Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 261624
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1224 PM EDT Tue Sep 26 2017

.SYNOPSIS...
Hurricane Maria will move slowly northward off the coast of the
Outer Banks through Wednesday before turning sharply east-
northeastward into the open Atlantic through the end of the
week. A cold front will move through the Mid-Atlantic on
Thursday, and a reinforcing front will progress through the area
Friday and Friday night. Cool high pressure will settle into
the Northeast this weekend through at least early next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
High pressure will remain over the northeastern states today.
Hurricane Maria will be located off the North Carolina coast.
The circulation around the two systems will keep an onshore flow
in our region for today.

Low clouds over the coastal plain continued to lift and erode
around midday. They may linger through the afternoon near the
New Jersey coast, in central and southern Delaware and in the
adjacent counties of eastern Maryland.

The wind should favor the northeast and east. Speeds are
expected to be around 10 MPH on the coastal plain and around 5
MPH inland. Gusts are expected along the coast.

Maximum temperatures are anticipated to be in the 70s near the
coast due to the onshore flow, and in the 80s inland.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
Low clouds and patchy fog will develop once again across the
coastal plain and the Delmarva late tonight. NE winds will
diminish to 5-10 MPH along the coasts and to 5 MPH or less
inland. Meanwhile, Maria will begin to weaken to a Tropical
Storm as it begins to sharply curve out to sea. A few outer rain
bands may move into southern portions of the forecast area, but
not expecting much, if any, QPF. Lows tonight generally in the
low 60s to the north and in the upper 60s to around 70 to the
south.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Main forecast concerns for the long-term period are
precipitation chances Wednesday and again Friday night and
Saturday and a sharp downward trend with temperatures versus
what has been observed recently.

Analysis of the 00Z operational model suite suggests that the
overall large-scale pattern is decently agreed upon, but rather
large discrepancies exist with the smaller-scale vorticity
maxima that will be progressing through and/or affecting our
region through the period. Additionally, poor run-to-run model
continuity continues, owing largely to uncertainty with the flow
pattern in the northern Pacific and the apparent substantial
influences downstream.

At 12Z Wednesday, Maria will be off the coast of the Outer
Banks. A potent vort max will be progressing through Ontario
with southwesterly midlevel flow between the two. Although the
main impacts from Maria will be well south/east of the area, the
pressure gradient will be tightening in the southern CWA, so
expect breezy conditions to develop in southern Delaware and far
southeast New Jersey. Northeasterly surface flow should keep
the area socked in clouds much of the day, with only gradual
improvement from northwest to southeast. Some showers are
possible, especially near the coast, as Maria makes its closest
approach to the area. However, a considerable portion of the
model output is dry through Wednesday night. The exceptions are
the 00Z CMC/ECMWF, which indicate some QPF close to the coast.
The totals from the CMC look overdone, but the geographic
agreement with the ECMWF suggests that precipitation chances
should remain. I did lower them considerably northwest of I-95,
where there is little remaining evidence of sufficient lift for
showers. One noteworthy development: the 00Z ECMWF is
considerably slower and continues on the west side of the
guidance with the track of Maria. By Thursday, it becomes an
outlier with the position of Maria, so I neglected to give the
new simulation strong weighting in the forecast.

As the Ontario trough moves rapidly east-northeast into the
Canadian Maritimes on Thursday, Maria should begin the eastward
turn. The ECMWF is weakest with this trough and considerably
more sluggish in advancing Maria farther offshore - this does
lower confidence in Thursday`s forecast a little bit. A surface
low will develop and progress rapidly eastward through Quebec
and New Brunswick, with a cold front sweeping across the Eastern
Seaboard Thursday. Cooler and noticeably drier air will seep
into the Mid-Atlantic upstream of the front, with forecast highs
Thursday around 10 degrees colder in the urban corridor and up
to 20 degrees colder in the southern Poconos. The change should
be less severe near the coast (which will be cooler on Wednesday
given the onshore flow). Kept most of Thursday dry given the
subsidence upstream of the front.

An upstream shortwave trough digs southeastward into the Great
Lakes Thursday night, and this is where much of the forecast
uncertainty lies. The GFS is much more progressive with this
trough than the CMC and especially the ECMWF. The GFS has
trended stronger with the first trough in eastern Canada and
weaker with the second, which does not agree with the recent
trends of the NAM or ECMWF. Additionally, there is very large
variation in the upstream flow run-to-run with the GFS. The
stronger agreement between the CMC/ECMWF made me lower the
weighting of GFS-based guidance Friday onward, and I may not
have done so enough.

The main effects on the forecast are with precipitation chances
Friday night and Saturday, with the GFS about 6-12 hours faster
in general (more Friday night versus early Saturday for the
CMC/ECMWF). For now, kept the highest PoPs late Friday night (a
compromise of the two camps), but my suspicion is that PoPs need
to linger into a portion of Saturday, especially for the
eastern zones. Trended this direction with the forecast, but
expect further (perhaps substantial) changes with PoPs during
this period.

Cool/dry high pressure moves in thereafter, with a prolonged
period of near to slightly below seasonal temperatures Saturday
onward. Models have large phase/amplitude variations with the
midlevel ridge developing in the East during this period, so the
duration of the cool/dry spell is a little uncertain at this
point.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Mainly VFR at our eight TAF sites through the balance of the
day. Fog/stratus looks to redevelop late tonight with sub-VFR
conditions likely once again.

E-NE winds will increase to 5-10 KT, except at KACY, where E-NE
winds will increase to 10-20 KT.

LGT/VRB winds tonight.

Outlook...

Wednesday: MVFR/IFR conditions possible, especially from the
urban corridor eastward, with gradual improvement to VFR from
northwest to southeast during the day. Some showers possible at
KMIV/KACY. Northeast winds 5 to 15 kts with stronger gusts near
the coast (up to 25 kts or so). Confidence below average.

Wednesday night and Thursday: Winds should become
north/northwest as a cold front sweeps through the region. May
see some lingering showers/low clouds at KMIV/KACY Wednesday
night. Speeds should reach 10 to 20 kts on Thursday, with higher
gusts possible. Confidence average.

Thursday night and Friday: Predominantly VFR. Northwest winds 5
to 15 kts. Confidence above average.

Friday night and Saturday: Showers and localized sub-VFR
CIGs/VSBYs possible. Winds generally north or northwest around
10 kts. Confidence well below average.

&&

.MARINE...
Small Craft Advisory continues today and tonight for the ocean
waters. Seas on the ocean will build to 8-9 feet. NE winds will
range from 15-20 KT with gusts up to 25 KT for southern NJ and
DE ocean waters. Otherwise, NE winds range from 10-15 KT.

With these increasing offshore seas, can expect breaking waves
at the shores of around 7 feet at southern NJ and DE beaches. A
high surf advisory has been issued for tonight through Wednesday
for southern NJ and DE coastal areas.

Outlook...

Wednesday: Northeast winds with gusts above 25 kts likely in
the southern NJ/DE coastal Atlantic, trending lighter in
Delaware Bay and the northern/central marine zones. Seas near 10
feet in the Atlantic, with rough surf likely. A high surf
advisory continues for the southern NJ/Delaware beaches. Showers
possible.

Wednesday night: Gradual decrease in winds, though advisory-
level gusts still possible south of Atlantic City. Seas will be
slow to decline. A chance of showers.

Thursday and Thursday night: Cold frontal passage will increase
winds to advisory level in the Atlantic waters and potentially
Delaware Bay. Seas well above advisory criteria.

Friday: Seas nearing but likely still above 5 feet, but winds
should diminish below advisory criteria.

Friday night and Saturday: Sub-advisory conditions expected,
but a chance of showers.

Rip Currents...

Due to swells emanating from Hurricane Maria, there continues
to be a HIGH risk for the development of dangerous and life
threatening rip currents for the ocean beaches today and
tonight.

The rip current risk will likely be in the high category
Wednesday and Thursday as long-period southeasterly swells from
Maria continue. Swell heights around or above 10 feet on
Wednesday will likely generate high surf/beach erosion issues
for much of the surf zone but especially for southern New Jersey
and Delaware. Overall, not a good time to spend at the beach
the next few days.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...High Rip Current Risk through Wednesday afternoon for NJZ014-
     024>026.
     High Surf Advisory from 6 PM this evening to 6 PM EDT
     Wednesday for NJZ024-025.
DE...High Rip Current Risk through Wednesday afternoon for DEZ004.
     High Surf Advisory from 6 PM this evening to 6 PM EDT
     Wednesday for DEZ004.
MD...None.
MARINE...Small Craft Advisory until 6 AM EDT Thursday for ANZ452>455.
     Small Craft Advisory for hazardous seas until 6 AM EDT
     Thursday for ANZ450-451.

&&

$$

Synopsis...CMS
Near Term...Iovino
Short Term...MPS
Long Term...CMS
Aviation...CMS/MPS
Marine...CMS/MPS


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