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fxus61 klwx 241541 
afdlwx

Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
1141 am EDT Mon Apr 24 2017

Synopsis...
low pressure over Georgia will gradually move northeastward,
reaching the South Carolina coast this evening, the North
Carolina coast Tuesday evening, and finally off the mid-
Atlantic Wednesday. A cold front will approach the region late
Thursday or early Friday and remain nearby through the weekend.

&&

Near term /through tonight/...
update...coastal flood advisory issued for St. Marys County this
afternoon. See tidal section below.

Intermittent light rain has been ongoing this morning across the
mid- Atlantic region. Cool conditions expected today as
northeast flow, clouds and periods of rain continue.
Temperatures will only climb a few more degrees today...staying
in the 50s. Coverage of rain will become more widespread by the
afternoon as the next wave of moisture moves northward from
southern Virginia. Rain will fight low- level dry air as apparent in
the 12z iad sounding so intensity will likely weaken some.



Previous discussion...

Cutoff low pressure over the southeastern Continental U.S. This morning
will slowly drift northeast into the Carolinas this afternoon
through tonight. High pressure remains over New England.

A northeast flow between these systems will continue to usher in
unusually cool conditions for this time of year. At the same
time...warm and moist air rotating around the low will overrun
the surface cooler air in place resulting in plenty of clouds
along with periods of rain.

The steadiest rain will be over the Potomac Highlands into
central Virginia through early this afternoon...where forcing
will be stronger closer to the cutoff low and overrunning will
be deeper. Farther north across the Washington and Baltimore
metropolitan areas into northern Maryland and Easter West
Virginia...rain will be light and more intermittent as it fights
dry air advection at the low-levels. However...as the low tracks
a bit farther north and east later today through tonight.
Overrunning will deepen across all locations and rain is
expected to become more widespread. Rainfall amounts across
central Virginia into the Potomac Highlands will be around one-
half to one inch today through tonight. Rainfall amounts around
one-quarter to one-half inch are expected across most other
locations.

&&

Short term /Tuesday through Wednesday night/...
low pressure will remain nearly stationary Tuesday over the
Carolinas before slowly moving off to the north and east Tuesday
night through Wednesday. A soaking rain is expected across most
locations Tuesday along with plenty of clouds. Tuesday will be a
bit milder compared to Monday...due to more of an easterly flow
instead of northeast but still well below climo. Max temps will
hold in the 50s for most areas with lower to middle 60s across
the Virginia Piedmont into southern Maryland. Rain will
gradually diminish in coverage Tuesday night through midday
Wednesday as the low weakens and slowly moves away. Wednesday
will turn out milder as weak high pressure builds overhead.
Clouds should give way to sunshine across the Potomac Highlands
and central Shenandoah Valley. Elsewhere...some breaks of
sunshine are possible later in the day but clouds will be
stubborn to break since they will be trapped underneath a strong
subsidence inversion. This will impact temperatures...and this
leads to high uncertainty. Tweaked Max temps down a few degrees
across central and eastern portions of the County Warning Area. If clouds hang
on all day...then Max temps will hold in the lower to middle
60s but if clouds break sooner then Max temps will be well into
the 70s.

The low will track well off to our north and east Wednesday
night and weak high pressure will build overhead. Areas of fog
are possible due to saturated soils and light winds.

&&

Long term /Thursday through Sunday/...
cold front forecast to be draped from Michigan southward to
Mississippi/Alabama by 12z Thursday...with blocking high over
the Atlantic. Front will try to push into the area...but will
likely stall out nearby...as it struggles to make progress into
the area due to the aforementioned blocking high. Front expected
to remain nearby through the weekend...with a persistent flow of
warm/moist air advecting into the region. This will allow for
warm/humid conditions to develop...with dewpoints climbing into
the M/u60s by the weekend and temperatures in the M/u80s.
Increasing instability will promote thunderstorm
activity...coverage initially appears to be isolated or widely
scattered for the end of the week...as capping inversion and
weak forcing reside over the area.

Juicy airmass remains through early next week...with diurnally-
driven thunderstorms chances remaining until moisture is scoured
out with next frontal passage. Continued moistening will lead
to moderate/strong instability over the weekend...with strong
storms possible. Front likely focus for development...so will
need monitor spatiotemporal forecast trends of front over the
next several days.

&&

Aviation /16z Monday through Friday/...
low pressure will track toward the Carolinas today and remain
nearly stationary through Tuesday before slowly moving away from
the area Tuesday night through Wednesday. Warm and moist air
will overrun cooler air near the surface...resulting in low
clouds along with periods of rain. Timing of the lower cigs
remains uncertain because drier air is advecting in at the lower
levels. For now...it looks like best chance of MVFR through
mid-morning will remain across kcho with VFR elsewhere.

Most guidance continues to hint at low-level drier air advecting
into the area through most of this afternoon. Therefore...VFR
conditions may persist for most of the terminals. Did allow for
MVFR conditions for kcho but even there confidence is low.
Intermittent light rain is expected during this time.

Overrunning will deepen late this afternoon through Tuesday and
this will increase rainfall coverage and lower cigs to IFR
levels. Low clouds will persist through at least Wednesday
morning before possibly breaking up Wednesday afternoon. Areas
of fog are possible Wednesday night due to saturated
soils and light winds as high pressure builds overhead.

VFR conditions Thursday. VFR should prevail Friday...though,
isolated to widely scattered thunderstorms possible (especially
during the afternoon/evening).

&&

Marine...
low pressure will track across the southeastern Continental U.S. This
morning before becoming nearly stationary over the Carolinas
this afternoon through Tuesday. The pressure gradient between
the low and the high will strengthen during this time. A Small
Craft Advisory is in effect for the waters. A Gale Warning may
be needed for middle portions of the Maryland Chesapeake Bay and
the lower tidal Potomac River Tuesday. The strongest winds will
be Tuesday morning through midday. The low will weaken Tuesday
night and slowly move away from the area Wednesday. A Small
Craft Advisory may need to be extended into Tuesday night for
portions of the waters. Winds will remain below Small Craft Advisory criteria
Wednesday through Wednesday night as weak high pressure builds
overhead.

Increasing southerly flow Thursday with wind gusts approaching Small Craft Advisory
criteria. Winds decrease some Friday...though isolated to widely
scattered thunderstorms possible (especially during the
afternoon/evening).

&&

Tides/coastal flooding...
update...
anamolies remain elevated this afternoon and minor coastal
flooding is expected for St. Mary's County this afternoon.
Advisory may need to be extended for the next high tide tonight.

Easterly flow around low pressure to the south will Foster
persistent onshore flow over the waters through Tuesday...which
will keep tidal anamolies elevated. Onshore flow will
strengthen a bit today through Tuesday...with minor flooding
possible near times of high tide. Though, it currently appears
that all sites will remain below minor tidal flood stage for the
next high tide cycle (still need to watch straits). Better
chance for minor tidal flooding this evening into
Tuesday...especially at the most sensitive sites (annapolis,
straits, and dc). The flow should gradually turn north Tuesday
night into Wednesday as the low moves away from the area. Water
levels should decrease during this time.

&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...coastal flood advisory until 3 PM EDT this afternoon for
mdz017.
Virginia...none.
WV...none.
Marine...Small Craft Advisory until 4 am EDT Wednesday for anz530>534-
537>543.
Small Craft Advisory until 6 PM EDT Tuesday for anz535-536.

&&

$$
Synopsis...bjl
near term...bjl/hsk
short term...bjl

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