Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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264
FXUS61 KPHI 252155
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
555 PM EDT Tue Apr 25 2017

.SYNOPSIS...
Low pressure will track northeast just off the Delmarva and New
Jersey coastline through Wednesday. Meanwhile, low pressure will
move from the upper Mississippi Valley into James Bay, and send a
weak cold front into our area Thursday night and Friday morning.
A backdoor front is expected to move through the region Saturday
evening. Another low pressure system moving from the lower Plains
into Quebec and its associated fronts will affect the area Monday
into Tuesday.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
The water vapor imagery early this evening shows a well defined
closed low centered near the coast of North Carolina. An area of
significant dry mid level air is working northward on its eastern
flank. An area of heavier rain is approaching our coast and this
appears to be with a strong short wave rotating northward on the
eastern side of the closed low. This batch of heavier rain will move
onshore this evening, although it may weaken some as it gets farther
inland. Some lightning has been occurring in this area, and a
mesoscale analysis shows an axis of mainly elevated instability
extending northwestward across our south/southeast zones. However,
the more significant instability remains offshore. As a result, a
few isolated thunderstorms cannot be ruled out for these areas this
evening. Otherwise, only changes made were to adjust the hourly
temperature, dew point and wind grids to keep them current. Some
tweaks were made to the PoPs this evening based on radar trends and
some assistance from the HRRR guidance.

Otherwise, surface low pressure centered over eastern North Carolina
will continue to slowly lift to the north and and east along the
East Coast this evening, and should be just east of the Delmarva
Peninsula by daybreak Wednesday.

The rain tapers off between 00-03Z over the Delmarva, southern NJ,
and SE PA, and then between 06-09Z over northern and central NJ and
the Poconos. Some lingering showers will be possible throughout the
region overnight, but additional rainfall will be light.

Once the main rain ends, the pressure gradient weakens as the low
lifts to the north. Winds take on more of a northerly direction, and
some cooler air will filter into the region. With abundant low-level
moisture in place, stratus will occur along with some fog. Not quite
certain that dense fog will develop, but there may be some patchy
areas where the visibility drops to one quarter of a mile. As of
now, do not think a Dense Fog Advisory would be needed.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
Low pressure off the NJ coast will continue to drift to the north on
Wednesday. Upper level low will be just to the west of it, and with
several strong shortwaves passing through the region, can expect
some showers throughout the day. Otherwise, fog gradually dissipates
through the morning, but clouds will remain across the region for
most of the day. Winds back a bit to the N/NW by afternoon, but the
gradient will be light, and winds will remain less than 10 MPH.
Highs generally top off in the 60s, but a few spots may come close
to 70.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
Overall - Warm pattern shaping up. The closed low that is
currently over the southeast US finally exits the area early
Wednesday night. With the polar jet displaced far to the north,
several disturbances traversing the southern stream jet will aid
in building a stout sub tropical ridge off the southeast US
coast from Friday into the early part of next week. As another
closed low ejects out of the midwest on Monday, a stronger and
more active cold frontal passage occurs in the late Monday into
early Tuesday period.

Wednesday night into Thursday should feature dry weather under brief
ridging aloft. With some partial clearing Wed night, light and
variable winds within the col between weather systems, and
abundant low-level moisture given recent rainfall, patchy fog is
likely. The fog will burn off early Thursday, with partly sunny
skies expected. A light southeast flow will maintain cooler
temperatures at coastal locals. Highs will be around 10 degrees
above normal.

A weak cold frontal passage Thursday night into Friday will lead to
considerable cloudiness, but moisture will be limited, with only a
slight chance of showers across the region. In fact, with the strong
subtropical high building over the southeast/orientation of the mid-
level flow, this front is expected to washout/stall in the
vicinity of the I-95 corridor. With deeper moisture and mid-
level support exiting the region, expect a return to partly to
mostly sunny skies by Friday afternoon. A weak synoptic flow
should encourage a robust sea breeze. Highs will be several
degrees warmer than Thursday, around 15 degrees above normal,
but far from daily record highs.

Clouds increase again Friday night into Saturday with another cold
front moving toward the region. There is considerable uncertainty
regarding the timing of this front, with model solutions varying
from Saturday morning to Saturday night, with a backdoor component.
In addition, there is also uncertainty regarding the precip
potential, with quite a bit of spread amongst the model
solutions. For now, we maintained chance PoPs, including thunder
on Saturday. Kept thunder out of the forecast for Sunday given
more stable maritime airmass, except Delmarva, with closer
proximity to the frontal boundary. Still some uncertainty with
high temperatures on Saturday, which will be dependent on the
location of the front, but expect them to be similar to Friday.

There is model agreement that the front will be just to our south-
west on Sunday, with the area under the influence of a northeast low-
level flow. This would favor a stratocumulus deck and perhaps some
sprinkles, mainly across the higher terrain. Kept thunder out of the
forecast for Sunday given stable maritime airmass, except Delmarva,
with closer proximity to the frontal boundary. High temps still be
several degrees above normal.

Moving into early next week, more showers and thunderstorms likely
Monday and Monday night with the next cold frontal passage. Expect a
return to drier weather on Tuesday. Continued above normal
temps.

&&

.AVIATION /22Z TUESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Tonight...IFR conditions this evening with periods of rain tapering
off to some showers from south to north later this evening.
Temporary LIFR conditions possible in heavier bands of rain during
this time, and an isolated thunderstorm may near ACY/MIV through
early this evening. Once the rain ends, stratus will continue along
with some fog, and IFR/LIFR conditions are expected. East-northeast
winds 15-20 knots with gusts up to 30 knots, diminishing to 10 knots
or less later this evening and overnight.

Wednesday...IFR/LIFR in the morning due to low clouds and some fog,
then conditions will be slow to improve to IFR/MVFR after about 15Z.
Scattered showers are possible especially through midday. Northeast
to north winds mainly 5-10 knots.

OUTLOOK...
Potential for fog leading to visibility restrictions Wed night
and MVFR ceilings persisting into Thursday. Predominantly VFR
Thursday night into Saturday. MVFR possible in low clouds Sat
night into Sunday.

&&

.MARINE...
Just a gale force gust or two across the northern waters but this
should be short-lived and therefore the Gale Warning has been
cancelled and changed to a Small Craft Advisory. On the ocean zones,
the advisory goes through Wednesday night given the elevated seas
that are forecast. On Delaware Bay, opted to go with a Small Craft
Advisory through 02z tonight as some gusts have been close to 25
knots thus far. The winds are expected to diminish from south to
north tonight.

Areas of fog should develop tonight, and localized VSBY less than
1NM is possible. Fog gradually dissipates Wednesday morning.

OUTLOOK...
SCA-level seas likely persist through Friday with a return to
sub-SCA conditions thereafter.

&&

.TIDES/COASTAL FLOODING...
The Coastal Flood Advisory remains in effect for coastal areas of
New Jersey and Delaware, including Delaware Bay, and for the far
lower part of the Delaware River.

Low pressure located over eastern North Carolina this afternoon will
drift northeastward reaching the waters off Delaware and New Jersey
on Wednesday. The tight gradient and resulting brisk onshore flow
will remain along the coasts of Delaware and New Jersey into this
evening before wind speeds begin to diminish.

The surge around this morning`s high tide ranged from +0.5 to +1.0
foot along the Delaware and New Jersey coasts. Based on the trends
this afternoon and on the forecast guidance it appears as though the
surge will be mostly in the +1.0 to +1.5 foot range for this
evening`s high tide. The surge is expected to result in widespread
minor flood in coastal areas of Delaware and New Jersey and along
much of Delaware Bay. Localized minor flooding is anticipated on the
far lower part of the Delaware River.

It appears as though the tidal Delaware River above the Commodore
Barry Bridge area may just reach the minor flooding threshold.
However, the impacts should not be widespread enough there to
warrant a Coastal Flood Advisory.

We are not anticipating any coastal flooding along the upper eastern
shore of Chesapeake Bay.

The diminishing wind overnight along with the lower astronomical
tides on Wednesday morning should preclude another round of minor
flooding with Wednesday morning`s high tide.  We will keep an eye on
the higher astronomical tides on Wednesday evening. If the water
does not drain away from the coast fast enough, we could see some
localized minor flooding at that time.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...Coastal Flood Advisory until midnight EDT tonight for
     NJZ012>014-020>027.
     Coastal Flood Advisory from 9 PM this evening to 1 AM EDT
     Wednesday for NJZ016.
DE...Coastal Flood Advisory until midnight EDT tonight for
     DEZ002>004.
     Coastal Flood Advisory from 9 PM this evening to 1 AM EDT
     Wednesday for DEZ001.
MD...None.
MARINE...Small Craft Advisory until 6 AM EDT Thursday for ANZ450>455.
     Small Craft Advisory until 10 PM EDT this evening for ANZ430-
     431.

&&

$$
Synopsis...Franck
Near Term...Gorse/MPS
Short Term...MPS
Long Term...Franck/MPS
Aviation...Franck/Gorse/MPS
Marine...Franck/Gorse/MPS
Tides/Coastal Flooding...Iovino



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