Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 281501
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1101 AM EDT Wed Sep 28 2016

.SYNOPSIS...
An area of low pressure will move across the Ohio River Valley
and combine with another low across the central and southern
Appalachians Thursday into Friday. This low will lift back across
the Ohio River Valley and into the eastern Great Lakes region
Saturday into Sunday and bring a frontal system across our area.
High pressure is then expected to build across the northeast late
Monday into Tuesday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
1040 am: Ceilings 800 to 1800 ft spreading southwest onto the NJ
coast now will eventually reach PHL around 19 or 20z. Cold air
damming minimal today since the big push of cool moist boundary
layer air doesn`t arrive most of our fcst area until after max
heating today. Cold air damming does become a much bigger player
tomorrow. Gusty e-ne wind 20-30 mph develops by this afternoon.

Showers and isolated embedded tstms MD e shore and w DE into
extreme sw NJ with diminishing intensity so the 1040 AM pop
adjustment was to account for the mid level rain dying out
midday/early this afternoon. while was not exactly sure how to
handle this CHES Bay organized system that dumped 3-4.5" on the
other side of the Bay and 1-2" parts of the MD e shore, it is on
the wane.

We have seen the SPC expanded marginal risk into MD E shore but no
action at this time for enhanced wording. Will reevaluate with 12z
GFS guidance. NAM has not handled this system very well so am
inclined to favor GGEM/GFS/EC/UK/WPC blend.

Forecast soundings support a deep enough moist layer for at least
patchy drizzle to develop late this afternoon, toward the NJ coast.

High temperatures range from the mid/upper 60s along and north of
the I-80 corridor to mid/upper 70s across southeastern PA,
southern NJ and Delmarva.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
The upper low is forecast to slow down over the Midwest states
tonight as it becomes cutoff. High pressure will continue to
remain anchored to our north while the frontal boundary remains
stalled to our south. The easterly low-level jet between these
systems over the mid Atlantic will strengthen further to around 40
kt.

Expect showers to expand northeastward across the entire area
tonight. Expect for the NAM-based guidance, models continue to
show a band of heavy rainfall developing very close to the western
most zones of our CWA. The trend has been to shift the axis of
higher QPF farther south and west, likely as models are more
accurately capturing the magnitude of the cold-air damming
pattern. Additionally, with the greatest low-level convergence
situated at nose of the low-level jet forecast to be positioned to
our west, the greatest risk for excessive rainfall may remain on
the western side of the Chesapeake Bay. QPF amounts of 1-2 inches
are forecast for eastern MD and out toward Chester/Berks Co. in
eastern PA. We opted to not issue a Flood Watch at this juncture.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Unsettled weather continues to be forecast through the first half
of the extended period, with generally fair weather for the second
half of the period.

For Thursday into Friday, an area of low pressure at the surface
will be located to our southwest, while high pressure remains to
our northwest. In between, a stationary boundary will be located
just to our south. This will keep an onshore flow across our area.
Meanwhile, a closed low aloft will be spinning to our west as
well. This will keep a persistent southerly flow aloft across the
area at the same time. With the southerly flow aloft, overrunning
precipitation is expected to continue area within the onshore flow
Thursday and Friday. It will not likely rain the entire time, but
several periods of rain will be associated with several short
waves/vorticity impulses rounding the closed low aloft to our
west.

By Saturday, and especially Sunday, the high to our north begins
to move eastward, and the low to our west will begin lifting back
toward the Great Lakes region. This will eventually pull the
frontal boundary to our south across our area on Sunday. There
will continue to be a chance of showers Saturday through Sunday as
several short wave/vorticity impulses move across the area as the
closed low aloft begins to break down and lift to the northeast.

By Monday into Tuesday, the surface low and low aloft pass to our
east, and drier air begins to move into the area as high pressure
tries to move in from the north. The high may remain to our north,
Tuesday which could produce an onshore flow which may end up
leading to more cloud cover and cooler conditions for Tuesday.

&&

.AVIATION /15Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

This afternoon...VFR conditions to start. Cigs 1000-2200 feet advect-
develop southwestward fm the NJ now and spread west to PHl by 18z.
showers vcnty KILG and KMIV may reach PHL but these showers are
tending to dry out as they move northeast. drizzle may develop
KACY late in the day. light n-ne wind early this morning becomes
gusty northeast 20-28 kt by this afternoon.

Tonight...widespread cigs 700-1700 ft with vsby occasionally aob
3 mi in stratus, mist and fog with showers. ne wind with scattered
gusts 20 kt.


OUTLOOK...

Thursday-Saturday...A prolonged period of MVFR or IFR conditions
with periods of rain is expected to affect the TAF sites through
the period. East to northeast winds are expected to gust 20-25
knots for much of the area.

Saturday night-Sunday...Some improvement, possibly to VFR by Sunday.
Showers remain possible.

&&

.MARINE...
SCA in effect for the NJ coast. A Gale Watch was previously in
effect for ANZ450/451 for tonight but we do not anticipate gales
to develop before Thursday. Winds of 15- 25 kt are expected with
gusts to 30 kt through tonight along the NJ coast. Seas in the
coastal Atlantic waters will subsequently build to 5-7 ft by
tonight. SCA continues with a 6 PM start for De Bay and the
Atlantic DE waters though could see a sooner start.


OUTLOOK...
Thursday...Gale Watch in effect for the northern half of the
Atlantic coastal waters. Small Craft Advisory in effect elsewhere.

Thursday night...Gale Watch in effect for ALL areas.

Friday...Gale force winds may remain across the northern half of the
Atlantic coastal waters. Small Craft Advisory level winds may remain
elsewhere.

Friday night-Saturday...Winds expected to drop below Small Craft
Advisory levels, however, seas may remain above advisory levels
through the period.

Saturday night-Sunday...Winds and seas expected to drop below Small
Craft Advisory levels.

Rip Currents...
There continues a moderate risk for the formation of dangerous
rip currents today as per a recheck of the ene sustained wind
~18kt and dominant 4 second short period swell.

Thursday: The probability for the formation of dangerous rip
currents is at least moderate to perhaps high.

&&

.TIDES/COASTAL FLOODING...
A persistent onshore flow is expected for the rest of the week
into the weekend. The strongest winds are expected Thursday through
Friday. Tides levels are forecast to increase through this period,
and we could reach minor coastal flood levels.

&&

.CLIMATE...
Will add extensive climate information late this morning or this
afternoon as time permits.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Watch from Thursday evening through late Thursday night
     for ANZ430-431-452>455.
     Small Craft Advisory until 6 PM EDT Thursday for ANZ452-453.
     Gale Watch from Thursday morning through late Thursday night
     for ANZ450-451.
     Small Craft Advisory until 6 AM EDT Thursday for ANZ450-451.
     Small Craft Advisory from 6 PM this evening to 6 PM EDT
     Thursday for ANZ430-431-454-455.

&&

$$
Synopsis...Robertson
Near Term...Drag/Klein  1101
Short Term...Klein
Long Term...Robertson
Aviation...Drag/Klein/Robertson 1101
Marine...Drag/Klein/Robertson 1101
Tides/Coastal Flooding...
Climate...



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