Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 120526

Area Forecast Discussion
National Weather Service Blacksburg VA
126 AM EDT Wed Aug 12 2020

A weak upper level trough to our west and surface boundary
nearby will allow for better coverage of showers and storms
over the next several days with some heavy rainfall possible.


As of 1015 PM EDT Tuesday...

No significant changes. Forecast remains on track with
convection waning on schedule, only a few lingering showers
remain over the NC mountains and far northern Rockbridge
County. One flood advisory issued for Grayson County was
cancelled with no reports of flooding.

This is the final AFD of my career, in all likelihood.

Previous valid discussion...
As of 700 PM EDT Tuesday...

Scattered thunderstorms as advertised continue early this
evening but will see a diminishing trend as the evening
progresses with the loss of heating. A few have produced modest
wind gusts along with pockets of heavy rainfall, up to 1 to 2
inches in isolated pockets but no notable issues. Patchy fog and
stratus likely again late tonight, varying somewhat depending on
where it rained.

No changes in what has been a very stable temperature regime
with low in the 60s across the mountains, to around 70 east.

Tomorrow is looking at more or less the same pattern but with
perhaps greater coverage of convection and a somewhat more
robust rainfall footprint. WPC introduced a Slight Risk for
Excessive Rainfall for 12Z Wednesday-12Z Thursday. Neighboring
office to northeast has issued a Flash Flood Watch up to our
northeast border but have refrained from joining at this time.
Wording in HWO was bolstered to mention some possible flood
issues. Hydro is discussed in somewhat more detail below.

Highs will still run at mid August normals with low-mid 80s west, to
upper 80s east.


As of 200 PM EDT Tuesday...

Looking to the second half of the workweek, weather forecast models
still in good agreement of a weak upper level trough, perhaps closed
low, stalling across the lower Ohio River Valley, while Bermuda high
pressure resides off the Atlantic coast. This will channel
increasingly deeper moisture along the Appalachian chain ahead of a
cold front drifting between the Ohio River and the Interstate 64
corridor. Uncertain at this point whether high pressure across
eastern Canada will have enough push to nudge the front southward
across our area Thursday, but still expecting abundant showers and
thunderstorms given precipitable water values in the 1.8 to 2.0+
inch range Thursday afternoon, again indicating a deeply moist

By Thursday night/early Friday, there is better indication that the
cold front will slide southward across the mid-Atlantic as a
backdoor front, resulting in a wedging pattern that remain through
the period. Given the close proximity of the upper level system
supporting increased rain chances and the east to northeasterly
windflow, lowered afternoon temperatures for Friday and Saturday by
several degrees below forecast guidance: mid 70s to low 80s for most
of the forecast area. Overnights will remain muggy as showers
diminish in coverage, but do not believe they will dissipate


As of 200 PM EDT Tuesday...

Little change in the pattern through the weekend will maintain
widespread shower/thunderstorm activity across the mid-Atlantic into
the central Appalachians, some of which may produce periods of heavy
rain and possibly localized flooding. By early Monday, still seeing
indications that a cold front will push across the central
Appalachians to bring drier air, though spotty showers will still be
possible Monday and Tuesday afternoons.


As of 120 AM EDT Wednesday...

Storms fading this evening, but mid and high level clouds are
hanging around into the early morning hours. Any clearing will
lead to fog/stratus towards sunrise (10Z-13Z), then lifting
during the rest of the morning.

More storms expected this afternoon and evening with possibly
greater coverage than yesterday.

Winds will be generally light through the period.

Extended Aviation Discussion...

Diurnally driven afternoon storms continue into Thursday, but a
front arriving with better upper support should lead to more
coverage by the end of the workweek into the weekend. Will have
to wait to see how far east the front makes it this weekend but
could see some high pressure wedge and lower cigs Sunday.


As mentioned above, tomorrow looks potentially more active
hydrologically with the a WPC Slight Risk (15-20 percent
probability) introduced for excessive rainfall, exceeding Flash
Flood Guidance (FFG). 1-hour FFG currently ranges from about 1.5
to 3 inches which is not especially low and 3-hr FFG from about
2 to 4 inches. Typically, rainfall needs to exceed FFG by a
substantial amount to produce actual flash flooding, especially
in mid-summer. Urban areas being a notable exception. It will
take some fairly slow-movers to get those kinds of amounts. As
noted in the WPC rainfall discussion a confluence of factors
point toward a favorable setup across parts of our area with
high PWATS, weak steering flow and an emerging short-wave from
the TN valley, along with peak heating and high instability. A
short-fused flash flood watch may the best solution for
indicating the flood potential for this scenario, although
midnight shift will be evaluating the latest CAMS and high-
resolution models. The latest available CAMs suggest that the
more organized heavy rainfall may be up toward northern VA/MD
area. And another round is possible on Thursday.




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