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627
FXUS61 KPHI 170732
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
332 AM EDT Tue Sep 17 2024

.SYNOPSIS...
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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.
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&&

.NEAR TERM /THROUGH TONIGHT/...
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Onshore flow continues this morning, bringing along higher dew
points in the upper 50s to low 60s with low clouds. A few
isolated showers will be possible across southern Delaware and
the eastern shores of Maryland towards daybreak.

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 the 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.
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&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
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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. This is
supported well by the Weather Prediction Center which now keeps only
southern NJ and the Delmarva in a Marginal Risk for Excessive
Rainfall on Wednesday.

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.
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&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
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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.
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&&

.AVIATION /07Z TUESDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Rest of tonight (through 12Z)...Widespread MVFR ceilings early
this morning look to drop to IFR at times a few hours prior to
daybreak. Guidance has snapped back to climatology with a
rather saturated boundary layer supporting more widespread low
stratus as a result of persistent onshore flow. Expecting all
terminals (with the exception of ABE) to see cig restrictions
early this morning. East to northeast winds around 10 knots
decreasing to around 5 kts overnight. Moderate confidence with
cig restrictions, lower confidence on exact timing and coverage
of any IFR ceilings.

Tuesday...Any lingering sub-VFR ceilings should improve to VFR
by 15Z. There is a slight chance (20%) of showers generally
after 18Z for most TAF sites, but have only mentioned them for
MIV/ACY where the greatest chance is. East winds increasing to
around 10 knots. Moderate confidence.

Tuesday night...VFR to start before sub-VFR conditions arrive
overnight with periods of rain, mainly south and east. ENE to NE
winds around 5-10 kts. Low confidence.

Outlook...

Wednesday through Wednesday Night...Periods of MVFR/IFR likely with
scattered showers and isolated thunderstorms. Low clouds hang around
in between periods of rain. Gusts near 20 kt possible at KACY.

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.
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&&

.MARINE...
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SCA conditions will continue across the waters, save the upper
Delaware Bay, Tuesday into Tuesday night. East to Northeast
winds around 15 kts with gusts around 25 kts expected. Seas
will be generally 5 to 8 ft. Scattered showers possible late
Tuesday into Tuesday night.


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 Tuesday, 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 Tuesday and
Wednesday.

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

.TIDES/COASTAL FLOODING...
Evening high tide cycle has passed for most of the Atlantic
coast and Delaware Bay (still coming for the tidal Delaware
River). Minor flooding has been confined to the advisory area
which seems to be working out well. Will let the advisory
continue through the expiration time of midnight as there is
some lingering flooding on back bays.

Not anticipating any tidal flooding with the overnight/Tuesday
morning high tide as it is the lower of the astronomical high
tides. However, additional minor flooding is possible starting
with the high tide cycle Tuesday evening, and may see a few
consecutive high tide cycles after that with minor tidal
flooding concerns as water is anticipated to continue to pile up
in the back bays and up the Delaware River with prolonged
onshore flow. This may result in further advisories. However,
will wait for the latest guidance to come in before making
decisions on future advisories.

No tidal flooding is expected along the northeastern shore of
Maryland currently.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None.
NJ...High Risk for Rip Currents through Wednesday evening for NJZ014-
     024>026.
DE...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.
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&&

$$

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