Area Forecast Discussion
Issued by NWS Blacksburg, VA-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KRNK 230549
Area Forecast Discussion
National Weather Service Blacksburg VA
149 AM EDT Sun Apr 23 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.
.NEAR TERM /THROUGH TODAY/...
-- Changed Discussion --As of 100 AM EDT Sunday...
Expect a lull in moderate to heavy showers this morning with
area of lighter showers moving across the foothills/piedmont and
lessening over the mountains. Surface low is over north central
Alabama early this morning and should continue to track slowly
east into the southern Appalachians by midday Sunday. Will have
to wait until upper low/trough over the mid-MS Valley starts to
pivot east and turn more neutral tilt and bring another area of
upper difluence and lift back over the area to see better rain
coverage and intensity increase, and this looks to take shape
more after midday Sunday. So any hydro issues look to be
Previous discussion from late evening...
Convective threat is over, now we move into the flood threat of
this large-scale synoptic system. First, the backdoor/wedge
front has slide generally south and southwest of the CWA, with
the possible exception of the far southwest corner of the CWA.
Solid cloud cover and rainfall will ensure that this remains in
place until the large-scale synoptic system moves out of the
area Tuesday. The upper low that will be the main culprit behind
the evolving pattern is currently located across the western TN
Valley. The low will intensify and deepen as southern stream
short wave energy translates from the desert SW into the
existing upper low. As the upper low moves into the
Midsouth/Southeast Sunday night, it will take on a negative
tilt, slow, and spin slowly in a northeastward direction from
GA/SC to off the VA coast in the Mon-Tue time frame. Thus, the
best dynamics for heavy rainfall and training of echoes will be
in the Sunday night/Monday morning time frame.
HRRR and radar trends suggest that rainfall overnight may tend
to be rather meager, and likely less than was previously
expected. Therefore, have decreased rainfall amounts some from
what was previously indicated. All threat of thunder has ended.
There may again be another lull in the precipitation Sunday
morning until the next wave of heavier rainfall arrives Sunday
afternoon. Again, the main concern from dynamics/synoptic
support will not arrive until Sunday night into Monday morning.
Heavy rainfall potential may linger into the area through much
of the day Monday.
Other than decreasing rainfall amounts in the near term, am not
making any changes to longer term rainfall or the flood watch.
Temperatures will remain unseasonably cool Sunday into Monday
as the wedge persists and is continually reinforced with
rainfall and solid clouds cover along with northeast flow.
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
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.SHORT TERM /TONIGHT THROUGH TUESDAY/...
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.
.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
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.
.AVIATION /06Z SUNDAY THROUGH THURSDAY/...
-- Changed Discussion --As of 140 AM EDT Sunday...
Continued to see poor flying conditions this period, but some
lull in showers occurring this morning as main batch heads to
the piedmont. With a very moist airmass have some fog issues,
especially BLF where it could get dense at times. Overall
leaning toward less pessimistic but still sub-VFR through the
morning, where vsbys/cigs vary from LIFR to low end MVFR.
During Sunday should see uptick in rain again, especially south
of a BLF-LYH line. Some heavier downpours likely, but thunder
threat is limited.
Look for IFR-LIFR cigs generally in place at all TAF sites
through the Sunday afternoon and evening. Visibilities will be
dependent on areas of heavier rainfall, but prevailing MVFR
visibilities in -RA/RA, with periods/pockets of IFR
visibilities. Winds will be NE 7-10kts with low end gusts
likely across the Piedmont through the TAF valid period.
Extended Aviation Discussion...
Poor aviation conditions expected to continue through Monday.
Some improvement will begin late Monday, but more likely into
Tuesday as the upper low moves northeast of the area and the
surface winds come around to the northeast as opposed to the
northeast. This will allow ceilings to improve to MVFR and
visibilities to VFR as the rain comes to an end or at least
becomes much more showery in nature. VFR conditions are expected
Wed-Thu as high pressure, but surface and aloft, build over the
area. Another weather system will bring showers and possibly
thunderstorms into the area late in the week.
-- End Changed Discussion --
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 through Monday evening for VAZ007-009>018-022-023-
NC...Flood Watch through Monday evening for NCZ001>006-018>020.
WV...Flood Watch through Monday evening for WVZ042.