations and expected trends into the early
afternoon. Have also removed the very small chance of light
rain showers across parts of western Greenbrier County based
upon the lack of returns the regional radar composite and trend
toward decreasing clouds.

As of 300 AM EDT Friday...

Upper low to the northeast will continue to pull away this morning
allowing brief shortwave ridging to slide east as the associated
surface high builds south of the area. This should bring a period of
drying as the flow turns more westerly and decent subsidence arrives
per drying seen on water vapor this morning. However expect will see
low clouds persist far west through around midday per northwest
upslope including trapped low level moisture with a few light
showers/sprinkles possible. Thus will keep some low pops going
before drying wins out and all break into sunny skies this
afternoon. Winds to remain gusty with some enhancement due to
heating/mixing so will keep the impact based wind advisory in
place this morning. Otherwise more insolation along with eastern
downsloping and exodus of the 850 mb cool pool should allow for
much warmer highs, with readings rebounding back into the 70s
most spots and perhaps around 80 southeast.

Upper ridge starts to flatten overnight in response to shortwave
energy passing north which will allow a warm front to shift
north passing west of the mountains late. An initial belt of
stronger winds aloft will ripple a weak sheared mid level
impulse east along the boundary with this feature approaching by
early Saturday. This along a baroclinic zone separating much
warmer/juicier air to the southwest and more of a capped
environment to the east. Latest guidance has a cluster/band of
showers zipping east with this wave perhaps reaching the far
northwest counties by daybreak Saturday within the sharp
westerly flow aloft. However latest soundings still indicate
some residual dry air aloft which should help to weaken
coverage. Thus only running with low chances northwest third
late. Otherwise more clouds north and remaining mostly clear
south overnight. Lows mostly in the 50s but could rise late
including the ridges where may stay above 60 as warm advection
aloft quickly returns.

Overall reprieve from persistent rainfall will be short as the
boundary stalls just northwest of the area Saturday with an
impressive return of moisture per jump in dewpoints and
subsequent humidity by afternoon. In return, models indicate
quite an unstable environment developing with MLCAPE values
better than 1500 J/KG given heating under decent jet energy
aloft ahead of a secondary wave aloft arriving over the
northwest late. This impulse could lead to an organized area of
convection riding east behind the initial wave with fast moving
storms impacting espcly northern sections, and perhaps elsewhere
pending organization under the fast flow aloft. Therefore will
keep likely pops northern sections with high/decent chance pops
elsewhere in the afternoon including severe mention in the HWO
per slight risk in the SWODY2 attm. Will feel quite warm given
the coolness of late as highs under strong 850 mb warming look
to zoom back well into the 80s east, and around 80 west pending
early debris clouds.


As of 415 AM EDT Friday...

Very active weather will continue through the period as a series
of upper-level disturbances track along/near a baroclinic
zone/frontal boundary that will be draped from west-east
bisecting the forecast area. Very unstable air will persist
south of the front Saturday night and continue Sunday as well.
Ongoing thunderstorm activity Saturday evening could well linger
into much of the night as the air mass south of the frontal zone
remains warm and unstable. Some of these could continue be
strong to severe, although the greater threat for severe will be
during the early part of this period, namely Saturday evening,
or before during Saturday afternoon. The axis of stronger
thunderstorms is expected to shift slightly further south for
Sunday, more into far southern VA and northern NC, as the
boundary sags further south in response to upstream troughing
and convective outflow boundaries from Saturday. Thermodynamic
and dynamics parameters suggest strong instability and moderate
shear. SPC continues to outlook much of this time frame in a
Slight Risk for severe.

Temperatures Saturday and Sunday will be warmer than in recent
days, with widespread 80s outside the mountains, where highs
will mostly be in the 70s. However, these warmer temperatures
will not be sustained long as showers, thunderstorms, and
associated clouds overspread the area during the afternoon. Low
temperatures will be on the mild side, mostly in the mid 50s to
lower 60s.

For Monday, the upper low remains well to the northwest keeping
a cyclonic flow in place across the region. The aforementioned
surface boundary will be push a little further south and is
expected to lie somewhere across the southeast states toward the
VA Tidewater region. Scattered convection will remain in the
forecast as another upper trough sweeps into the region from the
northwest. Instability is less impressive with this activity,
but with cooling temps aloft and good dynamics, even a few
strong to severe thunderstorms could even occur within this time
frame. Temperatures are expected to remain similar to what
noted above, namely lows in the 50s west to 60s east with highs
in the 70s west to the 80s east.


As of 430 AM EDT Friday...

Remaining active and wet more than dry. An upper-level low will
persist across the Great Lakes region with a series of embedded
disturbances rotating through the upper low keeping the threat
for showers and thunderstorms going in our area. In addition, we
will have yet another baroclinic zone, this time stalled just
to our south. At this point, Thursday appears to be the most
volatile day weather wise with widespread thunderstorms and
heavy rainfall possible. Temperatures during the period will
remain relatively close to climatological normals. Look mostly
for highs in the 70s west to 80s east and lows in the 50s west
to the 60s east.


As of 150 PM EDT Friday...

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.

Winds will remain gusty this afternoon from the west to
northwest with 15 to 25 kt gusts common until just before
sunset. Overnight light winds are expected, and the direction
will start to slowly back. On friday, winds will increase from
the southwest in advance of the approaching system.

Confidence levels high on all parameters for this near term
part of the aviation discussion.

Extended Aviation Discussion...
Saturday afternoon, shower and thunderstorm chances increase
across more of the area, with a greater abundance of the western
sections having sub-VFR conditions with the arriving convection.
Activity will continue into the evening hours.

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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