Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 292322

Area Forecast Discussion
National Weather Service Blacksburg VA
722 PM EDT SUN MAY 29 2016

Tropical Depression Bonnie has weakened. It is centered over
eastern South Carolina but will influence our weather over the
next several days as it slowly moves northeast along the coast.


As of 330 PM EDT Sunday...

clouds and scattered showers will remain over the forecast area
through Memorial Day as Tropical Depression Bonnie meanders over
northeast South Carolina.

Abundant cloud cover and tropical moisture extend north through
the Carolinas associated with Tropical Depression Bonnie. This
tropical system is forecast to move very little through Monday,
resulting in considerable cloudiness throughout the Mid Atlantic
Region along with scattered to numerous showers and thunderstorms.
PWATs east of the Blue Ridge are in excess of 1.50 inches, thus
any deep convection will be rain efficient, heavy downpours
potentially leading to poor drainage flooding. Coverage attm has
not justified the need for a flood watch. Main area of concern
within our CWA this evening and overnight will be Southside VA
into the Piedmont of NC where convergence axis will exist on the
north side of the tropical system, but even so, the deepest
moisture will remain confined farther south closer to the
circulation center.

For Monday and Monday night the circulation center is expected to
move very little...the Tropical Prediction Center showing a slight
jog to the north. Unlike today were there was abundant mid/upper
level moisture, models on Monday are suggesting some drying in the
mid levels allowing for some breaks in the overcast and subjecting
the forecast area to surface heating. This increase in solar
insolation is expected to increase the CAPE, allowing for
diurnally driven deep convection. Since airmass is of tropical
origin, anything that develops will contain heavy downpours.

Temperatures through Memorial Day will remain mild, elevated
dewpoints keeping the overnight lows above normal. The daytime
high is advertised in the 70s, but could easily eclipse 80 with a
few hours of sun.


As of 400 PM EDT Sunday...

Little change through mid week. TD Bonnie is forecast to slowly
move northeast just inland of the coast. Heaviest rain is forecast
to be aligned with the I-95 corridor, however our forecast area
will be subject to a daily threat of diurnally driven deep
convection aligned with the mountain ridges. Steering currents are
forecast to be weak, and with PWATs running AOA 1.5 inches,
expect scattered slow moving showers/storms with heavy downpours.

With more breaks in the cloud cover, temperatures will be warmer
than normal with 80s for highs. Lows generally in the 60s.


As of 400 PM EDT Sunday...

Thursday the air mass will remain warm and humid, with a
continuation of diurnally driven deep convection. A cold front is
forecast to approach the forecast area from the west late Thursday
into Friday. This boundary will result in an increased threat for
showers/storms...its passage bringing a less humid airmass to the
region for the weekend.

Until the frontal passage at the end of the week, temperatures,
especially the overnight lows, will favor above normal readings.
After the frontal passage, lower dewpoints and clearing skies will
support lower nightime lows. Daytime highs are advertised to be close
to normal.


As of 722 PM EDT Sunday...

High confidence in light winds but below average confidence in
cigs/vsbys during this taf period. The circulation of Bonnie continues
to spin over South Carolina. This is pushing moisture in our direction
from the southeast but we are lacking a good focusing mechanism to
realize significant precipitation. Orographic affects are being
minimized since the low level flow is not favorable for upslope along
most of the Blue Ridge so we are seeing showers move in our direction
and lose a lot of vigor as they enter the more stable environment that
resides over the TAF sites. Fully expect some degree of shower activity
to  linger on the radar east of the Blue Ridge through the overnight
hours but the overall trend will be for decreasing coverage tonight.

Guidance is very keen to develop low cigs and fog at all sites toward
daybreak. The atmospheric column is indeed quite moist but there are is
a non trivial amount of dryness shown in model soundings especially in
the boundary layer. While confidence is not high still believe the
degree and extent of vsby restrictions are overdone in the west as
mid/upper cloud cover looks sufficient to significantly restrict
radiational cooling and this should limit the changes for sub VFR
conditions at KBLF and KLWB. The chances for lower cigs and vsbys are
better further east where low level moisture will be higher so will
introduce some MVFR/IFR conditions there. However as stated earlier
this is not a high confidence situation so will monitor closely to see
if trends deviate from the expected forecast.


Low confidence on flight category for Monday-Thursday. Models
indicate low pressure will stall over the eastern mid-atlantic and
linger through mid week. Our forecast area will be on the western
periphery of this feature which suggests best flight conditions
will be over and west of the Blue Ridge. MVFR or lower ceilings
are possible at KLYH and KDAN Monday.




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