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000
FXUS61 KAKQ 280149
AFDAKQ

Area Forecast Discussion
National Weather Service Wakefield VA
949 PM EDT Thu Jul 27 2017

.SYNOPSIS...
Low pressure affects the region Friday through Saturday, with a
cold front pushing across the region Friday night. Low pressure
lingers near the Mid Atlantic Coast Sunday through at least
Monday night.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
Expect just sctd showers and isolated tstms overnight into Fri
morning, due to the combination of increasing low level moisture
and shortwave energy dropping acrs the region. Will have Pops at
generally 30-40%. Mild overnight with plenty of cloud cover and
light SSW winds. Lows in the lower to mid 70s.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH SUNDAY/...
Potent shortwave/compact upper low currently over southwest
Ontario, digs across the Great Lakes region into the Ohio Valley
Friday. An associated area of low pressure deepens and tracks
from the Ohio Valley into Northern Virginia Friday afternoon.
The trailing cold front is expected to sweep across the central
Appalachians late Friday, with a pre-frontal trough over the
Piedmont Friday afternoon. The air mass over the local area will
be warm and quite moist with dewpoints in the low to mid 70`s
and precipitable waters in excess of 2 inches (+2 standard
deviations). While mid-level lapse rates again will be rather
modest, theta-e advection and temperatures in the 80`s result in
mixed layer CAPE values of 1000-15000 j/kg. Mid-level flow
increases ahead of the shortwave, resulting in deep layer shear
of 25 to 30 knots. Expect scattered to numerous showers and
thunderstorms to spread over the northern local area Friday
morning through the afternoon as the low tracks into the region.
Some mid-level drying expected Friday morning into the
afternoon across central and southern Virginia, resulting in
some breaks in the clouds and plenty of opportunity to warm into
at least the mid 80`s. Latest short term guidance indicates
scattered to numerous showers developing ahead of the pre-
frontal trough mid afternoon, and quickly pushing eastward.
Another area of scattered to numerous showers and thunderstorms
possible along the cold front as it pushes through the region
Friday evening. Given the increasing mid level flow,
broken/bowing convective segments are possible with damaging
wind gusts possible. SPC maintains a slight risk for severe
weather across the region. Meanwhile, an extended period of
showers/thunderstorms expected across the northern local area as
the low stalls and deepens. There still remains some spatial
differences in the short term guidance with respect to the
surface low, but general consensus is across the Delmarva. The
Canadian remains a southern outlier. Given the pivot point and
strong 850mb u components over the Delmarva, expect periods of
heavy rainfall Friday afternoon through Friday night over the
Maryland Eastern Shore. Given 3 hour flash flood guidance of 2-3
inches and inter-office coordination, have hoisted a flash
flood watch for the Maryland Eastern Shore for Friday afternoon
through Saturday afternoon. Flooding could be exacerbated during
periods of high tide.

Mid-level dry air entrainment across inland Virginia late
Friday night will result in drying conditions inland. However,
likely PoPs persist along the coast. The surface low slides just
offshore Saturday as the upper low digs over the Mid-Atlantic.
Deepest moisture pushes offshore Saturday morning, but the
combination of strong upper level dynamics and plenty of remnant
moisture (precipitable waters of 1.5 to 1.75 inches) will
result in ongoing showers and embedded thunder through the day
Saturday. Best coverage again from the Northern Neck to the
Maryland Eastern Shore. Below normal temperatures forecast due
to clouds, showers, and a northwest winds of 10-15 mph. Highs
only in the upper 70`s to mid 80`s. Model consensus centers the
upper low along the Mid-Atlantic coast early Sunday morning,
with ongoing chances for showers along the coast, especially the
Maryland Eastern Shore. Cool Saturday night, with lows in the
low to mid 60`s.

Sunday temperature forecast will be a challenge given the
uncertainty in the guidance. Strong cold advection expected as
850mb temps drop to around 10-12C. Add in considerable
cloudiness, and daytime temperatures will be near 2 standard
deviations below normal. Highs forecast now in the mid to upper
70`s. If the GFS/MEX guidance pans out, a few record low max
temperatures may be in jeopardy. North to northeast winds could
gust to 25-30 mph near the coast Sunday afternoon. Chance PoPs
linger along the coast.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
Low confidence forecast early next week as the models continue to
offer up different solutions on just how slow and far off the coast
the cut off low tracks. Favoring a ECMWF track ivof the Gulf Stream,
combined with lingering upper level energy, decided to hold onto chc
pops generally along and east of I95 Sun night/Mon as the low slowly
lifts NE along the coast. Rather breezy over eastern sections with
continued marine issues early next week. Otw, high pressure over the
mts keeps the rest of the fa dry. Lows Sun nite from the upr 50s NW
to near 70 SE.  Highs Mon in the upr 70s-lwr 80s except mid 70s at
the beaches. Lows in the 60s to nr 70 SErn beach areas.

High pressure builds across the region for the mid week period.
Highs Tue in the mid-upr 80s except upr 70s at the beaches. Lows in
the mid 60s-lwr 70s. Highs Wed/Thus 85-90. Moisture returns from the
west Thurs afternoon. Added slght chc diurnal pops to the
Piedmont.

&&

.AVIATION /02Z FRIDAY THROUGH TUESDAY/...
Rather unsettled weather can be expected over the next 24-48
hours as a series of disturbances move across the region.
Showers and thunderstorms currently over the mountains will move
into the piedmont and coastal plain this evening and tonight.
A few light showers may affect RIC early while there will be a
chance for some patchy heavier rain during the overnight. Expect
only a slight chance for a thunderstorm at any of the TAF sites
tonight. MVFR cigs will be possible late tonight into early
Fri, then a chance for MVFR/IFR cigs and rain as a series of
weak disturbances move through during the day. A stronger
disturbance moves into the area from the NW by Friday
afternoon/evening, potentially bringing strong to severe storms
into the region.

This next disturbance/low pressure system will slow down just
off the coast for the weekend. This will allow for steady rain
especailly at SBY and expected MVFR or IFR cigs for much of the
weekend. Improving weather for early next week.

&&

.MARINE...
Some southerly channeling noted across the bay tonight but will keep
wind speeds below SCA levels for now. SW winds aob 15 KTS continue
through Fri nite ahead of a cold front.

A fairly strong cold front (by mid summer standards) will cross the
area early Sat shifting the winds to the N by the afternoon then NE
at nite. A decent surged noted behind this feature which will result
in SCA level winds late Sat/Sat nite. Meanwhile, another area of low
prs progged to develop along this boundary but uncertainity remains
as to this features speed and track. Will hold off on any headline
attm given a late 4th period starting time. Although some of the
data across the northern coastal waters Sun and Sun nite is rather
impressive for the summer, have capped gusts below gale force for
now. Plenty of time to adjust (higher or lower) if needed. Seas
build into the SCA range as well, but capped heights across the
northern coastal waters at 8 feet.

Contitions slowly improve by mid week but lingering NE swell will
likely result in marine issues through mid week.

&&

.TIDES/COASTAL FLOODING...
Water levels in the upper Bay (mainly eastern side adjacent to
the MD eastern shore) approached minor flooding thresholds
early this morning and will continue the coastal flood
statement mainly for Cambridge through 10 am. An additional
statement may be needed for tonight.

&&

.AKQ WATCHES/WARNINGS/ADVISORIES...
MD...Flash Flood Watch from Friday afternoon through Saturday
     afternoon for MDZ021>025.
NC...None.
VA...None.
MARINE...None.

&&

$$
SYNOPSIS...SAM
NEAR TERM...SAM/TMG
SHORT TERM...SAM
LONG TERM...MPR
AVIATION...JEF
MARINE...MPR/JEF
TIDES/COASTAL FLOODING...AKQ