Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 212016
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
316 PM EST Tue Nov 21 2017

.SYNOPSIS...
A cold frontal boundary will move east through our region tonight
and during the day on Wednesday. High pressure will then build
eastward into the region for Thursday and Friday before giving way
to an approaching cold front on Saturday. Another area of high
pressure will build into the region for the early part of next
week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
A deep upper trough, along with a sharp cold front out ahead of an
Arctic airmass, will continue to track eastward through the Great
Lakes and Ohio Valley tonight. Meanwhile, weak low pressure over the
eastern Gulf of Mexico will lift to the north and east and move off
the Mid-Atlantic coast tonight.

Several shortwaves will move into the Northeast and Mid-Atlantic
region out ahead of the approaching trough, and mesoscale banding of
precip will develop mainly across DE and southeast NJ. Models also
picking up on a new low forming just off the Eastern Seaboard
sometime late tonight and into daybreak Wednesday.

There are still differences among the global and short range models.
The first bands of precip will move into the Delmarva and southern
NJ just after midnight tonight, and then the bulk of the precip
moves in shortly thereafter.

The big question is where this banding sets up. Latest HRRR showing
bulk of precip holding off until after daybreak Wednesday, and even
much farther west than most models seem to indicate. HiRes-ARW keeps
the bulk of the precip across southeastern zones, generally to the
east of I-95. GFS, NAM, and ECMWF bring the precip a bit closer to I-
95 in southern NJ and southeast PA, but not as far west as the HRRR.

Another question will be the timing of onset of precip. It is
currently fairly dry with surface dewpoints in the upper 20s to
low 30s, so it will take some time to moisten up the column to
support rainfall. Temps will remain well above freezing where it
is expected to rain, so no ptype issues expected.

Based on these trends, will go ahead and bring the swath of likely
PoPs to and just west of the I-95 corridor and bring swath of
measurable QPF a bit to the west as well. For the most part,
heaviest precip should be on the order of 1/4" to 1/3" QPF.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
Rain continues, mainly along and south and east of I-95 corridor,
Wednesday morning as low pressure lifts out to sea. Rainfall should
wind down during the late morning hours, but with rapid
intensification of that low offshore, possible for some wraparound
precip to make it back to the NJ coast around lunchtime. Not much
additional QPF expected, but would not be surprised if there was
some additional couple of hours longer than forecast.

Meanwhile, cold front works its way through the region from late
morning through the afternoon. Front should be through the Fall Line
by noon, and then will clear the coast by mid-afternoon. With tight
NW pressure gradient developing and onset of CAA, winds will
increase to 15-20 MPH with gusts up to 30 MPH from midday on.
Temperatures will be turning cooler than today, especially in the
Poconos, but the coldest air will hold off on arriving until
Wednesday night.

Highs top off in the low to mid 50s, except in the Poconos, where
highs will be in the 40s.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
Wednesday night through Friday night:

High pressure will be in place during this period with mostly sunny
skies. Temperatures will slowly warm as winds go from northwesterly
ahead of the high pressure to southwesterly on the backside of the
high pressure. Highs will generally be in the 40`s with some spots
in the 50`s on Friday. Overnight lows will drop down into the 20`s
and 30`s. Model guidance may be a bit warm in the Pine Barrens and
in the Delaware Water Gap given a favorable radiational cooling
setup.

Saturday and Saturday night:

Some limited transport of moisture is possible ahead of an
approaching cold front. Some lift along the front may result in
a few showers. Another feature to watch is an area of low
pressure expected to track from near Florida northeastward to
just offshore of the Carolinas. Some ensemble members do bring a
period of rain northward into the region. However, the general
consenus is for the cold front to continue to push the low
pressure further out to sea. With warm air advection ahead of
the front, temperatures have the potential to be warmer than the
ensemble average which has been coming up the past few runs.
Highs could reach 60 in many locations Saturday before turning
colder Sunday behind the front. Northwest winds at night should
keep lows warmer and generally in the 30`s.

Sunday through Tuesday:

A fairly strong push of cold air advection will occur behind the
front. This will lead to another period of more winter like
temperatures across the region with stronger northwest winds,
gusting perhaps to 25 mph on Sunday. Some moisture associated with a
pocket of mid-level vorticity on the backside of a departing trough
may spark a few rain or snow showers across most of the region
Sunday and Sunday night. Higher terrain could act as another lifting
agent as well for the moisture. This outcome however depends on how
progressive the trough is moving through the area. Highs Sunday and
Monday will generally be in the 40`s and warming into the 50`s on
Tuesday. Overnight lows will be in the 20`s and 30`s.

&&

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

Tonight...SW winds 10-15 KT with gusts up to 20 KT rapidly diminish
to less than 10 KT this evening, and then become LGT/VRB late
tonight. VFR through most of tonight. BKN-OVC deck moves in from
south to north mainly after 06Z. MVFR conditions in -RA/BR expected
for terminals to the south and east of I-95, mainly at the KMIV and
KACY TAF sites, and there may also be a period of IFR conditions
late tonight as well. There may be a brief period of MVFR conditions
in -RA extending back towards the I-95 terminals of KTTN-KPNE-KPHL-
KILG after 07Z. Will use PROB30 for those sites given uncertainty.

Wednesday...Rain continues in the morning and gradually tapers off
through 12-15Z. Conditions lift to VFR. Winds become NW 5-10 KT
after 12Z, and then increase to 15-20 KT with gusts up to 25 KT
after 16Z.

Outlook...

Wednesday night through Friday night: VFR, northwesterly winds
becoming westerly by Thursday but at or under 10 knots. High
confidence.

Saturday and Saturday night: Sub-VFR possible with some showers.
Southerly wind gusts 10-15 knots. Medium confidence.

Sunday and Sunday night: VFR, winds shifting to northwesterly with
gusts from 20-25 knots in the afternoon. Medium confidence.

&&

.MARINE...
Will cancel the SCA for DE Bay as wind gusts have fallen below 25
KT.

Wind gusts should fall below 25 KT on DE Bay this evening, and then
SCA ends on DE ocean waters later this evening as pressure gradient
relaxes. Conditions on NJ ocean waters should remain above SCA
criteria for most of tonight.

Rain moves in from the south late tonight and will continue through
Wednesday morning before moving out to sea. A cold front moves
across the waters Wednesday afternoon, and then NW winds increase to
15-20 KT with 25-30 KT gusts. Went ahead and issued a SCA for DE Bay
as current SCA was cancelled, but a new SCA for the ocean waters
will be issued for at least Wednesday afternoon when current SCA
expires.

Outlook...

Wednesday night through Saturday night: Sub-SCA seas and winds from
Thursday onward. West winds shifting to southerly Friday into
Saturday with gusts of around 15 knots at times. Any westerly wind
gusts near 25 knots will diminish quickly Wednesday evening but seas
may linger near five feet through the entire night.

Sunday and Sunday night: Winds shifting to northwest with 25-30 knot
gusts possible at times. Seas building but look to stay under five
feet at this time.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 4 AM EST Wednesday for ANZ450>453.
     Small Craft Advisory until 10 PM EST this evening for ANZ454-
     455.
     Small Craft Advisory from noon to 6 PM EST Wednesday for
     ANZ430-431.

&&

$$
Synopsis...Gaines
Near Term...MPS
Short Term...MPS
Long Term...Gaines
Aviation...Gaines/MPS
Marine...Gaines/MPS


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