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Area Forecast Discussion (AFD)

Issued by Philadelphia/Mt Holly (PHI)

                            
843 
FXUS61 KPHI 200120
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
920 PM EDT Wed Sep 19 2018

.SYNOPSIS...
High pressure will continue building in from the north through 
Thursday before pushing off the New England coast through early 
Friday. A cold front will approach from the north and west on 
Friday before passing through the area Friday night and Saturday
morning. The front will then stall just south of our area 
through the end of the weekend as high pressure builds to our 
north. The high begins pushing offshore through the beginning of
next week.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
For the evening update, temp grids were adjusted slightly across
the Lehigh Valley, Berks Co, and Chester Co. Sky grids were also
adjusted along the coastal plain as more clouds have settled in
across Monmouth and Ocean Counties. Further west, west of the 
I95 corridor, skies were still mostly clear.

No change in the Wx grids as we continue to think tonight will
feature more low stratus vs. fog. We've kept the chance of 
drizzle in the grids east of I95. 

Previous discussion...
The remnant low of Florence lies about 250 miles SE of Atlantic
City, and will continue to move out to sea. An area of low 
clouds and stratus over New England continues to drift to the 
south and west towards New Jersey this evening, and should 
overspread the region from east to west later tonight. 
Meanwhile, a weak cold front lies just to the north and west of 
the local area and will drift through the region tonight.

Lower clouds will overspread the area from NE to SW overnight.
The metar at KBLM is already BKN014. Looks like the low clouds
may arrive early enough E/SE to preclude much in the way of 
fog, so I have cautiously removed it from the grids except NW 
where the mostly CLR skies will remain longest. The drizzle fcst
is tricky. Plenty of low clouds expected, but will the last 
1000 ft of the atmosphere saturate? Probably a better chc for 
drizzle back towards the coast, so trimmed the drizzle back to 
those areas, and hopefully won't have to re-expand back across 
the PA/NW NJ overnight. Fcst confid in fog and drizzle is rather
low. 

Lows tonight generally in the low 50s in the southern Poconos, and 
otherwise in the upper 50s to low 60s, except for Philly, which 
should be in the mid to upper 60s.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
Weak cold front washes out as it crosses the area, and whatever
remains of the front will become nearly stationary just south 
of Delmarva. With high pressure passing north of the area, winds
take on a northeast flow in the morning that eventually becomes
southeast in the afternoon. This onshore flow looks to keep 
abundant low clouds across the region for most of the morning, 
then those clouds will gradually lift in the afternoon.

A much cooler day on tap with highs in the low to mid 70s, and 
cooler along the shores.

&&

.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
By late Thursday, high pressure will be moving offshore to our north 
and east as and upper ridge axis passes to our north. This westerly 
flow and dry air aloft should keep the sensible weather dry and 
pleasant through early Friday with highs in the 70s to around 80 
degrees and lows in the 60s. By Friday evening, a cold front will 
begin approaching our region from the north and west bring the 
potential for scattered showers and thunderstorms primarily along 
and north of I-78. As the front begins moving in to our area, 
diurnal instability will be rapidly diminishing resulting in much 
lower PoPs through the overnight hours as it pushes toward the I-95 
corridor and points south. The exact timing of this front will 
dictate the evolution of convection across our area.

As the front stalls to our south on Saturday, chances for rain 
showers will exist across our southern zones along and north of the 
boundary. Coverage of showers will be dependent on just how far 
south the front stalls, but kept PoPs largely at slight chance now 
due to uncertainty.

Sunday through Wednesday... Stationary front will remain stalled to 
our south. There is still some discrepancy as to whether the rain 
will remain off to our south, or if it will override into the 
southern portions of our area. The ECMWF seems to push the front 
farther to the south than the GFS at this point. Walked the middle 
of the road and kept the chance for showers over the southern 
portions of the area into Monday for now. 

The front begins to nudge to the north as a warm front ahead of the 
next low pressure system late Monday into the day Tuesday as high 
pressure shifts out the Northeast and onshore easterly flow dies 
down. Temperatures gradually warm from the upper 70s to low 80s by 
Wednesday and Thursday as warm air advection takes over. Moist, 
southerly flow will continue, thus keeping the chance for showers 
through the middle part of the week. PWAT values look to remain near 
or just above 2 inches, so heavy rain could accompany any showers 
that do form. If added instability leads to elevated convective 
chances, we may need to watch the risk for flooding. Too far out to 
tell for now. Best chances for rain remain to our south, and 
gradually overspread the area by the middle of next week.

&&

.AVIATION /01Z THURSDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, 
KILG, KMIV, KACY and surrounding areas.

Tonight...MVFR CIGs developing across the terminals. There is a
low chance for IFR CIGs, especially in the pre-dawn hours. 
Patchy fog may also develop NW, but this looks less likely than
development of low stratus most other areas. Low confidence. NE
winds less than 10 kt, becoming LGT/VRB this evening.

Thursday...MVFR CIGs for most of the morning, gradually lifting
to VFR in the afternoon. LGT/VRB winds, becoming E-SE less than
10 kt in the afternoon.

OUTLOOK... 
Thursday night...mostly VFR expected with light south winds.

Friday...mostly VFR with increasing south winds.

Friday night...scattered showers and thunderstorms possible 
with brief MVFR conditions otherwise VFR. South winds less than 
10 knots becoming northwest late.

Saturday through Monday...generally VFR.

&&

.MARINE...
Although marginal, like for the NJ ocean waters, we have issued
a SCA for the DE ocean waters. The difference is timing. While
the SCA for NJ run from midnight tonight through 1 pm tomorrow,
the DE SCA runs from 6 am tomorrow through the day.

Previous discussion...
E-NE winds 15-20 kt will become SE 10-15 kt on Thursday. Ocean 
seas will build to around 5 feet on the NJ ocean waters tonight 
and will subside to sub-SCA levels Thursday afternoon. Will 
hoist a Small Craft Advisory from tonight through Thursday 
afternoon as the onshore flow should be strong enough to elevate
the seas. Opted for a general SCA as opposed to a SCA for 
Hazardous Seas as there may be some gusts to 25 kt tonight.

Outlook... 

Thursday night...generally sub-SCA conditions expected.

Friday through Saturday...SCA conditions possible with passage
of cold front.

Saturday night through Sunday night...sub-SCA conditions 
possible.

Monday...SCA conditions possible.


Rip Currents...

There is a moderate risk for the development of dangerous and life 
threatening rip currents at NJ and DE beaches Thursday, as there
is a medium-period easterly or southeasterly swell along with 
onshore flow.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 1 PM EDT Thursday for ANZ450>452.
     Small Craft Advisory until 6 PM EDT Thursday for ANZ453.
     Small Craft Advisory from 6 AM to 6 PM EDT Thursday for ANZ454-
     455.

&&

$$
Synopsis...Staarmann
Near Term...Kruzdlo/MPS
Short Term...MPS
Long Term...Davis/Staarmann
Aviation...Davis/Staarmann
Marine...Davis/MPS/Staarmann/Kruzdlo