Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 250138
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
938 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/...
An upper air analysis early this evening shows a closed low centered
over South Carolina and Georgia, and a narrow ridge axis is over our
region. There are areas of 850 mb warm air advection to the east and
north of the closed low. The main short wave energy is still to our
south, and therefore mostly just isentropic lift is occurring in our
area given the uptick in warm air advection aloft.

As we go through the overnight hours, the low-level warm air
advection is forecast to increase as a southeasterly low-level jet
of nearly 50 knots approaches from offshore. This should continue to
support waves of showers through the overnight, although coverage
may be less. Some elevated instability has been noted across the far
southern zones this evening, however it appears that any lightning
remains to our south and/or offshore. The PoPs were lowered some as
the coverage looks a bit less, however some showers are expected
through the night. The majority of the showers have been on the
lighter side thus far.

The dry low-level air up across the north is gradually eroding as
dew points have increased some, however there is still a pocket of
lower dew points remaining around the Philadelphia metro area. The
hourly temperature, dew point and wind grids were adjusted based on
the latest observations, then the LAMP/LAV guidance was blended in
for the next few hours. Temperatures should be more uniform across
the area given the northeasterly flow off the cooler ocean along
with some showers. Low temperatures were lowered some across the
southern zones with this update as current temperatures are very
near previous forecast lows.

&&

.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 /02Z 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 late this evening and
especially overnight. Some showers will move through, with the
intensity mostly on the lighter side. 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 LIFR at times, due to low clouds and
periods of rain. Some of the rain can 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...
A Gale Warning is in effect for all the coastal waters expect upper
Delaware Bay where a SCA remains in effect. The Gale Warning is from
10Z to 22Z Tuesday, and the SCA is 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
flooding. 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`s high tide.
The minor flooding should affect the back bays and the estuaries, as
well. We have issued a Coastal Flood Advisory for those areas for
Tuesday evening. The surge on top of the base astronomical tides
is expected to be a foot to a foot and a half at that time.

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.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...Coastal Flood Advisory from 6 PM Tuesday to midnight EDT
     Tuesday night for NJZ012>014-020>027.
     Coastal Flood Advisory from 9 PM Tuesday to 1 AM EDT Wednesday
     for NJZ016.
DE...Coastal Flood Advisory from 6 PM Tuesday to midnight EDT
     Tuesday night for DEZ002>004.
     Coastal Flood Advisory from 9 PM Tuesday to 1 AM EDT Wednesday
     for DEZ001.
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...Gorse
Short Term...Franck
Long Term...MPS
Aviation...Franck/Gorse/MPS
Marine...Franck/MPS
Tides/Coastal Flooding...Iovino


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