Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 262009
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
409 PM EDT Wed Jul 26 2017

.SYNOPSIS...
High pressure across the northeast will move offshore tonight. A
cold front will approach the region on Thursday before dissipating.
A warm front will move northward into the region late Thursday. Low
pressure will form along the frontal boundary, moving through our
area on Friday and Saturday. High pressure will then return later
Saturday and persist through Tuesday. Another cold front will
approach the region on Tuesday or Wednesday.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
High pressure centered over the Northeast will continue to drift
offshore this evening. Meanwhile, developing and strengthening low
pressure over the Midwest will begin to push east. A warm front out
ahead of that low will lift towards the Appalachians by daybreak
Thursday.

For this evening, skies will clear out as diurnally driven stratocu
will dissipate shortly after sunset with loss of diurnal heating.
This will yield mainly clear skies through midnight or so. With a
weak pressure gradient due to the high moving offshore, can expect a
light southeast flow (less than 5 MPH) to calm winds throughout the
forecast area. This combination will result in fairly decent
radiational cooling conditions for this time of year for the coastal
plain and the Pocono Mountains. With the onshore component to the
winds, however, this results in surface moisture increasing across
much of NJ, southeast PA, and the Delmarva with dewpoints slowly
climbing up through the low to mid 60s.

In the favored radiational cooling areas, can expect radiational
fog, stratus, and possible drizzle to develop after midnight tonight
as air temps fall close to the dewpoint.

Lows tonight will fall into the upper 50s to low 60s in the Poconos,
northwest NJ, the coastal plain, and the Delmarva, and in the mid to
upper 60s in the Lehigh and Delaware Valleys.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
Low pressure moves into the Ohio Valley on Thursday. The warm front
out ahead of the low will lift into the Lehigh Valley and Pocono
Mountains Thursday morning, and there may be just enough lift to
touch off a few showers with its approach. Will cap PoPs at slight
chance for areas west of the Fall Line. Any showers that develop
should taper off by late Thursday morning.

Otherwise, warm front continues its eastward approach throughout the
day, and seems to get hung up across southern NJ by late afternoon.
Latest models are fairly consistent in holding off on bulk of precip
until late in the day, so will adjust hourly PoP grids to hold off
on the higher PoPs until after 4pm-5pm. With SB CAPE values of 1000-
1500 J/kg expected across the region, there should be enough
instability for convection. The question will be if any convection
will be strong enough to overcome any capping inversions shown on
BUFKIT sounding profiles at 800-700 mb. Will keep PoPs and thunder
chances capped at chance through 7pm.

A warm and increasingly humid day on tap with higher generally in
the low to mid 80s and dewpoints rising into the 60s/low 70s.

&&

.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
Unsettled, wet weather will affect the region from Thursday night
through much of Saturday before we see a couple of dry days from
Sunday through the early part of next week.

Thursday night starts with a strong upper level shortwave moving
through the region. At the surface, a weak cold front dissipates as
it moves into the area while a warm front start to push northward
into the region. Limited instability does exist as the shortwave
crosses the region so there is the potential for thunderstorms to
develop and some may become severe. SPC has lowered the severe risk
for Thursday night to marginal.

Friday will be the day when the rains move into the area. Low
pressure will form along the frontal boundary and travels along it
and into our area on Friday. PWATS will increase in the southerly
flow and

The biggest threat from Thursday night through Saturday looks to be
mainly from heavy rain. Antecedent conditions are extremely wet and
the ground is not able to do much more than create runoff in many
areas across our forecast area. Precipitable water values will once
again rise to 1.5-2.0 inches and it is becoming much clearer that
parts of the area will see decent rainfall through Saturday. Models
continue to show a solid swath of 2-4 inches of rain around the I-
195 corridor to I-95 and points to the south and east, with 1 to 2
inches possible across the northern and western areas. Held off on
issuing any kind of flash flood or flood watch at this time due to
timing and location uncertainties but one will need to be
considered, mainly for Friday into Saturday. Not as sold on
convection for Friday but again, some instability is present and
some thunderstorms may develop across the region. Increased rainfall
totals will be possible wherever thunderstorms develop.

The low deepens and picks up speed as it moves to the northeast on
Saturday. Strong, gusty winds, especially along the coast, will be
possible. However, we should see conditions improve by the afternoon
hours as high pressure spreads into the region. With a decent
northerly flow, temperatures will remain cooler across the region,
struggling to reach 80 across much of the region.

Quiet weather will continue for Sunday through Wednesday as the high
remains in control. A cold front may approach the region on Tuesday
or Wednesday but it looks like we might have limited moisture with
it and have kept the forecast dry for now.

&&

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

Low clouds hard to scour out, and MVFR CIGs of 2500-3000 feet may
persist through sunset. Otherwise, stratocu deck dissipates after
sunset with loss of diurnal heating.

With increasing low level moisture through the night, MVFR CIGs in
stratus and MVFR VSBYs in BR/DZ likely for terminals away from the I-
95 corridor, though MVFR conditions possible at KTTN. Kept KTTN,
KPHL, KPNE, and KILG VFR with 18Z TAFs.

Conditions otherwise gradually improve to VFR on Friday, but
probably not until after 16Z

LGT/VRB winds become S-SW 5-7 KT late Thursday morning.

OUTLOOK...

Thursday night thru Saturday...Periods of MVFR/IFR conditions in
showers and thunderstorms. Strong northeast winds on Saturday with
gusts up to 25 knots possible, especially at KACY.

Saturday night thru Monday...Mainly VFR conditions expected.

&&

.MARINE...
Winds and seas will remain below SCA thresholds tonight through
Thursday. Seas are around 3 to 4 feet on the ocean with onshore
winds around 10 to 15 knots.

OUTLOOK...

Thursday thru Friday...Sub-SCA conditions expected. Locally higher
seas and gusty winds in the vicinity of thunderstorms through Friday
night.

Saturday and Sunday...both seas and winds will increase as low
pressure deepens along the Mid-Atlantic coast. A Small Craft
Advisory will likely be needed. Gale force gusts possible on
Saturday.

Sunday night through Monday...Sub-SCA conditions are expected.

RIP CURRENTS...

With onshore flow and waves in the surf zone around 3 feet, there is
a moderate risk for the development of dangerous rip currents at all
beaches through this evening.

A moderate risk may be in place on Thursday as well.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

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


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