Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS61 KPHI 270344
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1144 PM EDT Sat May 26 2018

.SYNOPSIS...
High pressure remains anchored well off the Southeast U.S.
coast through the weekend. A back door cold front approaches
tonight and then moves through the region Sunday, and will
become nearly stationary over the Mid-Atlantic and stretching
back to the south and west for the start of the new week.
Several waves of low pressure will impact the region through
Monday. Another cold front moves through the region on Tuesday.
Weak high pressure briefly builds in from the north on
Wednesday, and then the stationary boundary lifts north as a
warm front late in the week. Moisture from the remnants of
Subtropical Storm Alberto may affect the area late in the new
week.
&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...

1140 PM Update...Based on latest analysis and high res model
guidance, we have issued a Flash Flood Watch for most of our
eastern PA zones (except Carbon, Monroe Counties) through most
of central NJ north through the I-78 corridor. Expect that as
backdoor cold front moves south overnight into the region this
will act in tandem with upper support arriving from the west and
PWATs near 2 inches to trigger heavy showers and storms that may
tend to train repeatedly over the same areas. Biggest threat
area overnight will be across the north near I-78 with the
shower/storms and the associated threat shifting south by
morning.

Previous Discussion...The stronger thunderstorms from this
afternoon and earlier this evening are beginning to weaken as
daytime heating is waning. However, a surface trough remains
across the area and has actually pushed back northward this
evening. Meanwhile, a back door cold front continues to sink
southward toward the area later this evening and overnight. Even
through the stronger thunderstorms have ended, there will
remain a chance of showers and thunderstorms through the
overnight hours, especially as we go past midnight and through
daybreak. There will likely be a lull in activity for most areas
for the next few hours, then as the backdoor cold front
approaches, and additional short wave/vorticity impulses,
showers may become more concentrated across the northern half of
the area. With some instability across the area, a few
thunderstorms will remain possible. PW values remain 1.5-2.0
inches overnight, so some locally moderate to heavy rainfall is
possible.

Given the tropical air mass and heavy precipitation, patchy fog
will also be possible, but widespread dense fog is not
expected. It will be warm and humid, with low temperatures
ranging from the low-60s to low-70s, around 10 to 15 degrees
above normal.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/...
Quite a change on tap for Sunday as a backdoor cold front moves
from northeast to southwest across the area during the day,
with cold air advection in its wake. Meanwhile, above this very
shallow low-level feature, the mean tropospheric remains out of
the west-southwest, which will continue a transport of warm and
anomalously moist air atop this boundary. Precipitable Water
values will remain near 2.0 inches south of a PA Turnpike to
I-195 line, as a maritime air mass remains in place north of the
front. Meanwhile, several short wave perturbations will
traverse an increasingly west-east boundary, which will become
parallel to the mid-level flow. Closer to the front, there will
be sufficient instability for convection, with MU CAPE up to
1,000 J/Kg indicating thunder potential just to the north of
this feature as well. This all points to a continued heavy rain
potential though Sunday, with Delmarva again being the focus,
given the saturated ground and resultant lower threshold for
flash flooding discussed previously.

The greatest potential for thunder will be across Delmarva,
where surface based CAPE may approach 2,500 J/Kg during the
afternoon and evening just southwest of the front. While Bulk
Shear remains weak, generally AOB 30 kts, steeper 0-3 Km lapse
rates may favor gusty winds across this area with any convection
that develops.

There will be quite a contrast in high temperatures across the
region. The post frontal, maritime air mass will feature cloudy
skies and temperatures actually falling through the day (non-
diurnal trend), with highs most likely being reached during the
morning across northern NJ and northeast PA. Northward of the PA
Turnpike to Atlantic City Expressway, temperatures will not
rise much above overnight lows. To the southwest, high
temperatures will range from the mid-70s to mid-80s. Of course,
this is all dependent on the frontal passage timing, with the
higher resolution given greater weight given the shallow nature
of back door fronts.

To the north of the front, cloudy skies, northeast wind gusts
around 20 mph (closer to 30 mph along the northern NJ coast),
and the moist air mass will make it feel somewhat raw outside,
especially compared to the recent string of warm days!

&&

.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
Showers and thunderstorms taper off Sunday evening as low pressure
moves offshore. Another low develops to the west Sunday night and
passes through the Delmarva along a stationary boundary south of the
area late Sunday night and into Monday morning. This may keep
lingering showers in the forecast through Monday morning, and then
conditions should dry out in the afternoon as a back door cold front
sags southward through the Northeast. Skies remain cloudy for much
of the day as an onshore flow ushers a cool and damp airmass into
the region. This will also keep the temperatures down, as highs will
generally top off in the mid and upper 60s at the beaches and
coasts, and generally in the low to mid 70s elsewhere.

High pressure passes to the north on Tuesday, and then a back-door
cold front moves through the region late in the day. Ahead of that
boundary, though, a warm and humid airmass will be in place. Highs
top off in the mid and upper 80s with surface dewpoints well in the
60s. Scattered convection looks to develop over central PA and may
move into southeast PA, western NJ, and the Delmarva. Will follow
guidance and cap PoPs at slight chance. The moisture and the
instability will be there for convection over the area, it just
remains to be seen if there is enough of a trigger for storms to
develop.

During this time, what is currently Subtropical Storm Alberto over
the eastern Gulf of Mexico will lift north and affect the eastern
Gulf Coast states. By Wednesday, the remnant low of this storm will
lift into the Midwest. This also pushes that stationary front north
as a warm front. High pressure over the Northeast moves offshore by
Wednesday night, and then that warm front will move into the
Delmarva and Delaware Valley.

Unsettled weather then possible for the end of the week and start of
the weekend as the remnant low of Alberto meanders over the
Northeast and Mid-Atlantic. Best chances for precip will be Thursday
through Friday. Given high levels of uncertainty, will generally run
with chance PoPs for next weekend. However, given that this low will
arrive as a warm and tropical airmass will be over the area with
dewpoints well in the 60s, there is the threat for heavy rain and
flooding during this time.

&&

.AVIATION /04Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR conditions will continue for the first half of the
overnight period, unless a shower/thunderstorms affects a TAF
site. MVFR will develop across much of the area mainly in low
clouds as well as some patchy fog. Localized IFR restrictions
are possible due to lower ceilings and heavy downpours in
showers and thunderstorms, but were left out of the TAF at this
time due to the uncertainty of occurrence. Winds out of the
southwest 5 to 10 kts, becoming light and variable at times.

Sunday...Conditions are expected to continue to lower to IFR
during the morning Sunday. While ceilings and visibilities may
temporarily improve later in the morning or early afternoon,
they are expected to lower once again later in the afternoon and
evening. Winds will shift from the southwest to the northeast
during this time frame, and may gust up to around 20 mph,
especially at KPHL.

Outlook...

Sunday night...IFR/LIFR CIGs as marine airmass overspreads the
region. Showers and thunderstorms taper off in the evening, then
there is a chance for more showers late at night. VSBY
restrictions in fog/mist possible. NE winds less than 10 kt.

Monday...Lingering IFR CIGs and showers in the morning. MVFR
CIGs in the afternoon. NE winds less than 10 kt become SE late.

Tuesday...Mostly VFR. Potential for afternoon SHRA/TSRA. W-NW
winds less than 10 kt.

Wednesday...Mostly VFR. Light East winds become SE 10-15 kt.
Stronger winds possible at KACY. Potential for SHRA/TSRA late in
the day and SHRA at night.

Thursday...SHRA/TSRA possible throughout the day. S winds 10-15
kt.

&&

.MARINE...
Tonight...Sub-SCA conditions are expected at this time.

Sunday...A backdoor front will progress southward across the
coastal waters. Winds will shift from southwest to northeast
along with the frontal passage. The strongest northeast winds
are expected across the northern coastal waters of NJ
(ANZ450-51), where gusts of 25-30 kt are most likely to occur.
There could even be brief gale force gusts in the coastal waters
off Monmouth County (ANZ450). Seas in the ANZ450-51 may build
to 5 ft in response to the strengthening onshore flow. A SCA has
been issued in accordance with the frontal passage timing, as
northeast winds will rapidly increase behind the front, as
follows: ANZ450 at 10Z and ANZ451 at 13Z.

Outlook...

Sunday night...Any lingering SCA wind gusts will diminish in
the evening. NE winds 10-15 kt. Ocean seas 2-4 feet, and waves
on DE Bay 2 feet or less. VSBY restrictions in fog possible.

Monday through Thursday...Winds and seas below SCA levels.

Rip Currents...

Through this evening...Southwest winds 10 to 20 mph with waves
of 3 to 4 feet in the surf zone. Thus, a low risk for the
development of dangerous rip currents.

A back door cold moving southward across the area on Sunday
will cause winds to shift from the south-southwest to the
northeast. While swell periods are expected to range from 5 to 7
seconds, an onshore flow will develop along the NJ oceanfront
during the day, with wind gusts up to 20-30 mph. This will build
waves in the surf zone up to 3-5 feet. In particular, the rip
current risk will be greatest for the Monmouth and Ocean County
beaches, where the winds will be strongest (25-30 mph), waves in
the surf zone will be highest (4-5 ft), and where a northeast
flow yields more of an onshore component due to the orientation
of the coastline.

In terms of the risk for the development of dangerous rip
currents: a solid moderate risk of rip currents is expected for
the Monmouth and Ocean County beaches, perhaps approaching high
in some spots. Further south, a moderate risk for the Atlantic
and Cape May County beaches, and a low risk for the Delaware
beaches.

Remember, a low risk of rip currents does not mean no risk!
Life threatening rip currents often still occur near jetties,
reefs, and piers. The vast majority of rip current victims swam
at unguarded beaches. ALWAYS SWIM IN THE PRESENCE OF A
LIFEGUARD.

Sunday will certainly not be a `beach day` given cloudy skies
and the chance of showers, with cooler than average temperatures
in the mid-60s to mid-70s, along with northeast wind gusts
20-30 mph.

&&

.TIDES/COASTAL FLOODING...
Spotty minor tidal flooding is possible with the Sunday evening
high tide along the NJ and DE oceanfront, Delaware Bay, and the
late Sunday night high tide along the tidal portion of the
lower Delaware River. In particular, the potential for minor
flooding is greatest along northern NJ coastline, where onshore
flow will be stronger and more persistent.

&&

.EQUIPMENT...
The Sandy Hook NOS tide gauge (SDHN4) remains out of service.
CORMS is addressing the problem. There is currently no estimated
time for a return to service.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Flash Flood Watch until 10 AM EDT Sunday for PAZ060>062-070-
     071-101>106.
NJ...Flash Flood Watch until 10 AM EDT Sunday for NJZ007>010-
     012>015-017>020-022-025>027.
DE...None.
MD...None.
MARINE...Small Craft Advisory from 6 AM Sunday to midnight EDT Sunday
     night for ANZ450.
     Small Craft Advisory from 9 AM Sunday to midnight EDT Sunday
     night for ANZ451.

&&

$$
Synopsis...MPS
Near Term...Fitzsimmons/LF/Robertson
Short Term...LF
Long Term...MPS
Aviation...LF/Robertson/MPS
Marine...LF/MPS
Tides/Coastal Flooding...
Equipment...


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.