Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 220549

Area Forecast Discussion
National Weather Service Blacksburg VA
149 AM EDT Sat Apr 22 2017

A frontal boundary will stall out across our area through
Saturday with several waves of low pressure moving along it.
Will see a wet pattern with periods of moderate to heavier
showers through Monday morning. The front moves southeast by
Sunday night into Monday, with high pressure building in


As of 140 AM EDT Saturday...

Updated pops to account for radar and high-res trends showing
bulk of showers over the southwest this morning with less
coverage elsewhere, but still bands of lighter showers will
traverse the forecast area. A rumble of thunder possible in the
Mountain Empire of SW VA into the NC mountains.

Previous discussion from Friday evening...

The latest forecast adjustment will reflect an approaching gap
in the precipitation progressing through parts of eastern
Tennessee. This break in the showers will enter the southwestern
sections of the region around midnight, and progress across the
area heading into the early morning hours of Saturday.
Additional showers will arrive about three to four hours behind
this gap, thus bringing a return of showers to at least the
western sections of the area later tonight.

Thunderstorms will be likely limited to the far western and far
eastern sections of the region based upon the latest upstream
lightning trends.

Have also made minor tweaks to the hourly temperatures, dew
points, and winds based upon the latest observations and
expected trends into the early morning hours.

As of 635 PM EDT Friday...

Adjustments to the forecast this evening will reflect primarily
changes to geographical location of the best chance of
additional showers and thunderstorms through the evening hours.
This readjustment is based upon trends on regional radars of the
shower and thunderstorm activity, and expected movement through
the next few hours. The best coverage is expected to be along
and near the crest of the Blue Ridge from roughly Floyd, VA and
parts southwest into the Northern Mountains and Foothills of
North Carolina. This is expected to transition to more of a west
to east oriented location of the best coverage generally along
and south of Route 460. Have also made minor tweaks to hourly
temperature, dew point, and winds based upon the latest
observations and expected trends the next few hours.

As of 312 PM EDT Friday...

A cold front will travel southeast across the region this afternoon
into tonight and stall on Saturday. Scattered showers and
thunderstorms will develop ahead and along the boundary in the warm
unstable air. The Storm Prediction Center on SWODY1 highlighted a
marginal risk for severe thunderstorms for most of the area. With
CAPES around 1k J/kg, freezing level around 9 kft and a modest mid-
level flow of 30-40 kts, some of the stronger thunderstorms could
produce damaging winds and hail. As seen on SPC Mesoscale analysis
the best instability was located in the piedmont. Shaped the pops
tonight towards a blend of the HRRR AND NAM. The forecast area will
see a decent coverage of showers tonight, with thunderstorm chances
subsiding with loss of heating. Added some patchy fog overnight into
Saturday morning. Low temperatures tonight will range from the upper
40s in the northwest mountains of Greenbrier county in southeast
West Virginia to the lower 60s in the Piedmont.

An upper level trof will dig into the southeast State and develop
into a slow moving closed low by Sunday. as it drives a surface low
east through South Carolina. A wedge of high pressure will surges
into the region east of the Appalachians. For Saturday morning into
Saturday afternoon, isentropic lift from the surface low over the
lower Mississippi valley will bring widespread showers to the region
with a few thunderstorms. Across southern portions of the forecast
area, there will be some instability pool ahead of the wedge to fuel
development of thunderstorms, and shear along the backdoor front
will be sufficient to help organize the convection. The Day 2
Convective Outlook has placed southern portions of the forecast
area in a marginal risk for severe weather with wind and hail,
the primary threat. High temperatures Saturday will vary from
the lower 50s in the north to the mid 70s along the southern
Blue Ridge. Temperatures on Saturday will be highly dependent on
the strength of the wedge and placement of convection.


As of 312 PM EDT Friday...

A series of shortwaves in the northern jet stream from Colorado to
Montana will deepen the upper trough over the central United States
eventually closing off an upper low over the Tennessee Valley on
Sunday. This low continues to track southeast, reaching the Georgia
coast by late Monday. At the surface high pressure will be wedged
down the Appalachians on Sunday as a low tracks southeast out of the
Tennessee Valley. Inflow off the Atlantic is strong on Sunday night
and Monday. Upper jet supports good lift too on Saturday night and
Sunday. Forecast precipitable water values are above normal but not
excessively high, but will be watching the 1.25 to 1.5 inch values
advecting into the area because this would increase the heavy rain
threat. Any chance of thunder would be with elevated instability and
lift above the wedge. Will be keeping maximum temperatures at or
just below coolest guidance for Sunday and Monday.


As of 312 PM EDT Friday...

A long wave upper trough will develop over the western and central
United States Tuesday through Thursday. This will steer the weakened
upper low up the East Coast. Wide spread in the long range guidance
solutions after Thursday but overall looks like 500MB pattern is
amplifying with troughing in the west and ridging in the east.

By Tuesday the low will be off the southeast coast which will bring
surface and low levels winds around to the northwest and therefore
erode the wedge. Models have a cold front stalling through the
central United States Wednesday through Friday, with the GFS on the
east side of the differing solutions. Given the building southeast
ridge at the end of next week WPC preference was closer the the 00Z
ECMWF This keeps the Mid Atlantic region dry Tuesday night through
Friday with above normal temperatures.


As of 140 AM EDT Saturday...

Forecast will be of poor flying conditions, but this will not be
for the entire taf period as models are hinting at slower
frontal movement to the southeast thereby possibly keeping the
DAN area and possibly LYH more into VFR this morning, with less
rain coverage until this afternoon.

Needless to say amendments are likely given off and on again
rain. Thunder chances are there with slower movement and models
are showing ROA/LYH/DAN and BCB as best overall for seeing
thunderstorms in vicinity this afternoon. Could even see some
stronger storms east of ROA.

The front will slide southward tonight with winds shifting from
southeast and southwest to northeast. The boundary overall will
cause some wind shifts as will showers and storms, so again
flying wx is poor despite a period of VFR at times.

By Saturday evening, the models keep showers around with less
thunderstorm threat, but show cigs sinking to IFR especially
over all but BLF and possibly DAN. Will not sink it that far yet
given rainfall may be enough to mix the lower cigs keeping them
in the 1000-2000ft range.

Extended Aviation Discussion...

Weather pattern active into Monday with periods of
rain/showers, heavy at times. Should expect mostly sub-VFR
conditions when raining, but some VFR possible, especially
north of a LWB-LYH line.

Rain lingers into Monday as does lower cigs, and finally seeing
some VFR returning Tuesday. Drier weather continues Wednesday
into Thursday.


As of 300 PM EDT FRIDAY...

After 4 to 5 days straight days this week with spotty rainfall
mainly light across the area, we are still looking at the
possibility of a more significant hydro event this weekend. Despite
the semi-wet week, antecedent conditions are fairly dry across the
eastern 2/3 of the CWA and generally about normal in the west.
Moderate drought is still depicted on the U.S. Drought Monitor
across much of the piedmont and Abnormally Dry over most of the Blue
Ridge and surrounding area with near normal conditions in the west.
This suggests that we can absorb more water than might be typical. 3-
hour flash flood guidance from the River Forecast Centers ranges
generally from about 1.5 inches in the west up to around 3 to 3.5
inches in the west, reflecting well the drier conditions east of the

A slight risk for convective rainfall exceeding flash flood guidance
was issued by WPC earlier for Day 2 and Day 3 (through 12z Monday).
Pockets of minor advisory-type flooding cannot be ruled out in
convective storms which will be more likely today and Saturday and
further south in the CWA where instability may be much higher. Any
training convection over the same basins could produce pockets of
flash flooding. At this point the river forecast is a purely QPF
based. Model QPFs have shown some decent run-to-run consistency over
the last few cycles providing more confidence that this will fulfill
expectations. Current WPC QPF in the day 1-3 period (today through
early Monday) period is generally about 2 to 4 inches, with the bulk
of it falling Saturday and Sunday and current WFO grids are close to
these numbers. The prolonged nature of the rainfall (48 to 72 hours)
will lessen the risk of serious river flooding (and flash flooding)
as runoff will be more spread out and less efficient.

The two best St. Louis University CIPS analogs to this event are
April 9-12, 2003 and March 28-30, 2010 both of which featured upper
lows over the southeastern U.S. Both events resulted in minor to
moderate river flooding on the Dan River but not on the New, James,
upper Tennessee or upper Roanoke Rivers.  Ensemble river forecasts
from the GEFS ensemble are also highlighting the Dan River as the
most likely to flood, suggesting up to a 30 percent chance for
moderate flooding but the NAEFS has been consistently showing lower
probabilities. The situation will be monitored closely on future
shifts for a possible Flood Watch.




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