Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 281221
AFDPHI

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

.SYNOPSIS...
An area of low pressure will move across the Ohio River Valley
today 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/...
820 am: fog is done. Ceilings 800 to 1800 ft spread southwest
onto the NJ coast around 15z and eventually reach PHL around 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 this
afternoon.

showers and isolated tstms overspread md e shore this morning and
then a problematic timing issue on a further n and e spread this
afternoon. A strengthening 30-35 kt jet streak at 925 mb will
advect moisture off the Atlantic. There should be sufficient
isentropic lift to produce showers across southern/western zones
this afternoon. So, we expect showers to increase in coverage and
intensity later this afternoon.

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 /12Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Today...Basically VFR to start with any MVFR Fog dissipating by
1330z. Cigs 800-1800 feet advect-develop southwestward onto the
NJ coast in the 15z-18z time frame and spread west to PHl by 20 or
21z. drizzle may develop KACY late in the day. light n-ne wind
early this morning becomes gusty northeast 20-28 kt this
afternoon.

Tonight...widespread cigs 500-1500 ft with vsby occasionally aob
3 mi in stratus, mist and fog with showers developing. 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 goes into effect this afternoon for ANZ450/451 and the
remainder of the waters this evening as NEly winds surge down the
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 possible
tonight off the NJ coast. Seas in the coastal Atlantic waters will
subsequently build to 5-7 ft tonight.

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 is a moderate risk for the formation of dangerous rip
currents today. This is due to the combination of an increasing
onshore wind and building waves in the surf zone. Conditions may
warrant a high risk late in the day if winds and seas increase
quicker than currently expected.

The probably risk for the formation of dangerous rip currents is
at least moderate to perhaps high on Thursday.

&&

.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 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 from 6 PM this evening to 6 PM EDT
     Thursday for ANZ430-431-452>455.
     Gale Watch from Thursday morning through late Thursday night
     for ANZ450-451.
     Small Craft Advisory from 1 PM this afternoon to 6 AM EDT
     Thursday for ANZ450-451.

&&

$$
Synopsis...Robertson
Near Term...Drag/Klein 820A
Short Term...Klein
Long Term...Robertson
Aviation...Drag/Klein/Robertson 820A
Marine...Drag/Klein/Robertson
Tides/Coastal Flooding...
Climate...820A


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