Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 202102

Area Forecast Discussion
National Weather Service Mount Holly NJ
402 PM EST Mon Nov 20 2017

High pressure over the Tennessee Valley will track east and
move off the Mid-Atlantic coast tonight, and then will slowly
drift out to sea on Tuesday. A cold front approaches from the
west Tuesday night as low pressure moves along the Southeast
U.S. coast. The low passes south and east of the area Wednesday
morning, and then the cold front passes through the region late
Wednesday. High pressure builds through the region Thursday and
Friday. A cold front passes through the region on Saturday, and
then high pressure builds in from the west for the start of the
new week.


As high pressure moves offshore tonight, the wind will
gradually back from the west to southwest, and diminish this
evening with the loss of the diurnal boundary layer. A few high
clouds associated with a mid- level shortwave overnight,
otherwise mostly clear skies. We have lowered temperatures a
couple degrees, mainly over the favored radiators, as winds
become light. Overall, still near seasonal averages.


The area will be under the influence of weak shortwave ridging
aloft, with a cold front over the central Great Lakes and high
pressure well offshore, proving a southwest flow at the surface.
Expect some increase in mid and high level cloudiness during
the afternoon, mainly across Delmava and south Jersey, as a
southern stream jet advects moisture northward. Otherwise,
mostly sunny. It will be brisk, with model soundings supporting
wind gusts up to around 30 mph across the coastal plain, and 20
to 25 mph elsewhere. Temperature-wise, highs will be a couple
degrees above average.


The extended period starts out Tuesday night a bit rainy and
above average in temperature, but the remainder of the period
tends toward seasonably cooler and mostly dry weather. The
larger scale pattern over the CONUS shows ridging aloft in the
Eest and more troughing in the East, with a couple of shortwave
trofs and associated frontal systems moving through the mid-
Atlantic region mid-week and again next weekend.

A cold front is forecast to approach from the west Tuesday evening
and cross the forecast area overnight. The fropa will trigger a few
showers N/W of PHL. A sfc low pressure is also forecast to develop
off the NC/SC coasts with moisture spreading north ahead of it along
the DE/NJ coastal areas. The highest PoPs are thus over south NJ and
Delmarva, counter to climo. Overnight temps should remain warm
enought so that all precip will be in liquid form. However a few
showers may linger past sunrise, and with cold advection behing the
front a few snow showers or flurries are possible in the Poconos
Wednesday morning.

The weather for Wed afternoon through Friday night should be
dominated by hight pressure moving across the mid-Atlantic region
and a fairly dry airmass associated with it. Temperatures will be a
few degrees below normal on Thursday but should return to near
normal by Friday as the high shifts offshore and a more swly flow
develops. A cold front may push south in to NY state and New England
on Friday but probably will not affect our area to the south,

Another fast-moving frontal system is forecast to push through our
area on Saturday. In part becouse of its fast movement it will not
be able to pick up much moisture. The current forecast has chance
PoPs, higher N/W of PHL, but even this may be overdone. Again, temps
on Saturday appear to be warm enough to avoid any frozen precip.
However, the airmass behind this front seems to be colder than the
one mid-week, with strong low level cold advection beginning
Saturday night and continuing through the rest of the weekend. This
pattern of NW flow will favor lake effect snow with some streamers
possibly reaching the Poconos and vicinity.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Tonight...VFR conditions. West-southwest winds gusting up to 20
knots early this evening, diminishing to around 10 knots by 00Z.
Winds will also gradually from the west-southwest to southwest
through around 00Z. Confidence: above average.

Tuesday...VFR conditions. Southwest winds increasing with gusts up
to around 20-25 knots from late morning into the afternoon.
Confidence: above average. There is also some potential for Low-
Level Wind Shear (LLWS) to develop in the I-95 corridor terminals,
e.g., PHL, Tuesday evening. However, latest guidance is just at the
cusp of LLWS criteria, so no mention in the TAF at this time, with
low confidence with regard to LLWS.

Tuesday night and Wednesday...Mainly VFR. Rain possible south
and east of I-95 Tuesday night through Wednesday morning with
MVFR or lower conditions. SW winds 5-10 KT become NW Wednesday
afternoon and increase to 10-20 KT.

Wednesday night and Thursday...VFR. NW winds gradually diminishing.
A storm system is currently expected to remain offshore. Forecast
confidence: Medium to High.

Friday...VFR. NW winds 5-10 KT.

Saturday...VFR early...then MVFR or lower conditions possible in
rain showers later in the day. SW winds 10-15 KT.


SCA conditions prevailing across the NJ coastal waters through
Tuesday, with the combination of southwest winds and seas
meeting criteria.

Further south across the DE coastal waters, SCA conditions are
expected to return just before daybreak Tuesday morning and persist
through the day, with the combination of southwest winds and seas
meeting criteria.

The Delaware Bay waters are expected to remain below SCA criteria,
although the southern Bay will have to be watched for a possible
upgrade if winds strengthen a few more knots.


Tuesday night...SCA conditions should persist through Tuesday
evening but diminish overnight.

Wednesday...Lull in SCA conditions possible Wednesday morning.
Otherwise, winds shift to the NW and increase to 15-20 KT with 25-30
KT gusts.

Thursday through Friday...Sub-SCA conditions expected.

Saturday...SCA conditions are possible with a strong cold front


MARINE...Small Craft Advisory until 1 AM EST Wednesday for ANZ450>453.
     Small Craft Advisory from 5 AM to 7 PM EST Tuesday for ANZ454-


Near Term...Franck
Short Term...Franck
Long Term...AMC
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