Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 282018
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
418 PM EDT TUE JUN 28 2016

.SYNOPSIS...
Low pressure is forming on a cold front crossing the mid Atlantic
states late this afternoon and evening. That front drifts offshore
on Wednesday. Weak high pressure builds in for Thursday. A cold
front approaches from the west on Friday and moves offshore Saturday.
High pressure follows later Saturday into Monday. A strong warm
front should be moving northeast toward our area on Tuesday.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
A Severe Thunderstorms Watch has been issued for Eastern
Pennsylvania, portions of NJ along the Delaware River, as well as
New Castle and Kent, Delaware and Cecil, Kent, and Queen Anne`s
and Maryland through 10 pm.

A cold front is progressing through central Pennsylvania and
approaching our this afternoon. Showers and thunderstorms are
developing along and ahead of this front as instability has build
during the day, and a short wave/vorticity impulse is approaching
from the west as well. This activity will all move eastward through
late this afternoon and into this evening as the front continues to
slowly push eastward. MLCAPE has reached 1,000-1,500 J/KG, or will
increase, where the sun has come out and daytime heating has
occurred. Bulk shear values are around 30-35 knots, and mid level
winds are only 30-40 knots, but with the amount of available CAPE,
some storms could build high enough to produce some hail and strong
wind gusts. Another concern will be heavy rainfall. Even though PW
values are only ~1.5 inches, very heavy rainfall is occuring and
could produce over an inch in an hour in some areas, leading to
ponding in urbanized and poor drainage areas.

The convection should begin affecting the Poconos, Lehigh
Valley/Berks County, and Northwest New Jersey areas between 4 and 7
pm. Then continue eastward and reach the Delaware Valley/I 95/295
corridor between 7 and 10 pm, and then eastern New Jersey and
central/southern Delmarva by around 10 pm to midnight if it holds
together.

All shower activity will begin to diminish after midnight, and the
remainder of the overnight should end up mostly precipitation free
except a few isolated showers/sprinkles. Once the rain ends, some
low clouds and patchy fog could develop, especially in areas where
winds drop off and become light and variable.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
Wednesday should be a very nice day. The cold front will either be
offshore, or moving offshore at the start of the day. We are not
expecting any showers to be affecting the area, but if there was any
showers, it would be near the frontal boundary.

After any morning fog/low clouds burns off, a nice day should
develop across the area. Some scattered cumulus cloud build up is
expected during the day, but we`ll be precipitation free. We are
expecting close to normal, or slightly above normal temperatures for
Wednesday, even with the light northwest flow.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
500MB: A -1SD short wave trough over the eastern Great Lakes Tuesday
night weakens during mid week at the expense of a more significant
trough (-2SD) developing over southeast Canada late Friday and Saturday.
Eventually ridging follows into the northeast USA early next week.

Temperatures: June through the 27th has averaged 1 to 2 degrees above
normal at PHL, RDG, ABE, and TTN, near normal at ILG/ACY, 0.6F above
normal at GED and surprisingly a degree below normal at MPO. Calendar
day averages the next week or so should be within 3 degrees of normal...
nothing notable. So the monthly average should be very close to the
monthly departures posted above and also available in our
preliminary CF6 within the MT Holly climate web link.

Forecast basis: Unless otherwise noted, a 50 50 blend of the 12z/28
GFS/NAM MOS guidance Wednesday night - Thursday night, thereafter
the 12z/28 GFS MEX MOS for Friday-Friday night and WPC Guidance
Saturday-Tuesday. I did take into account the 12z/28 ECMWF op run
and added several degrees to some of the max`s Thu-Mon,
especially north of PHL near I78 north to the northern border and
then on Saturday, raised temps almost all the way to the coast
where near 90F possible.

The dailies...

Wednesday night through Thursday...High pressure briefly builds
in, in the wake of the cold front. Nice seasonable summer weather.
Confidence: Above average.

Thursday night...waa may still bring a shower or thunderstorm to
the DE coast. Patchy fog probable parts of the area, especially
favored nw NJ valleys.  Confidence: average.

Friday...a chance of showers and thunderstorms ahead of the next
cold front, otherwise partly to mostly sunny, very warm and more
humid. Heat index lower 90s. Confidence: average.

Saturday...Rainfree and very warm, lower dewpoints. Confidence:
above average.

Sunday through Monday Independence Day...Weak high pressure should
build in for Sunday and Independence Day. Thus at this point, we
still expect this period to be dry. Nice summer weather for the
outdoor celebrations. Confidence: Average. the reason its only
average is that am a little concerned about thunderstorms moving
into the area Monday afternoon ahead of a developing strong warm
front.

Tuesday...Depending on how quickly the high shifts off shore, low
level flow will shift to southerly sometime on Tuesday, bringing
warm air and moisture advection. Forecast is rainfree at this time
but warm frontal thunderstorms are a concern. Confidence: below
average.

&&

.AVIATION /20Z TUESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Conditions have improved to VFR and MVFR for all sites this
afternoon. These VFR/MVFR conditions will continue through this
afternoon and into this evening, with the exception of when any
showers/thunderstorms affect a TAF site. A period of heavy rainfall,
gusty winds, and low visibilities will occur with any thunderstorms.
We`ve updated ABE and RDG to include wind gusts and IFR conditions,
and will likely update ILG/PHL/PNE/TTN if the convection continues.

The convection should affect ABE/RDG between 21-01z. Then continue
eastward and reach ILG/PHL/PNE/TTN between 23z-03z, and then MIV/ACY
by around 00z-04z if it holds together.

After the rain moves through, low clouds and fog area again expected
to develop. Ceilings and visibilities are expected to lower to IFR
overnight through daybreak Wednesday. Once any low clouds/fog burns
off Wednesday morning, VFR conditions will prevail for the remainder
of the day.

Winds are mostly southeast across the area, but become variable at
times as some southwest winds from aloft mix down to the surface.
Once the front moves through overnight, winds will become southwest
to west, then northwest during the day Wednesday. Wind speeds are
expected to generally be 10 knots or less.

Outlook...

Confidence on the days listed below is generally above average.

Wednesday night through Thursday...VFR. Light northwest wind
become southerly late day.  Confidence: above average.

Thursday night through Friday...MVFR and brief IFR conditions
will be possible with any showers and thunderstorms that move
through the region during this time...probably mainly Friday.
Patchy IFR stratus/fog possible early Friday morning.

Saturday...VFR. west to northwest wind.

Sunday...VFR.

&&

.MARINE...
Sub-Small Craft Advisory conditions will prevail across the waters
tonight through Wednesday. A cold front will move across the waters
early wednesday, so southerly winds today/tonight will become
westerly/northwesterly on Wednesday.

A period of showers and possible thunderstorms may affect the waters
late this evening, before dissipating overnight. If any thunderstorms
do occur, they could locally increase winds and waves.

Outlook...
Wednesday night through Sunday...winds and seas should remain below
SCA criteria. Although gusty winds are possible in the vicinity of
any thunderstorms Thursday night or Friday.

RIP CURRENTS...The rip current risk will be low to possibly moderate
at times during the balance of this week through the weekend, in
part dependent on the amount of onshore wind. In other words, not
much change from the somewhat active month of June. AWARENESS:
weak swimmers for their own safety should swim only in the presence
of lifeguards. It is not worth being a rip current victim by swimming
near jetties, or after the lifeguards go home, or with untrained
weak swimming bystanders who wont be capable of saving a distressed
swimmer, except to call 911, which then could be too late.

&&

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

&&

$$
Synopsis...Drag
Near Term...Robertson
Short Term...Robertson
Long Term...Drag
Aviation...Drag/Robertson
Marine...Drag



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