Area Forecast Discussion
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FXUS61 KPHI 252209
Area Forecast Discussion
National Weather Service Mount Holly NJ
609 PM EDT MON JUL 25 2016
A cold front will approach from the west and move through the region
early Tuesday morning, before stalling near or just to the south of
our area. This front will gradually lift north by the end of the
week with areas of low pressure developing along this boundary.
.NEAR TERM /THROUGH TUESDAY/...
Ongoing SVR weather. Excessive heat flags have been taken down.
A new Severe Thunderstorm Watch (#416) was issued at 330 PM. This
watch includes eastern MD, DE and southern NJ and is in effect
until 10 PM. The remainder of the CWA is covered under Severe
Thunderstorm Watch #415, which is in effect until 8 PM.
The atmosphere continues to destabilize during peak heating this
afternoon. 19Z mesoanalysis from SPC indicates strong instability
(MLCAPE values ranging from 2000 J/kg across our northern most zones
to 3000 J/kg across Delmarva) supportive of robust convective
updrafts. The primary threat with these storms is damaging winds.
Even though mid-level lapse rates are weak and there isn`t an
abundance of dry air aloft, the environment is conducive for strong
downbursts with a inverted V sounding below the LCL.
The HRRR had a good handle on the location of convective initiation
with the lee trough just to our west as well as the more organized
convection that is tracking across southern NY. We expect the first
round of convection to organize along the lee trough and then
accelerate eastward toward the I-95 corridor between about 4-6 PM,
reaching the coastal plain of NJ before sunset. The second line of
convection with the upper shortwave trough is expected to move
through the I-78/I-80 corridors between 6 and 9 PM. A more
organized QLCS (and higher damaging wind potential) is likely with
this second line owing to enhanced bulk shear and deeper lift ahead
of the shortwave trough. You can see this starting to form upstream
across north-central PA. There is still uncertainty regarding the
strength of these storms in this line by the time it drops southward
toward the Philly metro since it will likely be around or just after
sunset. It will depend on how much the airmass is worked over from
the first round of storms this afternoon. The threat of severe
thunderstorms should be trending downward after sunset with the loss
of heating and as the airmass stabilizes in the cold pool behind the
storms. However, the severe threat may continue into the late
evening and overnight across the Delaware Bay region (southern NJ
and Delmarva) given this extremely unstable airmass will likely not
have been overturned by prior convection.
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. 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 southern end 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 heat headlines are set to expire at 6 PM. Heat indices this
afternoon were in the 100-110F across much of eastern PA and NJ, and
105-115F in Delmarva. The heat will temporarily break with the
Patchy fog may develop overnight, especially in sheltered river
valleys and rural areas, if clouds clear before the drier air moves
in behind a cold front.
.SHORT TERM /TUESDAY NIGHT/...
The cold front is expected to slow down as it moves southward
through the Delmarva early Tuesday morning. There is still
uncertainty regarding how quickly the front pushes southward. Fronts
tend to get hung up in the Delmarva this time of year and in this
pattern with zonal steering flow aloft. Added a chance for showers
and storms during the afternoon hours across the southern half of DE
and adjacent eastern MD with the front likely nearby.
Unfortunately, we won`t get much of a break from the heat tomorrow
as post-frontal CAA progged to be minimal. Forecast highs are in the
low to mid 90s across most of the area (except cooler in the higher
terrain of northeast PA/northwest NJ and along the coast). However,
it will be noticeably less humid across eastern PA and
central/northern NJ with dewpoints in the upper 50s and lower 60s.
Farther south, where the front gets hung up, dewpoints will still be
in the upper 60s and lower 70s. Afternoon heat indices should peak
near 100F in the Delmarva.
.LONG TERM /WEDNESDAY THROUGH MONDAY/...
The medium range models are in good agreement keeping high pwat
air just to our south as a sfc boundary stalls in the southern
portion of the forecast area. Higher humidity values and a slight
chance for storms will continue near the Delmarva. Heat index
values in the mid 90s are still expected from Philadelphia
In response to an approaching shortwave, heights build on Thursday
and 1000-850mb thicknesses increase resulting in temps climbing a
couple of degrees warmer compared to Wednesday, with heat index
values in the mid to upper 90s. As higher pwat air returns...and
shortwave energy moves through the region...widely scattered to
scattered storms also return Thursday.
The ECMWF/GFS/Canadian models are coming into better agreement
developing a broad upper trof over the eastern U.S. into the
weekend...bringing temperatures down closer to normal values for late
July/early August. With pwat values still 1.5-2.0" through
Monday, and weak shortwaves moving through the upper trof
axis, daily afternoon and evening showers and storms will be
possible through the remainder of the long term period.
.AVIATION /22Z MONDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
Rest of today...VFR conditions continue this afternoon. South to
southwest winds around 10 knots or less with a few gusts up around
20 knots through the rest of this afternoon.
Scattered showers and thunderstorms are developing to the north
and west of the terminals and will continue to push to the east
through this evening. Expect showers and thunderstorms to impact
the terminals, mainly between 20-02Z, with MVFR/IFR conditions
possible. Tempo groups have been added to indicate the most likely
timing of storms at the terminals.
Overnight...Behind the storms, winds will shift more to the west,
slightly northwest, but remain under 10 knots. Patchy fog will
develop in the more prone areas as winds go light and skies
Tuesday...Patchy fog early, clearing by around 15Z. Then VFR
conditions expected for the remainder of the day. Westerly winds
around 10 to 12 knots through the afternoon.
Tuesday night through Friday...Mainly VFR conditions expected.
Scattered showers and thunderstorms Thursday and Friday, with
MVFR/IFR conditions possible at times.
Winds and seas will be just below small craft advisory criteria
through tomorrow. Southern wind gusts to 20 knots will continue to
occur through this evening. Seas will increase from 2 to 3 feet to
4 feet in our coastal waters. Stronger thunderstorms developing on
land this afternoon could threaten the waters this evening.
Locally strong winds and higher seas are possible with these
Tuesday night 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...A moderate risk for rip currents continues through
this evening. Tuesday through the most of the week: 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
As of 3 PM EDT, the temp at PHL was 96 which has tied the record
high for this date. ABE was 95, also tying a record high, and ILG
was 96. This also has tied the record for this date.
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 Today through Thursday are below.
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