945
FXUS61 KPHI 170152
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
952 PM EDT Fri Aug 16 2024

.SYNOPSIS...
High pressure over the Northeast slides offshore late tonight.
Low pressure over the Great Lakes will slowly track east and
drag a cold front towards the region over the weekend. The cold
front will finally cross the region on Monday, and then high
pressure builds in from the west. A weak cold front may pass
through on Tuesday afternoon, and then high pressure gradually
builds overhead through the end of the new week.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
950 PM...No major changes were made to the forecast with the mid
evening update. Some echos can be seen on radar not too far away
from our western zones but there doesn`t appear to be a lot
actually reaching the ground on this leading edge as there`s lot
of mid level dry air. In other words, leading edge of what moves
in overnight may be virga initially. Previous discussion follows
below.

Expect clouds to increase through tonight. Guidance maintains a
slightly slower progression so most of the area should stay dry
tonight and any areas that do see some widely scattered
showers, likely won`t see them until well after midnight
tonight. PoPs 20-40% for the western third of the region
(increasing from east to west) with lower PoPs everywhere else.
With increasing clouds and continuing light southerly winds,
lows look to be in the upper 60s to near 70 across most of the
region.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH MONDAY/...
Unsettled weather on tap for the weekend and start of the new
week. Hurricane Ernesto will track from south to north along,
but well east of, the Delaware and New Jersey coasts. While
there will be no direct impacts from Ernesto (any thunderstorm
winds or flooding will be unrelated), large swells emanating
from Ernesto will result in dangerous rip currents at the
Delaware Beaches and Jersey Shore, as well as the potential for
minor coastal flooding around the times of high tide. Those
impacts will be discussed in the Marine and Tides and Coastal
Flooding sections below.

Broad low pressure will have a cold front trailing from it, and
will be associated with a deep upper trough that will dig down
through the Ohio and Tennessee Valleys and into the Gulf Coast
states. This system will meander in those areas Saturday through
Sunday, though several shortwaves will rotate through the base
of the trough, keeping conditions unsettled.

The first of these shortwaves will approach on Saturday,
followed by several stronger shortwaves Saturday night. Although
there are chances for showers in the morning, the bulk of the
activity will hold off until the afternoon. By then, southerly
flow will usher an increasingly humid airmass into the region
with highs in the 80s and surface dew points rising through the
60s and into the low 70s. Although there will be chances for
thunderstorms, the severe threat will be low on Saturday. PWATs,
however, will rise to 1.8 to 2.0 inches. Locally heavy rain
will be possible, mainly north and west of the I-95 corridor,
Saturday afternoon and evening. Showers and scattered
thunderstorms, mainly producing heavy rain, will really get
going Saturday night and into Sunday morning. The Weather
Prediction Center has a Marginal Risk (1 out of 4) for Excessive
Rain for southeast Pennsylvania and a portion of northern New
Jersey during this time.

A secondary low looks to form west of Delmarva Saturday night,
and then this low will lift to the northeast across New Jersey
Sunday. On Sunday, the broad low over the Great Lakes will move
into western New York, dragging the cold front closer. The
southerly pressure gradient will tighten over the area, ushering
tropical moisture into the region. As the front approaches and
that secondary low passes through, showers and thunderstorms
will develop in the afternoon and into Sunday night. Surface dew
points will rise into the low to mid 70s, resulting in PWATs
over 2 inches. SB CAPE values will rise to over 1000 J/kg, and
even 0-6 km Bulk Shear will rise to 35 to 40 kt. From Sunday
afternoon through Sunday evening, there is a risk for severe
thunderstorms. The Storm Prediction Center has a Marginal Risk
(1 out of 5) for severe storms for much of the forecast area
during this time, with damaging wind gusts being the primary
threat. Heavy rain producing flash flooding is also expected,
and the Weather Prediction Center has the region in a Slight
Risk (2 out of 4) for Excessive Rainfall for much of the region.

The secondary low should be northeast of the region by Monday
morning, and showers and thunderstorms will taper off then.
There should be a lull in the showers and thunderstorms for most
of the day until the cold front passes through in the
afternoon. This will trigger another round of showers and
thunderstorms over the area, though behind the departing low in
the morning, a somewhat less humid airmass will spread into the
region. The threat for flash flooding will be more based on if
storms pass over flooded areas than storms being capable of
flooding themselves.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
The base of a deep upper trough will settle over the East Coast
for the mid-week period. The cold front working through the
region late Monday will move offshore Monday evening. A
secondary cold front follows through on Tuesday afternoon, and
then surface high pressure builds over the Great Lakes and Ohio
Valley. The upper trough slides offshore by Friday. Meanwhile,
the surface high builds east and will be overhead by the end of
the work week.

In terms of sensible weather, any showers and thunderstorms
will taper off in the evening with the loss of diurnal heating.
A much more autumnal airmass will then spread into the Northeast
and Mid- Atlantic for the mid-week period. High temperatures
will be several degrees below normal with highs in the 70s both
Tuesday and Wednesday. The passage of that secondary cold front
Tuesday afternoon may spark off some afternoon showers and
thunderstorms, but based on the NBM, PoPs will be capped at
slight chance for most of the region, with some low end chance
PoPs in northern New Jersey as some shortwave energy passes
through.

Sunny, dry, and much less humid conditions on tap from
Wednesday through Friday. Radiational cooling conditions likely
at least Tuesday night and Wednesday night, so went with a blend
of NBM:NBM10 for low temperatures those nights. Using this
blend, lows will be mainly in the 50s both nights, but around 60
near the coasts due to the proximity of the warmer ocean
temperatures, and in the 40s in the southern Poconos! The
temperatures on Tuesday and Wednesday will be more similar to
late September than late August. Get your pumpkin spice lattes
ready!

Temperatures moderate a bit on Thursday and Friday with highs
in the upper 70s to low 80s. Humidity levels creep up a bit as
dew points rise to around 60 by Friday.

&&

.AVIATION /02Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR conditions expected with increasing mid to high level
clouds, particularly after 06Z. An isolated shower may approach
KRDG/KABE after 09Z. SSE winds around 5 knots or less. High
confidence on prevailing VFR conditions.

Saturday...Times of sub-VFR conditions possible as showers and
storms arrive from the west in the morning and become more
numerous through the afternoon into the early evening hours.
VCSH as early as 15-18Z for RDG/ABE with chances for TSRA
increasing after 18Z for all terminals. Some locally heavy
rainfall possible.

Outlook...

Saturday night through Sunday night...Sub-VFR in SHRA, TSRA,
fog, and stratus. Locally heavy rain Saturday evening, then
again Sunday afternoon and Sunday night.

Monday through Monday night...Lingering SHRA/TSRA Monday
morning, then another round of SHRA/TSRA Monday afternoon and
evening.

Tuesday...Overall, VFR. Isolated SHRA/TSRA in the afternoon.

&&

.MARINE...
For tonight, a Small Craft Advisory remains in effect for
Atlantic Ocean coastal waters beginning at 10 PM EDT. Seas
initially 1-2 feet this morning building to 2-4 feet later this
afternoon. Seas begin to exceed 5 feet late this evening and
continue to build towards 5-7 feet by late tonight. Winds of
5-10 kts with gusts around 15 knots expected throughout the
period.

Small Craft Advisory in effect for the ocean for Saturday as
seas build to 5 to 7 feet due to larger swells from distant
Hurricane Ernesto. Sub-SCA conditions expected for Delaware Bay.
VSBY restrictions in showers and thunderstorms in the
afternoon.

Outlook...

Saturday night through Sunday night...SCA is in effect for seas
of 5 to 7 feet due to larger swells from distant Hurricane
Ernesto. The conditions on Delaware Bay will stay below Small
Craft Advisory criteria. VSBY restrictions in showers and
thunderstorms, as well as patchy fog.

Monday...Lingering SCA conditions as seas will be slow to
subside on the ocean. Sub-SCA conditions on Delaware Bay.

Tuesday through Wednesday...Sub-SCA conditions.

Rip Currents...

For Saturday and Sunday, Hurricane Ernesto will track well east
of our region. However, larger swells arriving from this storm
and breaking into the surf zone will result in very dangerous
rip currents at the Jersey Shore and at the Delaware Beaches. As
a result, a HIGH risk for the development of dangerous rip
currents is in effect for the Jersey Shore and Delaware Beaches
through Sunday evening. Breaking waves could reach as high as 4
to 7 feet. Some beach erosion is possible. The high risk of rip
currents could linger into early next week as well.

For specific beach forecasts, visit weather.gov/beach/phi

&&

.TIDES/COASTAL FLOODING...
With long period swells and rough seas/surf expected to arrive
this weekend from distant Hurricane Ernesto coinciding with an
approaching full moon on Monday, at least some spotty minor
coastal flooding is anticipated to develop along the Atlantic
coasts of New Jersey and Delaware, Delaware Bay, and the tidal
Delaware River. The primary high tide cycles of impact will be
evening to nighttime high tides, perhaps beginning as early as
Saturday in the most vulnerable areas. As long period swells
persist and astronomical high tides increase Sunday and Monday,
more widespread minor coastal flooding is forecast with the
evening to nighttime high tides those days.

Anyone with interests in these areas that are susceptible to
minor coastal flooding should monitor the latest forecasts and
remain alert for any advisories that may be issued in future
updates as confidence in the forecast increases.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...High Risk for Rip Currents through Sunday evening for NJZ014-
     024>026.
DE...High Risk for Rip Currents through Sunday evening for DEZ004.
MD...None.
MARINE...Small Craft Advisory until 6 AM EDT Monday for ANZ450>455.

&&

$$

SYNOPSIS...MPS
NEAR TERM...Fitzsimmons/MJL
SHORT TERM...MPS
LONG TERM...MPS
AVIATION...Fitzsimmons/MJL/MPS
MARINE...Fitzsimmons/MJL/MPS/Staarmann
TIDES/COASTAL FLOODING...