Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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080
FXUS61 KPHI 280434
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1234 AM EDT Sun May 28 2017

.SYNOPSIS...
A weak surface low will move off the coast of Virginia tonight.
Weak high pressure will build into the Northeast on Sunday. A
stronger low pressure system will move into southeast Canada on
Monday, which will bring a cold front through the area. A couple
of reinforcing cold fronts will move through the region during
midweek before a surface high approaches the East Coast by the
end of the week.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
Weak high pressure will remain across New York and New England
overnight while weak low pressure passes off the coast between
the Virginia Capes and Cape Hatteras.

There were a few light showers off Cape May around midnight.
Otherwise, the precipitation had moved south of southern
Delaware.

Clouds were lingering across extreme southeastern Pennsylvania,
the upper Delmarva and southern New Jersey around midnight. The
northern edge of that cloud deck should erode a bit during the
night. Meanwhile, there was a deck of stratocumulus expanding
over northeastern Pennsylvania and northern New Jersey. The area
between the two cloud masses was mostly clear. Patchy fog may
develop during the night, especially where the clear sky exists.

The wind should be light and variable overnight with lows
mostly in the 50s.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TODAY/...
The area of weak high pressure will be offshore Sunday morning.
Low pressure will be advancing towards the area from the Ohio
Valley. Overall much of the daytime hours will be dry, but
clouds will be increasing over the area during the day. A couple
showers are possible across the southern Poconos or Berks
County by late afternoon. High temperatures tomorrow will be
much like today`s, with low/mid 70s in most areas. It will be
cooler across the shore areas and over the higher elevations up
north. Winds will be mostly E or SE at 5 to 10 mph.

&&

.LONG TERM /TONIGHT THROUGH SATURDAY/...
Primary forecast concern is chances for storms Sunday night
into Memorial Day.

Operational models have come in a little faster with the system
moving through the region to close out the holiday weekend.
Currently, the relevant vorticity maximum is in the Rockies and
adjacent Great Plains. The trough axis is strongly positively
tilted at this time, but with the aid of a kicker vort max in
central Canada, the western side of the central U.S. vort max
will pivot rapidly eastward on Sunday, with the trough axis
becoming more neutrally tilted Sunday night as it approaches the
Great Lakes and Ohio Valley. Attendant surface low will
intensify as it tracks northeastward from the southern plains to
the vicinity of Lake Erie by 00Z Monday, with a broad region of
warm air advection downstream into the Mid-Atlantic. Isentropic
ascent poleward of the associated baroclinic zone will permit
the development of widespread precipitation in the central
Appalachians, the Mid-Atlantic region, and portions of the
Northeast.

A predecessor vorticity maximum Sunday afternoon/evening, in
conjunction with increasingly favorable large-scale dynamics
(including favorable positioning of an upper-level jet streak
and downstream difluent flow), will allow for deep lift and
substantial cooling of the mid and upper levels of the
atmosphere. As a result, elevated instability will increase to
the north of a quasi-stationary boundary positioned in the
southern Mid-Atlantic. Though the strongest ascent and most
widespread precipitation should be to the north and west of the
area Sunday night (in closer proximity to the surface low),
models indicate 35 to 45 kts of 925-850 mb layer flow orthogonal
to the baroclinic zone in the Mid-Atlantic. With such strong
ascent in play, think chances are quite good for a round of
storms Sunday night into at least early Monday. Raised PoPs
quite a bit during this time frame, given the reasonable
agreement among the recent simulations of the NAM/GFS/CMC/ECMWF.

Less obvious is the residual environment within the warm sector
on Monday in advance of a cold front moving through the region.
Latest models swing the front through the region rather early
in the day, indicating that the time window for recovery of
instability will be limited at best. As such, think chances of
convection beyond Monday morning are somewhat more limited,
though given the model variability run-to-run regarding frontal
passage timing...not particularly confident of that assessment.
Nevertheless, felt it wise to reduce PoPs from southwest to
northeast late Monday morning onward, particularly given the dry
looking simulations of most of the operational guidance.

Tuesday looks generally like an "in-between-systems" day as the
main vort max swings northeastward into New England and
southeast Canada. The next perturbation will dig southeastward
into the Midwest on Tuesday and then eastward through the
Northeast on Wednesday as it rounds the semi-permanent longwave
trough in eastern Canada. Associated cold front should generate
another round of showers as it moves through, which looks to be
Wednesday (with timing uncertainty becoming even more
pronounced). Instability looks more limited with this system, so
kept mention of thunder out of the grids for now, though models
may be underplaying this a little bit (i.e., not sure if I`m
buying the pronounced inverted-V boundary layer the GFS is
depicting). The 12Z CMC, e.g., provides some evidence of more
vigorous convective development on Wednesday.

Thereafter, forecast looks a little drier as surface high
pressure becomes established. The 12Z GFS provides a prolonged
break in the active pattern, keeping somewhat drier
northwesterly flow in the region through the following weekend,
while past runs and the most recent 12Z CMC offer a strong low
developing in the Ohio Valley into the Mid-Atlantic by next
weekend. Latest ECMWF has a pretty dissimilar solution to either
camp. Given the large uncertainty during this period, stuck
quite close to the inherited forecast Thursday onward.

&&

.AVIATION /06Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Mainly VFR conditions are expected to continue through tonight.
Some fog may develop and affect KABE, KTTN, KRDG late tonight
into Sunday morning (most likely to occur between 08Z- 14Z). The
guidance hints at possible fog elsewhere but confidence was too
low to include at this time.

VFR conditions are expected to start for Sunday. Clouds will
increase through the day and gradually lower as we head into the
afternoon and evening, becoming MVFR late. Scattered showers
and thunderstorms are possible later in the day.

Winds have become light and variable in many spots. Winds will
become easterly tonight, turning slightly to the southeast
through the day on Sunday.

OUTLOOK...
Sunday night and Monday: MVFR/IFR CIGs/VSBYs are reasonably
likely, especially Sunday night. Scattered showers and storms
expected, especially Sunday night and Monday morning, with
conditions potentially improving during the afternoon. East to
southeast winds around 10 kts Sunday night transitioning to west
by Monday afternoon after cold frontal passage. Confidence
below average.

Monday night and Tuesday: Slight chance of showers/storms
north/west of KPHL. Some chance of low stratus or fog on Monday
night. Primarily VFR on Tuesday. Winds under 10 kts, likely
transitioning from west to north to east during the period.
Confidence below average.

Tuesday night and Wednesday: Generally VFR. Slight chances of
showers, primarily during the day Wednesday. Winds generally
west or southwest 5 to 15 kts, with perhaps some gusts to 20 kts
or so during the afternoon. Confidence below average.

Wednesday night and Thursday: VFR expected. Winds generally
west 5 to 15 kts. Confidence average.

&&

.MARINE...
Weak high pressure will remain across the waters tonight and
Sunday. An upper air disturbance will bring some showers across
Delaware Bay and the adjacent DE/srn NJ ocean waters through
this evening. Conditions will remain sub-SCA through the period
with winds mostly under 10 knots. Seas will be 2-3 ft on the
ocean and 1-2 ft across Del bay.

RIP CURRENTS...
A low risk for the development of dangerous rip currents exists
for Sunday. Onshore flow is forecast to strengthen to around 10
to 15 kts by Sunday afternoon and this may result in the rip
current risk approaching moderate, especially for the coast of
New Jersey.

OUTLOOK...
Sunday night through Thursday: Generally sub-advisory winds and
seas expected. However, winds/seas will be close to advisory
thresholds Sunday night and Monday as a storm system moves
through the area. In addition, there is some risk for visibility
restrictions Sunday night and Monday before a cold front moves
through the area later in the day.

A couple of chances for showers/storms exist through the
period, especially Sunday night and Monday and again on
Wednesday.

&&

.TIDES/COASTAL FLOODING...
A Coastal Flood Advisory remains in effect for tonight`s high
tide. It is in effect for the coastal counties of New Jersey
and Delaware, and for the counties along Delaware Bay and the
lower Delaware River up to near the Commodore Barry Bridge.

Minor coastal flooding may also occur during Sunday night`s
high tide, as astronomical tides remain high and onshore winds
should continue.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...Coastal Flood Advisory until 2 AM EDT early this morning for
     NJZ012>014-020>027.
     Coastal Flood Advisory until 3 AM EDT early this morning for
     NJZ016.
DE...Coastal Flood Advisory until 2 AM EDT early this morning for
     DEZ002>004.
     Coastal Flood Advisory until 3 AM EDT early this morning for
     DEZ001.
MD...None.
MARINE...None.

&&

$$

Synopsis...CMS
Near Term...Iovino
Short Term...O`Hara
Long Term...CMS
Aviation...CMS/Meola/O`Hara
Marine...CMS/Meola/O`Hara
Tides/Coastal Flooding...Staff



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