Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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225
FXUS61 KPHI 201007
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
607 AM EDT WED JUL 20 2016

.SYNOPSIS...
A front will remain stalled over the Delmarva region and southern
New Jersey today before finally pushing south of the area
tonight. Meanwhile, high pressure will build into the area. The
high is expected to move offshore Thursday and Friday. A cold
front may move into the area and dissipate Friday night into
Saturday. Strong high pressure then re-establishes itself into
next week, but another cold front will slowly approach from the
northwest late in the weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
High pressure was located over the Great Lakes region early this
morning. Meanwhile, a stationary front was analyzed over the
Washington DC area, extending eastward into the Delmarva and
southern NJ. The airmass south of the boundary is moist (dewpoints
in the upper 60s-lower 70s) and unstable (MUCAPE 1500-2000 J/KG)
while much drier air (dewpoints in the upper 50s to low 60s)
resides north of the boundary. Strong to severe thunderstorms
develop in the unstable warm sector during the night as an upper
shortwave trough moved southeastward toward the Chesapeake and
Delaware Bays. Additional showers and isolated storms will
continue to develop early this morning in the northern Delmarva
and southern NJ, where low-level convergence along the front will
provide enough lift for parcels to their LFC. The environment is
slightly more stable and thus not as favorable for severe storms
this far north as it is over the lower Delmarva.

There is quite a bit of model disagreement regarding how the front
moves over the course of the day. There will be a brief wind surge
out of the northeast this morning as high pressure builds in from
the north, so this may cause the front to drift southward a bit
through midday. However, the front may briefly waver back to the
north during the first half of the afternoon as light southerly
flow develops in response to a portion of the elongated surface
high moving offshore. We are confident that areas north of the PA
turnpike and I-195 should stay dry and see plenty of sunshine
today. Locations farther south into far southern NJ and the
Delmarva are at risk of seeing additional showers and storms that
form during peak heating this afternoon. Confidence for afternoon
storms is highest across the southern half of DE and adjacent
locations in eastern MD, since these areas will more than likely
reside in the warm sector south of the front. We think the 00/06Z
NAM is a little too bullish on low-level moisture and instability
(NAM forecast profiles show 2-3 kJ/kg of SBCAPE for this afternoon)
across our far southern areas. However, even if it`s overdone by a
factor of two (which is more in line with the RAP/HRRR), there
will be a significant dry layer present above the moist boundary
layer. Accordingly, there is a conditional threat (dependent on
thunderstorms developing) for pulse severe storms in this thermodynamic
environment that is conducive for wet-microbursts. PoPs mostly
follow a blend of HRRR/RAP/SREF with slightly modifications to
remove the noise caused by the hi-res model forecast grids.

It will feel very comfortable out today for a middle of summer day
in the mid-Atlantic region. Temperatures will be in the mid 80s
across most of the area; cooler along the shore and across the
higher terrain.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
The stalled front will finally push south of the area tonight as
high pressure moves overhead. Shower or storms that develop this
afternoon across the Delmarva and far southern NJ should dissipate
toward sunset, although cannot rule out an isolated shower or
storm in our southern most zones after dark where elevated
instability is forecast to reside. The setup will be favorable for
radiational fog with clear skies and light winds under the high.
Not expecting fog to be widespread across the entire area though
given how short the night is right now.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Sfc high pres will be over the area Thursday-Friday and there
will likely be few clouds. However, conds will be genly pleasant
but withe increasing humidity by Fri. Aloft, a large and strong h5
ridge will be parked over the mid-section of the country and will
gradually shift a bit ewd. A cdfnt will move acrs the region later
Fri aftn into Fri eve and bring probably the best chc of any
precip in the next 7 days.

There is still some uncertainty as to how much of the heat
associated with this ridge we get and when. Latest guid now
indicates the peak of the heat will be Sun and Mon, and the GFS
continues to be the hottest of the mdls and got even hotter with
its 12Z run. The ECMWF, which trended cooler last night has gotten
hotter again. Some for of heat headlines may be needed over the
weekend or into next week. The CMC a bit cooler.

Temps look to be well into the 90s Fri-Mon, with dewpoints
becoming worse by Sun and Mon. There is always the chc of some
diurnal convection in a humid ams, but the guid differs on its
extent especially around Mon.

&&

.AVIATION /10Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Isolated showers and storms in VC of MIV early this morning.
These should stay south of ACY and ILG. VFR expected for the TAF
sites today with most of the showers and storms remaining south of
the terminals. The convection could be close to ILG- MIV- ACY
though but confidence not high enough to mention with 06Z TAFs.

Light N-NE winds early this morning will veer more out of the NE
or E-N after sunrise. A brief wind surge near 10 kt could accompany
this wind shift. Winds then become light and variable around
midday before the direction favors more out of the S or SE late
in the day.

There is a chance for fog late tonight into the early morning
hours on Thursday. Winds will be light and variable tonight.

OUTLOOK...
Thu...VFR. High pressure. High confidence.

Fri...Mainly VFR. However, shra/tsra psbl with lcl mvfr and psbl
ifr late Fri aftn into Fri night. Moderate to high confidence.

Sat through Sun. Mainly VFR. High pressure. High confidence. A
slight chc of diurnal tsra n and w on sun aftn.

&&

.MARINE...
Storms are moving into the coastal waters off DE. These storms
may pose a threat to mariners (locally strong winds and higher
seas as well as lightning) early morning as they move eastward.
Otherwise, winds and seas will remain below Small Craft Advisory
criteria. Nly winds will veer out of the NE early this morning.
The could gust 15 to 20 kt this morning. Winds will slacken a bit
early this afternoon and then shift out of the S-SE late in the
day.

OUTLOOK...
Thu and Fri....Predominately sub-SCA conditions in S-SW flow.
However, there is a chance some wind gusts near 25 knots on Fri,
as well as increasing seas in advance of a cdfnt.

Sat and Sun...Brief elevated wind/seas are psbl early Sat before
subsiding to below SCA for the remainder of the pd.

RIP CURRENTS... The risk for the formation of dangerous rip
currents today will be low at the DE and NJ beaches, but very
localized moderate rips are possible, especially along the NJ
coast where onshore flow will be strongest this morning.

We have had some reports of upwelling along the coast, where
temperatures have dropped into the low 60s.

&&

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

&&

$$
Synopsis...Klein
Near Term...Klein
Short Term...Klein
Long Term...Nierenberg
Aviation...Klein/Nierenberg
Marine...Klein/Nierenberg



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