Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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024
FXUS61 KPHI 242221
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
621 PM EDT Wed May 24 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 THURSDAY MORNING/...
Quiet conditions early this evening. Will make minor
adjustments to hourly grids this evening based on current
surface obs. Clouds will continue to increase across the region
this evening as onshore flow draws marine stratus inland while
mid-level clouds move in ahead of the upstream low.

The boundary to our south will move northward as a warm front
tonight, likely reaching the lower Delmarva late tonight.
Enhanced isentropic lift along and to the north of the front is
expected to yield an organized band of showers that arrive in
our far southern zones (southern DE/eastern MD) around midnight
before moving northward through the rest of the forecast area
overnight. PoPs are categorical for a several hours with
widespread showers likely late tonight and early Thursday
morning. Given strong lift and increasing low-level moisture,
there may be enough elevated instability associated with a
pronounced warm nose south of the 850 mb front to see a few
embedded thunderstorms in our Delmarva zones toward the early
morning. Not anticipating much of a flooding risk tonight given
QPF below one inch thru daybreak.

Forecast low temperatures range from near 50F in the southern
Poconos to the upper 50s in eastern MD and southern DE.

An onshore wind may become breezy late tonight as the pressure
gradient tightens up briefly on the north side of the
approaching warm front. Gusts of 20 to 25 mph are possible.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
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 /THURSDAY NIGHT 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 /22Z WEDNESDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Clouds are increasing across the region but CIGs are currently
VFR early this evening. CIGs lower to MVFR from SE to NW as the
evening progresses with marine stratus rolling in. 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.

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 this
evening and 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 until midnight EDT tonight for
     NJZ012>014-020>027.
     Coastal Flood Advisory from 8 PM Thursday to 2 AM EDT Friday
     for NJZ012>014-020>027.
     Coastal Flood Advisory until 1 AM EDT Thursday for NJZ016.
DE...Coastal Flood Advisory until midnight EDT tonight for
     DEZ002>004.
     Coastal Flood Advisory from 8 PM Thursday to 2 AM EDT Friday
     for DEZ002>004.
     Coastal Flood Advisory until 1 AM EDT Thursday for DEZ001.
MD...None.
MARINE...Small Craft Advisory until 6 AM EDT Friday for ANZ450>455.

&&

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



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