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FXUS61 KRNK 240157

Area Forecast Discussion
National Weather Service Blacksburg VA
957 PM EDT Sun Apr 23 2017

Expect wet weather at least through Monday night, with surface low 
in the Tennessee Valley slowly moving eastwards to the South 
Carolina coast by Tuesday morning, while upper low slowly traverse 
the same general area.


As of 950 PM EDT Sunday...

Forecast update will reflect the addition of Rockbridge and
Ahmerst County, VA to the Flood Watch. Latest qpf guidance from
WPC offers a solutions of around two inches of rain falling
within an area of increasing upslope flow, especially late
tonight into early Monday.

Have attempted to depict a small decrease in the coverage of the
precipitation for a few hours after midnight tonight, before
increasing again close to 300 AM or 400 AM. Total rainfall
amounts now may be close to four inches along the crest of the
Blue Ridge south of Roanoke, VA, and eastward into the Piedmont
of North Carolina and neighboring parts of Southside Virginia.

Have adjusted hourly temperatures and dew points based upon the
latest observations and expected trends into the early morning 

As of 635 PM EDT Sunday...

The forecast update for early this evening will reflect mainly
one notable change. A gap of a few hours in the widespread 
rainfall, to more of a scattered coverage, is expected across 
the southeast portion of the area. This gap will translate 
northward from near the NC/SC border before additional 
precipitation fills the gap back in later in the evening. Have 
also made minor adjustments to the hourly temperatures based 
upon the latest observations and expected trends through the 
evening. Some rain-cooled areas across the Mountain Empire 
region of southwest Virginia are already cooler than the 
overnight forecast low. Have adjusted the forecast low 

As of 327 PM EDT Sunday...

Flood Watch continues for most of the region through Monday evening.

Positively titled upper level trough will deepen as it pivots to the 
southeast with a closed mid level low eventually traveling across 
the Southeast States. This pivoting will result in increasing 
difluent mid and upper level flow and associated divergence to the 
northeast of the low. A dual upper level jet structure may enhance 
upper level divergence over western North Carolina this evening into 
Monday morning. 

Rain, heavy at times will spread northeast across the region this 
evening into tonight and continue into Monday. Moderate to heavy 
rain is expected in the flood watch area, with 1.5 to 3 inches 
likely through Monday night. Locally higher amounts likely near the 
Blue Ridge, south of Roanoke. 

Initially the northern edge of the rain shield is sharp and utilized 
the 16z HRRR which captured the rain placement on the WSR-88d the 
best, then used a blend of GFS and NAM. The Day 1 excessive rainfall 
outlook placed an moderate risk across southwest portions of 
forecast area. This is supported by the better lift, jet 
dynamics, upper difluence and terrain enhancement. Models are 
hinting at higher qpf to our south across North Carolina and 
South Carolina. This may be suggesting potential for elevated 
convection to our south. Not expecting any thunderstorms or 
deeper convection for our area with wedge in place, and this 
should keep hourly rainfall rates limited. However, long 
duration rains will create a flood threat especially for 
creeks/streams and rivers. The flash flood threat will be 
isolated but cannot rule out some pockets across the southern 
Blue Ridge this evening into tonight. In contrast, for late 
tonight, there is some potential for dry slot to work toward the
Mountain Empire before the easterly fetch off the Atlantic 
pulls rain back to the west Monday. Complex forecast tonight, 
but models agree in general weather of cloudy and rain. Low 
temperatures tonight will range from around 40 degrees in the 
mountains to near 50 degrees in the Piedmont. 

Increased pops on Monday in the west as vertically stacked cyclone 
slowly pinwheel southeast. Timing looked a tad slower on the
movement. Good moisture transport continues Monday with PWAT 
around 1.25" along/east of the Blue Ridge and into the Piedmont.
Stable thermodynamic profiles owing to residual wedge should 
hinder any thunderstorms. Northeast to north winds especially in
the east will increase in speeds Monday afternoon into Monday 
night. This is a result of the strengthening pressure gradient 
between high pressure over the Northeast and low center over the
Carolinas. Played high temperatures a little cooler with rich 
cloud cover and rain with readings from the mid 40s in the 
mountains to the mid 50-s in the Piedmont.


As of 320 PM EDT Sunday...

Wedge is gone on Monday night as surface and low level winds respond 
to approaching low. Bufkit guidance showed easterly flow and a 
saturated sounding 6 to 9 thousand feet deep. This strong upslope 
and Atlantic inflow will prolong widespread moderate to locally 
heavy rain. Axis of 850MB jet crosses northern North Carolina and 
southern Virginia Monday night and will be pointing toward Maryland 
and northern Virginia on Tuesday morning. 

Little variation in GFS/NAM forecast track and timing of surface and 
upper low. Upper low moves off the southeast coast Monday night then 
tracks northeast, very close to the coast, passing over the Outer 
Banks on Tuesday night. On Wednesday the 500 mb low is far 
enough away from Virginia that moisture wrapping around the west
side of the system will be retreating from the Virginia, West 
Virginia and North Carolina. Will be slowing down the clearing 
on Tuesday. Similar to current forecast will have a low 
probability of showers in the Virginia piedmont on Tuesday 
night and a dry forecast for Wednesday. Stayed on the cooler 
side of guidance for maximum temperatures on Tuesday since more 
cloud cover is expected.


As of 300 PM EDT Sunday...

500MB heights rise Thursday and Friday over the southeast United 
States but with an overall deepening upper trough over the central 
United States. A deep closed low develops over the Rockies by 
the end of the week which will keep much of the east downstream 
in broad southwest flow. 

At the surface a low pressure system tracks into the Great Lakes. 
The front with this system stalls in the Ohio and Tennessee Valleys 
on Thursday and Friday. 850MB temperatures increase to as high 
as +18 on the ECMWF on Friday over southern Virginia and 
northern North Carolina. Will have Saturday and Sunday with 
continued above normal temperatures.


As of 725 PM EDT Sunday...

Only the area generally along and north of a KLWB-KLYH line
remains VFR this evening. The remainder of the area is
experiencing IFR/MVFR ceilings with some pockets of LIFR.
Visibilities have been variable between VFR and MVFR. 

Through the overnight hours, light to moderate rain will be
across the region along with primarily LIFR/IFR ceilings and
VFR/MVFR visibilities thanks to the rain and light fog. 

Little change in prevailing conditions is expected through the
day on Monday.

Winds will generally be from the northeast across the central
and eastern sections of the region at 5 to 10 kts, with western
sections having more of an easterly wind and similar speeds.
Higher elevations will have slightly higher prevailing winds
with some gusts in the 20 to 25 kt range.

Extended Aviation Discussion...

Poor aviation conditions expected to continue through Monday
night, and at times into Tuesday, thanks to the slow moving 
upper low and attendant sfc system. Look for gradual improvement
sometime Tuesday from west to east with better flying wx for by
midweek, though another front may bring scattered 
MVFR showers/storms toward late Thursday in the mountains. Drier
weather will return for Friday.


As of 300 PM EDT Sunday...

No changes to the flood watch, with still some model differences
in placement of highest rainfall threat. High-res models and
analogs along with pattern recognition favor the southern Blue
Ridge from Roanoke to Boone, east to the piedmont of VA/NC as
those areas which appear to be in line to receive 2-3" through 
early Tuesday. This amount of rain combined with the 1 to 2+ 
inches that has already fallen since Friday should lead to hydro
issues on rivers, as well as smaller creeks and streams.

Looking at the analogs this pattern favors April 5th, 1993, and
March 27th, 1993 which produced some minor to moderate flooding
along portions of the Dan and Roanoke River mainly downstream of
Danville and Altavista. Still will have to see how this sets up
as we had been dry prior to this event.

WPC has kept the moderate risk of FFG exceeded over the 
southern Blue Ridge. As mentioned in the near term, deep 
convection seems limited, thereby will have to rely on upslope 
component to enhance rainfall rates, so isolate flash flooding 
is possible, but more longer term >6hr flooding is more likely.

Saturated ground may also result in downed trees and rock or 
mud slides.


VA...Flood Watch through Monday evening for VAZ007-009>018-022>024-
NC...Flood Watch through Monday evening for NCZ001>006-018>020.
WV...Flood Watch through Monday evening for WVZ042.



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