Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 222052

Area Forecast Discussion
National Weather Service Mount Holly NJ
452 PM EDT Tue Aug 22 2017

A cold front crossing the upper Ohio Valley this afternoon will
move into our region late tonight and slowly exit off the
Delmarva coast Wednesday afternoon. Canadian high pressure
builds into the region on Thursday and should continue to be the
primary influence on our weather into early next week. Two low
pressure systems should be near the Gulf coast states this
weekend and eventually turn northeast early next week, possibly
affecting our weather around Tuesday.


Heat indicies have reached 100-102 ILG to PHL PNE TTN.

Severe Thunderstorm Watch 464 is in effect until midnight for
all our eastern Pennsylvania counties except Delaware and
Philadelphia, as well as the New Jersey counties of Sussex,
Warren, Morris and Hunterdon.

Initial issue this afternoon is the heat across the area in
broad but increasing southerly flow ahead of approaching low
pressure. Moisture advection has been tempered only slightly by
deep mixing allowing dew points to drop a few degrees this
afternoon. None the less, heat indices are close to 100 F across
portions of northern Delaware, the Philadelphia metros
northeastward into NYC. A heat advisory remains in effect
through 8 pm.

Cold front and pre-frontal convection reach the area this
evening. Convection has already formed into several short line
segments with severe wind gusts reported. Given veering wind
profiles with 30-35 kt of deep layer shear, any discrete cells
ahead of the main lines may be capable of rotation and perhaps a
brief tornado. Otherwise, deeper mixing east of higher terrain
may enhance damaging wind gusts in part due to evaporative

Given the warm/moist conditions ahead of the front, diurnal
cooling will be slow, which may allow convection to continue in
severity for at least a few hours after sunset before gradually
diminishing in intensity. The cold front is expected to very
slowly move through from northwest to southeast starting after
midnight, but will be slower to move through coastal areas. As
such, a chance for showers and thunderstorms will continue
through early morning.


Any ongoing showers and thunderstorms near the coast will
slowly move eastward towards the waters through the afternoon as
the cold front continues its surge eastward. Humidity should
fall to more comfortable levels later in the day, more so by
evening as a secondary front pushes through the region.


500 MB: a -2SD trough begins the long term period in the Great
Lakes region, then weakens slightly as it moves into the
northeast Friday- Monday. Thereafter, we will monitor the
northeastward progress of what should be a split flow low of
tropical origins, having moved ashore along the Gulf Coast by
early next week.

Temperatures: Calendar day averages Thursday...near or slightly
below normal, then 2 to 6 degrees below normal daily Friday-

Forecast basis: a 50 50 blend of 12z/22 GFS/NAM MOS was used
Wednesday night-Thursday night, then the 12z/22 GFS MEXMOS was
applied Friday and thereafter the 15z/22 WPC D4-8 gridded
elements of max/min T, 12 hr POP 6 hrly dew/wind/sky.

Wednesday night...Fair and more seasonable with light wind and
possible patchy countryside fog.

Thursday...Scattered light showers possible late Thursday in e
PA with considerable afternoon cloudiness.

Friday-Monday...High pressure shifts southeastward into the
Great Lakes Thursday through Saturday,then eastward into
southeastern Canada/Northeast U.S. Sunday and Monday. This
expansive high will control our regional weather pattern across
the Mid Atlantic during this time. The trough aloft could result
in a shower north of I-78 Friday afternoon and there should be
considerable cloudiness at that time. Otherwise, for now, good
weather is predicted.

Tuesday...WAA overrunning clouds, if not rain, from a potential
tropical remnant moving northeastward from the Gulf coast.


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

Cloud bases have risen to around 4000 ft this afternoon, and may
occasionally go broken across TAF sites through the remainder
of the afternoon and evening.

Primary concern at this point will be a significant impact to
aviation as multiple lines of thunderstorms form well west of
the area, then gradually move into the TAF sites from northwest
to southeast this evening into tonight. Brief heavy downpours
reducing visibility and strong gusty winds will be possible in
association with the thunderstorms.

The cold front moves through overnight with a wind shift from
southwest to west-northwest. There may be a few lingering
showers and thunderstorms ahead of the front through Wednesday
afternoon, primarily for KPHL, KPNE, KTTN, KMIV and KACY before
conditions clear from west to east by evening.

Wednesday night...VFR except possible mvfr or ifr fog patches
toward dawn Thursday with nearly calm wind.

Thursday through Sunday...VFR. Generally light winds from the N
or NW, possibly becoming N-NE on Saturday and Sunday. small
chance of a light shower vicinity KABE/KTTN/KRDG Thursday
afternoon and Friday afternoon.


Strengthening southerly flow is allowing seas to gradually
build this afternoon, which should peak around 3 to 6 feet
across the ocean waters and 3 to 4 feet across the Delaware Bay.
Strongest gusts are expected through this evening around 25-30
kt, and will gradually diminish overnight. Seas will gradually
subside into Wednesday as a cold front moves through the area
and the pressure gradient begins to subside.

There will be a chance of showers and thunderstorms tonight
into tomorrow afternoon. These storms may locally reduce
visibility and produce wind gusts to near gale-force.

Wednesday night through Saturday...Winds and seas below SCA

Sunday...For no SCA headline but chance that northeast flow
will increase and cause hazardous seas to develop along the S
NJ and DE coasts Sunday (5 feet).

There is a moderate risk of rip currents at area beaches this
afternoon. If swimming in the waters the remainder of today, use
extra caution and common sense. Swim only in the presence of
lifeguards, and do not swim alone! Heed any and all restrictions
by area beach patrols.

Outlook for Wednesday... Low risk, possibly bordering moderate
based on what develops this evening for residual effects
carryinginto Wednesday.


Atlantic City and Allentown were within 2F of record today.
Around 91F for a max T.


PA...Heat Advisory until 8 PM EDT this evening for PAZ070-071-102-
NJ...Heat Advisory until 8 PM EDT this evening for NJZ010-012-015-
DE...Heat Advisory until 8 PM EDT this evening for DEZ001.
MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ430-431-


Near Term...99
Short Term...99
Long Term...Drag
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