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FXUS61 KPHI 170133

Area Forecast Discussion
National Weather Service Mount Holly NJ
933 PM EDT Mon Jul 16 2018

A low pressure system moves across Ontario and Quebec tonight and 
Tuesday. An associated strong cold front will cross our area Tuesday 
afternoon and Tuesday night. High pressure from the Great Lakes will 
follow for Wednesday and remain over our area into Friday night. A 
series of disturbances will then be across the Mid-Atlantic for the 
upcoming weekend.


The mostly isolated convection has dissipated, as of 01z. The main 
forcing was terrain and a sea breeze which was enhanced inland by 
convective outflow. The boundary was well defined on radar however 
it has dissipated now since is has moved well inland. As a result, 
scaled the PoPs back accordingly for the remainder of this evening.

As we go through the overnight though, an amplifying upper-level 
trough moves across the Great Lakes and the leading edge of the 
height falls arrive into our northern and western areas. It appears 
though the bulk of the forcing is farther to the west closer to the 
cold front. Therefore some uncertainty exists on how much shower or 
thunderstorm activity redevelops or arrives across mainly our 
western zones late tonight. Some guidance suggest some convection 
occurs mostly in portions of eastern Pennsylvania to northern New 
Jersey late tonight while other guidance is dry. For now, carried 
slight chance PoPs for the western areas (low chance PoPs for the 
Poconos) late tonight. The southerly flow at the surface diminishes 
tonight and therefore patchy fog is possible given lots of low-level 
moisture. We are more in the warm sector given a cold front to our 
west (plus some more mid to high level clouds late), and therefore 
any fog may be rather light/limited and thus did not add to the 
forecast at this time.

It will be a warm and rather muggy night ahead of a cold front (dew 
points mostly in the 70s). The hourly temperature, dew point and 
wind grids were adjusted based on the latest observations, then the 
LAMP guidance was also blended in to help assist. The cloud cover 
has mostly dissipated outside of some cirrus. Some increase in the 
clouds is anticipated through the night though especially across the 
western areas as cloud debris arrives.


The day may start with a few showers or thunderstorms across the 
western areas, however an active afternoon is expected as an 
amplifying upper-level trough approaches from the west and a surface 
cold front arrives. The expectation of more clouds and eventually 
widespread convection will result in a cooler day but still rather 
humid. Did slow down the eastward increase in PoPs though through
about early afternoon.

The SPC maintained the I-95 corridor on westward in a marginal risk 
for severe thunderstorms for Tuesday afternoon. The guidance has 
PWATS of 2-2.25 inches, therefore quite a bit of moisture is 
available. Given enough instability and where updrafts are able to 
be more robust, frequent cloud to cloud lightning is a good 
possibility. The low and mid level flow/shear however ahead of the 
cold front is somewhat limited, with this greatest to our north. 
There should be some multicells though ahead of the front, and if 
these can organize enough and generate a cold pool then local strong 
surface wind gusts can occur. There is the potential for at least 
some isolated severe thunderstorms, with locally damaging winds 
(water loaded downdrafts) the main threat. The mid level flow is 
more perpendicular to the cold front, therefore convection should be 
on the move. Given the high PWATS though, downpours could result in 
localized flooding.


Tuesday night...The strong cold front will be moving offshore and 
the showers and thunderstorms associated with it will be ending 
across the region. Cooler and drier air will arrive across the NW 
areas, but it may take until Wednesday to finally arrive across 
Delmarva and srn NJ. Overnight lows will drop into the upper 50s/low 
60s N/W and mid/upper 60s S/E. Winds will become NW overnight at 5 
to 10 mph. 

Wednesday thru Friday... High pressure will be across the Great 
Lakes Wednesday and it will build across the Middle Atlantic 
Thursday before moving offshore Friday. The high will bring fair 
weather along with seasonable temperatures. Highs will be in the 
low/mid 80s in most areas, but the overnight lows will be cool for 
July with 50s across the north/west and low/mid 60s for Delmarva and 
srn NJ. These lows will be around 5 degrees below normal. 

Next weekend/Monday...Unsettled with an unusually strong upper 
trough moving towards the region. A deep south to southwest flow
aloft will become established and abundant moisture from the 
south will arrive across the region. Showers and thunderstorms 
will probably be around Sat thru Mon, so we have kept the high 
chance pops for the weekend. The latest NMB has some likely pops
for Sunday, so we went along an put those in the grids attm.
Locally heavy rains are possible, but we'll get more details
later this week before getting any more specific.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, 
KILG, KMIV, KACY and surrounding areas.

Rest of tonight...VFR overall. An isolated shower or thunderstorm 
cannot be ruled out mainly west of PHL. Some MVFR visibilities due 
to light fog is possible at some terminals overnight, however this 
is of lower confidence. South to southwest winds less than 10 knots, 
or even becoming locally light and variable to calm.

Tuesday...Local MVFR visibilities early due to light fog, otherwise 
VFR for awhile. Showers and thunderstorms are expected to develop 
from west to east and become more widespread mainly in the afternoon 
ahead of a cold front. There will be ceiling and especially 
visibility restrictions (MVFR/IFR) with the showers and 
thunderstorms, and locally brief gusty winds may accompany some 
stronger thunderstorms. South to southwest winds increasing to 
around 10 knots, becoming west-northwest late in the afternoon at 
RDG and ABE and closer to the early evening farther east. 

Tuesday night...Showers and thunderstorms ending from west to 
east, with VFR returning. Patchy fog possible across Delmarva 
and southern New Jersey. 

Wednesday through Friday...VFR expected. 

Friday night and Saturday...Mostly VFR. A few showers and 
thunderstorms possible.


Tonight...Sub SCA conditions expected. Mainly SW winds 15 to 20 kt. 
Seas 3 to 4 ft. 

Tuesday...No headlines south of Little Egg Inlet with SW gusts up to 
20 kt and seas up to 4 feet. North of Little Egg, gusts up to 25 kt 
and seas up to 5 feet are expected. A SCA has been issued north
of Little Egg between 12z/Tue through 04z/Wednesday. 

Tuesday night...SCA diminishing and showers/tstms ending from W to

Wednesday thru Friday...Sub-SCA with fair weather. 

Friday night thru Saturday...Scattered showers and tstms.
SCA psbl. 

Rip Currents... 
For Tuesday, opted to go with another moderate risk of rip 
currents for both the New Jersey and Delaware beaches. This is 
due to a lingering underlying southeasterly long period swell 
(although it looks weaker than previous days), the seas building
some and a 10-25 mph south-southwest wind along the coast again
especially late morning through the afternoon.


MARINE...Small Craft Advisory from 8 AM Tuesday to midnight EDT Tuesday 
     night for ANZ450-451.


Near Term...Gorse

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