Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 230108
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
908 PM EDT Thu Jun 22 2017

.SYNOPSIS...
Low pressure will track well north of our area Friday and Saturday.
An associated cold front crosses our region during Saturday as some
tropical moisture moves ahead of and along it. As high pressure
builds well to our southwest Sunday into Monday, a weak cold front
moves through our area later Monday. High pressure then builds
across the Mid Atlantic Tuesday and Wednesday, before shifting
offshore Thursday.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
While the showers and thunderstorms have weakened and
consolidated across central PA, they continue to track to the
east. POPs were raised to likely adjacent to the CTP CWA. Chance
POPs and the threat of showers were also painted further east
into the western Philly suburbs as the HRRR continues to show
showers approaching I95.

Previous discussion...
The 6:30 pm ESTF update will include a few tweaks with temps
and dewpoints. Sky, weather, and POPs were also adjusted across
our far western zones as the showers and thunderstorms are
dissipating a bit more slowly. The HRRR brings some precip to
I95 corridor between 03-04z, but we feel this over done.

Previous discussion...
The first challenge of the near term forecast will be if and
how quickly any convection associated with a low level trough to
our west reaches our region by early this evening. Showers and
thunderstorms have already developed across western PA, but it
is unclear if they will reach our region before sunset, when we
should see activity diminish (this convection appears to be very
much diurnally driven as there is limited forcing in the mid
and upper levels and model soundings show a low level inversion
developing quickly this evening.

Through the overnight hours, the closed low over south central
Canada is expected to continue gradual progress east. As it does so,
we may begin to see the next round of showers and thunderstorms move
into our region in the pre dawn hours, though latest model timing
indicates it is unlikely we will see this before daybreak.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
As the low and it`s associated cold front make slow progress east,
it will interact with remnant moisture of TD Cindy. Through much of
the day tomorrow, it looks like the axis of the deepest moisture and
best lift generally stay to our north and west. Having said that
though, it is likely that some convection will move into our region
through the day especially west of the fall line. It is also likely
that we will keep mid level clouds over our region for much of the
day as the mid level cloud shield associated with this is already
moving into SW VA. Cloud cover will be crucial in determining our
severe potential tomorrow afternoon. Model soundings indicate that
if we can see some clearing by mid day, we could have CAPE values in
our region over 500 J/kg. Bulk shear remains limited (generally less
than 30 kt) until very late in the day. Thus, any severe threat
(hail and strong winds) will be conditional on clearing early enough
in the afternoon.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
Summary...Tropical moisture moves through Friday night and the first
half of Saturday with a cold front, then lowering humidity levels
during the second part of the weekend and next week. Some cooling
next week as a trough settles into the East for a time.

Synoptic Overview...An upper-level trough amplifies eastward from
the Midwest and Great Lakes to the Northeast through Sunday. This
will send a cold front our way, however the remnants of tropical
system Cindy interacts with it. This feature, while possibly less
defined at the surface, should have its mid level energy enhance our
chances for locally heavy rain later Friday night and Saturday
morning. The trough is then forecast to sharpen in the East through
the first half of next week, before lifting out later Wednesday or
Thursday. The presence of this trough will result in cooler
temperatures for a few days, with perhaps some instability
showers/thunder especially early in the week. We used a
model/continuity blend for Friday night through Saturday night, then
blended in the 12z WPC Guidance. Some adjustments were then made
following additional collaboration with our neighboring offices.

For Friday night and Saturday...As a trough amplifies eastward from
the Midwest and Great Lakes region, surface low pressure tracks well
to our north. An associated cold front however will be moving
through our area Saturday. While this occurs, the remnant energy
from tropical system Cindy gets caught up in the faster flow aloft
as it moves northward and then accelerates east-northeastward. While
the surface feature should be less defined, the model guidance shows
the mid level energy holding together as it crosses especially the
southern half of our area Saturday morning. There is an axis of very
high moisture content air moving east with PW values of 2.0-2.5
inches. This moisture in combination with lift from the front and
remnant tropical system Cindy will result in locally heavy rain.

While there is some potential for localized flooding, the system
should be moving through fairly quickly. As a result, we held off on
a Flash Flood Watch at this time. This may have to be revisited
though especially for the urban I-95 corridor. The main area of
concern looks to be from the Philadelphia metro area on south and
eastward. The forecast soundings generally show a tall and thin CAPE
profile, which within a very moist air mass tends to lead to areas
of locally very heavy rain. Backing up a bit, there should be
ongoing convection to our west that spreads eastward Friday night
with the front and then the tropical system remnants. We have an
area of likely PoPs across the western/northern areas Friday
evening, then these spread south and eastward overnight and increase
to low end categorical toward daybreak for some areas. For Saturday,
we confined an axis of likely to low-end categorical PoPs to mostly
the morning hours. At least some clearing should then occur from
west to east Saturday afternoon and especially at night with dew
points dropping in the wake of the cold front. It is possible that a
few showers or thunder develops in the afternoon especially across
the eastern and southern zones where lingering instability is
present before the front clears the coast.

For Sunday and Monday...An upper-level trough is forecast to amplify
across the East during this time frame. This should be accompanied
by a secondary cold front at the surface later Monday. While Sunday
overall looks dry with lowering dew points, some instability showers
or low-topped thunderstorms will be possible Monday especially in
the afternoon to early evening. The presence of the upper-level
trough and accompanying cooling aloft will result in cooler surface
temperatures especially more noticeable on Monday.

For Tuesday through Thursday...The upper-level trough in place
Tuesday into Wednesday may begin to lift out later Wednesday or
Thursday. This will allow surface high pressure to build in from the
west-southwest before shifting offshore Thursday. As the main part
of the trough is overhead Tuesday, there will be the chance for some
instability showers during the daytime. The dew points are
anticipated to be in the comfortable zone due to the presence of the
trough and associated cooler air mass. Some moderating of the
temperatures is expected Wednesday and especially Thursday as high
pressure moves offshore and a warm air advection pattern should
begin.

&&

.AVIATION /01Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

With showers and isolated thunderstorms still holding together
and now working through Berks and Lancaster counties, VCSHs were
added to the I95 terminals.

Previous discussion...
Tempo -SHRA was added to both KRDG and KABE between 23z and 02z
to account for the showers and thunderstorms moving in from the
west.

Previous discussion...
VFR conditions are expected to continue through at least 00Z, and
likely through 06Z. After 00Z lower clouds and showers and
thunderstorms are expected to approach the region from the
southwest. Though the chance for showers and thunderstorms begins
after 00Z, it looks more likely that precipitation will hold off in
our region until closer to 12Z. With any showers and thunderstorms,
MVFR conditions are likely, and IFR conditions are possible,
especially with heavy downpours which are possible tomorrow morning.
Conditions may improve to VFR for several hours after 18Z.

OUTLOOK...
Friday night and Saturday...Times of MVFR/IFR conditions with
showers and some thunderstorms. Some heavier rain can significantly
reduce the local visibility at times. The greatest coverage of
showers should be late Friday night and Saturday morning, with
improving conditions Saturday afternoon and night.

Sunday...VFR overall.

Monday and Tuesday...VFR overall, however a few mainly afternoon and
early evening showers or a thunderstorm are possible each day.

&&

.MARINE...
Winds and seas should stay below SCA criteria through Friday
morning. By mid day seas and southwesterly winds will  increase on
the coastal waters and SCA conditions are likely to develop by early
afternoon. Strong winds are possible in the vicinity of any
thunderstorms that may develop especially Friday afternoon.

OUTLOOK...
Friday night and Saturday...A cold front will approach Friday night
with some increase in a southwesterly flow. The forecast soundings
indicate that the mixing is not all that efficient due to a very
warm and humid air mass, however times of 25-knot gusts can occur
and this should build the seas on the ocean to around 5 feet. As a
result, a Small Craft Advisory was issued for only the ocean zones
through 10z/6 AM Saturday. This advisory may need to be extended
through much of Saturday prior to the frontal passage.

Sunday through Tuesday...The conditions are anticipated to be below
Small Craft Advisory criteria during this time frame, with subsiding
seas during Sunday.

RIP CURRENTS...
We are forecasting that the low risk for the formation of dangerous
rip currents will continue into tomorrow. However, even with a low
risk...the bigger diurnal difference in the tide cycle as we
approach the date of this months new moon could mean some rapidly
changing conditions.

&&

.TIDES/COASTAL FLOODING...
High astronomical tide levels are expected in association with the
new moon on Friday. That, combined with increasing southerly flow,
is expected to result in minor coastal flooding with the afternoon
and evening high tide for the Atlantic coastal areas and the shores
of the lower Delaware Bay. There may be minor coastal flooding
further up the bay, but it is unlikely, so will not expand the
coastal flood advisory at this time.

Minor coastal flooding may occur again with the high tide cycle
Friday afternoon and evening, though the latest guidance has backed
off slightly from previous runs. We will have a better idea of the
potential tomorrow once we see the tidal departures this evening.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...Coastal Flood Advisory until 9 PM EDT this evening for
     NJZ012>014-020-022>027.
DE...Coastal Flood Advisory until 9 PM EDT this evening for DEZ003-
     004.
MD...None.
MARINE...Small Craft Advisory from noon Friday to 6 AM EDT Saturday for
     ANZ450>455.

&&

$$
Synopsis...Gorse
Near Term...Johnson
Short Term...Johnson
Long Term...Gorse
Aviation...Gorse/Johnson
Marine...Gorse/Johnson
Tides/Coastal Flooding...



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