Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 240719
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
319 AM EDT Thu Aug 24 2017

.SYNOPSIS...
High pressure will drift from the Great Lakes today to the
northeastern United States this weekend. Meanwhile, an upper
level trough will sweep southeastward across the area this
afternoon and evening. High pressure will remain in control into
early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Although the cold front has passed east of the area, dewpoints
in the 60s still hanging on from DC southeast. The threat for
showers has ended, but there is still a decent amount of cloud
cover, mainly across Virginia. Anticipate this may increase a
little through dawn due to nocturnal cooling. Am leaning a bit
more toward clouds vs fog, which should be held in check not
just by clouds but also due to dry advection and light gradient
flow.

During the daylight hours, a vorticity max will pivot across the
area...especially late in the day. As was my concern at the
beginning of the week, guidance now picking up on the PVA and
steep lapse rates. Broken clouds and scattered showers will
result, especially in the Appalachians and along I-81. The loss
of daytime heating will allow these showers to fall apart as the
vort continues east this evening, but will still hold onto a
small chance for north/central Maryland and DC suburbs.

Since high pressure will be building into the area at the
surface, winds should be light north. And with a Canadian air
mass comes Canadian temperatures-- high will be in the 70s west
of the Blue Ridge and across northern Maryland. It will be
equally crisp tonight...in the 50s to lower 60s DC-Baltimore
metro.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
High pressure will be passing north of the area Friday and
Saturday. There will be no significant weather features through
this time, just lower heights northeast as the trough axis
slowly lifts. Expect Friday to be dry, but not completely sunny
as flow will be off the water and there will be a window for
diurnal clouds.

By Saturday, the overunning/lift will manifest in a developing
inverted trough over the Appalachians. Anticipate a fairly thick
cumulus field, with sufficient moisture and lift for scattered
showers. While the precip should dissipate soon after sunset,
the clouds may leak east Saturday night.

High temperatures shouldn`t get much higher than the source
region of the airmass...which is in the 70s. Low temps should
stay fairly consistent too.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
Surface high pressure lingers over New England Sunday and at least
into Tuesday. This will produce an onshore flow for the central Mid-
Atlantic and maintain below normal temperatures.

00Z operational GFS/ECMWF consensus is for low pressure to develop
off the southeast CONUS coast early in the week and drift northeast
with the Gulf Stream off Cape Hatteras into the midweek. At a
minimum this would enhance the onshore flow over the central Mid-
Atlantic and allow stratiform rain or drizzle to develop.

This is a complex scenario in the midweek as there will be remnants
of Harvey to contend with. Global models suggest we are over a week
away from any impacts in the Mid-Atlantic.

&&

.AVIATION /07Z THURSDAY THROUGH MONDAY/...
While there is a fairly thick cloud field early this morning,
especially across Virginia, VFR conditions remain as these
clouds have bases 050-070. That should remain the case through the
day and into the evening as a shortwave crosses the area. There
may be a few showers too late day (MRB) or early evening (rest
of terminals). However, the risk of restrictions low...and if
they did occur it would be brief.

VFR will primarily continue Friday-Saturday as well. Patchy
early morning fog possible Friday morning, but coverage should
be limited to valleys. If a terminal impacted, best bet would be
CHO. A couple of mtn showers possible on Saturday. Again, these
should mainly avoid the airfields.

High pressure persists over New England Sunday and Monday with
onshore flow and VFR conditions. Rain may move into the area
Tuesday/Wednesday.

&&

.MARINE...
Have a Small Craft Advisory in effect until noon for most of the
waters due to mixing associated with the change of air mass. It
has not materialized yet. Will give it until sunrise; will
consider a cancellation at that time if it does not manifest by
then. After that, high pressure will build to the north of the
waters. Winds should generally be 10 kt or less this afternoon
into Saturday morning. East flow may pick up to 10-15 kt on
Saturday.

High pressure persists over New England Sunday and Monday with
onshore flow through this time. SCA level onshore flow looks to
begin by Monday. This onshore flow may be further enhanced by low
pressure developing over the Gulf Stream off the Carolinas in the
middle of next week.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until noon EDT today for ANZ531>534-536-
     537-539>541-543.

&&

$$
SYNOPSIS...HTS
NEAR TERM...HTS
SHORT TERM...HTS
LONG TERM...BAJ
AVIATION...HTS/BAJ
MARINE...HTS/BAJ


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