Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 271717
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
117 PM EDT Tue Sep 27 2016

.SYNOPSIS...
A cold front will stall offshore from this afternoon through
Wednesday. Low pressure systems over the Great Lakes and Southeast
U.S. are expected to drift toward the region for the later half of
the week before finally lifting into New England for the weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
At 117 PM EDT...Light rain showers continues to impact SE NJ,
across DE Bay, and into S DE. Most of the rain is light, and
resulting in minimal rainfall. Widespread clouds persist along the
Jersey Shore and down towards the Delmarva, but skies are clearing
to the north and west. Where skies have cleared out, temps have
risen into the upper 60s to lower 70s. Those areas on track for
highs in the mid 70s. For areas where it is cloudy and raining,
temps are currently in the mid 60s, but there is still enough time
for some breaks in the clouds to allow temps to rise to near 70.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
The closed low will push slowly southward through the upper Midwest
tonight. Meanwhile, a wave of low pressure is expected to develop
along the stalled front to our south and east. We should be mainly
dry tonight, even along the coast as the secondary low tracks far
enough offshore.

Good radiational cooling conditions may occur this evening across
inland areas under clear skies and calm winds. Therefore, temps
should drop quickly after sunset. Forecast lows are in the 40s for
the valleys in northeast PA and northwest NJ. However, there is some
uncertainty with the temperature forecast as high pressure builds
southward. A light northeasterly wind may develop overnight, which
could prevent temps from bottoming out.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Strong ensemble signal for a heavy rain event somewhere in the Mid-
Atlantic region late this week with uncertainty on the location.
Current indications are our CWA will be on the eastern edge of it.

Wednesday through Friday:
An area of low pressure (most noted in the upper levels) is
expected to move southeastward toward our region from the Great
Lakes and another area of low pressure will be forming in the
Carolinas ( main surface low). The exact locations of these
systems will determine where the highest risk for an excessive
rainfall event will be. The GFS and Canadian operational models
are more progressive with the upper low to our northwest which in
turn forms the surface low just to our south. In turn a deep but
narrow moisture axis ahead of these features tracks from the
Eastern Gulf and Atlantic Oceans northwestward into DE and Eastern
PA sending PW values from 1.5-2 inches, very high for this time of
year. Sufficient lift and dynamics are present coupled with
marginal instability to allow for heavy showers within this
moisture axis. A few thunderstorms would be possibile as well with
a tall skinny CAPE profile normally seen with more aggressive
heavy rainfall events. A hindrance of the heavy rainfall threat
looks to be fast moving convective elements in the axis though.
This outcome would bring several inches of rainfall to areas south
and west of Philadelphia.

However, the NAM and the ECMWF model have all of these features
further westward which would shift the moisture axis further inland
to areas like Baltimore and State College. Waffling on the locations
of the players and their respective locations is evident in the
ensembles. Many members do suggest an axis of heavy rain sets up
but are variable on location. The axis looks to be narrow, maybe
only 100 miles wide.

Overall, model trends have been to the west with the 00z cycle. So
the forecast features heavy showers from Chester county PA south
into Delmarva with just lighter showers from Philadelphia and
points east.

Showers will likely begin to develop from south to north across the
region during the afternoon Wednesday. Precipitation is then likely
to become more widespread along and near the main axis of moisture
Wednesday night. This axis then could move little through Friday but
gradually become cutoff from the deeper moisture. By Friday, the
precipitation should become much lighter regionwide even with both
areas of low pressure lingering over the Mid-Atlantic and Ohio
Valley.

Rainfall amounts from the individual ensembles are as impressive as
the operational runs, so some totals over a few inches are likely
just west of our region. It should be highlighted that areas outside
of the heavy axis Wednesday night and Thursday may not see much more
than lighter showers with rainfall under an inch total eventhough
it rains off and on for a couple of days.

Diurnal temperature swings should be very limited with clouds and
rainfall with temperatures staying in the 60`s for most locations
Thursday and Friday. Ahead of the rain, temperatures will be in the
70`s Wednesday.

Winds will be out of the east from 10-15 mph on average Wednesday
through Friday.

Friday night through Monday:
Lingering low pressure both at the surface and mid-levels could
keep a few light showers around. Some breaks in the clouds are
likely as well. The clearing process will be gradual. This will
allow for high temperatures to get back into the 70`s for most of
the region, with lows falling back into the 50`s. Ensemble
guidance is likely a few degrees to cool by Sunday, not accounting
for the more abundant breaks and warmer boundary layer
temperatures.

&&

.AVIATION /17Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Some light -RA possible for KMIV/KACY, but conditions should
remain VFR. For all other terminals, VFR conditions expected
through tonight. Winds are light, generally 5 KT or less. With
skies clearing out to the north and west, winds should increase by
a couple of KT or so, but should remain less than 10 KT. Winds
become LGT/VRB this evening and into Wednesday morning.

VFR this evening. Patchy fog may develop overnight at the fog-
prone terminals, resulting in MVFR or locally IFR VSBY early
Wednesday morning.

MVFR CIGs possible at KMIV/KACY late Wednesday morning.

OUTLOOK...
Wednesday night through Friday night: Mainly MVFR or IFR with
periods of showers. easterly winds around 10 knots.

Saturday: Improving to VFR with isolated showers. East winds 5-10
knots.

&&

.MARINE...
Occasional rain showers will pass through the waters through this
afternoon. Most rainfall will be light, but a few moderate showers
are possible.

W to NW winds will become SW going into this afternoon and
evening, and wind speeds will diminish to 5-10 KT.

OUTLOOK...
Seas building to five feet by Wednesday night with low pressure
nearby. Seas will likely be very slow to drop as the low pressure
systems linger. Winds will be easterly with SCA gusts by Wednesday
night. Some gusts may reach gale force from ACY northward
Wednesday night and Thursday. Winds should decrease by Saturday
below SCA criteria.

Rip Currents... The moderate risk for the formation of dangerous
rip currents is forecast at the NJ/DE beaches today. Conditions
may improve to yield a low risk by late afternoon and winds turn
offshore and decrease.

&&

.TIDES/COASTAL FLOODING...
Spotty minor tidal flooding on the oceanfront is possibile with
the Thursday and Friday high tidal cycles.

&&

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

&&

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


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