Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 180459
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1259 AM EDT Sun Jun 18 2017

.SYNOPSIS...
A warm front will remain in the vicinity of northern New Jersey
tonight, then lift northward on Sunday. Return flow around a
strong Bermuda high will result in warm and humid conditions
through Monday. A cold front is expected to move through the
area Monday night and stall just to our south midweek. A
secondary cold front passes through Wednesday night. The
boundary may return back northward as a warm front late in the
week. Another cold front could approach from the west early next
weekend.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
1230 am update: Only remaining showers are occurring generally
north of a Jim Thorpe to Morristown line. Updated PoPs to
diminish them through the night as they continue to weaken.
Removed thunder from the forecast tonight as well, and updated
patchy fog in the grids based on latest expected temperature/dew
point spreads. There has been a recent uptick in lower
visibilities northwest of the Fall Line and in northern/central
New Jersey, so may need to include a little bit more emphasis on
fog in subsequent updates.

Previous discussion...

930 PM ESTF: Updated PoPs for showers and thunderstorms that
continue to redevelop in the I-95 urban corridor. These are
slowly propagating northwest, and that trend should continue
through the overnight, as moisture axis and more unstable air
shifts west into the Lehigh Valley and southern Poconos. This
activity should then gradually diminish after midnight.

Scattered showers and a few tstms will continue across much of the
area into the evening. A warm front lifting across the area will
cause these showers and there will also be some lingering fog
across the coastal areas. Breaks have develop across Delmarva
and SE NJ, but overcast conditions continue over the southern
Poconos, lehigh Valley and much of nrn NJ attm. Overnight,
showers will diminish but more low clouds and some fog will once
again be across the region. Lows overnight will remain mild
with readings in the upper 60s north and low/mid 70s south/east.
It will remain humid overnight.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM MONDAY/...
Winds will become more SWrly on Sunday and some drier air will be
over the area. We expect some sunshine by late morning and into
the afternoon. Then skies will become cloudy again as some
daytime showers/sct tstms develop. It will be a warm and humid
day with highs reaching the mid/upper 80s in many areas. We will
hold pops in the chc range for now, with the highest chcs N/W.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Sunday night through Monday night...An active weather pattern looks
to be in store with an upper trough amplifying over the Great Lakes/
Midwest region. The trough may become negatively tilted by the time
it progresses through the region Monday night. Deep southerly flow
between the downstream Bermuda ridge and upstream cold front will
continue to the feed of warm, moist air up the eastern seaboard.
Monday afternoon and night contain the highest PoPs (60-80 percent)
in the period. Convection is expected to initiate during peak
heating Monday afternoon over the higher terrain along a pre-frontal
(either just to our west or in our far western zones in E PA). The
orientation of the deep-layer shear vectors relative the boundary
favors convection growing upscale into multi-cellular line segments
or possibly even a more organized line. Models indicate a stronger
wind field and deep lift arriving just ahead of the cold front
Monday evening, which could enhance the intensity and coverage of
storms and sustain convection past sunset. Damaging winds and
excessive rainfall/flash flooding will be the primary threats Monday
afternoon and evening. The D3 Severe Thunderstorm Outlook from SPC
depicts areas west of I-95 in an enhanced risk for while east of I-
95 is in a slight risk. The D3 Excessive Rainfall Outlook from WPC
places the entire CWA in a marginal risk.

Tuesday through Thursday...Drier air moves into the area in wake of
the cold fropa. Isolated showers cannot be ruled out on Tuesday or
Wednesday with a series of weak shortwave troughs passing through
(especially later on Wednesday when a secondary cold front arrives).
However, think most areas should stay dry. There is more uncertainty
for Thursday as models disagree on how much lift and shower activity
will arrive when a warm front returns northward toward the region.
Accordingly, PoPs were capped at 30 percent. Temperatures will be
near normal with afternoon highs in the low to mid 80s and nighttime
lows in the 60s.

Friday and Saturday...The heat and humidity starts to build (but not
to excessive levels) after a warm front lifts north of the region.
Diurnal showers and storms may develop each day, but the better
chances for precip of the two days looks to be Saturday with a cold
front possibly approaching from the west.

&&

.AVIATION /06Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

For the 06Z TAFs...Spotty/occasional MVFR to IFR conditions are
occurring at KABE, KTTN, and KILG with more prevalent IFR at
KACY. Conditions will likely deteriorate somewhat at all
terminals through the rest of the overnight, but latest guidance
has come in a little more optimistic in general. Nevertheless,
expect more widespread MVFR/IFR in the 06Z to 13Z time frame.
VFR conditions should develop across the region by 15Z with
isolated/scattered storms possible this afternoon, especially
north/west of KPHL. Coverage is expected to be low, and
confidence in timing is too limited for TAF mention anywhere at
this point. Winds will pick up from the S or SSW to speeds of
10-15 kts with gusts to 20+ kts this afternoon.

OUTLOOK...

Sunday night...Highest probability for showers and storms will
be during the evening and inland toward RDG/ABE. Brief/localized
restrictions to MVFR or IFR possible if the convection directly
impacts a terminal. IFR also possible overnight in fog/low
clouds, but this may be confined to coastal sites (ACY).

Monday...A period of VFR in the morning and early afternoon
before showers and storms arrive from the west. Storms are
likely to bring at least temporary restrictions to the terminals
but despite the high confidence, there is still uncertainty in
timing of storms. SWly winds could gust to 25 kt during peak
heating.

Monday night...Showers and storms continue. Temporary LIFR
visibility possible in heavy rain.

Tuesday through Thursday...Generally VFR. Low chances for
showers each day.

&&

.MARINE...
Marine dense fog advisory issued for the northern NJ waters
into mid-morning Sunday where areas of dense fog will reduce
visibility to 1 NM or less at times.

Winds and seas pick up tonight and therefore the SCA issued
earlier for the ocean will continue. Seas will slowly increase
to around 5 ft. Scattered showers tonight and Sunday afternoon.
Slight chc for a tstm.

Sunday night and Monday...The SCA was extended into Monday for
our coastal zones with S-SW winds gusting to 25 kt and seas 5-6
ft.

Monday night and Tuesday...Winds will weaken but seas will
likely remain around 5 ft. Thus, a SCA will likely be needed.

Tuesday night through Thursday...Winds and seas expected to
remain below SCA criteria.

RIP CURRENTS...
Rip current risk for Sunday is HIGH. We continue to expect a
period w/frequent oscillations between a 6-8 second easterly
swell and a 15-17 second swell, with the latter containing
the majority of the wave energy. Winds and seas will be more
elevated compared to Saturday, with low tide occuring during
the mid-morning and late evening.

The primary wave period is forecast to be 5 to 7 seconds from
the south during the period from Sunday though Wednesday.
However, the underlying 14 to 18 second southeasterly swell is
expected to linger into the mid week period. As a result, the
risk for the development of dangerous rip currents is
anticipated to be at least moderate through the period along the
coasts of Delaware and New Jersey.

&&

.HYDROLOGY...
Locally heavier rainfall amounts from the showers that moved through
this morning exceeded an inch in a few spots in E PA and NJ. With
the same moisture-rich airmass remaining in place through the
weekend and into Monday/Monday night, heavy downpours with any
showers and storms that develop will could produce locally
heavy rainfall. The flooding threat looks to be very localized
and predominately limited to poor-drainage areas through Sunday
night.

The potential for heavy rainfall increases heading into Monday
afternoon with the greatest concern falling roughly within a
12-h period from about 18Z Monday - 06Z Tuesday. Models
generally indicate widespread rainfall with 24-hr QPF ending 12Z
Tuesday between 0.75-1.50 inches while some of the higher-res
models like the NAM that go that far out show locally higher
amounts of 2-3 inches. However, confidence in QPF amounts is
low since convective-allowing models have better skill with
convective setups like this but they do not yet run this far
out to cover this event. A Flash Flood Watch may be needed as
we get closer to the event depending on the antecedent
conditions as areas that receive locally heavy rainfall this
weekend will have a heightened risk for flooding.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...High Rip Current Risk through this evening for NJZ014-024>026.
DE...High Rip Current Risk through this evening for DEZ004.
MD...None.
MARINE...Small Craft Advisory until 6 PM EDT Monday for ANZ450>455.
     Dense Fog Advisory until 10 AM EDT this morning for ANZ450.
     Dense Fog Advisory until 8 AM EDT this morning for ANZ451.

&&

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



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