Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 240907
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
507 AM EDT Sat Aug 24 2019

.SYNOPSIS...
High pressure across the upper Great Lakes region this morning
will continue to move east across southern Canada today and
tonight. The high will try to spread down the east coast, but
not fully make it into the area. The high will retreat north and
east next Tuesday and Wednesday. This will allow an area of low
pressure to lift northward off the eastern seaboard during the
mid week period and a cold front to track through the area
later next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
A trough of low pressure will remain aloft while a surface high
pressure area moves across southern Canada today. This will
bring more cooler and drier air from the north into our region.
We`ll see more sunshine than Friday, so highs will be 4 to 6
degrees warmer that yesterday, but highs will be below normal
for mid-late August. Readings will top out in the low/mid 70s
N/W and close to 80 across SE PA, south NJ and Delmarva. Winds
will be mostly N to NE at 5 to 10 mph this morning and the NE to
E at 10 to 15 mph this afternoon. Some mid/high level clouds
will move across the srn areas this morning and into the
afternoon, but overall a nice day expected.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/...
High pressure will continue to strengthen while it moves to
ME/srn Quebec. This will bring an increasing NE to E flow across
our area and could result in some increasing low clouds and
perhaps some light rains near the coast towards daybreak Sunday.
We have kept the slight chc pops and increased the clouds for
these areas. Low will be cool with readings dropping into the
mid/upper 50s N/W and over the Pine Barrens of NJ. Elsewhere,
readings will be in the 60s with mid/upper 60s near the shore
with the nearby warmer waters. Winds will be NE to E at 5 to 10 mph.
The winds will be higher near the shore with 10 to 15 mph more
common there.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Main concerns for the medium-range forecast include the effects
of prolonged onshore flow Sunday through Tuesday and a cold-
frontal passage during the middle of next week.

A fairly quiet period is in store for the area, but as usual,
some subtle details are uncertain and may end up making a big
difference in sensible weather "quality" the next few days.
Perhaps the biggest question is the ultimate result of a
lengthy period of east to northeast winds across the area late
this weekend into early next week. Operational models have been
hinting for a while at the development and inland penetration of
lower clouds and possibly some light precipitation as the
surface high to our north/northeast lingers the next few days.
It appears the primary window for this potential is Sunday and
Monday, as an upper low lingers off the coast on Sunday before
drifting off to the northeast Monday. This will promote some
ageostrophic acceleration southward on the east side of the
Appalachians, allowing the surface high to build southward a bit
through this period. Eventually, the flow should become
somewhat more northeasterly or even north-northeasterly Sunday
night as the acceleration peaks, and this will aid in shunting
the higher low-level moisture content southward. By Monday, the
effects may primarily be in lower Delmarva and perhaps the far
southeastern portions of New Jersey with improvement in
conditions to the north. Far western portions of the area may
be much sunnier during this entire period.

Confidence is low on just how far inland the thicker clouds and
even light precipitation make it, but decided to go a little
bit more pessimistic than consensus (which is close to the
previous forecast). Kept temperatures close to continuity and a
statistical blend during this period as well, but there may be
a more pronounced gradient between the coast and the western CWA
depending on how far inland the clouds make it (and how long
they last).

By Tuesday, the effects of a tropically-induced low off the
coast will generally become most pronounced. The surface high to
the north is expected to retreat northeastward, permitting a
return to more easterly surface flow. Anticipating an increase
in clouds and low-grade chances of showers. Think most of the
precipitation will be to the south of the area, but given
uncertainty with the track of the low, felt some low PoPs remain
warranted. Additionally, should see some warming temperatures
as low-level flow begins to tap into warmer/moister air in
association with the low.

The good news is a well-timed trough will deepen and progress
eastward into the Great Lakes on Wednesday, which will force the
low off the coast well away from the area. There will be an
attendant cold front associated with the trough, with timing
Wednesday night (plus or minus) for our area. The ECMWF is more
aggressive with generating some storms along the front. The GFS
is drier, which appears to be tied to how close the low off the
coast tracks toward the area and the residual influence of the
retreating surface high. The GFS is closer with the low track
and stronger with the residual effects of the surface high,
which precludes convection from maintaining itself as it reaches
the area. The ECMWF is farther offshore with both, allowing
more substantial large-scale lift to reach the area. Suspect the
ECMWF is a little too aggressive, particularly with the retreat
of the high to the north. For now, maintained chance PoPs
Wednesday and Wednesday night given the uncertainty (only
somewhat higher than climatology).

Should see improving conditions late next week as high pressure
builds in from the west.

&&

.AVIATION /09Z SATURDAY THROUGH WEDNESDAY/...
Today...VFR expected. N winds 5 to 10 knots early, then NE 8 to
12 knots late morning and into the afternoon. Some gusts close
to 20 knots near KACY late morning and afternoon. Sct-Bkn
050-060 KMIV/KACY this afternoon.

Tonight..VFR continues. Low clouds possible late near the shore
towards dawn Sunday. Confid is not high enough to include in
TAFS attm. E winds 5 to 10 knots.

OUTLOOK...

Sunday...Mainly VFR, though sub-VFR conditions may occur near
the coast (including MIV/ACY) as low ceilings move in with the
persistence of onshore flow. Cannot rule out some drizzle or
light rain/showers near the coast as well. East to northeast
winds 7 to 15 kts with stronger gusts possible near the coast.
Low confidence.

Sunday night through Tuesday night...Conditions possibly
varying from VFR to MVFR as lower ceilings move farther inland
and retreat at times during the period. May see some drizzle/fog
as well near the coast. Winds mainly east or northeast 5 to 15
kts, with higher gusts possible near the coast. Directions may
become more southeasterly by Tuesday afternoon. Very low
confidence.

Wednesday...Mainly VFR as winds become more
southerly/southwesterly (up to around 10 kts). A chance of
storms, especially north/west of PHL. Low confidence.

&&

.MARINE...
High pressure moving north of the waters today and tonight will cause
winds to be mostly N or NE today and then NE to E tonight. Winds
and seas will slowly increase to near SCA conditions for awhile
both this afternoon and late tonight. Not really expecting the SCA
conditions to persist too long with only perhaps a gust or two and
a few seas nearing 5 ft. We`ll hold on the SCA flag for now, although
a short term SCA is possible later. Fair weather today and a slight
chc for a shower late tonight.

OUTLOOK...

Sunday through Wednesday...Advisory conditions possible at times
through the period as steady east to northeast winds persist,
with seas building to around or slightly above 5 feet. Some
drizzle or light precipitation is possible at times as well.

RIP CURRENTS...

Increasing northeasterly winds and seas building to around 4 ft
by the afternoon will result in a moderate risk of rip currents
today.

This elevated rip current risk (either moderate or at times
high) is expected to persist until at least Tuesday as a
prolonged period of onshore flow and elevated seas looks likely.

&&

.TIDES/COASTAL FLOODING...
A prolonged northeast to east flow is expected today through at
least Tuesday. This persistent onshore fetch, combined with increasing
astronomical tides due to an approaching new moon on Thursday,
will likely lead to increasing coastal water levels. Guidance is
already indicating that minor coastal flooding is becoming more
likely Monday and Tuesday over Delaware Bay and portions of the
NJ shore.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...Kruzdlo/O`Hara
Near Term...O`Hara
Short Term...O`Hara
Long Term...CMS
Aviation...CMS/O`Hara
Marine...CMS/O`Hara
Tides/Coastal Flooding...CMS/Kruzdlo/O`Hara/Robertson


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