Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 141936
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
336 PM EDT Sat Sep 14 2019

.SYNOPSIS...
A cold front will pass through the region tonight and into the early
part of Sunday, before stalling to our south. High pressure will
weaken through the day Sunday as an area of low pressure glides
eastward across the Great Lakes and into the Mid-Atlantic by Monday.
High pressure will build across southeastern Canada and into the
Northeast through the middle to end of next week.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
An upper-level trough will continue to shift eastward through
tonight across the Great Lakes region and adjacent Canada. The
southern extent of this trough will works its way across our region
overnight, and that is when an elongated stronger short wave is
forecast to move through. At the surface, a cold front will march
its way across the region through tonight with it being near the
coast toward daybreak.

The combination of ongoing warm air advection and moisture advection
is sustaining a wealth of clouds across nearly the entire region. A
south wind however is helping to keep some of the immediate coastal
areas sunnier than just inland. There is no real organized and
strong forcing for ascent with the front to our west thus far,
therefore some showers and at times isolated thunderstorms are
rolling eastward. The cloud cover has helped to keep the instability
on the rather low side.

A mesoscale analysis this afternoon shows a lack of MLCAPE and
SBCAPE across our region, however an axis of increasing instability
while still to our west is slowly shifting eastward. It is in and
near that axis where some lightning has been detected at times. The
model guidance overall suggests the axis of instability continues to
shift eastward (mostly in the form of elevated instability) through
the night ahead of the cold front. This in combination of sustained
warm air advection and the arrival of some better forcing with an
associated short wave should result in an uptick in showers for a
time later this evening into the overnight hours. While some
thunderstorms are certainly possible, the overall instability may
not be enough to result in much lightning production. We will
maintain an isolated thunder mention through this evening especially
near and west of I-95. Severe thunderstorms are not expected given
the more unfavorable timing and the forecast magnitudes of the
instability/shear.

Otherwise, a more muggy night for much of the area ahead of the cold
front with areas of low clouds expected. Low temperatures are mainly
a blend of MOS and continuity.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/...
An upper-level trough remains in place from the Great Lakes and
adjacent eastern Canada and the Northeast Sunday. At the surface, a
cold front is forecast to be very near the coast at daybreak then it
shifts east and southward through the day and ultimately stalls to
our south by days end. Some short wave energy within the modest
cyclonic flow will move through, and this combined with the front
initially near the coast could produce some showers. The chance of
showers ends for the coastal areas as the day wears on.

In the wake of the cold front, drier air will be advecting
southeastward however the drop in the dew points may be slowed some
especially for areas closer to the coast. The flow overall looks
lighter and therefore a sea/bay breeze is expected to develop from
about midday onward. The main change with the passage of the cold
front is more with the dew points and not temperatures. Many areas
are forecast to top out into the lower 80s in the afternoon.
Sunshine should be on the increase from northwest to southeast
Sunday as drier air builds in.

&&

.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
Overview...a fairly quiet extended forecast through the upcoming
week with ridging across the Great Lakes and troughing across the
Pacific Northwest and across the Canadian Maritimes. There is some
uncertainty in how exactly Tropical Storm Humberto will interact
with the pre-existing longwave trough as it moves northeastward
across the Atlantic, but it is becoming less likely that we will see
any significant impacts from this system.

A shortwave trough will continue to cross the region Monday, driving
a Clipper-type low as it does so. Moisture looks to be more limited
with this system while the latest guidance from the GFS and ECMWF
keeps the greater chances for rain to our north. The GFS is much
wetter of the two.

High pressure will build southeastward out of Ontario Tuesday and
into Wednesday as the longwave trough amplifies over the Canadian
Maritimes. A shortwave trough will pivot southward along the western
edge of this trough, with the GFS being considerably more bullish
regarding its intensity. The ECMWF keeps what will likely be
Hurricane Humberto by that time south of this trough, while the GFS
suggests ongoing interaction and possible extratropical transition
by Wednesday. This process will likely influence our weather in how
the aforementioned trough amplifies by midweek. Highs at this point
look to remain in the mid to upper 70s both Tuesday and Wednesday.

Surface high pressure looks to remain anchored across the eastern
U.S. through the remainder of the week, slowly building southwest
into the mid-Atlantic by Saturday. There is considerable
disagreement in the placement and intensity of the next area of low
pressure and cold front by this point. The GFS brings a cold front
slowly eastward into Pennsylvania by Friday night, while the ECMWF
erodes this feature by Saturday morning, likely due its western
placement of Humberto. Thus, kept PoPs low for Saturday with no
mention of precip. Highs will remain gradually warm from the mid 70s
to mid 80s by Saturday.

&&

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

Rest of this afternoon...Ceilings mainly between 3500 and 7000 feet,
however MVFR ceilings (1500-2500 feet) will be around at times.
Confidence is on the low side regarding the duration and extent of
the MVFR ceilings. Some showers around especially north and west of
PHL, with a thunderstorm possible (though chances are too low for a
mention). Southerly winds around 10 knots.

Tonight...Periods of MVFR to perhaps IFR conditions as a cold front
moves through the area. Some showers and perhaps a few
thunderstorms, though the chances of lightning are too low for a
mention in the TAF at this point. South to southwest winds 5-10
knots, becoming light and variable. Low confidence on the coverage
of showers and also sub-VFR ceilings (went as low as MVFR for now).

Sunday...MVFR ceilings near and south/east of PHL to start then
improving to VFR. Some showers possible in the morning near ACY and
MIV southward. North to northwest winds increasing to 5-10 knots,
becoming west to west-southwest in the afternoon (southerly at ACY
due to a sea breeze).

OUTLOOK...
Monday...VFR, with some restrictions possible with scattered showers
and thunderstorms. Westerly winds 5 to 10 knots.

Tuesday...VFR expected with northeasterly winds 5 to 10 and gusts to
15 knots.

Wednesday...VFR expected with easterly winds 10 to 15 knots.

Thursday...VFR expected with easterly winds turning southeasterly
from 5 to 10 knots.

&&

.MARINE...
The seas continue to gradually subside this afternoon, and therefore
the Small Craft Advisory for the northern two New Jersey Atlantic
coastal waters is cancelled. The remainder of the coastal waters
advisory remains as is through this evening. Southerly winds gusting
to about 20 knots will diminish through this evening then shift to
west and northwest overnight as a cold front moves through. The flow
looks lighter Sunday and therefore wind shifts are expected;
starting out northerly in the morning then becoming southeasterly
through the afternoon.

Outlook...
Monday...Sub-SCA conditions expected with southerly winds 5 to 10
knots. Seas from 1 to 3 feet.

Tuesday...SCA conditions possible with northeasterly winds to 15
knots and gusts to 25 knots possible. Seas building from 3 to 5 feet
across the Atlantic zones and from 1 to 3 feet across the Delaware
Bay.

Wednesday...SCA conditions possible, especially across the southern
Atlantic zones with northeasterly winds from 15 to 20 knots and
gusts up to 25 knots. Wind gusts look to remain below 20 knots for
the northern zones. Seas building from 3 to 5 feet across the
Atlantic Zones and 1 to 3 feet across the Delaware Bay.

Thursday...SCA conditions possible with northeasterly winds from 10
to 15 knots and gusts to 18 knots. Seas from 3 to 5 feet across the
Atlantic Zones and from 1 to 3 feet across the Delaware Bay.

Rip Currents...
A HIGH Risk for the development of dangerous rip current continues
through this evening, with modest southerly winds along with waves
in the surf zone around 4 feet.

For Sunday, the wind is forecast to be shifting from northerly to
start the day to southeasterly in the afternoon however it looks to
be around 10 mph. Waves should be about a foot lower, however some
roughness in the surf zone may linger enough resulting in a moderate
risk of rip currents.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...High Rip Current Risk until 10 PM EDT this evening for NJZ014-
     024>026.
DE...High Rip Current Risk until 10 PM EDT this evening for DEZ004.
MD...None.
MARINE...Small Craft Advisory until midnight EDT tonight for
ANZ452>455.

&&

$$
Synopsis...Davis
Near Term...Gorse
Short Term...Gorse
Long Term...Davis
Aviation...Davis/Gorse
Marine...Davis/Gorse



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