Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 272032
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
432 PM EDT Tue Jun 27 2017

.SYNOPSIS...
High pressure over the Ohio Valley will build east across the
mid-Atlantic region through Wednesday. The high pressure will
move offshore for Thursday and Friday, returning our area to
southwest flow. Another weak cold front is expected this
weekend, bringing a chance of showers through early next week,
especially if the front stalls.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
A potent vorticity maximum is moving through New York at this
time and will progress toward New England and adjacent far
southeast Canada tonight. Meanwhile, an elongated northeast- to-
southwest oriented vorticity lobe will swing through the CWA
late this afternoon and this evening. Substantial heating on the
upstream side of the perturbation that generated far greater
coverage of convection last night than I expected has allowed
for a few storms to develop this afternoon in portions of
southeast Pennsylvania and New Jersey. Meanwhile, to the
northwest (in closer proximity to the aforementioned vort max in
New York), greater coverage of scattered convection has
developed. Convection-allowing models continue to hint at the
developed of scattered storms in the far northern CWA become
spottier southward to I-76 through the afternoon. Instability
will be marginal but seems to be a little bit more than
advertised owing to residual low-level moisture. Combined with
strong vertical wind shear, there is a chance for a stronger
storm or two through early evening. Cold midlevel temperatures
may allow for some small hail in the stronger updrafts, which
has been reported with storms both last night and early this
afternoon. Generally have the highest PoPs north of I-78 with a
transition eastward to far northern New Jersey by early evening.
The vort max moves rapidly off to the east after dark, so
precipitation should generally end in this region by late
evening.

Finally, an upstream vort max is rapidly pushing eastward through
the central Appalachians and adjacent southern Mid-Atlantic at this
time. Convection is beginning to develop east of the higher terrain
in VA and adjacent southern MD. High-resolution guidance spreads
this convection eastward into portions of the Delmarva Peninsula
late this afternoon and early this evening. Have locally higher PoPs
in this region as well, given the fairly strong consensus for
greater coverage of storms in this area. Precipitation may linger
through mid-evening before being shunted east as the vort max
rapidly progresses offshore.

Thereafter, large-scale descent should begin in earnest, with skies
rapidly clearing out and the low levels drying out through the rest
of the night. With relatively light winds, temperatures should fall
well below average overnight. Lows are forecast to be near 50 in the
Poconos and near 60 on the coast and in the urban corridor.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
Northwest flow becomes established tomorrow on the upstream
side of the shortwave trough moving through the area tonight. A
surface high will build into the eastern U.S. This sets the
stage for a very pleasant day, with skies mostly sunny and
temperatures slightly below seasonal averages. With some fairly
pronounced mixing, winds may gust to around 20 mph or so during
the afternoon. May also see a cumulus field develop during the
day, but subsidence above the boundary layer will keep these of
the "fair weather" variety.

MOS is in generally good agreement with highs tomorrow, but low-
level cold air advection will cease during the morning, with ridging
beginning to nose into the region as the day progresses. If this
happens sooner, forecast highs may be a bit on the low side. For
now, did not stray too far from a statistical MOS blend, given the
strong consensus. Highs will be around 70 in the Poconos and near 80
southeast of the Fall Line.


&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
The long term starts out with surface high pressure just off
the mid-Atlantic coast and associated fairly pleasant weather.
However as the high moves farther offshore and low pressure
moves into the Great Lakes, a more typical summertime SW flow
will set up over the region. This in turn will cause increasing
heat and humidity over the forecast area through the end of the
week and into the weekend. At this point it appears that
Saturday may be the most uncomfortable day with heat indices
reaching the mid 90s along the I-95 urban corridor.

Along with more heat and humidity will be gradually increasing
chances for afternoon showers and thunderstorms. For Thursday
and Friday it looks like any precip should remain well to the
northwest of PHL. By Saturday an upper-level trof and associated
cold front are forecast to approach the mid-Atlantic and also a
surface trof is forecast to develop from eastern PA south
to near the Chesapeake Bay. Enhanced low-level convergence is
expected to result in more widespread afternoon convection on
Saturday.

For Sunday and beyond the forecast becomes more cloudy, as the
models seem to disagree on whether a surface cold front can push
through our area or whether it will stall out. Given the
uncertainty, the forecast maintains at least a slight chance for
showers/t-storms through early next week. Temperatures are
expected to be near or slightly above normal at that time.

&&

.AVIATION /20Z TUESDAY THROUGH SUNDAY/...
VFR through the period. Spotty convection possible through this
evening, especially north of KPHL, with local reductions in
CIGs/VSBYs likely if a storm affects the terminal. Chances
should end shortly after 00Z, with mostly clear skies likely to
develop by tonight. West winds around 10 kts with gusts to 20
kts late this afternoon becoming light and potentially variable
tonight. Tomorrow, winds will generally be W or WNW backing
slowly to SW during the day with speeds around 10 kts with gusts
to 20 kts. A cumulus deck around 5000-7000 feet may develop
during the afternoon.

OUTLOOK...
Thursday and Friday...VFR conditions expected. Southwest winds
gusting up to 20 kt in the afternoon both days.

Saturday and Sunday...Mostly VFR conditions but lower at times
in scattered mainly afternoon showers and thunderstorms.

&&

.MARINE...
Sub-advisory conditions are expected through the period. A
slight chance for storms exists through this evening, with
locally gusty winds and higher waves possible should a storm
move through. Winds should switch from southwest to northwest
through the nighttime hours, with speeds generally 10 to 15 kts
with gusts to 20 kts. Winds will switch back to southwest by
afternoon tomorrow, generally remaining around 10 to 15 kts with
higher gusts. May see gusts approach 25 kts by late afternoon,
but predominantly winds should stay below advisory criteria.


OUTLOOK...
Thursday and Friday...SCA conditions may develop Thursday
afternoon and continue into Friday due to increasing SW winds
and building seas.

Saturday and Sunday...Winds and seas are expected to be somewhat
below SCA levels.

&&

RIP CURRENTS... Currently, the forecast is for a low risk of
rip currents through tomorrow. However, southwest winds will
increase to around 15 kts with higher gusts by late afternoon.
With a 9-second period from a southeasterly direction, this may
increase the rip current risk level, especially on the New
Jersey coast.

&&

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

&&

$$
Synopsis...CL/AMC
Near Term...CMS
Short Term...CMS
Long Term...AMC
Aviation...AMC/CMS
Marine...AMC/CMS


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