Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 240040
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
840 PM EDT THU JUN 23 2016

.SYNOPSIS...
A front will remain nearly stationary near Delmarva through Friday,
as waves of low pressure move along it. The front will move
southward Friday night, as high pressure over New England builds
into the region for this weekend. As the high moves offshore
Monday, a slow moving cold front will approach from the northwest
Tuesday, and move through the region Wednesday.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
Front has stalled out directly over the region, almost perfectly
bisecting Philly in half (at 23Z, the wind direction at KPNE and
KPHL were exactly 180 degrees different). A cluster of showers and
thunderstorms is moving along the front now resulting in locally
heavy rain. Expect showers and thunderstorms to continue for a few
more hours before becoming more limited in coverage, especially
north of the front where onshore flow will lead to a stable
boundary layer.

High pressure will begin to build in from the north
later tonight. Patchy fog could develop with visibilities
possibly dropping into the 3-4 sm range in some areas. It will be
seasonably cool overnight with lows in the low to mid 60s across
the metro areas and the Delmarva and 50s across the Srn Poconos
and nrn NJ. Winds will be mostly light from the N or NE.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
The high will continue to build in Friday. A E to NE surface winds
may bring some low level moisture off the ocean, so there may be
extra cloudiness across the area during the morning. The op models
and mos indicate some low chc pops for showers across the region.
It appears that the better chc will be across the Delmarva, away
from the onshore flow. I have therefore mentioned some chc pops
for those areas during the late morning and afternoon.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
A mid-level ridge will build over the middle Atlantic this weekend,
giving way to cyclonic flow aloft early next week, as the trough is
reestablished over the northeast conus. After a pleasant weekend,
there will be a renewed chance of showers and thunderstorms in the
Monday thru Wednesday time frame. Temperatures are expected to be at
or slightly above normal through the period, with uncomfortable
humidity levels anticipated by Tuesday.

Friday night thru Sunday...Dry. Clouds Saturday morning,
particularly over Delmarva, will move offshore for a mostly sunny
day. The column will dry out rapidly, given confluence aloft, as
the mid-level flow goes anticyclonic with the region within the
right front quadrant of the upper jet. In fact, it will be a
struggle to reach convective temperatures on Saturday and Sunday,
so cumulus should be limited. With the dry air mass in place and
clear skies, went below guidance on low temperatures Sunday
morning, especially at the typically good radiators. The surface
flow will generally be out of the east this weekend, which may
advect some slightly higher dew points well inland by Sunday
morning, with the setup favorable for patchy ground fog.

On Monday, the surface high moves southeast, as a lee trough is
established in advance of a cold front over the Great Lakes. This
will favor a southerly flow, with time sections indicating a rapid
increase in low level moisture during the day. The flow aloft also
becomes diffluent, with weak shortwaves traversing the mid Atlantic,
as the southern edge of the westerlies approach the region. This
will favor a chance of diurnally driven showers and thunderstorms,
particularly northwest of I-95 in the evening, closer to the lee
trough and instability axis.

There will be a renewed chance of showers and thunderstorms on
Tuesday, as a cold front moves into the region. There is general
model agreement that this front will become stationary across our
region on Tuesday as the mid-level trough approaches, and the
tropospheric flow veers, becoming increasingly parallel to the
front. As the right rear quadrant of the upper level jet approaches,
Bulk Shear will increase. However, there is uncertainty on the
placement of the front, and thus the instability axis. In fact,
the more favorable shear and instability may be dislocated. Thus,
there is still a lot of uncertainty with regard to how organized
and robust the convection will be.

The front is forecast to move offshore by Wednesday, as the mid-
level trough axis crosses the region, turning the large scale flow
westerly. There is disagreement in the 12Z/23 model suite with
regard to the timing of the front exiting the coast. For now, we
have maintained a slight chance of thundershowers on Wednesday,
in continuity with the previous forecast.

Fair weather is expected to return for Thursday.

&&

.AVIATION /01Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Area of shra and tsra could affect KRDG, KPHL, KILG, and KMIV over
the next few hours. After that, focus shifts to fog potential
overnight (generally between 08 and 12Z). Threat for fog and low
clouds has decreased with the latest guidance, but still think
patchy fog is likely primarily for the more rural TAF sites. Once
the fog dissipates, expect VFR conditions to continue for the
remainder of the TAF period.

There is a front bisecting the region currently (as of 23Z, it was
in between KPHL and KPNE). North of this front, winds are
northerly. South of the front, winds are southwesterly. Expect the
front to slowly sag south, resulting in northerly and
northeasterly winds at all the TAF sites by 12Z.

Outlook...
Friday night...There is the potential for MVFR in low clouds and
SHRA at ACY and MIV, otherwise, VFR at the remainder of the TAF
sites.

Saturday and Sunday...Predominantly VFR. Patchy ground fog may lead
to sub-VFR conditions late Saturday night and early Sunday morning,
mainly at ABE, MIV, and RDG.

Monday and Monday night...Predominately VFR. MVFR possible in
isolated SHRA/TSRA northwest of ACY and MIV, mainly Monday evening.

Tuesday...More widespread SHRA/TSRA across the TAF sites may lead
to MVFR at times. Otherwise, predominantly VFR.

&&

.MARINE...
Winds tonight should remain mostly around 10 knots or less and
seas (ocean) mostly 2 to 3 ft tonight. Friday will become NE to E
and increase to near 15 knots and seas will increase a bit to 3
to 4 ft.

RIP CURRENTS...With an onshore flow developing, and seas near 4
feet, the risk for the formation of dangerous rip currents could
be moderate by tomorrow afternoon.

Outlook...
A persistent onshore flow will be established in the Friday night
thru Sunday period. This will peak Friday night into Saturday,
particularly the southern NJ and DE coastal waters. The guidance
has trended upwards over the past 24 hours in terms of winds, but
is still sub-SCA. Nevertheless, northeast winds may briefly reach
SCA criteria late Saturday morning into Saturday afternoon, as
3-hourly pressure rises maximize, particularly over the southern
NJ and DE coastal waters. Confidence is not high enough for a SCA
at this point in the forecast, but this potential will need to be
reevaluated. Otherwise, with the persistent onshore flow, seas
will build to the 3-4FT range.

Looking ahead to the Sunday night thru Tuesday period, winds are
anticipated to remain below SCA criteria attm.

RIP CURRENTS: Based on the expected wind, wave and swell conditions
a moderate risk for the development of dangerous rip currents may
persist into Saturday along the New Jersey shore and at the
Delaware Beaches.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...Franck
Near Term...Johnson/O`Hara
Short Term...O`Hara
Long Term...Franck
Aviation...Franck/Johnson/O`Hara
Marine...Franck/Johnson/O`Hara



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