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Area forecast discussion 
National Weather Service Wakefield Virginia
207 PM EDT Wed Sep 28 2016

Synopsis...
low pressure tracks north across the North Carolina and Virginia
Piedmont this afternoon and tonight. An upper level low slowly
drops south from the Great Lakes region resulting in unsettled
weather conditions into the weekend.

&&

Near term /through tonight/...
latest GOES water vapor imagery centers a deep/anomalous upper
low over the western Great Lakes region with a plume of high water
vapor lifting into the mid-Atlantic region. At the surface, a weak
quasi-stationary boundary has located over the forecast area.
Scattered showers over the nrn neck/lwr Maryland ern shore will
continue to lift NE along the boundary late this morning.

The upper low drops swd today, centering over in late today. The
associated surface low drops over the Ohio Valley. Diffluent flow
over the local area and increasing winds aloft will provide strong
forcing for ascent along the stalled frontal boundary. Deep layer
southwesterly flow will advect anomalous pw values into the
region. Scattered to numerous showers expected to develop by mid-
afternoon as the better forcing arrives, and persists through the
day and into the overnight period. Continued with high end likely
and categorical pops from mid-aftn through the overnight period.
Theta-E advection and dewpoints in the upper 60s will result in
increasing instability. Instability will be dependent on how much
(partial) clearing occurs as current vis imagery depicts some
breaks in the cloud cover. Given this, there is the potential for
an axis of 1200-1800 j/kg of 0-1km MLCAPE centered over the Virginia
Piedmont by this aftn. Deep layer shear is progged on the order
of 30-40 knots. Meanwhile, there will be some low-level turning in
vicinity of the lingering sfc boundary. In summary, this will
result in a slight risk for severe tstms this aftn/early evening
from the I-95 corridor westward into the Piedmont, with the main
threats being strong wind gusts and iso tornadoes. Highs range
from the mid 70's northwest to low 80's southeast.

The upper low slowly drifts southward tonight as another area of
surface low pressure approaches from the northwest. Height falls
over the region progged to develop a wave of low pressure along
the frontal boundary tonight, with an area of strong isentropic
lift across the Piedmont to the Maryland Eastern Shore. The result
will be ongoing categorical pop's and the potential for moderate
to locally heavy rainfall. Lows forecast in the mid to upper 60's
under a cloudy sky.

Rainfall totals through the near term expected to range from 1-2
inches across the Piedmont and central Virginia to one quarter to
one half inch northeast North Carolina.

&&

Short term /Thursday through Friday/...
anomalous upper-level low centers over Kentucky Thursday, with
deep layer southerly flow over the mid-Atlantic. At the surface,
an associated area of low pressure and attendant cold front drop
into north central North Carolina. High pressure located over the
northeast will help keep low pressure south of the region, with
onshore flow north of North Carolina. Deepest moisture and
Theta-E/instability axis begin to pivot northward, but showers
with embedded thunder persist. Best coverage and heaviest
precipitation forecast Richmond northward through the day,
tapering off to solid chance pops far southern Virginia and
northeast North Carolina late Thursday. Highs Thursday range from
the mid 70's northwest to low 80's southeast. The upper low moves
very little Thursday night as a frontal boundary pivots northward
from North Carolina, locating over the forecast area. By that
time, the deepest moisture progged to be north of the region, but
precipitable waters around 1.5 inches and strong upper level
dynamics will keep solid chance to likely pops in the forecast.
Lows in the mid to upper 60's.

Forecast has trended wetter and cloudier Friday than 12 hours ago
as the stubborn upper low remains centered in the vicinity of
eastern Kentucky. Deep layer southerly flow prevails, but the
deepest moisture continues to pivot north and east of the region.
Strongest onshore moisture flux progged north of the region,
keeping the heaviest rain away from the local area. However, the
combination of continued upper vertical motion and the presence of
the frontal boundary will keep solid chance to likely pops across
the region through the day Friday. Sky remains mostly cloudy to
cloudy. Highs forecast in the mid 70's northwest to low 80's
southeast.

Per sref and gefs mean guidance, storm total rainfall amounts
expected to range from 3-4 inches north and west of Richmond to
around one half inch in northeast North Carolina. Rainfall amounts
generally 1-3 inches elsewhere. No Flood Watch products issued on
this shift as the anomalous flow remains north of the region.
However, localized minor flooding from several periods of showers
(some heavy at times) is possible.

&&

Long term /Friday night through Tuesday/...
long term period starts off Fri night/Sat with an upper-level cutoff
low and associated sfc low centered over the Ohio Valley. Better rain
chances will stay north of the area, but included at least a 20-30%
chance of showers especially over NE areas where the highest
moisture will be located. Mainly dry conditions thereafter into sun
and Mon as the cutoff low weakens and slides NE into Canada. As for
temps, highs will generally be in the mid/upper 70s with lows
ranging from the mid/upper 50s over the Piedmont to the mid/upper
60s near the coast.

&&

Aviation /18z Wednesday through Monday/...
a very challenging aviation forecast for the next 24 hours. An east-
west stationary front will remain across Virginia through the
period. This will serve as the focus for showers and thunderstorms
through Thursday. Some of these storms will likely contain very
heavy rain especially across Maryland and inland Virginia allowing for areas of
IFR at times especailly tonight. Could not rule out a severe storm
either across interior Virginia late this afternoon into this evening.
Lower chances for MVFR across the Virginia Tidewater and NE NC as this
region will remain further away from the front. Slowly improving
conditions Thursday but still chances for showers and storms
especailly across the interior.

Outlook...unsettled weather persists into Friday with continued
chances for showers and thunderstorms due to the frontal boundary
remaining nearby. Improved weather for the weekend.

&&

Marine...
latest analysis shows a broad area of surface low pressure over the region,
with another area of low off the coast. Strong/anomalous surface high
pressure is situated over eastern Canada. The pattern over the next few
days will feature this strong high building over eastern Canada, ridging
into New England and the northern mid-Atlc region. This will make for
a persistent and rather strong east/NE flow over northern portions of the
marine area. Have added Bay zones north of new pt Comfort to the Small Craft Advisory as well
as the coastal waters from Cape Charles to Parramore Island (beginning
late tonight or Thu morning). Seas are expected to build to 6-8 ft over
northern waters, while remaining around 4 ft across the south). It is
possible that marginal Small Craft Advisory conditions could make it to NC border for
5 ft seas by Thu aftn and that Lower Bay could also get to marginal
Small Craft Advisory levels (but confidence is rather low at this occurring and given
that it would be 3rd period did not go that far with the headlines).

Seas will slowly subside Fri-Sat (though the headlines will likely need
to be extended through at least Fri night eventually). A lighter
south-southeast flow expected Sat leading to sub-Small Craft Advisory conditions with 1-2 ft waves
over the Bay and 2-4 ft seas over coastal waters.

&&

Tides/coastal flooding...
the synoptic pattern with strong high pressure centered over
eastern Canada with a slow moving boundary/sfc low pressure inland
over the Carolinas/mid-Atlantic will bring persistent east-northeast flow to
the area Thu-Fri. Strongest winds expected for the northern areas,
with minor tidal/coastal flooding becoming a concern for Ocean
City/Chincoteague/Wachapreague by Thu evening and Fri morning high
tide cycles. Also some potential for flooding into the Bay (both
lower and mid-upper bay) by Fri-Sat as seas remain elevated and
water is expected to struggle to exit the Bay for several days.
No coastal flood watches have been issued as chance for moderate-
severe flooding is low, but we may eventually need to issue
coastal flood advisories.

&&

Akq watches/warnings/advisories...
Maryland...none.
NC...none.
Virginia...none.
Marine...Small Craft Advisory from 7 am Thursday to 6 am EDT Friday for
anz654.
Small Craft Advisory from 7 PM this evening to 6 am EDT Friday
for anz650-652.
Small Craft Advisory from 7 am Thursday to 1 am EDT Friday for
anz630-631.

&&

$$
Synopsis...Sam
near term...ajz/Sam
short term...Sam
long term...mas
aviation...mrd
marine...lkb
tides/coastal flooding...

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