-- Discussion containing changed information from previous version are highlighted. --
401
FXUS61 KPHI 171732
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
132 PM EDT Tue Sep 17 2024

.SYNOPSIS...
An area of low pressure tracks northwestward into the Carolinas
while a secondary low passes off to our east, bringing increasing
chances for showers across our area through midweek. Unsettled
conditions persist through the end of the week and possibly into the
weekend as an area of low pressure lingers offshore though high
pressure will be trying to build in.

&&

.NEAR TERM /THROUGH TONIGHT/...
A cut off low pressure system that we have been monitoring the
past few days continues to shift NNW across the Carolinas while
an elongated area of high pressure over coastal New England
gradually shifts offshore. The resultant onshore flow will keep
things cloudy and cool for Tuesday with increasing chances for
showers through this afternoon and into tonight as a secondary
coastal low is slated to begin organizing near Cape Hatteras.
Initially, this low will actually help cut off an otherwise more
organized plume of tropical moisture coming in from the
Atlantic as it crawls up the eastern seaboard. How close it
tracks to land will determine just how much rainfall we get, but
at the moment it is forecasted to remain offshore, allowing a
dry and cool northeast flow on the back side of the low to
prevent more organized precip from pushing inland Tuesday night.
As a result, overall QPF values have diminished to around a
half inch or less across the Delmarva and South Jersey, with
highest amounts along the coastlines. That said, locally higher
amounts will be possible. If the tropical airmass does shift
inland further, we could be looking at a few heavy tropical
downpours late afternoon and into the nighttime hours.

Cool temperatures will persist with this pattern, keeping
afternoon highs in the upper 70s to around 80 and overnight lows
in the low to mid 60s.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
Tropical moisture will be advected north from the remnants of an
area of low pressure that moved ashore over the Carolinas yesterday.
This will result in some showers and scattered heavier downpours,
mainly near the coast and over the Delmarva peninsula supported by
several shortwaves embedded in the upper flow.

The first wave still looks to be on pace to affect the region late
Tuesday night into the day on Wednesday. This will be associated
with a secondary coastal low that develops off the Carolinas
separating itself from the upper low off to the west. This secondary
low will lift northward paralleling our coastline while remaining
offshore. The biggest difference at this point amongst models, is
just how far offshore this secondary low will be. If the low, takes
a closer track to the coast, then more in the way of heavy rain
looks plausible during the day on Wednesday. If the track is further
offshore as indicated by some guidance, there may be enough dry air
filtering in from the north to keep any beneficial rainfall
offshore. Regardless, PWATs are forecast to be around 2 inches, so
some tropical related downpours are possible for Wednesday. In terms
of the flood risk, the risk really isn`t that high due to the dry
conditions that have occurred over the past couple of weeks.
Given how the flood risk looks low, WPC removed the entire area
from the Marginal Risk of Excessive Rainfall.

Heading into Wednesday night and Thursday, guidance is overall
struggling but the 00z suite fell more in line with the 12z GFS,
which has the upper level low transitioning into an open trough
and hanging over the region through Thursday night. This will
result in unsettled weather, with some showers around with an
area of low pressure meandering off the New Jersey coast. Given
the positioning of the low, thinking showers are mainly from the
Fall Line on east, with drier conditions expected in the Lehigh
Valley and Poconos. Overall though, PoPs are only around 20%
Wednesday Night through Thursday Night as things have trended a
bit drier. Rainfall amounts have trended down as well, with a
half inch to an inch and a quarter expected for lower Delmarva
and the NJ coast, with amounts decreasing pretty quickly as you
go away from the coast. Currently have around a tenth to a
quarter of an inch in the forecast for the Philly metro, with
minuscule amounts north and west of the city.


In terms of temperatures, we are looking at highs in the 70s on
Wednesday and Thursday with lows in the 60s with limited
diurnal variation thanks to the tropical airmass in place.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
The blocking pattern aloft will likely continue as a deep trough
will remain within the vicinity of the East Coast through the
weekend. This will yield unsettled and cool conditions through the
end of the week, before turning potentially more tranquil to kick
off the weekend into next week.

As of now, most of the deterministic guidance keeps the deep upper
trough over the East Coast through the weekend before an upper ridge
begins to move over the top into early next week. Guidance wants to
develop another coastal low somewhere off the Mid-Atlantic coast
later this week, and have it meander within the stagnant pattern as
well. This will keep the threat for showers and cool weather to
occur through the weekend, though high pressure tries to build in
from the north and east by the second half of the weekend. This will
result in some showers chances, especially for the southern half of
the area and areas along the coast, with drier conditions to the
north and west. However, the placement of the these features will
have large implications on how Friday onward turns out and guidance
generally struggles with blocking patterns and the location of
developing coastal lows, so there likely will be some variance to
the forecast in the coming days.

In terms of temperatures, most of the region does appear to stay on
the cooler side of normal with temps mostly in the 70s during the
day and in the 50s at night.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
-- Changed Discussion --
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Rest of Today...A mix of MVFR/VFR ceilings expected for the
rest of the day with a cloud deck around 2500-4000 feet. East
winds around 5-10 kt occasionally gusting up to 15 kt. Moderate
confidence.

Tonight...MVFR/VFR ceilings to continue before dropping to MVFR
at all terminals by 03-04Z. Ceilings should fall to IFR for
almost all terminals by 08-09Z. Occasional showers possible.
East-northeast winds around 5-10 kt. Moderate confidence, but
lower confidence with regard to timing.

Wednesday...MVFR/IFR ceilings expected to continue with periods
of rain and showers through the afternoon. Northeast winds
around 10 kt gusting up to 15-20 kt in the afternoon. Low
confidence.

Outlook...

Wednesday Night...Periods of MVFR/IFR likely with scattered
showers and isolated thunderstorms. Low clouds hang around in
between periods of rain.

Thursday through Thursday Night...Primarily VFR, though some
showers around could result in some restrictions.

Friday through Friday Night...Sub-VFR conditions possible (30-40%)
with scattered showers. Low clouds hanging around as well in between
periods of rain.

Saturday...Primarily VFR. Some scattered showers around. Gusts near
25 kt possible at KACY.
-- End Changed Discussion --

&&

.MARINE...
SCA conditions will continue across the waters, save the upper
Delaware Bay through tonight. East to northeast winds around 15
kt with gusts around 25 kt expected. Seas will be generally 5
to 8 ft. Scattered showers possible later today into tonight.

Outlook...

Wednesday through Wednesday Night...Small Craft Advisory was
extended for the ocean through Wednesday as seas will be around 5 to
6 feet. Gusts near 25 kt are possible at times, mainly on Wednesday
morning on the ocean. Seas begin to relax on Wednesday night. May
need an extension to the SCA though. No marine headlines expected on
the Delaware Bay

Thursday through Thursday Night...No marine headlines expected.

Friday through Saturday....Small Craft Advisory conditions expected
on the coastal waters as seas build toward 5 to 7 feet. Gusts near
25 kt as well, which may necessitate an SCA for the Delaware Bay.

Rip currents...

For today, east winds will average 15 to 20 mph with breaking
waves of 2 to 4 feet with an 8 to 9 second period. This results
in a HIGH risk for the development of dangerous and life
threatening rip currents for the Delaware Beaches and Jersey
Shore. For Wednesday, east winds will decrease slightly to
around 15 mph but breaking waves remain 2 to 4 feet with an 8 to
9 second period. Therefore, the HIGH risk for the development
of rip currents was maintained for Wednesday. As a result, a Rip
Current Statement is now in effect for both today and
Wednesday.

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

&&

.TIDES/COASTAL FLOODING...
Coastal Flood Advisories were posted for tonight`s high tide for
areas where the advisory was last night (Cape May, Atlantic, and
Cumberland in NJ and Sussex and Kent in Delaware) as minor tidal
flooding is expected. Not anticipating any tidal flooding with
this morning high tide as it is the lower of the astronomical
high tides though.

Given the uncertainty with the upcoming pattern, have elected to
issue the advisory for just tonight`s high tide, though
extensions remain possible if not likely. We also will likely
need to issue further advisories for the rest of the NJ coast at
some point later this week with prolonged onshore flow and water
piling up, but tonight`s high tide should only produce spotty
minor flooding within Ocean, Monmouth, and Middlesex Counties.

For the upper Delaware Bay, only spotty minor tidal flooding is
expected with the high tide tonight and beyond.

No tidal flooding is expected along the northeastern shore of
Maryland currently and within the tidal Delaware River at the
moment.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT
     Wednesday for NJZ021>025.
     High Risk for Rip Currents through Wednesday evening for NJZ014-
     024>026.
DE...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT
     Wednesday for DEZ002>004.
     High Risk for Rip Currents through Wednesday evening for DEZ004.
MD...None.
MARINE...Small Craft Advisory until 10 PM EDT this evening for ANZ431.
     Small Craft Advisory until 6 PM EDT Wednesday for ANZ450>455.

&&

$$

SYNOPSIS...DeSilva/Hoeflich
NEAR TERM...DeSilva/MJL
SHORT TERM...DeSilva/Hoeflich
LONG TERM...DeSilva/Hoeflich
AVIATION...DeSilva/Hoeflich/MJL
MARINE...Hoeflich/MJL
TIDES/COASTAL FLOODING...