Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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641
FXUS61 KPHI 291624
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1224 PM EDT Sat Apr 29 2017

.SYNOPSIS...
A cold front arrives early this evening and is expected to
settle southward tonight before stalling over Delmarva and south
Jersey. The front is expected to return north as a warm front
Sunday night. Another cold front is expected Monday night as an
area of low pressure moves into the Great Lakes region. This low
will drift northeastward through eastern Canada through mid
week. Another low pressure system will then track towards the
Mid Atlantic by late in the week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Incoming showers from the west have dissipated with less instability
farther east. An analysis shows the most instability currently
across portions of Delmarva with this dropping off quite a bit
with a northward extent.

There is quite a bit of uncertainty regarding additional convective
development through the afternoon as earlier activity pushed
some outflow southward. The main dew point front is farther
northwest and our region may now be within a zone of subsidence
following the earlier convection. Additional convection is
tracking across Ohio however this appears to be tracking more
northeastward. The low-level jet is forecast to weaken over our
area through the afternoon along with a mid level ridge building
some. This may be enough to limit convective development.
However, there remains a chance across the southern areas mainly
and therefore we adjusted the PoPs accordingly. Given the
uncertainty, kept PoPs no higher than chance. Across the
northern areas, it may remain dry. If anything of significance
develops, we may have to wait closer to early evening as
convection may develop in the Washington DC and Baltimore areas.
Given the building instability across the Delmarva area especially,
there remains a chance for a strong/severe thunderstorm or two
there late this afternoon.

Otherwise, the convective cloud debris is gradually thinning out
leading to more sunshine. This is allowing the temperatures to
start increasing faster than earlier. A very warm/hot afternoon
is expected with humid conditions for most, although some areas
should have the dew point mix out a bit. The flow is turning
more westerly and therefore enough wind should keep the coastal
areas warmer overall. The 12z Sterling, VA RAOB showed a temperature
of +22C in about the 960-900 mb layer. Some locations look to
be within a couple of degrees of record highs (GED is forecast
to reach a record).

The hourly temperature, dew point and wind grids were adjusted based
on the latest observations, then the LAMP/LAV guidance was blended
in for the next few hours. The sky cover was adjusted based on
satellite trends.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/...
The backdoor cold front will continue to push through the area this
evening and overnight. The round of showers and thunderstorms
mentioned for the late afternoon and evening will slide east and
move offshore during the evening hours as the short
wave/vorticity impulse associated with it moves to our east as
well.

As the front sinks to our south, a northeast-east flow will develop
across the area. There could be some light fog develop overnight,
but we do not expect much if any dense fog at this time as winds
will remain around 5-10 mph through the night and dewpoints are
expected to drop enough behind the front that RH values do not reach
high enough for widespread fog for most places.

With the front near the area, it is possible that some
isolated/scattered showers may develop overnight through daybreak
Sunday north of the frontal boundary.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Sunday...there is some question on how far south the front will
reach before stalling. The general trend with the 00Z models was
to bring the front further south (possibly entirely south of our
forecast area). Where the front stalls will have implications
for both high temperatures Sunday as well as precip chances. As
mentioned by the previous shift, the GFS appears to be
overestimating the precip north of the front given relatively
dry air in the cool sector. Therefore, focused the PoPs through
this period on Delmarva closer to the front. The front is
expected to return north as a warm front either late in the day
tomorrow or tomorrow night.

Monday...With the cold front still over the Ohio Valley, expect
a mostly sunny and dry day. A brisk south- west flow will
develop, with wind gusts up to around 30 MPH in the afternoon.
Temperatures will be well above normal, with highs in the low
80s across the urban corridor. With the cold frontal passage
Monday night, showers and thunderstorms are likely. While the
timing is not ideal for severe weather and instability is
limited, the wind field is strong and Precipitable Water values
approach 2.0 inches, so gusty winds and heavy downpours are
likely with any thunderstorms, the primary focus being northwest
of the NJ Turnpike/I-95.

Tuesday and Wednesday...in the wake of the cold front,
temperatures will be about 5 to 10 degrees lower, with highs
generally in the 60s and 70s. Models have trended faster with
the dry air advection behind the front, so I trimmed back on
the PoPs on Tuesday. It is looking mostly dry both days.

Unsettled weather during the Thursday and Friday time frame. A broad
trough to our west will maintain a southwest flow aloft, along with
moisture of Gulf of Mexico origins moving up the eastern
seaboard, within an overrunning regime. Heavy rainfall is
certainly possible, with WPC guidance indicating the potential
for 1-2 inches across our region, but there is still inherent
uncertainty given it`s Day 6-7. Stay tuned.

&&

.AVIATION /16Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

This Afternoon...VFR with clouds at or above 5000 feet. There
is a chance for a few showers and thunderstorms to develop
mainly late this afternoon mostly south and west of PHL. The
coverage and timing of showers/thunderstorms is low. West-
southwest winds around 10 knots, with some local gusts to 20
knots at times.

Tonight...A few showers or thunderstorms possible mainly in the
evening especially near and south and east of PHL with locally
reduced ceilings/visibilities, otherwise VFR. However, a cold
front settles southward and the winds will turn to the northeast
allowing for cooler air to move. This may result in MVFR
ceilings developing toward morning. Northwest winds mostly 10
knots or less this evening, becoming north and northeast.

OUTLOOK...
Sunday...Mostly VFR conditions expected.

Sunday night...MVFR or IFR conditions are possible with low clouds
and fog.

Monday...Mostly VFR conditions are expected although MVFR conditions
are possible with any showers or thunderstorms.

Monday night...Widespread MVFR or IFR conditions with thunderstorms
likely. An abrupt wind shift to westerly is expected with a cold
front Monday night.

Tuesday and Wednesday...Mostly VFR conditions expected.

&&

.MARINE...
Conditions on the coastal waters are expected to remain below Small
Craft Advisory levels through tonight. Even though there are some
strong winds above the surface, there is a strong inversion which
will keep those winds from mixing to the surface. However, winds
could gust around 20 knots and seas reach 4 feet at times through
the day.

There is a chance of showers and thunderstorms across the waters.
Any thunderstorms mainly late today and this evening will contain
frequent lightning and may produce strong gusty winds.

OUTLOOK...
Sunday...winds and seas are expected to be below SCA criteria.

Monday and Monday night...building seas and increasing winds are
expected by Monday morning. Southerly gusts near or above 30 KT
are possible Monday evening.

Tuesday and Wednesday...winds will shift to southwesterly and
then westerly behind a cold front expected early Tuesday. Gusts
on Tuesday may linger near 25 kt. Elevated seas may linger into
early Wednesday.

&&

.CLIMATE...
Record high temps listed today where it is forecast to be
within 2 degrees of record.

PHL 90-1974

GED 88-1990

TTN 88- 1974 & 1888

**Record or number 2 warmest April on record expected**

Presuming our forecast temps these last 2 days of April are
accurate, we are assured of a top 2 warmest April on record in
much of our forecast area. Sunday is probably the critical day
for determining record or not.

Below: April projected within the top April average temps, the
normal for April and the period of record (POR).

This includes Friday`s (28th) high and low temperatures through
4 PM.

Sundays max temps may occur at 1 am and the min temps Sunday
evening.


PHL normal 54.0 POR 1874

59.5 2017 Projected warmest ever: still  could slip .2 degrees.
59.4 1994
58.5 1921
58.4 2010
57.9 1941

ABE normal 49.9 POR 1922

56.9 projecting record and almost cant miss a record.
56.4 1941
54.7 1994

ACY normal 51.7 POR 1874

57.6 projecting record and almost cant miss.
56.3 2010
56.1 2011

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...Gorse/Johnson
Near Term...Gorse
Short Term...Robertson
Long Term...Franck/Johnson
Aviation...Gorse/Johnson
Marine...Johnson/Robertson
Climate...



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