Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 261602
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1202 PM EDT TUE JUL 26 2016

.SYNOPSIS...
A cold front was located to our northwest near the Pennsylvania
and New York border. This front will gradually push southward
into the area tonight before stalling nearby to our south Wednesday
into Thursday. Several waves of low pressure developing along
this boundary will result in periods of showers and thunderstorms,
mainly toward the end of the work week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
A cold front was still to our north and west over upstate New York
and back toward the Ohio River Valley this morning. The pre-frontal
trough that was the focus for yesterday afternoon`s round of severe
storms was positioned just to our south across southeast VA. The
cold front is expected to weaken as the boundary catches up with
the pre-frontal trough this afternoon. As this happens, the
primary surface trough may actually reform slightly farther
northwest.

West-northwest winds behind the trough axis has advected drier air
into the northern two-thirds of the region. Although dewpoints are
still high (upper 60s/lower 70s) in eastern PA and central/
northwestern NJ, dewpoints will lower 5-10F from what they began
at this morning as the drier air mixes down during peak heating
late this morning and afternoon. Heat indices this afternoon will
be close to dry-bulb temperatures (generally mid 90s) across this
part of the forecast area.

The drier air will have a difficult time reaching our south zones
with the surface trough lifting back northward today into extreme
southern NJ, central and southern Delaware, and eastern Maryland.
The combination of high temperatures in the mid 90s and dewpoints in
the low to mid 70s will yield heat indices between 103-107F across
mid Delmarva. A Heat Advisory was issued for Kent and Sussex
Counties in DE and Queen Annes, Carolina and Talbot Counties in MD.

The same area covered in the Heat Advisory will also be at risk for
an afternoon thunderstorm. Locally heavy rainfall cannot be ruled
out with the airmass remaining moist (PWATs still around 2 inches),
slow storm motion (under 15 kt) and the potential for multiple
storms to develop along the quasi-stationary surface trough. The low
coverage of storms (mainly isolated) and sandy Delmarva soils will
limit the overall flash flooding risk.  One or two storms may also
contain locally strong winds given they are forming in an
environment conducive for wet microburst.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
Weak high pressure is forecast to nose into our region from the
west for tonight. As a result, we are expecting a mainly clear
sky. The only exception will be our far southern counties where
there may be some lingering scattered shower and thunderstorm
activity.

Temperatures are expected to fall into the 60s in our northern
counties and into the 70s elsewhere with a light and variable
wind.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
The stalled frontal boundary across the delmarva will be the main
focus of attention over the next few days. Available medium range
guidance all keep the high pw air just south of the boundary with
values in excess of 2". Heat indices Wednesday look to top out in
the lower to middle 90s climbing to the middle to upper 90s from
the Philadelphia metro area southward to the delmarva Thursday.
Precipitation chances remain confined to southern portions of the
forecast area Wednesday with increasing chances Thursday into
Friday for the remainder of the area.

Temperatures Wednesday on track to be a couple of degrees cooler
than today, but with building heights Thursday temps rebound into
the mid 90s in the metropolitan areas with upper 80s/lower 90s
near the water.

Forecast for the Thursday through Saturday period remains
complicated. Medium range deterministic guidance is highly
divergent as to the evolution of the shortwave trough spinning up
a surface wave on the retreating warm front. The ECMWF remains on
the slower end of the solution while the GFS is faster and
generates a lot more QPF. The Canadian is in between. At this
point, forecast will lean heavily on ensemble means are probably
the most sensible approach until more agreement develops across
the medium range guidance. In the sensible weather this would
suggest keeping pops fairly constrained during the day Thursday
with the best chances for storms late Thursday night through
Friday.

Precipitation chances will continue through the weekend before
dryer air moves into the mid-Atlantic early next week.
Temperatures this weekend and early next week expected to be
closer to climatological normals.

&&

.AVIATION /16Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

VFR conditions are anticipated through the TAF periods under a
mainly clear sky. We are not forecasting any precipitation at or
around our TAF sites.

A light and variable wind early this morning should settle into
the west and northwest around 10 knots for today. The wind is
forecast to become light and variable for tonight.

Outlook...
Wednesday through Saturday...Mainly VFR conditions expected.
Scattered showers and thunderstorms Thursday and Friday, with
MVFR/IFR conditions possible with any shower and thunderstorm
activity.

&&

.MARINE...
The wind direction should favor the northwest and west this
morning before backing toward the southwest and south for this
afternoon and evening. The wind may turn around toward the north
late tonight. Speeds should remain 15 knots or less. Wave heights
on our ocean waters are expected to favor the 2 to 3 foot range.
Waves on Delaware Bay should be 2 feet or less.

Outlook...
Wednesday through Saturday...Sub-advisory conditions are expected
to continue on the area waters through most of the work week.
Expect higher winds and waves in and near scattered showers and
thunderstorms that will be more likely at the end of the week.

Rip Currents...
There is a low risk for the development of dangerous rip currents
for today. We are anticipating an offshore wind this morning
around 10 mph becoming south to southwest for this afternoon.
There will likely be a southerly 2 foot swell with a period of
about 5 or 6 seconds.

The probably risk for the development of dangerous rip currents
on Wednesday is also low. Conditions will be similar to those of
today except the wind should be mainly onshore during both the
morning and afternoon hours.

&&

.CLIMATE...
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     26th       27th       28th
----     ----       ----       ----

PHL...   101-1892   101-1940   100-1941

ABE...    98-1940    98-1955    97-1949

ACY...    96-2011    99-2005    98-1999

ILG...    99-1894   100-1894   101-1894

TTN...    99-1892   100-1894   101-1894

GED...    97-2012    98-2005    99-1949

RDG...    99-1940    98-1955    99-1941

MPO...    89-1949    91-1955    93-1949

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...Heat Advisory until 7 PM EDT this evening for DEZ002-003.
MD...Heat Advisory until 7 PM EDT this evening for MDZ015-019-020.
MARINE...None.

&&

$$
Synopsis...Manning/Klein
Near Term...Klein
Short Term...Iovino
Long Term...Manning
Aviation...Iovino/Klein/Manning
Marine...Iovino/Manning
Climate...Staff


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