Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 220742

Area Forecast Discussion
National Weather Service Blacksburg VA
342 AM EDT Sun Jul 22 2018

Upper level low over the Ohio Valley continues to drop south
toward the southern Appalachians over the next few
days. Scattered to numerous showers and storms will accompany
this system through the first part of next week.


As of 330 AM EDT Sunday...

The unusual mid-summer upper level closed low will continue to
sink south toward the southeastern U.S. today with a slow-
moving surface boundary pushing across the area. Plenty of
moisture is expected ahead of and along the front and combined
with short-wave energy rounding the upper low will maintain a
high chance for rainfall today and tonight. Despite extensive
cloudiness there should be some diurnal forcing to the
precipitation and have shaped pops accordingly. The entire CWA
is under a marginal risk (5 to 10 percent probability) for
flash flooding per WPC outlook which is low enough to preclude a
flash flood watch at this time. Severe risk also quite limited
due to the clouds limiting instability with only a marginal risk
per SPC across the far eastern counties of the CWA.


As of 200 AM EDT Sunday...

This portion of the forecast will be characterized a much wetter
than normal pattern. A review of the NAEFS mean 500-hPa Geopotential
Height and Climatological Percentile for the upcoming couple of days
shows a strong blocking pattern of record proportions. All time high
heights for this time of year are forecast in the form of ridges
centered across both western Texas/north central Mexico and also
across Maine and the Canadian Maritimes. In between, a record
setting minimum in heights is progged over AL/GA/FL with a nearly
stationary low/trough progged to linger in this region for most of
the portion of the forecast. It`s not until Wednesday night that a
low pressure heading southeast along the U.S./Canadian border near
Minnesota helps to lift the low/trough across the southeast U.S. to
the northeast as a more progressive open wave.

While the southeast U.S. low/trough remains nearly stationary, our
region will be within a prolonged period of deep south to southeast
moist flow. The result will be multiple waves of periods of showers
and storms that will advect across the region. The progged southeast
flow in the low level will allow for even greater amounts along the
crest of the Blue Ridge thank to the effects of upsloping. NAEFS
850hPa specific humidity values are in the 90 to 97.5 percent range
as compared to climatology across much the area Monday night through
Wednesday. This is paired with the V component of the wind pegged at
97.5 to 99 percent values Monday night through Tuesday night.

Given the above, a high probability of a significant rainfall across
the area, especially areas along and east of the crest of the Blue
Ridge, is warranted. Taking into accounts amounts forecast both
today and tonight, expected rainfall through Wednesday is expected
to be in three to four inch range within this region. Amounts of one
to three inches are more likely west of the crest of the Blue Ridge,
with the low end of that range across southeast West Virginia.

WPC has offered a slight, 10 to 20 percent, chance of excessive
rainfall significant enough to produce flash flooding across most of
the forecast area Monday through Monday night. The axis of this
projection centered near the crest of the Blue Ridge. At this point
in the forecast time frame, will continue to make reference to the
potential flooding situation within our Hazardous Weather Outlook.

Given the abundance of rainfall and clouds across the region during
this portion of the forecast, expect high temperatures cooler than
normal by roughly an average of five degrees across the forecast
area. This same cloud cover will help limit how much temperatures
fall during the overnight hours. Expect low temperatures to average
a few degrees above average across the forecast area.

Forecast confidence in the above scenario is moderate to high.


As of 300 PM EDT Saturday...

Once the closed low reaches the Gulf coast, it will open up and
become assimilated into the mean flow as another closed low moves
across southern Canada to near Hudson Bay by the end of next week.
However, the net result will keep us in a wet pattern as a stalled
frontal boundary develops over the Appalachians and central mid
Atlantic region on the periphery of high pressure over the Atlantic.
This is a more typical summertime pattern and will yield ample
amounts of instability with lower amounts of shear in southwesterly
low level flow. Expect daily rounds of thunderstorms with locally
heavy rain and a severe threat generally dependent on storms that
can organize into lines and/or clusters. Will have to see where
the heaviest rain occurs early in the week to get a handle on
where a flood threat may materialize later in the week.

Temperatures will warm steadily through the middle of next week
with highs by Friday in the upper 80s/near 90 east of the Blue
Ridge and in the low/mid 80s to the west.


As of 130 AM EDT Sunday...

Forecast for aviation has changed little with this update.
Expect highly variable cig/vsbys through the valid TAF period as
a slow-moving upper level trough over the Ohio Valley produces
scattered to numerous showers and thunderstorms across the
entire region. Expect greatest coverage of storms during the
afternoon and evening hours. In addition, shallow easterly low
level winds may allow for the formation of fog/stratus along the
Blue Ridge late tonight and early Sunday, especially where
rainfall has been significant.

Extended Aviation:

Wet weather pattern is forecast for most of the upcoming week
with a nearly stationary upper level trough of low pressure
remaining over the eastern CONUS. As such, forecast will reflect
Scattered to numerous showers and thunderstorms, but mainly
during the afternoon hours coinciding with peak heating and


The wet pattern described above leads one to the possibility of
hydrologic concerns both flash and riverine. The short-term
portion of the discussion deals with WPC probabilities and
looking at the river forecasts from MMEFS does not suggest much
cause for concern at this time with nearly all the ensemble
members showing below flood stage crests. River flooding under
a mostly convective regime in July is simply not very common. An
isolated river flooding threat could develop where rainfall
maximizes over a smaller river basin such as the upper Roanoke,
but larger basins such as the New, James and Dan should avoid
any flooding issues.




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