Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 262013
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
413 PM EDT Fri May 26 2017

.SYNOPSIS...
Weak high pressure will briefly affect the area tonight, before
building to our northeast over the weekend. A weak low is
forecast to move along a developing stationary front to our
south Saturday into Saturday night. Another area of low pressure
is forecast to combine with a larger low across southern Canada
as it moves into the eastern Great Lakes Sunday into Sunday
night. An occluded frontal system is expected to affect the area
around Monday. A series of cold fronts or surface troughs are
forecast to move across the area Tuesday and Wednesday.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
Light showers have developed this afternoon in eastern Pennsylvania
and New Jersey in advance of a northwest-to-southeast oriented vort
max swinging through the Mid-Atlantic region. With the combination
of sufficient surface heating and subtle convergence along a surface
trough moving through the area today, showers should continue
through late afternoon before quickly dissipating near sunset. A
tight pressure gradient to the southwest of a surface low off the
coast of New England has also enabled fairly strong mixing today,
which has allowed for the downward transport of momentum to the
surface. Gusts of 20 to 30 kts have occurred in much of the area
this afternoon. Along with the shower coverage, wind gusts should
diminish quickly near/after sunset.

As the vort lobe moves east of the area overnight, transient
subsidence upstream of the shortwave trough will allow for clearing
skies. With the lowering winds, temperatures should cool fairly
readily overnight. However, a stronger vorticity maximum will be
approaching from the west late tonight. Increasing cloudiness should
begin to occur near daybreak, which adds a little uncertainty to the
temperature forecast. MOS has been relatively consistent the past
few cycles, however, so stuck close to a consensus blend overnight.
Generally, lows are expected to be near 60 in the urban corridor,
with lower 50s in the Poconos and northwest New Jersey and the lower
60s in southern Delmarva.

There is some chance of some radiation fog developing in the favored
locations in the Poconos/Lehigh Valley, but increasing cloud cover
late may prevent this from occurring. For now, left this chance out
of the grids, but this will be something to monitor overnight.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
The aforementioned vort max upstream will move through the region on
Saturday. Models have sped up the timing of this system by quite a
bit, generally on the order of 6 hours. Most operational guidance
and some of the longer-range high-resolution simulations indicate
precipitation moving into the area by tomorrow morning. Increased
PoPs for the morning hours based on this trend.

Cyclogenesis will occur east of the Appalachians (probably in
Virginia) during the day, with a baroclinic zone rapidly developing
to the east of the low. As a result, the warm sector will likely be
south of the area as the zonally-oriented surface boundary will
remain to the south. Not overly excited about thunder chances in
this regime, but there are at least some indications that elevated
instability will exist south of the Mason-Dixon Line as large-scale
lift cools the midlevels atop the near-surface stable layer. Kept
thunder chances during the afternoon hours for this area as a
result.

The vort max will be in the process of being sheared out in the
confluent flow upstream of the upper low in the northwestern
Atlantic. This will result in relatively weak (and weakening) lift
as the system moves through. Thus, precipitation will generally be
light, and given that the warm sector is expected to be south of the
CWA, not expecting more robust thunderstorm development. However, if
the warm sector sneaks into the southern CWA, then a strong storm or
two is possible with the vertical shear being favorable for storm
organization. This is considered to be a very low probability at
this point.

Temperature forecast is tricky with the approaching system and
complications from cloud cover. MOS has come in colder tomorrow (by
about 3-7 degrees), and have generally followed, though I did tweak
temperatures a bit using higher-resolution guidance, which may have
a better handle on how any precipitation affects proximity ambient
temps. Forecast highs range from the low-mid 60s in the Poconos to
the mid 70s in southern Delmarva - with lower than average
confidence given the aforementioned complications.

&&

.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
An unsettled weather pattern continues for much of the extended
forecast with several periods of rain possible.

On Saturday night, the area of low pressure will be passing to
our south and southeast as it moves eastward along the the
frontal boundary to our south. Any showers associated with this
low are expected to pull away from the area during the evening
hours, with the remainder of the night likely staying dry.

High pressure is forecast to build across the northeast
Saturday night into Sunday, which would keep the frontal
boundary to our south and keep an east southeasterly flow across
the area. This easterly flow would keep cloudy and cool
conditions.

On Sunday night, the front to our south is forecast to begin
lifting north toward the area, while an occluding frontal
boundary approaches from the west as low pressure moves through
the Great Lakes Region and combines with a larger low across
southern Canada. The occluded front may not actually move into
the area until Monday or Monday night, but the chance for rain
will begin to increase from the west overnight Sunday into
Monday.

A series of cold fronts or surface troughs are forecast to move
across the area basically each day from Tuesday through Friday
as low pressure slowly drifts eastward across southern and
eastern Canada. There will be a chance showers or thunderstorms
each day, which will be focused around the time of any short
wave/vorticity impulse passages.

High pressure tries to build back into the area for Thursday, which
would provide a dry forecast.

&&

.AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

BKN CIGs are hovering around 3500-6000 feet this afternoon and
should continue to rise slowly before scattering out early this
evening. Shower coverage is spotty, so any expected impacts this
afternoon at the terminal should be minimal at best. CIGs return and
lower steadily tomorrow morning, with chances of showers increasing
through the day. Periods of MVFR/IFR conditions possible in
proximity to showers. Stray lightning strikes are possible in the
afternoon, but chances appear low at this time. West winds 10 to 15
kts with gusts 20 to 25 kts decreasing below 10 kts this evening.
Winds should remain light tomorrow with primarily a southwesterly or
southerly direction.

OUTLOOK...

Saturday night...MVFR to IFR CIGS possible. Chance of scattered
showers in the evening.

Sunday...MVFR to IFR CIGS in the morning, improving during the day.
Isolated showers during the day.

Sunday night...MVFR to IFR conditions possible with periods of low
clouds and rain.

Monday...MVFR to IFR conditions forecast to continue through the
day, but some improvement by afternoon. Scattered showers and
thunderstorms possible during the day.

Monday night...Conditions may lower to MVFR or IFR with low clouds
and fog.

Tuesday-Wednesday...Generally improving to VFR during the day
Tuesday and continuing through Wednesday. Chance of showers or
thunderstorms on Tuesday and Wednesday afternoon/evening.

&&

.MARINE...
Small craft advisory remains in effect through 6 pm for the Atlantic
waters of southern New Jersey and Delaware and through 10 pm for the
Atlantic waters of northern/central New Jersey, mainly for high
seas. Significant wave heights have varied considerably this
afternoon, generally ranging from 4 to 6 feet. Wave guidance
continues to show a downward trend this evening, but given the
variability in the buoy observations, not confident enough to change
the expiration times or to cancel the advisory early.

Winds should remain below advisory criteria through tomorrow, with
directions primarily westerly tonight. Directions should switch to
the southeast quadrant by tomorrow afternoon. A chance of showers
(with a slight chance of thunderstorms) exists tomorrow, especially
during the afternoon.

RIP CURRENTS...

Rough seas continue this afternoon and evening on the Atlantic
coastal waters, where a moderate risk of rip currents exists.
However, seas will lower tonight, and winds should generally be
around 10 kts or less through most of the day tomorrow. Current
expectation is for the risk of rip currents to be in the low
category for tomorrow. However, steadier east winds will develop
late in the day and continue through Sunday, so the risk of rip
currents may approach the moderate category later this weekend.

OUTLOOK...

Saturday night-Wednesday...Conditions expected to remain below
advisory levels, although winds may gust around 20 knots at times.

&&

.TIDES/COASTAL FLOODING...
Astronomical tides continue to run high, and despite an offshore
flow, tidal departures remain about 0.5 to 1.0 feet above
astronomical tide. Therefore, minor coastal flooding is expected
to occur on the coastal areas of New Jersey and Delaware and
areas along Delaware Bay.

As we go into the weekend, winds are expected to become onshore
again, which could keep tidal departures elevated, possibly
leading to additional coastal flooding with the evening high
tides Saturday and Sunday, especially on Saturday.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...Coastal Flood Advisory from 8 PM this evening to 2 AM EDT
     Saturday for NJZ012>014-020>027.
DE...Coastal Flood Advisory from 8 PM this evening to 2 AM EDT
     Saturday for DEZ002>004.
MD...None.
MARINE...Small Craft Advisory until 6 PM EDT this evening for
     ANZ452>455.
     Small Craft Advisory until midnight EDT tonight for ANZ450-451.

&&

$$
Synopsis...Robertson
Near Term...CMS
Short Term...CMS
Long Term...Robertson
Aviation...CMS/Robertson
Marine...CMS/Robertson
Tides/Coastal Flooding...


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