Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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642
FXUS61 KPHI 251957
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
357 PM EDT Tue Jul 25 2017

.SYNOPSIS...
High pressure centered over the Great Lakes will track east and
move overhead on Wednesday before drifting offshore on
Thursday. Low pressure developing over the Great Lakes and Ohio
Valley will push a warm front towards the region Thursday night.
That low slowly moves across the Northeast through the end of
the week. High pressure eventually builds in from the west for
the weekend and moves offshore early next week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
High pressure is centered over the Great lakes and will
continue its journey to the Northeast tonight, moving over over
our area by early Wednesday morning.

Flow around the high will keep us in a more north and east flow
regime. Winds will generally become light overnight and any
directional component will favor the east. With easterly flow
enmeshed over the area, we will see more clouds through the
overnight period. Additionally, moisture trapped below the
inversion will allow for some light showers/drizzle and some fog
to develop.

Overall, it will remain cool tonight with lows in the mid to
upper 50s over the Poconos and northern New Jersey and low to
mid 60s the Lehigh Valley, Delaware Valley, central/southern New
Jersey and Delmarva.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
The high will slide across the region on Wednesday. As the high
shifts overhead, winds should become more southerly through the
day. Skies will remain pretty cloudy at the start of the day as
the marine layer wins out. However, we should start to see some
clearing out later in the morning and some sun will be present
in the region.

Temperatures will remain on the cool side and in the 70s. A few
locations may break out, especially across the southern and
western areas and some lower 80s will be possible there.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
Overview...Unsettled weather on tap for most of the late week
period as intensifying low pressure slowly works across the
region. High pressure returns for the new week.

High pressure continues to drift offshore Wednesday night.
Behind the departing high, a warm front lifts north through the
region, and a warmer and more humid airmass will spread to the
north and east. Highs on Thursday will climb into the low to mid
80s, and surface dewpoints begin to creep back up into the
upper 60s and low 70s.

Upper trough with several strong shortwaves then approaches
ahead of a developing system upstream. That trough should be
enough to trigger some late afternoon and evening showers and
thunderstorms.

From here, there is considerable uncertainty among the models.
Low pressure develops over the Great Lakes and deepens over the
Ohio Valley. This pushes another warm front into the region
Thursday night, and then strong S-SW flow develops along much of
the East Coast. Low level moisture increases throughout the
region, and PWATs increase to over 2". The main problem is that
the placement of the low varies among the models. The GFS keeps
the low well south of the region Friday, and then that low
passes through the Delmarva and moves offshore Friday night
through Saturday morning. The NAM is a bit quicker and a bit
farther north, bringing a slug of heavy precip across much of NJ
Friday before a secondary low develops and slides across the
Delmarva Friday night. It is key to note that the final frame of
the 12Z NAM is 00Z Saturday, and the secondary low is right
over DE Bay at that time. The 12Z CMC-GDPS is similar to the NAM
in that it brings one low across the region during the day
Friday with a slug of heavy rain, and then a secondary low sits
over the region Friday night through Saturday, finally departing
Saturday night. The ECMWF also features a round of precip
Friday as one low passes through the region, followed by a
secondary low with more precip Friday night and into Saturday.

Given the uncertainty, will follow closely to the Superblend
guidance, as well as WPC long range guidance. This means likely
PoPs Thursday night and Friday, and then chance PoPs Friday
night and Saturday. Will continue to mention a chance for
thunderstorms, as well as heavy rain.

There is the potential for some 2-4 inches of rain, which, in
and of itself, could pose flooding problems. But given the mount
of rain that fell over the last couple of days, it is important
to follow this system, as additional flooding problems may occur
during the late week period.

Important to note the NAM showing a strong LLJ, generally on
the order of 40-50 KT, that does not show up on the GFS.

WPC Day 3 outlook (12Z Thursday to 12Z Friday) has most of the
Delaware Valley and the Delmarva in a Slight Risk for Excessive
Rainfall, and SPC Day 3 outlook (12Z Thursday to 12Z Friday) has
the whole CWA under a Slight Risk for Severe Weather.

Once this system departs, high pressure builds in from the west
by Sunday, and gradually moves offshore through Tuesday.
Temperatures will be near to just below normal with relatively
comfortable humidity levels.

&&

.AVIATION /20Z TUESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

VFR conditions this afternoon will start to deteriorate later
this evening overnight as the onshore flow continues to stream
low level moisture in the region. MVFR conditions overnight in a
mix of low clouds and fog. Isolated showers are possible
through this afternoon and tonight.

Conditions will improve through Wednesday morning becoming VFR
everywhere between 13Z-16Z. The winds will pick up out of the
east before become more south to southeast through the day.

OUTLOOK....

Wednesday night...Potential for MVFR or lower CIGs.

Thursday...Potential for MVFR or IFR CIGs in SHRA/TSRA, mainly
in the afternoon and evening.

Thursday night through Friday night...IFR or lower conditions in
moderate to heavy rain. Chance for TSRA as well.

Saturday...Lingering sub-VFR conditions possible through the day
in -SHRA.

Saturday night...Improving conditions.

Sunday...VFR.

&&

.MARINE...
Relatively quiet conditions are expected to continue on the
area waters through Wednesday. Seas are around 3 to 4 feet on
the ocean with winds around 10 to 15 knots. Winds will lighten
up overnight and again be around 10 to 15 knots on Wednesday.

OUTLOOK...
Wednesday night...Sub-SCA conditions.

Thursday through Friday night...Generally sub-SCA conditions
with gusts to 20 KT likely. Heavy rain with VSBY restrictions
possible.

Saturday...SCA conditions possible as low pressure works its way
across the waters.

Sunday...Improving conditions on the waters.

RIP CURRENTS...
A moderate risk for the formation of dangerous rip currents is
expected for the New Jersey beaches in Atlantic, Ocean and
Monmouth Counties owing to an underlying longer period swell and
a relatively strong northeast wind (15-20 mph). Farther south
along the coast, where the winds will be lighter and backed out
of the north or northwest, the risk is lower.

&&

.TIDES/COASTAL FLOODING...
Astronomical tides will continue to decrease as we get further
from the new moon. However, onshore flow continues. Therefore,
minor tidal flooding is expected along the New Jersey shore and
Delaware Beaches and the nearby back bays. It is much less
certain if minor flooding will occur along the shores of the
upper Delaware Bay and the tidal Delaware River. For these
locations the wind direction is not favorable for minor tidal
flooding and unlike the last few days, there won`t be any heavy
rain exacerbating already high tides. Therefore, will hold off
on an advisory for the tidal Delware River, but will watch tidal
levels along the lower Bay very closely this evening.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...Coastal Flood Advisory from 9 PM this evening to 2 AM EDT
     Wednesday for NJZ012>014-020-022>027.
DE...Coastal Flood Advisory from 9 PM this evening to 2 AM EDT
     Wednesday for DEZ003-004.
MD...None.
MARINE...None.

&&

$$
Synopsis...MPS
Near Term...Meola
Short Term...Meola
Long Term...MPS
Aviation...Meola/MPS
Marine...Meola/MPS
Tides/Coastal Flooding...



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