Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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578
FXUS61 KPHI 271326
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
926 AM 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/...
926 AM EDT...The bulk of the rain has moved offshore, and latest
KDIX radar indicating some light rain showers in/around S NJ. Not
anticipating much rainfall from these showers, and they should
move offshore within the next hour or so. Some patchy fog has
developed across the region, so will go ahead and add that to the
forecast this morning.

Clearing conditions expected for this afternoon as some dryer air
works its way into the region behind the front and ahead of an
upper low digging into the Great Lakes.

The warmest temperatures this afternoon (highs in the mid 70s)
will likely reside along the I-95 corridor and the warmer valleys
farther to the north and west as these areas will clear out early
enough in the day to receive strong daytime heating. Farther south
and east toward the southern NJ coast and lower Delmarva, clouds
will hold on longer into the afternoon, resulting in highs near
70F.

&&

.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 /13Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

MVFR VSBYs, along with IFR CIGs, will persist for several more
hours through late morning. Dryer air will filter into the region
behind the departing cold front, and conditions will lift to VFR.
W-NW winds will back to the SW later this afternoon, but wind
speeds will be 10 KT or less.

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

OUTLOOK...
Wednesday 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...
Winds and seas have fallen below SCA criteria on the waters, so
will go ahead and cancel the SCA that was in effect until 11 AM.

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, especially
this morning. Conditions may improve to yield a low risk by this
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...Klein/MPS
Short Term...Klein
Long Term...Gaines/MPS
Aviation...Gaines/Klein/MPS
Marine...Gaines/Klein/MPS
Tides/Coastal Flooding...



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