Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 250747
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
347 AM EDT Mon Sep 25 2017

.SYNOPSIS...
High pressure over the Northeast will weaken a bit and lift to
the north and east tonight through Tuesday. Meanwhile, Hurricane
Maria will track to the north and will approach the eastern
Carolinas on Wednesday before a cold front curves it out to sea
on Thursday. Another cold front moves through the region on
Friday. Surface high pressure and an upper trough digs into the
East Coast for next weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Despite a quiet day overall, several minor forecasting
challenges exist today.

The first challenge is the patchy fog that has developed early
this morning. Fog is a bit more widespread compared to the last
few days, given the increased dew points and the midlevel ridge
parked overhead. Have expanded the coverage of fog a little bit
with the morning update. A second related concern is the
advancement of a fog/low stratus deck offshore today. Currently,
this deck is almost 100 miles offshore but has made slow/steady
progress westward overnight. Models are insistent in bringing
this deck close to the coast by late afternoon, but so far, they
have been much too aggressive in the westward progress (despite
the presence of long- duration easterly flow). I slowed the
progress of this cloud/fog deck today, but this may still be too
pessimistic for immediate coastal locations during the
afternoon. This will have some impact on hourly/max
temperatures, which are lower-than-average confidence along the
immediate coast.

Other concern is forecast max temperatures today. Reviewing
yesterday`s forecast, guidance was a solid 3-5 degrees too low
for highs (My numbers last night were only about a degree
better, on average, thus underplaying the record high
potential.), strongly tied to operational models continuing to
struggle with the intensity of the aforementioned midlevel
ridge. This ridge will be pivoting northeastward into New
England today, so heights are expected to lower somewhat.
However, given the strong insolation expected today, a forecast
that accounts for persistence seems appropriate. Though I think
temperatures will be a degree or two cooler than yesterday
overall, forecast highs remain at or above the highest
statistical guidance values.

Record highs look at least reachable, with forecast highs about
1-3 degrees below them, in general. That said, the statistics
of forecasting would suggest that the probability is decently
high that a record or two might be reached.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
Forecast becomes a little more interesting tonight, as onshore
flow to the north of Maria begins to have more of an influence
on sensible weather across the region. The 00Z RGEM is bringing
in light showers to the far southern CWA late tonight (mostly
offshore, but possibly into southern DE). The 00Z NAM Nest is at
least suggestive of this (though far less aggressive), and
there is marginal support from the latest WRF-ARW and WRF-NMM
simulations. Some drizzle or light showers may occur as this
deck moves inland, especially near the coast. Have kept mention
of PoPs to account for this possibility, though any
precipitation amounts should be quite light. Notably, forecast
soundings do not moisten the atmosphere above an inversion
around 975-950 mb quickly, and such a profile strongly favors
drizzle versus showers.

More likely is the increase in low stratus and potentially fog
from east to west through the area tonight. Not completely sold
on the fog potential -- model forecast soundings show an
elevated inversion, which is strongly suggestive of stratus
versus fog. However, the presence of a fog deck offshore now
certainly suggests its possibility, so may need to include
mention of this in the grids in subsequent updates.

Meanwhile, farther west, skies should remain clear for longer,
and this should promote the development of fog as winds relax
and temperatures plummet to surface dew points. Have more
widespread mention of fog in the grids tonight to the west of
the Delaware River (mainly in more rural/valley locations),
though kept coverage as patchy given antecedent dry conditions
and the general overforecasting of fog the past few nights.

Low temps are generally a blend of MAV/MET, with some
incorporation of hi-res guidance to accommodate for the expected
increased cloud cover overnight.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Impacts from Hurricane Maria of high surf, beach erosion, and a
high risk for rip currents are likely for the DE and NJ coasts
for most of the mid-week period. Tropical Storm force winds,
however, should stay south and east of the area.

High pressure weakens a bit and lifts to the north and east on
Tuesday. Hurricane Maria tracks to the north and approaches the
Carolina coast Tuesday through Wednesday. Outer bands of precip
will lift to the north and may move into southern and eastern
portions of the forecast area, but the high looks to be strong
enough to keep most of the outer bands at bay. Will go ahead and
lower PoPs a bit from previous forecast, and will carry slight
chance/low chance PoPs for Tuesday through Wednesday. Northeast
winds during this time will range from 5-10 MPH inland and 10-20
MPH along the coasts. This brings a cooler airmass to the
region. Highs will range from the upper 70s to low 80s along the
coast to the low to mid 80s most areas inland. On the western
side of the Fall Line, highs may even get into the mid and upper
80s.

A cold front passes through the region Wednesday night through
Thursday. Mid-level shortwaves will pass well to the north, so
without much upper level support, not expecting much in the way
of showers. Will cap PoPs at slight chance inland to low chance
along the coasts, mainly due to the onshore influence.

This cold front will help curve Maria out to sea. It also
ushers a much cooler airmass into the region, as highs drop by
almost 10 degrees on Thursday. Highs range from the mid 70s to
around 80, with cooler temps up in the Poconos.

A secondary cold front will pass through the region Friday, and
this ushers a much cooler airmass into the region with below
normal temperatures for the weekend.

Surface high pressure builds into the Eastern U.S., but this
will also be accompanied with an upper trough for the weekend.
This trough may result in some isolated showers, but the surface
high should keep conditions dry for the most part. Highs should
be in the 60s to around 70.

&&

.AVIATION /08Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Patchy fog may occur this morning at KRDG, KABE, and KMIV, but
VSBYs should remain predominantly VFR. Another nearly cloud-free
day is on tap with light northeast or east winds. A stratus
deck may move from east to west slowly over eastern portions of
the area tonight, affecting primarily KACY and KMIV, with
MVFR/IFR conditions possible by or shortly after midnight with
potential to reach the urban corridor near daybreak Tuesday.
Meanwhile, fog should redevelop west of the urban corridor,
especially after midnight, with coverage more widespread and
VSBYs potentially reduced more frequently to MVFR/IFR
categories.

Outlook...

Tuesday and Wedensday...Mostly VFR conditions expected.
Scattered showers are possible, mainly at KACY. These showers
may result in MVFR conditions if they pass over a terminal. NE
winds 5-10 KT during this time, except 15-20 KT with gusts to 25
KT along the coast, including KACY.

Wednesday night and Thursday...A cold front will bring a shift
to northwesterly winds and a slight chance for showers.
Otherwise, mostly VFR conditions are expected.

Friday...Mostly VFR conditions expected.

&&

.MARINE...
A fog bank is lurking 70-100 nautical miles offshore this
morning, though models have been too fast in moving this bank
westward. Nevertheless, expect a slow westward trend today and
tonight (though this should be tempered during the late morning
and afternoon hours). Tonight, low stratus should encompass the
marine waters, and there may be some fog as well with the
accompanying visibility restrictions.

Seas have been overforecast by 1-2 feet from model guidance,
and this is expected to continue today. Thus, small craft
advisory conditions have been slow to materialize so far on the
Atlantic waters. Nevertheless, the trend in seas will be upward,
with long- period (11-15 seconds) 3-6 ft southeasterly swells
emanating from Hurricane Maria making for gradually rougher
marine conditions. Small craft advisory for hazardous seas
continues for the northern/central New Jersey waters but has
been changed to a small craft advisory (from effects for both
winds and seas) for the southern NJ and DE coastal waters (as
winds will increase on Tuesday in advance of Maria; see the
Outlook section below).

Outlook...

Tuesday and Tuesday night...SCA conditions for both winds and
seas expected on DE and southern NJ ocean waters. Otherwise, SCA
for Haz Seas for northern NJ ocean waters. Seas building to
9-11 feet. NE winds gusting to 25-30 KT over DE and southern NJ
ocean waters.

Wednesday...An SCA will likely be needed for the entire period
on the ocean waters as elevated seas due to swells associated
Hurricane Maria will be observed through this time. Seas will
gradually build to 8-11 feet by Wednesday. Gusts above 25 KT
will be possible especially on the Delaware Coastal waters
Wednesday. Elevated waves will be possible at the mouth of the
Delaware Bay. Otherwise, winds and waves should stay below SCA
criteria on the Bay.

Thursday and Thursday night...A cold front will bring a shift
to northwesterly winds and gusts above 25 KT likely for part of
the period on the Atlantic Coastal waters. In addition, seas
will likely still be elevated due to swells from Maria.

Friday...Winds and seas will be diminishing through the day,
though it is uncertain how quickly they will diminish.

Rip Currents...

The rip current risk remains high as long-period southeasterly
swells continue to build thanks to Hurricane Maria. Conditions
should only deteriorate today, with swells expected to build
slowly through the day. Given the observed rip currents this
weekend along much of the NJ/DE surf zone, swimming is not
recommended, especially in the absence of lifeguards/beach
patrols.

Dangerous surf conditions will continue for much of the new
week. A HIGH risk for the development of dangerous rip currents
is expected from Tuesday through at least Thursday. High surf
conditions and beach erosion likely to develop Tuesday and
Wednesday.

&&

.CLIMATE...
High temperature records for today:

ACY...91(1970)
PHL...92(1970)
ILG...93(1970)
ABE...92(1970)
TTN...92(1970)
GED...92(2010 and 1970)
RDG...92(1970)
MPO...85(1970)

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...High Rip Current Risk through late tonight for NJZ014-024>026.
DE...High Rip Current Risk through late tonight for DEZ004.
MD...None.
MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ452>455.
     Small Craft Advisory for hazardous seas until 6 AM EDT
     Wednesday for ANZ450-451.

&&

$$
Synopsis...MPS
Near Term...CMS
Short Term...CMS
Long Term...MPS
Aviation...CMS/MPS
Marine...CMS/MPS
Climate...


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