Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 191646
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1246 PM EDT Fri May 19 2017

.SYNOPSIS...
A cold front will cross our area late this afternoon and
tonight. High pressure across Canada will then build over the
area for the weekend. Low pressure will move into the upper
Great Lakes region Sunday and cause a slow moving front to
affect our weather Sunday night and Monday. More high pressure
will develop across the area next Tuesday. Another disturbance
will arrive for Wednesday and affect the weather through
Thursday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Hot and humid airmass remains in place over the region. Cold
front, currently extending from northern and central NY back
through central and western PA, continues to sage southward and
will works its way south into the region this afternoon.

Currently, widespread temps in the mid to upper 80s along with
dewpoints in the mid to upper 60s. Temps should warm up by a few
more degrees or so before leveling off in the lower 90s. Cooler
temps expected near the shore and in the Poconos.

As a result of the heat and humidity, SB CAPE currently
2500-3000 J/kg across SE PA, southern NJ, MD, and DE and
1500-2000 J/kg CAPE across central and northwest NJ and the
Poconos. Along with a Lifted Index as low as -5C, and a
Showalter index as low as -3C shows the potential for
thunderstorm development this afternoon as that front pushes
south through the region. Highest 0-6 km bulk shear lies north
of I-78, where it ranges from 35-40 kt. Where the highest
instability lies, 0-6 km bulk shear generally ranges from 20-25
kt.

Latest RADAR indicates showers developing across central and
southern PA, and other showers beginning to fire up across WV
and W MD. Those showers should develop into thunderstorms, and
will become more widespread.

12Z NAM has the bulk of the activity firing up after 21Z over
the southern third of the CWA, mainly including SE PA, S NJ, and
the Delmarva area. Latest HRRR quite progressive as well, also
focusing the bulk of the activity across southern portions of
the forecast area. Will trim back on PoPs across northern zones
to put the focus on southern zones, and will increase PoPs to
likely there.

Locally heavy and potentially damaging winds are biggest threats
this afternoon and evening.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/...
The cold front is expected to push south of DE shortly after
midnight. Showers may linger in our Delmarva zones overnight but the
expectation is for convection to decrease in coverage and intensity
overnight as instability wanes.

Temperatures do not follow a typical diurnal curve with the front
moving through. Temperatures will fall at a much faster clip in wake
of fropa as N-NE winds advect cooler air into the region. Low
tonight range from the mid 40s north of I-80 to the low 60s in
downtown Philadelphia and points southward.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
An area of high pressure will build over the area this weekend. It
will be a break in the recent heat and humidity with a seasonably
cool air mass prevailing across the area. High temperatures both
days will be around 3 to 5 degrees below normal. Little
precipitation is expected with the ridge across the area. If the
next system arrives a little sooner, a couple of Sunday afternoon
showers are possible far N/W.

A slow moving front attached to low pressure over the Great Lakes
will push across the area Monday. A swath of showers and some tstms
will accompany the system on Monday. Temperatures will be around
normal, or a degree or so above. Rainfall totals of an inch are
possible with the system. The convective elements could create a
situation with highly variable totals however.

A fair weather day is expected Tuesday, but it will be followed by
another wet system for Wed and Thu. This system is being carved out
by another upper low developing across the SE states. This could
produce a situation with the possibility for a prolonged period of
showery weather late next week. No big swings in temperatures
expected, with readings near or a little below normal expected.

&&

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

This afternoon...Showers and storms expected to develop after
18Z, primarily from RDG-TTN south. These storms are then
expected to move eastward thru the coastal plain late in the
day, perhaps lingering after sunset this evening (01-03Z) S/E of
PHL. Currently only advertising a PROB30 TSRA group for most
TAF sites with coverage of storms generally expected to be
widely scattered. We will be able to add more details as we get
closer to the event. Winds veer slightly out of the west during
the day with speeds of around 10 kt. Gusts to 20 kt favored
during the afternoon hours.

Tonight...Wind shift to NW and the N-NE expected to occur this
evening as a cold front progresses southward through the terminals.
Winds may briefly become breezy with gusts to around 20 kt this
evening in concert with the wind shift out of the N-NE.

OUTLOOK...
Sat thru Sun...Mostly VFR. SE to E winds much of the time.

Sun night thru Mon evening...Lower CIGS/VSBYS psbl in showers and
    sct tstms. S to SE winds Sun night becoming SW Mon.

Mon night and Tue...Mostly VFR.

&&

.MARINE...
Today...S winds between 10-20 kt today in the coastal waters;
weaker in the DE Bay.

Tonight...SCA was issued for the coastal waters adjacent to S NJ and
DE for late tonight. High pressure quickly building in behind a cold
front looks to be accompanied by a wind surge of 15-20 kt with gusts
near 25 kt as winds turn out of the NE. Seas will build in response
to the onshore winds to 4-5 ft as well. The pressure surge does not
appear to be quite as pronounced farther north up the NJ coast and
in the DE Bay, where we did not issue a SCA at this time.

OUTLOOK...
Saturday...SCA conditions on the southern ocean
  waters...diminishing late.
Sat night thru Sunday...Sub-SCA with fair weather.
Sunday night thru Monday...Sub-SCA with showers.
Tuesday...Sub-SCA with fair weather.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory from 2 AM to 6 PM EDT Saturday for
     ANZ452>455.

&&

$$
Synopsis...O`Hara
Near Term...MPS
Short Term...Klein
Long Term...O`Hara
Aviation...Klein/MPS/O`Hara
Marine...Klein/MPS/O`Hara



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