Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 251727
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
127 PM EDT MON JUL 25 2016

.SYNOPSIS...
A warm front will slowly push northward across
the region today.  meanwhile, a cold front will approach from the
west, moving into the region tonight and Tuesday, before stalling
near or just to the south of the area through the midweek period. A
couple of areas of low pressure may develop and move along this
boundary as it remains over the area for the end of the week.

&&

.NEAR TERM /UNTIL 8 PM THIS EVENING/...
1 PM update...Severe Thunderstorm Watch 415 was just issued. The
watch includes all of eastern PA and all of NJ except the four
southern most counties in the state and is in effect until 8 PM.

Updated hourly Pop and Weather grids through tonight were already
published around noon. Added severe thunderstorm wording to the
forecast, highlighting the potential for damaging winds and
locally heavy rain. Also, added more timing details of the
convection based on the expected downshear progression of the
storms. The HRRR seems to have the best handle on convective
initiation with the lee trough just to our west as well as the
more organized convection that is tracking across southern NY.
It should also be noted that the SVA 415 may need to be locally
extended in time by an hour or two across the southern and
eastern fringe of the watch box.

Mid morning discussion...
The MCS that moved through the area early this morning continues
to move off the coast mid morning. Clouds and precipitation had
delayed heating so far this morning as temps were still in the 70s
in parts of central and eastern NJ as of 10 AM. However,
subsidence in wake of the exiting mid-level shortwave trough has
resulted in extensive clearing of the debris clouds farther to the
west. Temperatures have quickly jumped into the mid 80s to near
90F across eastern PA and the Delmarva region where the clearing
has already persisted for more than an hour. The humidity will be
a factor into the excessive heat today with a very muggy airmass
is in place. Dewpoints are currently in the low to mid 70s across
the majority of the area (upper 60s in northeast PA and northwest
NJ; upper 70s in the mid Delmarva region). There is quite a bit of
model spread regarding how these dewpoints readings change over
time . We think typical model biases are showing up in the
moisture fields with the RAP/HRRR overdoing the amount of mixing
of drier air aloft (they show dewpoints dropping into the mid 60s
across most of the area this afternoon) while the GFS/NAM are too
moist (the GFS has an 80F dewpoints in eastern PA this afternoon).
We took a middle solution of these two extremes, which would
result in dewpoints remaining near currently readings (maybe
lowering a couple of degrees). It`s important to nail down the
dewpoints to determine how extreme our heat index rises this
afternoon as well as how much instability will be available to
fuel afternoon storms. After updating the hourly grids at 930 AM,
heat indices are similar to previous forecast so there are
currently no plans to make changes with the heat headlines.

We anticipate a lull in precipitation across the region thru at
least midday before the boundary layer full destabilizes during
peak heating. Modified OKX and IAD RAOBs for this afternoon shows
about 1500-2500 J/kg MLCAPE for this afternoon). Convective
initiation will likely occur closer to 1 or 2 PM along a pre-
frontal/lee trough just to our west. If the convection is able to
quickly grow upscale and organize along this lee trough, then this
line of storms may race eastward into our far western zones in PA
between about 2-4 PM, the Delaware Valley/I-95 corridor between
about 4-6 PM and the coastal plain between about 6-8 PM. Morning
runs of both the HRRR and HRRRx are indicating a second line of
storms trailing back to our north and west that would be
associated with a mid-level shortwave trough moving thru Upstate
NY. If this were to happen, then another round of storms could
threaten the same areas in eastern PA and NJ this evening. The
coverage of storms is expected to be more isolated in the Delmarva
initially until the line of storms makes it that far south closer
to sunset (and may not until after dark in our farthest southern
zones).

The latest D1 outlook from SPC places all of our eastern PA and NJ
zones as well as northeastern MD and northern DE in a slight risk
for severe storms. A marginal risk extends farther south into the
Delmarva due to lower coverage of storms. The primary threat is
damaging winds. Even though mid-level lapse rates are weak, there
isn`t an abundance of dry air aloft, and the wind field in the
boundary layer is weak (less than 25 kt), the environment will be
supportive of strong downbursts with almost an inverted V sounding
below the LCL. Storms that form in such an anomalously hot, moist
and unstable environment such as the one in place tend to produce
severe episodes.

The other concern will be locally heavy rainfall that could result
in flash flooding as storms develop in a moisture-rich environment
(PWATS 2-2.25), especially if multiple rounds of storms affect
the same area.

&&

.SHORT TERM /8 PM THIS EVENING THROUGH 6 AM TUESDAY/...
The threat of severe thunderstorms should be trended downward
after sunset although we will still have to watch out for
additional strong to severe storms toward the Delaware Bay region
(far southern NJ and Delmarva) during the late evening if the
unstable airmass has yet to be worked over by prior convection.
We`ll also have to watch how the orientation of the convective
line evolves over time. If the strongest part of the line bows
out, the trailing portion of it may become oriented more west to
east and allow for storms to train along it. The threat would then
transition from winds to flash flooding across these southern
areas later this evening and maybe into the overnight.

The cold frontal boundary will push through tonight with
thunderstorms clearing the coast early in the evening. However,
another muggy night is expected with lows still in the mid and upper
70`s for most of the region.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
The cold front will be through much of the area before it stalls in
the vicinity of Delmarva Tuesday and Tuesday night. It will then
remain nearly over the Delmarva through the middle of the week.
Drier and less hot air will try to filter into the region from the
north. Areas north of Philadelphia will see some relief from the
heat and humidity with high temperatures trending down to the lower
90s by Wednesday with dew point temperatures dropping into the lower
to mid 60s. Philadelphia metro and southern New Jersey will also see
high temperatures drop into the low 90s, but will likely see dew
point temperatures remain in the middle to upper 60s. Finally,
further south across the Delmarva, the only relief will be the lower
temperatures as dew point temperatures will remain in the 70s. This
area will also likely continue to have a chance of a few showers and
thunderstorms due to the more humid airmass and the proximity to the
frontal boundary. Areas further north should stay dry for the middle
of the week.

As the week progresses, the frontal boundary will shift slightly
northward resulting in a return to more humid conditions and a
threat of showers and thunderstorms across the entire region. The
models are starting to hint that a couple of areas of low pressure
will develop and move along the boundary. This will increase
the amount of clouds, humidity levels and the chance of showers and
thunderstorms for the end of the week and the weekend.

&&

.AVIATION /18Z MONDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

VFR conditions early this afternoon. Showers and thunderstorms
are currently developing to our north and west. These storms are
expected to increase in coverage/strength and move southeast
through the early evening. Tempo groups have been added to the
TAF`s to account for this potential of lower ceilings and VSBYS.
Storms should only last for an hour or two at any given location.
However, the exact timing is still uncertain. There could be more
than one round of storms as well. The first round of storms may
affect the Phila area terminals between 20-22Z with another line
possibly moving thru between about 00-02Z.

Outside of the storms, southwesterly wind gusts up to 20 knots
could occur in the afternoon as well.

Mainly VFR tonight after thunderstorms end with winds shifting to
westerly late under 10 knots. However, patchy fog may develop
late, especially at the fog-prone terminals, if an extended
period of clearing occurs.

Outlook...
Tuesday through Friday...Mainly VFR conditions expected.
Scattered showers and thunderstorms Thursday and Friday, with
MVFR/IFR conditions possible at times.

&&

.MARINE...
Seas will start the day near two feet likely rising in
the southern waters during the afternoon and evening hours getting
close to four feet. Some southwest wind gusts around 20 knots could
occur in the afternoon hours today.

Outlook...
Tuesday through Friday...Sub-advisory conditions are
expected to continue on the area waters through much of the week.
Expect higher winds and waves in and near scattered showers and
thunderstorms that become more likely at the end of the week.

Rip Currents...Today`s risk for formation of dangerous rip currents
is low. However if the wind were to turn south 15 kt and a 4 ft 5
second southerly swell were to develop late in the day, we`d
probably experience a low enhanced risk along the NJ coasts. A
south to north longshore current will prevail. Beyond Monday: with
the Atlantic Basin continuing quiet tropically, unless we get a
strong onshore flow...RC risk this coming week should be generally
low. Tstms and pockets of chilly upwelling water may the greater
concerns. There is some sign of colder upwelling along the coasts
this evening.

&&

.CLIMATE...
The forecast high temperature for Philadelphia today, July 25th,
is currently 98F. The last time the temperature reached or
exceeded 98F at PHL was three years ago on July 18, 2013. The last
100-degree day at PHL occurred four years ago on July 18, 2012.


Record high temperatures for Monday through Thursday are below. We
may tie or break daily record highs tomorrow at several of our
climate sites.

KABE and KRDG had record equaling highs on Sunday the 24th.


Site    25th      26th       27th       28th
----    ----      ----       ----       ----

PHL...  96-1899   101-1892   101-1940   100-1941

ABE...  95-1999   98-1940     98-1955    97-1949

ACY...  99-2010   96-2011     99-2005    98-1999

ILG...  96-1987   99-1894    100-1894   101-1894

TTN...  97-1999   99-1892    100-1894   101-1894

GED...  99-2010   97-2012     98-2005    99-1949

RDG...  96-1999   99-1940     98-1955    99-1941

MPO...  90-1999   89-1949     91-1955    93-1949

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Heat Advisory until 6 PM EDT this evening for PAZ054-055-
     060>062-101-103-105.
     Excessive Heat Warning until 6 PM EDT this evening for PAZ070-
     071-102-104-106.
NJ...Heat Advisory until 6 PM EDT this evening for NJZ001-007>010-
     012>014-016-020>027.
     Excessive Heat Warning until 6 PM EDT this evening for NJZ015-
     017>019.
DE...Heat Advisory until 6 PM EDT this evening for DEZ002>004.
     Excessive Heat Warning until 6 PM EDT this evening for DEZ001.
MD...Heat Advisory until 6 PM EDT this evening for MDZ008-012-015-
     019-020.
MARINE...None.

&&

$$
Synopsis...JK
Near Term...Klein
Short Term...Gaines/Klein
Long Term...JRK
Aviation...Klein/JK
Marine...Gaines/JK
Climate...Staff


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