Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 212029
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
429 PM EDT Fri Jul 21 2017

.SYNOPSIS...
A very warm and humid air mass will remain over the area
through this weekend and possibly into Monday. Meanwhile, low
pressure along a roughly west to east frontal boundary will
result in periods of showers and thunderstorms. By Tuesday, high
pressure will build over the area bringing somewhat milder and
drier conditions through through the middle of next week. A
seasonally strong cold front may cross the area later in the
week.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
The Excessive Heat Warning will remain in place for the urban
corridor until 8:00 PM.

We are anticipating an increase in high level clouds this evening,
especially over the southern and central parts of our region, as a
convective complex slides from West Virginia into Virginia. Most of
the shower and thunderstorm activity with that system is expected to
slide to our south. However, we will mention a low or slight chance
of showers and thunderstorms for tonight in parts of eastern
Maryland, central and southern Delaware and far southern New Jersey.

The sky is expected to be mostly clear over Berks County, the Lehigh
Valley, the Poconos and northern New Jersey for much of the night
allowing some fog to develop in those areas.

A light and variable wind is forecast for tonight. Low temperatures
should range from the middle 60s in the elevated terrain up north to
the 70s in the Philadelphia metropolitan area, southern New Jersey
and on the upper Delmarva.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
We are forecasting some sunshine for the morning hours with an
increase in high clouds ahead of an approaching convective
complex in the afternoon.

The convective complex was located over Wisconsin and vicinity this
afternoon. It will continue to work its way to the east southeast.
It should begin to arrive in eastern Pennsylvania late on Saturday
afternoon. As the system moves into very warm and humid air on
Saturday it is expected to strengthen. The thunderstorms may become
strong to severe as they approach our region.

Conditions are anticipated to be slightly less hot on Saturday than
they were today. Maximum temperatures are forecast to range from the
lower 80s in the elevated terrain of the Poconos and northwestern
New Jersey to the lower 90s in our central and southern counties.
Maximum heat index values will likely remain below 100 except in
interior section of the Delmarva where heat index values may reach
100 to 103. All locations should remain below their excessive heat
criterion, so no warnings or advisories should be necessary.

A light west to southwest wind is anticipated to develop on
Saturday.

&&

.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
The extended period starting Saturday night features a
transition in the flow aloft from roughly zonal flow across the
northern US to a somewhat more amplified pattern with troughing
over the east US by the latter part of next week. In terms of
weather this translates to very warm/hot and humid conditions
this weekend transitioning to a milder and somewhat drier period
for at least part of next week.

A quasi-stationary east-west frontal boundary is expected to
persist across the area through the weekend, although it may
lift north a bit on Sunday. Low pressure along the front will
favor low-level convergence and areas of showers and
thunderstorms. PWAT values near 2 inches will support some
heavy downpours and possible flooding. Also SPC has a slight
risk of severe for Saturday and Sunday. Timing is tricky but
there is some indication of more likely precip for Saturday
night and possibly again Sunday night.

By Monday morning a fairly pronounced shortwave trof is forecast
to be over the Great Lakes and move east to New England by
Tuesday morning. The GFS is a bit slower than the ECMWF, but
either way the trof should force a cold front across the mid-
Atlantic and offshore by Tuesday morning. With the frontal
passage on Monday there will be a continued good chance for
aftn/eve t-storms. Max temps on Monday ahead of the front look
to be still a few degrees above normal.

Tuesday and Wednesday should be relatively pleasant with shrtwv
ridging aloft and surface high pressure moving across the area.
The high is associated with a somewhat cooler and drier air
mass. Looks like some low-level warm advection develops
Wednesday night and the forecast carries a chance of showers at
that time. Chance of precip increases on Thursday as another
shrtwv trof and associated cold front approach from the NW.

&&

.AVIATION /20Z FRIDAY THROUGH WEDNESDAY/...
VFR conditions are expected through the balance of the daytime
hours and into much of tonight. Showers and thunderstorms moving
across West Virginia and Virgina this afternoon and forecast to
remain to the south of our TAF sites. Patchy low clouds and fog
are anticipated during the late night and early morning hours.

VFR conditions are forecast for much of Saturday. Showers and strong
thunderstorms are expected to approach from the west late in the
day. They should begin arriving around KRDG and KABE about 2100 to
2200Z, and they are expected to reach KTTN, KPNE, KPHL and KILG
around or shortly after 2300Z.

A light west wind today is anticipated to become light and variable
for tonight, then west to southwest on Saturday.

OUTLOOK...

Saturday Night through Monday...Generally VFR conditions
expected, but temporarily lower cigs/vsbys in scattered showers
and thunderstorms are likely at times.

Tuesday and Wednesday...VFR conditions.

&&

.MARINE...
The wind is expected to favor the southwest around 10 knots
tonight and it should back to the south for Saturday. Wave
heights on our ocean waters will remain around 2 to 3 feet and
waves on Delaware Bay should be 1 to 2 feet.

Scattered showers and thunderstorms may affect the coastal waters of
Delaware and far southern New Jersey, as well as Delaware Bay,
tonight.

OUTLOOK...

Generally sub-SCA conditions are expected Saturday night through
Wednesday. However winds and seas may be locally higher in
scattered thunderstorms over the waters from Saturday night
through Monday.

RIP CURRENTS...

The energy associated with the underlying 10 to 14 second
southeasterly swell continues to weaken and the 6 second south
southeast swell has become prominent. As a result, we will continue
to carry a low risk for the development of dangerous rip currents
for the balance of today and the same should be true for
Saturday.

&&

.TIDES/COASTAL FLOODING...

The unusually high astronomical tides associated with the upcoming
new moon may result in some minor coastal flooding around the time
of high tide on Saturday evening.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Excessive Heat Warning until 8 PM EDT this evening for PAZ070-
     071-102-104-106.
NJ...Excessive Heat Warning until 8 PM EDT this evening for NJZ015-
     017>019.
DE...Excessive Heat Warning until 8 PM EDT this evening for DEZ001.
MD...None.
MARINE...None.

&&

$$
Synopsis...AMC
Near Term...Iovino
Short Term...Iovino
Long Term...AMC
Aviation...AMC/Iovino
Marine...AMC/Iovino
Coastal Flooding...Iovino



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