Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 260752
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
352 AM EDT Wed Apr 26 2017

.SYNOPSIS...
Low pressure off of the DelMarVa will move northeastward away from
the Mid-Atlantic coast today. A cold front will approach the region
and weaken over the area Thursday night. A backdoor front may slide
into the area late Saturday into Sunday. A cold front will pass
through the region early next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Vertically-stacked low that has been plaguing our weather the past
several days is currently analyzed over the DelMarVa coast. Low
clouds and some patchy drizzle/fog will remain through daybreak
today...with improvement expected from south-to-north through early
afternoon as low finally shifts off to the northeast. Primary
forecast challenge today will be timing when clouds break...and
subsequent high temperature forecast. Most confident in highs across
central VA...where clouds break early and insolation is strongest.
Expect highs in the U70s to near 80F across this region. Slightly
less confidence from the DC metro northward to the Mason-Dixon
line...where clearing is expected near midday (DC) and then into the
afternoon (Mason-Dixon line). If clearing is slower (faster) then
highs would be lower (higher) across these areas.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH FRIDAY NIGHT/...
Ridging develops over the area tonight...leading to light winds and
relatively clear skies. With dewpoints in the U50s/L60s...expect
lows near 60F...with the possibility of fog due to radiational
cooling.

Very warm with increasing humidity across the area Thursday as
southerly return flow strengthens ahead of a weak cold front. Front
will begin to washout as it approaches our area...bringing the
possibility of showers/elevated thunderstorms to the higher terrain
late Thursday. Only a low chance that a shower or storm pushes east
of the Blue Ridge...as forecast thermodynamic profiles suggest
capping inversion would generally suppress convection.

Warm and humid weather continues Friday. Warm air aloft will lead to
a capping inversion near 800 mb that will likely suppress convection
across the much of the area...especially with a lack of a focusing
mechanism. One exception could be across the higher terrain where an
isolated shower/thunderstorm could develop by late afternoon.
Though, poor mid-level lapse rates across this area would likely
limit parcel accelerations and prevent necessary ingredients for
lightning.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
There is (unfortunately) tremendous spread in the weekend forecast.
The synoptic setup by that time will be for a warm (or backdoor
cool) front stretched east-west across the Mid Atlantic and the Ohio
Valley, to low pressure which will be in the southern Plains. The
quandry is how far south will the front make it. The GFS, supported
by the GEFS mean and a number of its ensemble members, suggests
somewhere around southern Pennsylvania or northern Maryland...which
would obviously keep the forecast area on the warm side of the
boundary. Some GEFS members are still further north than the mean.
The ECMWF, on the other hand, represents the furthest south of the
ensemble spread, although some GEFS members support it, too.  Based
on this solution, the forecast area will be impacted by a backdoor
front, which would drop temperatures by around 10 degrees, most
notably across the northern portion of the forecast area. Will seek
some sort of middle ground, although will still favor warmer
solutions until forecast mean comes into some sort of consensus.

By Sunday, it seems apparent that we will be under the influence of
a backdoor front. The only question here will be how cool will it
get. Errors from the Saturday forecast will only multiply, so will
not deviate from the model blend at this time.

There are precip implications to the above, as warmer weather will
yield greater instability, and the presence of a surface boundary
potentially could yield thunderstorm development. Will be keeping
the Saturday forecast as chance showers/thunderstorms. Am thinking
it may be dubious for storms to have a chance on Sunday (they`d be
shunted to the south instead). Will utilize showers instead.

Either way, by Monday the front would be dragged north of the area
again as low pressure intensifies and heads toward the Great Lakes.
That would pave the way for a late Monday or Monday night cold
frontal passage.

&&

.AVIATION /08Z WEDNESDAY THROUGH SUNDAY/...
Low cigs/vis will cause IFR (to at times MVFR) conditions across
much of the area through daybreak...with clearing (and a return to
VFR) expected from south to north through the morning into midday.
VFR expected by afternoon at all TAF sites. Sub-VFR could return
tonight as fog develops over the area (especially across the NE).
VFR returns during the day Thursday. Cold front approaches the area
late Thursday with the chance for isolated showers/thunderstorms.
Dry Friday with VFR conditions.

There is a large degree of uncertainty in the weekend forecast, as a
frontal boundary will be meandering around the area. Showers and
thunderstorms will be likely, but areal coverage will be a question.
If the front does make it south of the terminals, then there will be
the opportunity for low clouds to work inland. Lots of opportunities
for flight restrictions, but details unclear at this time.

&&

.MARINE...
Light winds expected today/tonight as low pressure continues to move
away from the area. Strengthening southerly flow Thursday ahead of a
cold front could lead to low-end SCA wind gusts. Low confidence
precludes issuance of SCA at this time range. Light winds and dry
weather expected Friday.

A front likely will be meandering across the waters. As a result,
the pressure gradient likely to not be strong. Direction of winds
unclear, dependent upon what happens with the front. If the front
does sink south, then a northeast wind may pick up Saturday
night.
&&

.TIDES/COASTAL FLOODING...
Elevated water levels continue, but departures are not as high as
last evening. Have cancelled the Coastal Flood Advisory for Calvert
County, as tides have come in an inch or two under minor threshold.
Straits Point definitively above minor, so that Advisory in good
shape. The theme of water levels hovering around the minor threshold
will be working north, which does reduce confidence.  Have opted to
issue a Coastal FLood Advisory for Anne Arundel County nonetheless.
No action north of there.

On the Potomac, the Advisory for Alexandria and Southwest Washington
DC seems to be on track. However, base on what`s happening
downriver, am not completely sold on this outcome.

The overnight tide cycle is the higher of the two astronomically.
Since these forecasts are right at threshold, that will be the only
cycle of concern. Will need to monitor trends for tomorrow
night/Thursday morning, and perhaps again Friday morning.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory until noon EDT today for DCZ001.
MD...Coastal Flood Advisory until 8 AM EDT this morning for MDZ014.
     Coastal Flood Advisory until 6 AM EDT early this morning for
     MDZ017.
VA...Coastal Flood Advisory until noon EDT today for VAZ054.
WV...None.
MARINE...None.

&&

$$
SYNOPSIS...MSE
NEAR TERM...MSE
SHORT TERM...MSE
LONG TERM...HTS
AVIATION...MSE/HTS
MARINE...MSE/HTS
TIDES/COASTAL FLOODING...HTS



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