Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 222050

Area Forecast Discussion
National Weather Service Blacksburg VA
450 PM EDT Sat Apr 22 2017

A frontal boundary will stall out across our area through
Sunday with a wave of low pressure moving along it. Low
pressure will slowly track along the front through the
Tennessee Valley and into the Carolinas. The front moves
southeast by Monday, with high pressure building in Tuesday.


As of 450 PM EDT Saturday...

Severe thunderstorm watch remains in effect until 9 PM this
evening for southeast portions of Forecast Area.

Flood Watch has been posted for midnight to Monday evening for
most of the region.

Frontal boundary continues to slowly drop south across the region
this afternoon, before it stalls tonight into Sunday. Scattered
showers and thunderstorms will develop ahead and along the
front. With the instability and shear, a few strong to severe
thunderstorms capable of damaging wind gusts and large hail will
be possible this afternoon and evening.

Localized flash flooding may be possible this afternoon into tonight
with any training of thunderstorms with heavy rainfall. Look for a
southward shift in convection this evening into tonight, then as
next upper trough moves east into Tennessee. A surface low will
shift to eastern Tennessee, then eastward to South Carolina by
Sunday night. This will result in a northward shift in the rain
with the wedge reinforcing. For pops used a blend of HRRR and
NAM. Low temperatures tonight will range from around 40 degrees
in the northwest mountains of Greenbrier county to the lower
50s in the Piedmont. Cloudy and cooler Sunday with occasional
rain, heavy at times.

As the rain continues to fall, the ground will become saturated
and the potential for flooding will increase with time. The
widespread potential starts at midnight tonight and continues
into Sunday. High temperatures Sunday will be cool with readings
from the upper 40s in the mountains to the mid 50s in the


As of 440 PM EDT Saturday...

The upper low tracks southeast and will be off the coast Monday
night before making a turn to the northeast on Tuesday. Wedge at the
surface remains in place through Monday night then pressure lower
enough as the surface low reaches the North Carolina coast to bring
winds around to the north then northwest and erode the wedge. Models
showed a prolonged period of decent easterly winds which will aide
in widespread to heavy rain Sunday night. The low is far enough
offshore that a bulk of the moderate to heavy rain will move east of
Lynchburg and Danville by Monday morning. However, there is enough
moisture wrapping around the low that chance of showers in the
foothills and piedmont extends through Tuesday. Stayed close to MAV
guidance for temperatures through the period.


As of 100 PM EDT Saturday...

Brief period of upper ridging and higher 500MB heights Tuesday night
and Wednesday. By then the long wave trough deepens in the central
United States putting the region in broad southwest flow Thursday
through Saturday. Models were showing varying degrees of strength of
the southeast ridge. Low pressure tracking northeast into the Great
Lakes will move a front into the Ohio and Tennessee Valleys on
Thursday. This boundary stalls on Friday before another low moving
out of the southern Plains pushes a warm front into the Ohio Valley
on Saturday. This pattern keeps the rain potential to the west of
the Mid Atlantic states.  Temperatures will be above normal
Wednesday and Thursday.


As of 152 PM EDT Saturday...

Poor Flying conditions expected during the taf period with
showers and thunderstorms with pockets of low clouds and fog.
Quickly changing conditions and gusty winds are possible near

MVFR conditions with pockets of IFR in convection will prevail
this afternoon into tonight, as the cold front drops southeast.
Some strong to severe storms may be possible along the Virginia
and North Carolina border. The KDAN has the best chance for a
stronger storm this afternoon into tonight.

Tonight into Sunday morning, keeping showers around with less
thunder along with vsbys in the MVFR to IFR range as well as
cigs. Winds shift from the southeast to southwest to northeast
as cold front sags south behind first area of convection late
this afternoon.

Beyond midnight expect off and on rain heavy at times, with
generally sub-VFR conditions. IFR conditions will become more
widespread Sunday into Sunday evening.

Low to moderate confidence in ceilings, visibilities and winds
during the taf period.

Extended Aviation Discussion...

No change in the pattern and looking wet with predominant sub-
VFR Sunday night into Monday, with finally some improvements
from west to east Tuesday. Drier and warmer mid to late next
week so mainly VFR after early week. Some moisture may return
towards the end of the week.


As of 330 PM EDT SATURDAY...

Flood watch has been posted for much of the region from Midnight
tonight until Monday evening.

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 mountains.

A slight risk for convective rainfall exceeding flash flood guidance
was issued by WPC earlier for Day 1 and Day 2. Pockets of minor
advisory-type flooding cannot be ruled out in convective storms
which will be more likely tonight and Sunday 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 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
(this afternoon through early Monday) period is generally about
2 to 4 inches, with the bulk of it falling 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 and portions of
the Roanoke basins as the most likely to flood, suggesting a greater
likelihood for moderate or higher flooding but the NAEFS has been
consistently showing lower probabilities.


VA...Flood Watch from midnight EDT tonight through Monday evening
     for VAZ007-009>018-022-023-032>034-043>046-058-059.
NC...Flood Watch from midnight EDT tonight through Monday evening
     for NCZ001>006-018>020.
WV...Flood Watch from midnight EDT tonight through Monday evening
     for WVZ042.


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