Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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695
FXUS61 KPHI 260928 AAA
AFDPHI

Area Forecast Discussion...UPDATED
National Weather Service Mount Holly NJ
528 AM EDT Sat May 26 2018

.SYNOPSIS...
High pressure will be centered well east of the Mid-Atlantic coast
this holiday weekend. A cold front moves southward across our area
late tonight and Sunday, then stalls just south and west of our area
on Memorial Day. A secondary cold front moves through the area on
Tuesday. High pressure builds in from the north Tuesday night before
moving offshore Wednesday. Moisture from the remnants of Subtropical
Storm Alberto may affect the area late in the week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
High pressure over the western Atlantic is maintaining a SW
flow bringing in warmer and more humid air with time. This could
also result in a bit of patchy fog early this morning.

Heading into the day Saturday, a hot and humid day on tap as
the southwest flow really allows for a tropical airmass to build
into the region throughout the day. Meanwhile, an upper trough
with several shortwaves will approach during the day and move
across the region in the afternoon into Saturday night.

Highs will generally top off in the mid and upper 80s, with
highs around 90 along the I-95 corridor from Newark to Philly,
and in inland areas of the Delmarva. Surface dewpoints will
climb into the 60s in the southern Poconos and northern NJ, and
in the upper 60s to around 70 across southern NJ, southeast PA,
and the Delmarva.

As the upper trough approaches, scattered showers and
thunderstorms will develop by early to mid afternoon over our
eastern PA zones..aided by bay breezes from the Chesapeake and
terrain circulations in the southern Poconos. Forecast models
indicate ML CAPE values climbing to 1500-2000+ j/kg with deep
layer shear values approaching 30 knots. There also looks to be
a dry layer in the mid levels...at least through the afternoon
however PWATs will rise past 1.75 inches. So this all said,
heavy rain looks to be the biggest threat with storms and this
could lead to urban and small stream flooding with even the risk
of localized flash flooding. However given the fairly high CAPE
and mid level dry layer, isolated damaging winds may be
possible as well as precip loading may aid in downdraft
accelerations along with DCAPES already progged to be 700-1000+
j/kg. Scattered Storms initially developing over eastern PA
will track east and may affect the I-95 corridor E/SE into NJ
and the Delmarva by the late afternoon into the early evening.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/...
Convection will be ongoing across the region in the early
evening. Heading through time, the upper trough and associated
shortwave energy detailed above will continue advect east toward
the region providing upper level support for continuing lift
through the night. Meanwhile, the tropical moisture feed from
the south will continue to feed into the area. These factors
along with just enough elevated instability look to keep showers
and storms going across the region through most of the night
rather than dying off through the evening. Of concern is the
anomalously high PWATs progged to rise over 2 inches. Also,
forecast profiles indicate a modest LLJ with slight veering in
the profile and light winds aloft...suggesting slow moving
and/or backbuilding and training of cells. Thus, though severe
threat should diminish through the evening very heavy rainfall
with the threat of urban, small stream and even flash flooding
will continue with these storms into the overnight. In fact
pattern even looks to resemble Maddox Frontal Pattern identified
in some flash flood cases. Highest threat area for heavy
rainfall and associated hydro issues looks to be Philly to
Trenton and points south.

Most of the area will see conditions remain quite warm and muggy
through the night with lows in the upper 60s to low 70s. However
across northern NJ and the southern Poconos, a backdoor cold
front will begin to work south into the region overnight so it
will turn cooler here with lows by morning in the low to mid
60s.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Sunday... A backdoor cold front will continue to push south across
the area as high pressure over southeastern Canada builds down the
eastern seaboard. The front will be the divide between warm and
humid air (temps 75-80F and dewpoints near 70F) to the south and
southwest and notably cooler air (temps 55-65F) to the north and
northeast. The steadiest/more widespread showers on Sunday will be
focused along and to the south of the boundary, which includes much
of eastern PA and NJ for the morning and locations along and south
of the Mason-Dixon line for the afternoon. The risk of thunderstorms
and locally heavy rain/flooding will increase in the warm sector
during the afternoon and evening across far southern NJ and
Delmarva. Coverage/intensity of showers should diminish north of the
boundary during the afternoon. There is cautious optimism for a rain-
free period for part of the afternoon to the north and especially to
the northeast of Philadelphia (before light rain returns during the
evening) even though skies are expected to remain cloudy. Temps will
not follow a typical diurnal curve due to a notable drop in
temperatures that occurs from north-northeast to south-south west in
wake of fropa.

Monday...A relative lull in the unsettled weather pattern is
expected for Memorial Day. The holiday may be our best shot over the
next week at a precip-free day with support from latest model
consensus. However, could still see a scenario where the backdoor
front stalls nearby to our south, prolonging rain chances over mid
Delmarva on Monday. The cool airmass in place to start the day on
Monday looks to moderate a bit as onshore flow is replaced by a
light south-southwest wind in the afternoon. Unfortunately for those
looking to take advantage of the start of pool/beach season,
stubborn low clouds will limit how much warming can occur- forecast
highs are only in the 60s at the coast and generally in the 70s
inland.

Tuesday...Another backdoor cold front looks to approach from the
north. Strong heating in the pre-frontal warm sector will allow for
temperatures to rise well into the 80s Tuesday afternoon. Scattered
convection is also likely to develop during peak heating over
northeastern PA and northern NJ as the cold front arrives late in
the day. It is still unclear if this activity will propagate
southward into southeastern PA, southern NJ and northern Delmarva
Tuesday night or if convection will dissipate after dark.

Wednesday through Friday...The remnants of Subtropical Storm Alberto
looks to get steered northward up the Mississippi Valley by a weak
mid-latitude trough early in the week and then northeastward around
the periphery of the downstream ridge during the second half of the
week. Chances for showers and storms increase initially across far
western zones Wednesday afternoon and night and then area wide
Thursday and Friday as the remnants of Alberto track into the
eastern Great Lakes. There is also a signal in the extended range
for the low to stall over the Mid Atlantic if it becomes cutoff by
the end of the work week or next weekend. The risk for yet another
heavy rainfall event increases late in the week in concert with the
arrival of deep tropical moisture but the extent of a flood threat
will ultimately depend on the track of the remnant low, including
if and where the circulation becomes a cutoff.

&&

.AVIATION /09Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...Mainly VFR. Scattered afternoon showers and
thunderstorms may briefly reduce the visibility and lower
ceilings to MVFR or IFR with the highest threat of this at KRDG
and KABE. Southwest to west wind around 5 to 10 knots.

Tonight...Showers, heavy at times, with the chance of embedded
storms will eventually result in restrictions developing
overnight to at least MVFR for most sites with IFR possible by
morning.

Outlook...

Sunday...Low clouds will advect S-SW across the region behind a back
door cold front, resulting in at least MVFR restrictions for most of
the day. Periods of IFR restrictions possible, especially toward the
coast and elsewhere in heavier showers. Thunderstorm activity looks
to mainly setup S/SW of Phila terminals. Light and variable winds
prior to fropa will become NE and then E behind it. CIGs likely
lower to IFR Sunday night as marine air becomes increasingly
established.

Monday...IFR CIGs to start the day should gradually improve to MVFR
late morning-early afternoon and possibly to VFR late in the day.
Light E winds become S in the afternoon.

Tuesday and Wednesday...Mainly VFR. S-SW winds shift out of the NW
on Tuesday as a cold front moves southward. Winds then veer out of
the E-NE Tuesday night and then SE Wednesday. Wind speeds generally
under 10 kt through Wednesday morning before increasing to 10-15 kt
Wednesday afternoon.

&&

.MARINE...
Today...Conditions remain below SCA with SW winds generally
around 15 knots with a few gusts up to 20 knots. Seas remain
2-4 feet on the ocean and 2 feet or less on DE Bay.Showers and
thunderstorms could affect the waters this afternoon.

Tonight...The conditions are anticipated to be below
Small Craft Advisory conditions with a southwest breeze ahead of
a backdoor cold front. Seas should be 4 feet or less.

Outlook...

Sunday...A sharp wind shift from SWly to NEly is expected with a
cold front moving southward. Expect this wind shift to occur across
the northern coastal waters of NJ early in the day, then southern
coastal waters of NJ and DE Bay either late morning or early
afternoon, and possibly not until late in the day for the DE coastal
waters. The strongest NE winds look to develop across the northern
coastal waters of NJ on Sunday where gusts to 25 kt are most likely
to occur. Seas in these northern zones could also build to 5 ft in
the northern coastal waters later in the day in response to the
strengthening onshore flow. A SCA has been issued for
ANZ450-451 for Sunday afternoon and evening.

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

Rip Currents...
There is a low risk for the development of dangerous rip
currents today. The wind is forecast to be southwest around 10
to 15 MPH with waves in the surf zone building to 3 to 4 feet.

A cold front sinking southward across the area on Sunday will
cause winds to shift from S-SW to NE. The setup may favor a
moderate rip current risk by Sunday afternoon, particularly for
Monmouth and Ocean County beaches in NJ, where winds will be
strongest (15-20 mph), waves in the surf zone will be highest
(4-5 ft) and where a NE flow yields more of an onshore component
due to the orientation of the coastline (relative to the
shoreline farther south in NJ).

&&

.TIDES/COASTAL FLOODING...
Spotty minor tidal flooding is possible with the Sunday evening
high tide along the northern coastline of New Jersey and late
Sunday night along the tidal portion of the lower Delaware
River.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory from noon Sunday to midnight EDT Sunday
     night for ANZ450-451.

&&

$$
Synopsis...Klein
Near Term...Fitzsimmons
Short Term...Fitzsimmons
Long Term...Klein
Aviation...Fitzsimmons/Klein
Marine...Fitzsimmons/Klein
Tides/Coastal Flooding...Klein



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