Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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856
FXUS61 KPHI 240718
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
318 AM EDT Sun Sep 24 2017

.SYNOPSIS...
High pressure entrenched over the Northeast gradually builds to
the north and east through the middle of the week. Meanwhile,
Hurricane Maria will track to the north over the western
Atlantic waters and will approach the Carolina coast on
Wednesday. A cold front moves across the East Coast on Thursday
and curves Maria out to sea. High pressure returns for the end
of the week, and then low pressure may impact the region late in
the weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Strong midlevel ridging will continue to move slowly eastward
into the Northeast today. Result will be very warm temperatures
in an environment of strong subsidence and clear skies. With
abundant diabatic heating from insolation, today`s maximum temps
should be a couple degrees warmer than those seen yesterday.
Guidance was too cool yesterday, by several degrees in some
locations. Given a similar environment today, was on the high
side of guidance (and in some cases, exceeded all guidance) for
temperatures this afternoon.

There may be a few limiting factors for temperatures getting
too far out of hand (i.e., well above guidance). The first is
that winds will be a little weaker today, so boundary-layer
mixing will be somewhat more limited. As such, surface dew
points may not mix out to the degree observed yesterday, and
this may temper near-surface heating to some degree.
Additionally, the component of the wind should be more easterly
today, which may temper warming via advection from a cooler
source of air and will reduce the effects of downsloping. These
factors were enough for me not to go well above guidance, but
the current forecast may be too conservative should these
factors prove to be overestimated.

As far as records are concerned, think most of them are safe,
especially Philly`s and Georgetown`s. Wilmington`s 92 and Mount
Pocono`s 85 are at least plausible, but the current forecast is
a couple degrees shy in both locations.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
Main forecast concern overnight is fog formation. Dew points
will continue to creep upward, and with clear skies and calm
winds thanks to ridging parked near or over the area,
temperatures will likely decrease sharply after dark. Latest
guidance is more aggressive with fog formation overnight (both
in terms of areal coverage and persistence) compared to the past
couple of nights. Included mention of patchy fog in the grids
for most locations outside of the urban corridor.

Used a blend of MAV/MET/ECMWF guidance for lows tonight.
Forecast temps are expected to be in the low 60s in the southern
Poconos to near 70 in the urban corridor. For hourly
temperatures, incorporated 2-meter temperatures from a blend of
higher-resolution guidance, given the rather poor performance of
statistical guidance the past couple of evenings as
temperatures begin their nocturnal decline.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Surface high pressure over the area on Monday weakens and lifts
to the north and east on Tuesday. Temperatures remain well
above normal with highs ranging from the low 80s along the
coasts and in the Poconos to the mid and upper 80s for most of
NJ, SE PA, and the Delmarva, and around 90 for the I-95
corridor. By Tuesday, winds take on more of a NE flow and winds
increase to 10-15 MPH. This ushers a cooler airmass into the
region, and highs will be about 10 degrees cooler than Monday.
With onshore flow and some weak mid- level shortwave energy
passing through the region, there may be some showers, possibly
a thunderstorm, Tuesday afternoon and evening.

Meanwhile, Hurricane Maria continues to track northward and
approaches the Carolina coast on Wednesday. High pressure should
be strong enough to keep Maria suppressed to the south, and
then a cold front passing through the East Coast should take
Maria out to sea on Thursday. Some outer rain bands associated
with Maria may lift into the region during this time, but there
is a better chance for showers and possible thunderstorms with
approach and passage of the cold front. Tropical Storm force
winds also should stay south and east of the region.

Tide levels are running low since we are in between tide
cycles, so the chances for minor coastal flooding are low at
this time as well.

Autumnal weather builds into the region to close out the week.
Highs will be in the low 70s Friday, and then into the upper 60s
to around 70 over the weekend.

&&

.AVIATION /07Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

VFR through tonight. Light/variable winds will become
northeasterly around or below 10 kts during the day, veering to
more southeasterly during the afternoon, especially near and
east of the urban corridor. Winds should become light/variable
again tonight. Few or no clouds are expected today. Some patchy
fog may occur early this morning and again tonight, with chances
tonight a little higher than the previous couple. A fog bank
may also approach the New Jersey coast late tonight. However, no
sub-VFR fog is mentioned in the TAFs at this point, as
uncertainty in coverage/persistence is too high for inclusion.

Outlook...

Monday...VFR conditions are expected.

Tuesday and Wedensday...Mostly VFR conditions expected, but
there is a chance for showers and possibly a thunderstorm both
days. Showers could produce brief MVFR or lower conditions. NE
winds 10-15 KT, higher at KACY and possibly KMIV.

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

&&

.MARINE...
Small Craft Advisory for hazardous seas will remain in effect
for the southern NJ and DE coastal waters, and has been reissued
for the northern/central NJ coastal waters beginning at 4 pm
this afternoon. Seas are generally around or just below 5 feet
currently, but are expected to build slowly through the day.
There will be a bit of a lag off the coast of northern/central
NJ, but all Atlantic coastal waters should have seas greater
than 5 feet by the evening hours. Winds will remain well below
advisory criteria through the period.

Long-period easterly to southeasterly swells will continue
today, and there may be considerable chop from the combined
swells of what remains of Jose and Hurricane Maria.

One concern tonight is the potential for a fog bank to move
westward into the coastal waters of the Atlantic. A rather large
fog bank has persisted overnight well offshore, but has moved
into far eastern Long Island and the adjacent waters. Latest
guidance suggests this will advance farther west tonight, and
given that surface flow will have a more easterly component
today/tonight, this seems reasonable. Included mention of areas
of fog tonight, mainly for locations away from the immediate
coast. Confidence is not particularly high, but this may promote
fairly widespread visibility restrictions should this threat
materialize.

Outlook...

Monday through Wednesday...A prolonged SCA will likely be
needed due to increasing seas due to swells associated with
Hurricane Maria. Seas will gradually build to 8-12 feet by
Wednesday. Gusts of 25-30 KT are possible, especially on the
Delaware Coastal waters, Tuesday and 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...A cold front will bring a shift to northwesterly
winds and gusts above 25 KT likely on the Atlantic Coastal
waters. In addition, seas will likely still be elevated due to
swells from Maria.

Rip Currents...

Today...Early morning observations indicate longer-period
swells are occurring on the coastal waters, with 8-10 second
northeast swells from Jose being overcome by 12-16 second
southeast swells from Maria. Wave heights will increase (albeit
slowly) today and tonight. With yesterday`s observations
suggesting rip currents were prevalent, today`s conditions are
not expected to be any better and will likely get worse by late
afternoon into the evening. As such, the risk of rip currents
has been upgraded to high for today.

Dangerous surf due to swells emanating from Hurricane Maria will
likely result in a high risk for the development of dangerous and
life threatening rip currents from Monday through at least Thursday.

&&

.CLIMATE...
Here are the records highs for today and Monday.

Today

ACY...92(2010)
PHL...95(1970)
ILG...92(2010)
ABE...92(1970)
TTN...92(1970)
GED...97(2010)
RDG...95(1970)
MPO...85(2010)

Monday

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 from 8 AM EDT this morning through this
     evening for NJZ014-024>026.
DE...High Rip Current Risk from 8 AM EDT this morning through this
     evening for DEZ004.
MD...None.
MARINE...Small Craft Advisory for hazardous seas until 6 PM EDT Monday
     for ANZ452>455.
     Small Craft Advisory for hazardous seas from 4 PM this
     afternoon to 6 PM EDT Monday for ANZ450-451.

&&

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



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