Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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260
FXUS61 KPHI 242239
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
639 PM EDT Mon Apr 24 2017

.SYNOPSIS...
Low pressure over the Southeast States will slowly lift north along
the Atlantic Seaboard tonight through Tuesday night, then track east
on Wednesday. High pressure builds along the coast on Thursday. A
cold front approaches on Friday, but may become stationary north of
our area into the weekend as high pressure becomes entrenched over
the western Atlantic Ocean. A warm front lifts north through the
area for early next week.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
For the 630 PM update, made some adjustments to PoPs to reflect some
lower values initially this evening as showers are in waves but are
mainly light. Dry low-level air hanging on across the far northern
areas with dew points much lower compared to southern Delaware and
far southeastern NJ. This may result in some sprinkles for awhile
before actual showers settle in. It looks more showery tonight
overall. The hourly temperature and dew points were adjusted based
on the latest observations, which included showing initially warmer
values up north and cooler across the south and coast. Low
temperatures were lowered some mainly across the northern areas
where dew points are currently near freezing.

Otherwise, surface ridge axis will be situated across the area
overnight, with a cold front to the north and low pressure to the
south. This will maintain an overunning regime with rain showers
across much of the area. Precip will have a tougher time making it
north of I-78, with drier air in place. Cloud cover and an easterly
flow will not allow temperatures to drop off much. Generally upper
40s to mid-50s.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...
Closed mid-level low lifting northward along the eastern
seaboard will entrain some tropical moisture as it does so.
There is model disagreement between the NAM/GFS, with most of
the international models supporting the wetter NAM solution.
This brings a strong easterly 850 hPa jet of 50 to 60 knots
across the region, which will lead to heavy downpours. Stability
indices also decrease, as a plume of steep mid-level lapse
rates promotes elevated instability. As a result, we have
inserted thunder into the forecast across much of the area.
Precip amounts of 1-2 inches are expected, especially southeast
of the NJ Turnpike/I-95, with resulting urban and poor drainage
flooding the biggest concern. High temperatures will be in the
50s over PA and NJ, with low to mid 60s in Delmarva.

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
By Tuesday evening, low pressure should be centered just over
the Delmarva Peninsula. Swaths of moderate to locally heavy rain
will continue to track northeast along the coast through
Tuesday evening, gradually tapering off from southwest to
northeast as the low slowly tracks along the NJ coast.

Wednesday afternoon, the surface low will be just east of the
region, moving into NYC and western Long Island. Although the
bulk of the rainfall will be tapering off, the upper level low
will be just to the west of the surface low. This will keep at
least some showers in the region through Wednesday, but for now,
will keep PoPs capped at chance through the morning,
diminishing to slight chance as the afternoon progresses.

Upper level ridging will build into western NY/PA Wednesday
night as weak high pressure builds towards the East Coast. With
an abundance of moisture over the area due to the rainfall,
patchy fog is possible Wednesday night as skies clear out and
winds become nearly calm. That ridge builds east on Thursday,
and although there will be an onshore component to the winds,
temperatures will rise into the 70s for much of the area, and
possibly into the lower 80s over the Delmarva. Right along the
coasts, however, highs should stay in the 60s.

For the end of this week and into the start of the new week,
Bermuda high pressure looks to become entrenched over the
western Atlantic waters. There is a cold front that will
approach on Friday, but the strength of the offshore high should
be enough to keep the front from making too much eastern
headway. Southwest flow develops ahead of this front, and this
will allow a very warm and humid airmass to spread into the Mid-
Atlantic and Northeast. With H8 temps between 14C-16C, highs on
Friday will climb into the low to mid 80s for most of the area,
although the Poconos will top off in the mid 70s, and coastal
NJ/DE will remain in the 70s due to the sea breeze.

That frontal boundary will make a slow eastward progression
over the weekend as the high remains over the western Atlantic
waters. Conditions remain warm and humid into the weekend with
highs generally in the 70s and 80s. Although the main system
will remain to the west, several weak upper level shortwaves
will spin off ahead of the front, and with a warm and humid
airmass in place, there should be enough instability to touch
off some showers and thunderstorms over the weekend and into the
new week. For now, will keep PoPs capped at slight
chance/chance.

&&

.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Tonight...VFR conditions lower to MVFR/IFR this evening and
especially overnight. Rounds of showers will move through, light
initially then some heavier downpours are possible overnight.
Northeasterly winds in the 5-15 knot range overall, with local gusts
at times to around 20 knots especially closer to the coast.

Tuesday...IFR conditions, with occasional LIFR, due to low clouds
and periods of rain. Some of the rain will be heavy at times,
allowing for times of even lower visibility. Northeast to east winds
around 15 knots with gusts to around 25 knots. The highest winds are
expected to from about PHL south and east.

OUTLOOK...
Tuesday night...IFR and lower conditions in rain. E-NE winds
10-15 KT with gusts up to 25 KT, diminishing towards daybreak.

Wednesday...Lingering showers through Wednesday afternoon with
MVFR/IFR conditions. Improvement expected late. NE winds become
N at 5-10 KT.

Wednesday night through Thursday morning...IFR and lower
conditions possible in fog/stratus.

Thursday afternoon...VFR. SE winds 5-10 KT.

Friday through Saturday...VFR. SW winds 5-10 KT.

&&

.MARINE...
We have posted a Gale Warning for all the coastal waters expect
upper Delaware Bay where a SCA remains in effect. Gales run from 10Z
to 22Z Tuesday, and the SCA through 10Z Wednesday. The strongest
east winds are expected from late morning into the afternoon on
Tuesday. Seas will build and range from 2 to 4 feet on Delaware Bay
and 7 to 10 feet on the ocean.

OUTLOOK...
Tuesday night through Wednesday...Gale force winds will
diminish to 25-30 KT Tuesday evening, and then winds diminish to
less than 20 KT by daybreak Wednesday. However, seas on the
ocean will remain elevated at 7-10 KT, gradually diminishing to
4-6 FT by late Wednesday.

Wednesday night through Friday...Lingering 3-5 ft seas on the
ocean.

Saturday...Sub-SCA conditions.

&&

.TIDES/COASTAL FLOODING...
The cumulative effects of the onshore flow into Tuesday night
and the approaching new moon will likely result in minor
coastal. The minor flooding is anticipated along the New Jersey
shore, at the Delaware beaches and along Delaware bay and on the
far lower part of the Delaware River around Tuesday evening`
high tide. The minor flooding should affect the back bays and
the estuaries, as well.

We will take a look at the trends around this evening`s high
tide. We will likely issue a Coastal Flood Advisory for Tuesday
evening with our forecast update late this evening.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Warning from 6 AM to 6 PM EDT Tuesday for ANZ431-450>455.
     Small Craft Advisory until 6 AM EDT Wednesday for ANZ430.

&&

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



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