-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KAKQ 220156
AFDAKQ

Area Forecast Discussion
National Weather Service Wakefield VA
956 PM EDT Mon Aug 21 2017

.SYNOPSIS...
High pressure becomes centered off the Mid Atlantic coast
through Tuesday. A cold front crosses the area late Wednesday,
with strong high pressure building north of the area from
Thursday into the upcoming weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
Update...Showers and storms from earlier this evening have all
but dissipated with the loss of daytime heating and instability.
Have trimmed POPs back accordingly and made minor adjustments to
dewpoints that are running a degree or two warmer. Otherwise,
forecast is on track for stratus and potential fog development
overnight.

Previous discussion (410 PM)...
Latest surface analysis centers ~1023mb high pressure just
offshore, resulting in light south to southeast flow over the
local area. A weak wave visible on water vapor and in the RAP
analysis is located over the Delmarva this afternoon. The wave
coupled with increasing moisture, afternoon destabilization, and
a lee/thermal trough over the northern Piedmont has helped fire
off scattered showers and thunderstorms generally north and
west of Richmond. Overall storm strength has been limited due to
westerly flow and modest lapse rates. However, cannot rule out
some gusty winds and brief heavy downpours. Showers and
thunderstorms push northeast through the afternoon and into the
evening hours as the wave pushes offshore. Coverage wanes
tonight with the loss of daytime heating/destabilization and
upper level support.

Warm and humid tonight with lows mainly 70-75 F tonight.
Fog/stratus possible again tonight over the Piedmont into
central Virginia, but not expected to be as dense as this
morning.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH WEDNESDAY/...
Increasing return flow on the back side of departing high
pressure and ahead of an approaching cold front will result in
warm and humid conditions Tuesday. Highs generally in the low to
mid 90`s, cooler along the coast. Dewpoints in the upper 60`s
to low 70`s will produce heat indices in the 100 to 103 degree
range. While the atmosphere will be quite moist with
precipitable waters around 1.75 inches, warm air aloft, weak
lapse rates, and a lack of appreciable forcing will limit the
convective potential Tuesday afternoon. Will retain silent PoPs.
Mild Tuesday night with lows in the mid 70`s. Clouds increase
across the north ahead of the approaching cold front.

Better forcing arrives along the front Wednesday as a lead wave
drops over the Mid-Atlantic. The cold front progged to slowly
push across the area Wednesday afternoon into Wednesday night.
PoP`s increase during the morning, becoming likely Wednesday
afternoon with the arrival of the better dynamics. The air mass
ahead of the front will become modestly unstable, with mixed-
layer CAPE values progged around 1000-2000 J/kg. Deep layer flow
also progged around 25-30 knots as mid level flow increases,
resulting in better organization and overall storm strength. Mid
level lapse rates also improve to around 6 to 6.5 C/km. SPC
maintains a marginal risk for severe weather, with damaging
winds and heavy rainfall the main threats. Highs generally in
the mid 80`s northwest to around 90 south.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Strong Sfc cold front drops south across the area Wednesday night
into Thursday morning. Modest ascent accompanies the front within
the RRQ of a ~110kt jet over New England, along with pw values to
around 2.0" (around +1 st dev). By Wed evening, have maintained
highest (70%/Likely) POP across srn VA/NE NC, with slight to low end
rain chances by Wed night NW of RIC, where drier air arrives first.
PoPs across SE portions gradually diminish to 20-40% by Thursday
aftn. Thunder will be maintained into Wednesday evening, with
showers thereafter as stable NE flow prevails in the wake of the
front. Drier air gradually filters in from the NW late Wednesday
night into Thursday with partial clearing across the nrn tier
counties.

Thereafter, the late week period will be characterized by the gradual
development of a longwave upper level trough over the eastern United
States for the late week period into next weekend. Meanwhile, broad
1024+ mb surface high pressure builds south over the interior
northeast from Great Lakes Thursday and Friday, eventually setting
up over New England and ridging down the east coast over the weekend
into early next week.

Resultant E-NE flow will result in markedly cooler and drier
conditions over the region, with early morning lows in the 50s
inland to mid 60s along the coast. Maintained a dry forecast through
the forecast period. High temperatures Thu-Mon will be at or just
slightly below normal through the period, mainly in the upper 70s to
low 80s.

&&

.AVIATION /02Z TUESDAY THROUGH SATURDAY/...
Showers and storms from earlier this evening have all but
dissipated with the loss of daytime heating and instability.
Otherwise, forecast is on track for stratus development across
SE VA/NE NC and potential fog development overnight (initially
inland after 22/0600Z and then closer to the coast around
sunrise). Any fog/stratus that develops should dissipate/lift
after 22/1300Z. A cold front pushes into the Ohio Valley on
Tuesday with the region remaining dry with SCT cumulus
developing in the afternoon. The front approaches the region
Tuesday night and is expected to cross the area during
Wednesday. Flight restrictions will be possible Wednesday...
mainly in the afternoon and through Wednesday night due to
showers and thunderstorms. The front should exit the coast
Thursday morning with lingering showers possible across
far SE VA/NE NC. Conditions are anticipated to improve Thursday
afternoon into Friday as the front shifts farther southeast and
high pressure builds north of the region. Breezy NE winds are
also anticipated near the coast Thursday and Friday.

&&

.MARINE...
Latest surface analysis has a high pressure centered just off the
coast. The high will continue to move out to sea overnight and
Tuesday. A cold front approaches the waters Tuesday night and into
Wednesday morning. The front crosses the area late Wednesday and
moves SE of the area Thursday with high pressure returning for the
remainder of the forecast period. Obs indicate generally S/SE winds
of 10 to 15 knots over the waters with waves of 1 to 2 feet and seas
around 2 feet. Winds increase out of the SSW Tuesday through Tuesday
night as the front approaches the waters. Wind speeds may approach
15 to 20 over the Chesapeake Bay and ocean with seas building to 3
to 4 feet and potentially 5 feet north/20nm out, and waves building
to 2 to 3 feet. A Small Craft Advisory may be needed for the
Chesapeake Bay and northern two ocean zones Tuesday night into early
Wednesday. Winds shift to the N/NE behind the front late Wednesday
through Thursday around 10 to 15 knots.

Strong surface cold front drops across the waters Wed afternoon and
evening...pushing well SE of the area during Thu. Winds shift to the
N/NE behind the front late Wed through Thu around 10 to 15 kt.
Post-frontal, look for onshore (E-NE flow) for the remainder of the
forecast period as high pressure builds NNW of the local area. Winds
increase to 15-20 kt Thu night as a couple of waves move across the
stalled front to the south across the Carolinas. Seas will remain
choppy for the late week period into the weekend with the persistent
onshore flow, remaining 3-4 ft northern waters...4-5ft southern
waters.

&&

.AKQ WATCHES/WARNINGS/ADVISORIES...
MD...None.
NC...None.
VA...None.
MARINE...None.

&&

$$
SYNOPSIS...LKB
NEAR TERM...BMD/LKB/SAM
SHORT TERM...LKB/SAM
LONG TERM...AJZ/MAM
AVIATION...BMD
MARINE...SAM