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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KPHI 211958
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
358 PM EDT Thu Sep 21 2017

.SYNOPSIS...
Tropical storm Jose is expected to weaken as it meanders off the New
England coastline through this weekend. Meanwhile, high pressure
over eastern Canada will expand southward into the Middle Atlantic.
During the early to middle part of next week, Hurricane Maria is
forecast to move northward over the Atlantic Ocean waters between
Bermuda and the Middle Atlantic states. A cold is expected to
move through our area in the Wednesday and Thursday time frame.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
Tropical Storm Jose is expected to continue its gradual weakening
through tonight while remaining nearly stationary about 150 miles
southeast of Nantucket, MA or about 330 miles due east of LBI, NJ.

Looking upstream, drier air with dewpoints about 10-15F lower than
in our area was noted over interior New England this afternoon.
The pressure gradient between the offshore tropical system and high
pressure over southeastern Canada that extends down along the
Appalachian spine will allow this dry air to surge southward into
our region tonight. Not sure if the dry air makes it here as fast as
the GFS is indicating with surface winds becoming very light after
sunset, but it should reach NE PA and NW NJ during the overnight.
The setup for radiational fog in the river valleys of NE PA and NW
NJ would not be nearly as favorable as in recent days (and as climo)
assuming the dry air arrives by early morning.

Low temperatures range from the mid 50s along the I-80 corridor to
the mid 60s along the I-95 urban corridor and near the coast.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
The NHC forecasts Tropical Storm Jose to undergo post-tropical
transition by Friday. The center of Jose will either continue to
stall near its current position or start to drift ever so slowly
westward but still remain comfortably offshore by several hundred
miles.

A westward drift would allow for denser cloud cover from Jose to
spread back toward the northern mid-Atlantic coast on Friday and
perhaps as far inland as the Delaware Valley late in the day.
Temperatures will have a dependency on how far west the cloud cover
progresses. For now, think clouds shouldn`t have a huge impact on
temperatures across inland locations. Forecast highs range from the
upper 70s in the higher elevations of NE PA/NW NJ and near the coast
to mid 80s near Phila and points S/W.

Friday is still expected to remain dry with outer rain bands from
Jose staying offshore. However, could see scenario, albeit an
unlikely one, where one of these bands reaches the coast during the
morning.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
The forecast challenges during this time frame will be high temps
Saturday through Monday and the eventual track of Hurricane Maria.

A complex mid-level pattern will evolve over the eastern US and
southwest Atlantic, with a southern stream rex block, eventually
breaking down with the approach of height falls associated with a
northern stream trough late in the period. Meanwhile, Hurricane
Maria is forecast to move northward through a weakness in the
subtropical ridge, under the increasing influence of a mid-level
trough along the eastern fringe of the rex block, e.g., remnants
of Jose, by the middle of next week.

In terms of high temperatures, the first full week of fall will
average above normal across our area. In fact, high temps in the
Saturday through Monday period will be around 10 degrees above
normal! The airmass will be generally dry with a light surface flow
in place. It will be cooler near the coast with a sea-breeze on
Saturday, and a more pronounced synoptic onshore flow Sunday into
Monday. There is some uncertainty in high temps, given the spread
exhibited in the ensembles, although a similar air mass lends to
more of a persistence forecast.

With the synoptic flow becoming onshore during the second half of
the weekend and persisting into the midweek, low-level moisture will
gradually increase, with perhaps some contribution from the remnants
of Jose as well. We are noting some of the GEFS ensemble members
bringing measurable precipitation into the coastal plain on Tuesday,
well in advance of the anticipated cold frontal passage Wednesday
into Thursday. Therefore, we have maintained slight chance PoPs in
the coastal plain beginning Monday night, expanding to chance PoPs
over the entire region on Wednesday.

The forecast also becomes uncertain in the Tuesday through Thursday
period and will be highly dependent on the eventual track of Maria,
including the extent of its interaction with the approaching cold
front. We continue to stay closer to a consensus of the guidance,
which takes the cold front offshore Wednesday night, thereby
introducing a drying trend by Thursday.

Stay tuned to the latest advisories and forecasts from the
National Hurricane for more information on Jose and Maria.

&&

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

VFR thru the valid TAF period.

Winds will primarily have a northerly component but vary slightly in
direction between NW (~320 deg) and NE (~040 deg).  Wind speeds for
the remainder of the afternoon and again for the daytime on Friday
will generally range between 6 and 12 kt while tonight will average
around 6 kt or less.

Outlook...

Predominantly VFR conditions are anticipated Friday night through
Monday across the terminals regionwide. There is the potential for
MVFR at ACY and MIV in -SHRA Monday night into Tuesday. Winds north
10 knots or less Friday night into Saturday night, then becoming
east-northeast on Sunday and continuing through Tuesday, generally
around 10 knots with gusts 15 to 20 knots at times.

&&

.MARINE...
Seas have dropped below 5 ft and are in the 3-4 ft at all of our
nearby offshore buoys. Northerly winds generally in the 5-10 kt
range through this evening. Accordingly will cancel the Small Craft
Advisory for Hazardous Seas for the coastal waters thru 2 AM
tonight. Thereafter, northerly winds are forecast to increase to 15-
20 kt during the early morning. We may need to go back to a Small
Craft Advisory for winds and seas for the NJ coastal waters with the
potential for wind gusts near 25 kt between during the early and mid
morning hours. Seas should also respond to the strengthening wind on
top of the E-NE swells from Tropical Storm Jose by building to
around 5 ft toward the early morning. Winds will gradually relax
during the afternoon but seas will likely still be in the 4-6 ft
range.

Both NWPS and WaveWatch guidance have initialized/verified too high
by about 1 ft. This bias correction was applied to the forecast for
this afternoon, tonight and Friday.

Outlook...

The Small Craft Advisory for hazardous seas was extended through
Saturday after collaboration with surrounding offices. However, we
anticipate this may eventually need to be extended further, with
guidance indicating waves remaining at or above 5 feet through at
least Tuesday.

Winds are expected to remain below 25 knots through Monday, but the
forecast becomes uncertain beyond as Maria moves northward off
the east coast per latest NHC forecast. The GEFS probabilistic
guidance suggests at least a 50 pct chance of wind gusts around
25 knots in the Tuesday - Wednesday time frame. This may well be
due to Maria`s wind field expanding at this point. Stay tuned.

Rip Currents...

This late in the day, will let High Rip Current Risk (RP.S) headline
expire this evening though the risk has likely dropped to moderate
or even low in some spots with the surf zone falling to 2-3 ft this
afternoon (based on data at BTHD1 and OCSM2 and surf cams in the
area) and N-NE winds around 10 mph or less. Factoring in the new
moon yesterday and longer-period (9-10 sec) swells from Jose, there
will still be a risk for the formation dangerous rip currents.

Outlook:

Friday...A northerly wind surge is expected during the morning,
which should cause waves in the surf zone to build. This should
bring the rip current risk solidly in moderate range, possibly
approaching high. Will let the evening shift re-evaluate the
need for another RP.S tomorrow.

Saturday...A continued moderate rip current risk.

Sunday through Wednesday are outlooked high risk days with the
greatest risk Tuesday or Wednesday of next week, due to leftover
Jose swells combined with newly arriving southeast swells from
Maria.

&&

.TIDES/COASTAL FLOODING...
Expect any tidal flooding with the two upcoming high tide cycles
(tonight and Friday morning) to be spotty with levels at NOS gauges
expected to reach minor threshold but just short of advisory.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...High Rip Current Risk until 9 PM EDT this evening for NJZ014-
     024>026.
DE...High Rip Current Risk until 9 PM EDT this evening for DEZ004.
MD...None.
MARINE...Small Craft Advisory for hazardous seas from 2 AM Friday to 8
    PM EDT Saturday for ANZ450>455.

&&

$$
Synopsis...Franck
Near Term...Klein
Short Term...Klein
Long Term...Franck
Aviation...Franck/Klein
Marine...Franck/Klein
Tides/Coastal Flooding...Klein