Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 230937

Area Forecast Discussion
National Weather Service Mount Holly NJ
537 AM EDT Mon Jul 23 2018

A large upper atmosphere low pressure system will circulate
moisture across our area the first part of the week. Several
weak disturbances will touch off showers and thunderstorms at
least through Wednesday. A weak cold front will cross the area
Thursday followed by another on Friday. High pressure should
settle in for the first part of next weekend.


Area of showers lifting into northern NJ, and more showers over
the ocean will follow behind. Otherwise, scattered showers
elsewhere, with a cluster of heavier showers just outside of
Berks county.

Blocking high remains over the central Atlantic ocean waters,
centered some 750 miles east of the 40/70 benchmark. Meanwhile,
an elongated closed upper low with several strong shortwaves
associated with it will remain blocked from the Great Lakes down
to the Southeast U.S.

Deep south-southeast flow will usher abundant moisture into the
region, with surface dewpoints well into the 70s throughout much of
the CWA. This also will yield PWATs generally 2-2.25". So showers
that develop, especially this afternoon, will have the potential to
produce heavy rain.

Based on latest model guidance, with the 00Z NAM having the heaviest
rainfall, the heaviest of the rain looks to fall just outside of the
western portion of the forecast area. Will issue a Flash Flood Watch
for most of Delmarva and SE PA, as this lines up with the axis of
heaviest rainfall. Flash Flood Guidance is higher over much of
NJ and southern DE, so will leave those areas out for now.

Several rounds of showers and thunderstorms will develop throughout
the day today. There should be decent CAPE values, upwards of 1500
J/kg, along with a LI of -3C. But shear is limited. A few stronger
cells could produce strong, gusty winds.


Pattern continues. With loss of diurnal heating, thunderstorm
activity diminishes through the evening. However, showers should
continues through the overnight, with locally heavy rain.


No changes were made overnight. The long term discussion from
Sunday afternoon is below.

This story remains the same...our unsettled wet weather pattern
continues through most of the week, with several rounds of
showers and storms likely through at least Thursday, possibly
continuing into next weekend.

A strong Bermuda High to the east will keep a deep mid-level trough
stalled to our west over the upcoming week. This trough will
continue to dig over the eastern U.S., slowly filling as it does
so. Several short wave vorticity maxima will pivot around the
trough, bringing persistent rounds of showers and thunderstorms
to the area through the week. By Thursday, the GFS and ECMWF
show the trough having mostly filled and beginning to finally
lift off to the northeast as a deepening trough moves over the

Tuesday through Wednesday...Deep southerly flow is expected to
persist through Wednesday, drawing tropical moisture well into
our area. Forecast soundings are nearly saturated through the
troposphere, which make forecast PWATs near 2 inches
unsurprising. There will likely be some instability each day,
especially where any breaks in the cloud cover occur.
Thunderstorms that do develop will further progress (flash)
flood concerns, especially if training occurs. That is a real
possibility, as guidance suggests a single axis of heavy
rainfall will meander over our area through Wednesday. The humid
airmass will limit the diurnal pattern, keeping highs in the
upper 70s to low 80s and overnight lows in the upper 60s to low

Thursday...The developing trough over the Midwest will aid in
the formation of a surface low pressure system and cold front to
our northwest. A strong jet max directly to our north will
position us in the entrance region alongside the front. With
ample moisture already in place, more heavy rainfall is likely
to occur Thursday. The front will attempt to move through
Thursday night, but may washout/stall just offshore.

Friday through Sunday...Another shortwave may then move east
along the remnant boundary by next weekend. Highs Friday and
Saturday will likely be slightly warmer, making it to the upper
80s. Lows are likely to be in the upper 60s to lower 70s.

Exact rainfall amounts are still uncertain, but there is increasing
potential for several inches of rainfall to accumulate over much of
the area through the week. WPC has 3-5 inches forecast over our
area, with highest amounts over eastern Pennsylvania. However,
amounts may wind up being much higher than this in a few spots.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...MVFR CIGs may briefly dip to IFR early this morning, then
will lift to MVFR by this afternoon. Showers and isolated
thunderstorms will result in locally heavy rain with a few storms
capable of gusty winds as well. SE flow around 10 kt.

Tonight...MVFR CIGs redevelop. Lingering showers through the night
with sub-VFR conditions. Light SE flow.

Tuesday through Friday...Variable conditions with numerous
rounds of showers and thunderstorms. This activity may be more
organized Tuesday across our western terminals (RDG and ABE)
while terminals near the coast (MIV and ACY) have a better
opportunity to see breaks in the rain. Showers and storms could
become more widespread heading into Wednesday and Thursday. S-SE
winds generally 10-15 kt each day and 5-10 kt each night.
Forecast Confidence: Low for timing of each round of showers and


The Small Craft Advisory remains in effect for the Atlantic Ocean
coastal waters today and tonight as seas remain 5-8 feet. Winds will
likely also begin gusting 25-30 knots later today. SCA up for DE Bay
today for winds gusting to 25 kt as well.

Tuesday...SCA for our coastal waters with seas expected to remain
around 5 ft. Winds could gust to 25 kt at times.

Tuesday night through Thursday...SCA conditions expected. Wave
heights on our ocean waters are expected to remain in the 4 to 7
ft range with S winds in the 10-20 kt range. Winds may
occasionally gust to 25 kt, particularly on Wednesday and

Friday...Seas may finally drop below 5 ft as southerly winds


High risk of rip currents is in effect today. The surf is expected
to remain agitated with southeasterly winds gusting to 25 knots, and
seas remain around 5-8 feet with swells around 8-10 seconds.


The KDOX radar remains out of service. An update on repair time
is expected later today.


PA...Flash Flood Watch through late tonight for PAZ060-061-070-071-
NJ...High Rip Current Risk through this evening for NJZ014-024>026.
DE...High Rip Current Risk through this evening for DEZ004.
     Flash Flood Watch through late tonight for DEZ001-002.
MD...Flash Flood Watch through late tonight for MDZ008-012-015-019-
MARINE...Small Craft Advisory until 6 PM EDT Tuesday for ANZ450>455.
     Small Craft Advisory until 6 PM EDT this evening for ANZ430-


Near Term...MPS
Short Term...MPS
Long Term...Davis/Klein
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