Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 200430 AAA
AFDPHI

Area Forecast Discussion...UPDATED
National Weather Service Mount Holly NJ
1230 AM EDT Sat May 20 2017

.SYNOPSIS...
A cold front will continue to cross our area tonight. High
pressure across Canada will then build over the area for the
weekend. Low pressure will move into the upper Great Lakes
region Sunday and cause a slow moving front to affect our
weather Sunday night and Monday. More high pressure will develop
across the area next Tuesday. Another disturbance will arrive
for Wednesday and affect the weather through Thursday. This will
be followed by another possible disturbance at the end of the
week.

&&

.NEAR TERM /THROUGH SATURDAY/...
Considerable changes were made to the forecast for the overnight
and Saturday, mainly adding scattered showers to what previously
was a dry forecast.

Currently the only precip over the forecast area was a complex
of showers and thunderstorms that were crossing the MD
Chesapeake Bay around midnight. This activity will move eastward
through the Delmarva and into the DE Bay during the overnight.
A second areas of convection (mostly showers though there are a
few embedded thunderstorms) associated with a MCS over western
PA will move into eastern PA and NJ during the morning. Although
the surface cold front will continue to progress south of the
region, the W-E oriented frontal boundary aloft will remain hung
up to the north near the I-78/I-80 corridor as a result of
zonal steering flow. Backed, southerly flow ahead of the
aforementioned MCS/shortwave trough will strengthen the
isentropic lift (and low-level convergence) across the frontal
boundary. The result will be the expansion of anafrontal
precipitation across eastern PA and NJ at least through midday,
if not later into the first part of the afternoon. For now
raised PoPs to the 30-50 percent range, but may need to go even
higher later this shift, especially if we can hone in on timing
and location of the greatest lift. Did add a slight chance of
thunder to the grids through the early morning in our northern
zones and mid morning farther south with forecast soundings from
the HRRR/RAP/NAM nest showing 400-600 J/kg of elevated
instability south of the 850 mb front.

At the surface, high pressure builds southward from eastern
Canada during the day. An onshore flow around the high and
cloudy conditions will result in much cooler temperatures (a
change from summer temperatures back to spring). Highs should
get to around 70 inland, but right along the coast, highs will
range from the low to mid 60s.

&&

.SHORT TERM /SATURDAY NIGHT/...
High pressure centered over eastern Canada will continue to nose
southward into the Mid-Atlantic states. Persistent easterly
flow will keep temperatures from tanking night. Clouds above
the marine layer should clear from N to S during the evening as
drier air advects southward behind the elevated cold front.
Despite winds off the ocean, the marine layer looks very dry
with the airmass source originating from the Canadian high.
Therefore, we do not expect marine stratus to develop tonight.
Low temperatures generally range between 45 and 55 degrees.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Overview...The period begins with surface high pressure flexing
southward from Quebec. The high will be with us through the day
on Sunday, but weaken Sunday night. On Monday, a surface low
will drift into Ontario, first tracking across the Great Lakes.
Both a warm front and cold front, associated with this low, will
be in play for the Eastern Seaboard. Both fronts will move off
the coast Monday night allowing high pressure to build in across
the region on Tuesday. Another surface low will track into the
Great Lakes region Tuesday night and Wednesday. As this low
tracks into Quebec, a warm front/cold front will move through
the region and should be near or just off the coast Thursday
morning. Another frontal system is possible late in the week.
Tough to pinpoint any timing at this point, but Friday into the
Memorial Day weekend looks unsettled as mid-level troffing is
favored across the eastern U.S.

Temperatures...For comparison, normals for Philadelphia are in
the mid 50s and mid 70s. Temps across the area will be several
degrees below normal on Sunday, a couple degrees above normal on
Monday, and about normal Tuesday through Friday.

Precipitation...There will be about a 24 hour stretch from
Sunday night to Monday night with POPS painted in the grids.
General amounts in the 0.50 to 1.00 range cannot be ruled out.
There will be a convective contribution, so some higher amounts
are certainly possible. We should dry out Tuesday, before
another shot of rain Wednesday into Thursday. Another s/w
rotating around the mid- level trof is forecast for late Friday
or just outside this 7 day forecast period.

Winds...Outside of gusty winds possible in and near any
thunderstorms that develop, winds during the period will be 20
mph or less.

Impacts...Widespread impacts are not expected through the
period.

&&

.AVIATION /04Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

VFR conditions are anticipated through the TAF period.

The wind is expected to shift to the north with the arrival of
a cold front. The front had already reached KABE and was about
to arrive at KRDG and KTTN around 0130Z. The front should pass
through KPNE and KPHL between 0200Z and 0300Z, and KILG, KMIV
and KACY between 0300Z and 0400Z. The wind is forecast to veer
toward the northeast overnight and to the east on Saturday at
speeds around 10 knots.

OUTLOOK...
Saturday night and Sunday...Mainly VFR. East to southeast
winds.

Sunday night through Monday night...Lower CIGS/VSBYS likely in
showers and thunderstorms. Southeast to south winds Sunday night
becoming southwest on Monday, and northwest on Monday night.

Tuesday...Mainly VFR. North winds in the morning, becoming
southeast later in the day.

Wednesday...MVFR likely in showers and/or thunderstorms.
Southeast winds.

&&

.MARINE...
Remainder of this evening...Showers affecting DE ocean waters,
as well as DE Bay. Sub-SCA conditions.

Overnight...SCA conditions develop on DE ocean and southern NJ
ocean waters late tonight as E winds increase to 15-20 KT with
gusts up to 25 KT. Seas build to around 5 ft.

Saturday...SCA conditions on all ocean waters and on Lower DE
Bay for E winds 15-20 KT with gusts up to 25 KT. Conditions will
be just short of SCA levels on Upper DE Bay.

OUTLOOK...

Saturday night and Sunday...SCA conditions possible across the
southern ocean waters. No precipitation expected. Southeast
winds gusting to 20 kt at times.

Sunday night through Monday night...SCA conditions possible
into Monday. Showers and thunderstorms possible. Southeast winds
to 20 kt Sunday night, becoming southwest Monday with
decreasing speeds, and northwest on Monday night.

Tuesday...Sub-SCA conditions. Northwest winds at or below 15 kt
becoming southeast.

Wednesday...Sub-SCA conditions. Southeast winds at or below 15
kt. A chance of showers and/or thunderstorms.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 2 AM EDT Sunday for ANZ452>455.
     Small Craft Advisory from 6 AM to 6 PM EDT Saturday for ANZ431-
     450-451.

&&

$$
Synopsis...Kruzdlo
Near Term...Klein
Short Term...Klein
Long Term...Kruzdlo
Aviation...Iovino/Kruzdlo
Marine...Kruzdlo/MPS



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