Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 240806
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
406 AM EDT Sat Sep 24 2016

.SYNOPSIS...
A cold front will move southward through the Delmarva region
early this morning. High pressure over eastern Canada will build
southward toward the mid-Atlantic states through the weekend. The
center of the high will retreat off the East Coast Monday
afternoon. A cold front is expected to move in from the west
Monday night before pushing offshore sometime on Tuesday. An area
of low pressure may cutoff to our west midweek. High pressure
looks to eventually build back into the region late in the week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
A cold frontal boundary will continue to push southward through
the region this morning. As a result, a thick overcast will
continue to build southward ahead of the front. The overcast
should slowly lift and thin out this afternoon with the front
moving well south of the region. Due to the overcast, temperatures
will struggle to rise into the low 70`s today. The NAM and HRRR
indicate enough lift for a brief light shower or two across
southeastern PA, central and southern New Jersey during the
morning hours. We have some slight chance pops across these
regions. Any rainfall will be light.

Winds will increase out of the north and northeast around 10-15 mph
with slightly higher gusts today centered in the late morning and
early afternoon hours. High pressure building toward the region
later today will actually allow winds to decrease some by days end.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/...
High pressure will continue to build southeastward into the region
tonight eroding away any remaining clouds. Conditions will be
favorable for radiational cooling across the region. More rural
locations should fall a few degrees below MET and MAV statistical
guidance with most of the region in the 40`s for lows. Still to
warm for any frost concerns.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
High pressure centered over Hudson Bay and along the Ontario-Quebec
border will continue to build southward into the mid-Atlantic region
on Sunday. The CAA pattern that develops behind the cold front today
will persist over the area on Sunday but start to weaken. Even with
abundant sunshine, high temperatures in the lower 70s (60s in the
higher elevations) will likely fall shy of climo but only by a
couple of degrees. Nonetheless, it will certainly feel like autumn
after the summer-like warmth that has occurred across the region
through much of September.

On Sunday night, there will be a very favorable setup for
radiational cooling with the surface ridge axis in the area. With
clear skies, calm winds, a very dry airmass in place and
increasingly longer nights this late in the year, Sunday night has
the potential to be the coolest night of the month and possibly
since late May in spots. Under this type of pattern, sided with the
colder MOS guidance, if not below it, for low temperatures. Patchy
frost may form in the sheltered valleys of northeastern PA and
northwestern NJ before sunrise early Monday.

High pressure and H8 ridge axis will shift offshore on Monday.
Southerly return flow around the high will establish a WAA pattern.
However, the WAA will be relatively weak and brief in duration,
precluding a significant warm up early next week. This is because a
progressive cold front will move through the region Monday night.
PoPs were increased to likely for Monday night into Tuesday morning
with the 00Z model runs coming into much better agreement with the
timing of the rainfall along and ahead of the front. While any
rainfall would be much welcomed, it won`t be a drought buster for
portions of eastern PA and NJ that are under a D1 drought with QPF
amounts generally around one-half inch.

There is a significant degree of forecast uncertainty for the second
half of the work week with the potential for the upper low over the
Great Lakes to become cutoff. Models are having a difficult time
handling this system, exhibiting a considerable amount of run to run
spread. For example, the latest 00Z ECMWF showed a cutoff low
forming over the Ohio Valley Wednesday night and stalls it near the
central Appalachians for several days (into next weekend). This run
gives us an extended period of wet weather. Meanwhile, the 12Z ECMWF
from yesterday showed a much more progressive solution as the low
harmlessly kicks out to sea midweek, followed by high pressure in
control late in the week and next weekend. The GFS followed the
opposite trend. Both the GEFS and ECMWF ensembles exhibit large
spread. For now, introduced slight chance PoPs Wednesday afternoon
through Thursday but did not go any higher due to the low
predictability. Temperatures should be near normal mid to late
week.

&&

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

A MVFR stratus deck will continue building southward through the
morning hours. This deck has already reached KABE and is encroaching
upon KTTN and KRDG. Timing uncertainties of a couple of hours are
present with the arrival of MVFR ceilings, so TEMPO groups were
included to represent this in the 06Z TAFS. This deck will slowly
thin out and lift from north to south during the late morning and
afternoon hours.

North-northeasterly winds from 7-15 knots are possibile today with
any gusts in the late morning or early afternoon hours. Winds will
decrease tonight.

OUTLOOK...

Sunday through Monday...VFR. Light winds Sunday. Southerly flow
strengthens Monday with gusts to 20 kt possible during the afternoon.

Monday night and Tuesday...SHRA likely as a cold front moves in from
the west. MVFR conditions probable with the potential for IFR in
heavier/steadier rain. Gradual improvement from west to east during
the day Tuesday.

Tuesday night and Wednesday....Predominately VFR.

&&

.MARINE...
Wave heights will gradually build to around five feet on the
ocean waters by this afternoon after the passage of a cold front.
Winds will also increase from the north and northeast around 15
knots with some gusts to 25 knots possibile. A SCA is in effect
through tonight as the five foot seas may linger.

OUTLOOK...

Sunday...SCA extending into Sunday morning for the coastal Atlantic
zones. Long-period southeasterly swells from Tropical Storm Karl is
expected to continue along the coast during the morning. Seas around
5 ft are expecting in the morning before subsiding to 3-4 ft in the
afternoon.

Monday through Wednesday...No marine headlines expected. Winds may
gust out of the south to around 20 kt Monday afternoon and evening
ahead of a cold front.

Rip Currents...Both long period swells and building seas suggest
conditions may be more representative for a moderate risk of rip
currents.

A moderate risk for the formation of dangerous rip currents will
likely continue into Sunday with long-period southeasterly swells
from Tropical Storm Karl expected to impact the eastern seaboard.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until noon EDT Sunday for ANZ452>455.
     Small Craft Advisory until noon EDT Sunday for ANZ450-451.

&&

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


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