Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 272127
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
527 PM EDT Thu Jul 27 2017

.SYNOPSIS...
A warm front will move through our area later tonight. Low pressure
will develop along the frontal boundary across the Ohio Valley, and
it will move towards the area early Friday through Saturday. High
pressure will return over our area for the late weekend and will
persist into the week.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
Weak cold front moving through the eastern Great Lakes will continue
to track east this evening. Some light showers may continue to pop
up across areas mainly along and north of I-80 into this evening.
Meanwhile, low pressure continues to organize and develop over the
Midwest. A warm front extending out ahead of that low will lift
towards the Mid-Atlantic this evening, and scattered showers and
thunderstorms look to develop as that warm front approaches from
the south and west. Think best chances for any convection will be
from around sunset through around midnight this evening. THereafter,
with loss of heating, CAPE values go down.

With abundant low level moisture in place and increasing across the
region, can expect patchy fog to develop late tonight.

Weak cold front sags across northern zones, generally north of I-78,
and skies may somewhat clear out. However, with that abundant
moisture, do not think any clearing will last long.

Models have slowed the storm down enough to warrant removing the
mention for heavy rain tonight. Outside of some evening showers and
thunderstorms, and then some lingering showers thereafter, most
widespread QPF is generally 0.25" or less. Though the NAM is showing
the potential for convection with spotty locally higher amounts,
those amounts seem a bit overdone.

Warm and muggy tonight with lows in the 60s and low 70s.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
The start of a prolonged period of unsettled weather begins on
Friday. Warm front ahead of the developing low seems to get hung up
across the Mid-Atlantic. Several waves of low pressure will develop
and intensify over the OH/TN Valleys along the boundary as closed H5
low digs into the OH Valley from the Great Lakes. The first surface
low will lift into the Mid-Atlantic and Delmarva Friday afternoon,
and showers and scattered thunderstorms will move into the Delmarva,
DE Valley, and southern NJ during this time.

Onshore flow ahead of the low will usher abundant moisture into the
region. Surface dewpoints will rise into the upper 60s/low 70s
throughout, and PWATs will range from 2-2.25".

This first wave of showers and thunderstorms will produce heavy
rain, mainly to the south of I-78. Hardest hit areas will be across
northeast MD, DE, and southern NJ, where between 3/4-1" rain is
possible during the day. Locally higher amounts are possible in
training of thunderstorms. These areas are the areas already hit by
heavy rain early this week.

With 3 hour and 6 hour FFG values on the order of 2-4 inches, do not
think this will be hard to reach, especially given the potential for
training of thunderstorms. Therefore, have issued a Flash Flood Watch
for much of southern NJ, parts of southeast PA, northeast MD, and DE
starting Friday afternoon, though the bulk of the rain comes Friday
night and Saturday.

SPC has most of southern NJ and northern parts of the Delmarva in a
Marginal risk for severe weather on Friday, and southern portions of
the Delmarva in a Slight Risk for severe weather. Thunderstorms with
damaging winds are possible, but big concern will be for the
flooding.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
Summer nor`easter this weekend with the potential for flash
flooding increasing across portions of the region...

A shortwave over the upper midwest as of this afternoon will carve
out an an anomalous cutoff low across the mid Atlantic on Saturday
which will persist into Tuesday. This will be the primary weather
maker during this time frame with a summer nor`easter leading to
heavy rainfall. The potential for flash flooding has increased
and a flash flood watch is now in effect generally south of
Interstate 78.

Heavy rainfall...Shortwave energy rounding the base of the mid
Atlantic trough will lead to cyclogenisis along a stalled frontal
boundary near Delmarva from late Friday into early Saturday. As
the mid-level trough closes off, the system will become vertically
stacked on Saturday, with the surface low stalling off the
Virginia Capes, before moving gradually northeastward on Monday.
While there is model consensus through Saturday with the track
of the low, the Canadian and NAM take the low further south and
east on Sunday, while the GFS and ECMWF keep it locked in closer
to the coast. We favored the GFS and ECMWF over the Canadian
and NAM, as they have a better handle on the shortwave impulse
associated with this system.

That said, there is still considerable spread in the GEFS and
EPS 500 hPa heights with the placement of the cutoff low, and this
is leads to uncertainty in the track of the surface low. Both
the GEFS and EPS means low positions are close to the op runs at
00Z Sunday. This is further evident in the ensemble spread in
mean precipitation amounts, which is most acute in the I-195 to
PA turnpike corridor. We have higher confidence in the potential
for flash flooding southeast of this line, and lower to the
northwest.

Quite an impressive setup develops across the flash flood watch
area on Saturday with an easterly 850 hpa jet of 50 kts
developing (+3 to +4 STD DEV above normal), transporting deep
later moisture into the region with Precipitable Water values
in excess of two inches (+2 to +3 STD above normal). There are
two potential bullseyes of heavy rainfall, one near the NJ
coast, and another extending from southeast PA into Delmarva.
With a frontal boundary stalled across Delmarva, rainfall across
this area into southern NJ will also be convective in nature,
as a broad area of instability sets up, as evidenced by MU
CAPE, along with negative lifted and Showalter Indices.

Additional rainfall is possible on Sunday, mainly across
southern NJ and Delmarva.

A return to fair weather is expected in the Mon - Wed time
frame, with the next chance of showers on Thursday with an
approaching front. Temperatures will be below normal from
Saturday thru Monday, especially closer to the coast with an
onshore flow. A return to near normal is expected thereafter.

&&

.AVIATION /22Z THURSDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Weak cold front approaches from the north this evening, and then a
warm front approaches from the south and west late tonight. Low
pressure approaches on Friday.

MVFR CIGs look to continue into this evening. Weak cold front sags
to the south, and then there may be some clearing in the evening,
but isolated showers and thunderstorms are possible as well.
Confidence low on any given storm impacting a terminal, so will
carry VCTS to cover this potential.

Conditions somewhat clear out in the evening, but with abundant low
level moisture in place, expecting MVFR CIGs/VSBYs to develop late
tonight and into Friday morning. IFR conditions are possible at
southern terminals (KPHL/KMIV/KACY/KILG).

Showers develop at those southern terminals Friday morning.

For KPHL, showers and thunderstorms with heavy rain will move across
the airfield after 18Z Friday.

Light winds, generally 5 KT or less, on tap through Friday morning.
East winds then increase to 5-7 KT.

OUTLOOK...

Saturday...Widespread IFR expected in low clouds and heavy
downpours. Northeast winds may gust up to 25 knots and there is
the potential for low-level wind shear across the terminals.

Sunday...IFR may continue at ACY, MIV, and ILG. Improvement to
MVFR elsewhere. Northeast winds may continue to gust up to
around 20 knots.

Monday and Tuesday...MVFR conditions may persist into Monday at
ACY and MIV, otherwise VFR conditions. North winds generally
around 10 knots both days.

&&

.MARINE...
South to SW winds 10-20 KT this evening will become light by Friday
morning, then will become E-SE 10-15 KT Friday afternoon. Occasional
gusts to 25 KT possible at DE and southern NJ ocean waters this
evening, but do not think they will be widespread or frequent enough
to warrant a Small Craft Advisory. Seas will range from 2-4 feet.

Widespread showers and thunderstorms will impact the waters Friday
afternoon with reduced visibilities, gusty winds, and rough
seas.

OUTLOOK...
We issued a Gale Watch for the New Jersey waters, including the
waters east of the mouth of Delaware Bay, from Saturday morning
into Saturday night. SCA conditions may persist thereafter from
Sunday into Monday.

RIP CURRENTS...

A moderate risk for the development of dangerous rip currents
continues through this evening due to the combination of onshore
flow and an underlying long (10 second) period southeasterly
swell.

A moderate risk for the development of dangerous rip currents
is outlooked to continue into Friday as well.

A high risk is possible from Saturday through Monday.

&&

.HYDROLOGY...
A Flash Flood Watch has been issued for much of southern NJ, parts
of southeast PA, MD, and DE. 3 hour and 6 hour FFG values range from
2-4", and with widespread showers and thunderstorms producing heavy
rain, these values are likely to be met.

Will forecast a Storm Total of 2-4" inches of rain from tonight
through Saturday, however, with training of thunderstorms,
significantly higher rain amounts are likely, especially in areas of
DE and southern NJ that already received heavy rain and flooding
early this week.

Please be aware of road conditions, and be aware for Flash Flood and
Flood Warnings that may be issued, especially Friday night and
Saturday.

&&

.TIDES/COASTAL FLOODING...
Minor coastal flooding is possible Saturday night and Sunday
morning along the oceanfront. While astronomical tides lower,
a surge potential of 1-2 feet may put some oceanfront sites
into the minor category at the time of high tide.

There is the potential for high surf with breakers of at least
8 feet along oceanfront beaches on Saturday and Sunday. A high
surf advisory may be needed. Beach erosion is also possible.

&&

.CLIMATE...

Record daily rainfall

        7/28       7/29
ACY 2.05 2012  2.21 1884

PHL 8.02-2013  3.53-1980

ILG 2.34-1914  1.85-1913

ABE 3.00-1969  1.64-1979

TTN 2.35-2012  2.84-1961

GED 2.80-2016  1.07-1969

RDG 3.57-1969  2.51-1961

MPO 2.15-1969  4.59-1969


July total
ABE 8.21 #8 wettest          10.42 is the record in 1969

RDG 8.02                     13.85 is the record in 2004


July average temps: Projecting, for now,  PHL 0.9 above normal,
losing 1.3 positive departure from the values through the 26th.
Ditto approximately the same loss at our other CLI sites will
show MPO a little below normal, TTN and ACY near normal, and all
other stations above normal.

&&

.EQUIPMENT...
Dew point readings at KDOV continue to measure too high compared to
surrounding locations and should be treated as unrepresentative of
the area.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Flash Flood Watch from Friday afternoon through Saturday
     afternoon for PAZ060-070-071-101>106.
NJ...Flash Flood Watch from Friday afternoon through Saturday
     afternoon for NJZ012>027.
DE...Flash Flood Watch from Friday afternoon through Saturday
     afternoon for DEZ001>004.
MD...Flash Flood Watch from Friday afternoon through Saturday
     afternoon for MDZ008-012-015-019-020.
MARINE...Gale Watch from late Friday night through late Saturday night
     for ANZ450>454.

&&

$$
Synopsis...99
Near Term...MPS
Short Term...MPS
Long Term...Franck
Aviation...Franck/MPS
Marine...Franck/MPS
Hydrology...
Tides/Coastal Flooding...
Climate...527P
Equipment...



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