Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 250458
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1258 AM EDT Thu May 25 2017

.SYNOPSIS...
As an area of low pressure moves into the Ohio Valley tonight
into Thursday, an occluded front and warm front will lift toward
our region. A triple point low will develop and take over as
the dominant low as it moves across our area Thursday night.
This low will move away from the area Friday, with a weak
frontal boundary or surface trough crossing the area during the
day. Weak high pressure may briefly move across the area Friday
night into early Saturday. A weak low may move along a
developing warm front to our south Saturday into Saturday night.
An occluded frontal system is expected to affect the area
Sunday into Monday, followed by another frontal boundary on
Tuesday.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
We have made some adjustments to the timing of the initial band
of rain associated with the approaching warm front. The leading
edge of the rain was into eastern Maryland and southern
Delaware around midnight. It should reach a Reading-
Philadelphia-Atlantic City line around 2:00 AM and it is
expected to arrive in the Poconos and far northern New Jersey
around 4:00 AM.

The rain is forecast to become moderate and locally heavy at
times. We are anticipating maybe about 0.40 to 0.70 inches
through 8:00 AM in much of southeastern Pennsylvania, southern
New Jersey, Delaware and northeastern Maryland. Amounts in the
Lehigh Valley, the Poconos, and central and northern New Jersey
should range from around 0.10 to 0.40 inches.

The wind is forecast to remain out of the east around 8 to 14
MPH. The wind direction may begin to veer toward the southeast
in parts of northeastern Maryland and Delaware around daybreak
with the arrival of the warm front.

The onshore flow is anticipated to keep temperatures mainly in
the 50s for the balance of the night.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TODAY/...
Low pressure is expected to slowly lift northward through the
Midwest/Ohio Valley on Thursday. Meanwhile, its attendant warm
front will also move northward through the Delmarva region
before stalling in eastern PA-NJ (likely near the Philadelphia
metro). Models are indicating the development of a secondary low
during the afternoon on the lee side of the Blue Ridge
Mountains. This low would then approach the forecast area late
in the afternoon.

PoPs are highest initially in the morning hours when the
southeasterly low-level jet interacts with the approaching warm
front. This band of steady precip will eventually lift north of
the region late in the morning. Coverage of showers will
generally be more widely scattered during the afternoon.

Breaks in the cloud cover will promote strong heating south of
the warm front across Delmarva and perhaps southeastern
PA/southern NJ. The environment will be conducive for
thunderstorms south of the warm front later in the afternoon as
the boundary-layer destabilizes and strong lift arrives ahead of
the negatively-tilted trough. A few storms in these southern
zones could potentially become severe with damaging winds being
the primary threats. These storms will also pose a risk for
localized heavy rainfall that could lead to urban/poor drainage
flooding.

&&

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

By Thursday night, the triple point low will begin moving
across the area, pulling an occluded frontal system across the
area as well. Showers will likely be ongoing during the evening
and continue into the overnight hours. By Friday morning, the
low will continue to move to our northeast and offshore of New
England, and showers associated with the low will move to our
northeast as well. However, a weak frontal boundary or surface
trough is forecast to move across the area during the day. With
help from the trough aloft and any short wave/vorticity
impulses, there will be another chance of scattered showers
during the afternoon.

Friday night through most of Saturday is expected to be dry as
weak high pressure quickly moves across the area. However, a
warm front is expected to develop to the south of the area,
while a weak area of low pressure moves along this boundary
during the day. Also, a short wave/vorticity impulse may move
across the area during the day, so there will be a chance of an
isolated shower during the day Saturday.

On Saturday night, the warm front to the south will begin
lifting northward, but not likely make its way into the area
until Sunday. A triple point may form near the area by Sunday
night as well, which could slow the progression of the frontal
system as it begins to occlude. The exact timing of these
features is still a little uncertain, but Sunday into Sunday
night look to have the best chance of showers, and possible
thunderstorms if enough instability builds on Sunday.

The actual cold front is forecast to move across the area
Monday, with another cold front possible on Tuesday. This could
lead to additional showers or thunderstorms, especially during
the day Monday and Tuesday.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

CIGs will continue to lower overnight. Rain is expected to
arrive from S to N between 05Z and 08Z. Expect IFR conditions
shortly after the rain arrives with CIGs dropping below 1 kft.

The steadier rain ends from S to N between approximately 11Z
and 14Z Thursday morning. However, CIGs may lower to LIFR during
the morning hours with the probability of an extended period of
LIFR higher for northern terminals (TTN-ABE-RDG). Farther
south, CIGs will try to slowly improve as a warm front lift
northward.

E-SE winds this afternoon around 10 kt with occasional gusts
15-20 kt thru sunset. Winds look to become more easterly tonight
with speeds increasing to 10-15 kt and gusts 20-25 kt late
tonight and early Thursday morning. A wind shift out of the S-SE
is possible during the afternoon from about PHL southward
assuming a warm front makes it this far north.

OUTLOOK...

Thursday night...MVFR to IFR conditions possible with low
clouds and fog/drizzle possible.

Friday-Friday night...Improving to VFR during the day and into
the night, scattered showers possible during the daytime. Gusty
northwest winds 20-25 knots.

Saturday...Generally VFR. Showers moving into the area later in
the day, which will lead to lowering ceilings.

Saturday night-VFR conditions early, possibly lowering
overnight.

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

Monday...Generally improving to VFR during the morning.
Scattered showers and thunderstorms possible during the day.

&&

.MARINE...
SCA in effect for the coastal waters for tonight and Thursday.
Although E-NE winds are generally 10-20 kt this afternoon, seas
have been building to around 5 ft at our offshore buoys. Winds
may reach SCA criteria of 25 kt early Thursday morning when the
pressure gradient increases ahead of a warm front. Seas will
remain in the 5-6 ft range through the day on Thursday.

There is a moderate risk for the development of rip currents at
ocean beaches on Thursday.

OUTLOOK...

Thursday night...Small Craft Advisory in effect.

Friday...Winds may drop below advisory levels, but seas may
remain above 5 feet into Friday.

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

&&

.TIDES/COASTAL FLOODING...
Some of the highest astronomical tides of the year are occuring
through the end of the week. As a result of that and onshore
flow, minor coastal flooding is expected with the high tide
again on Thursday evening. One source of guidance shows water
levels along the northern NJ shore approaching moderate flooding
thresholds with the Thursday evening high tide, but this seems
uncertain at this time. Minor coastal flooding may again be
possible with the Friday evening high tide.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT
     Friday for NJZ012>014-020>027.
DE...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT
     Friday for DEZ002>004.
MD...None.
MARINE...Small Craft Advisory until 6 AM EDT Friday for ANZ450>455.

&&

$$

Synopsis...Robertson
Near Term...Iovino
Short Term...Klein
Long Term...Robertson
Aviation...Klein/Robertson/MPS
Marine...Klein/Robertson/MPS
Tides/Coastal Flooding...Staff


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