Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 171954
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
354 PM EDT Tue Apr 17 2018

.SYNOPSIS...
Low pressure over the Canadian Maritimes will gradually pull
away from our area tonight and Wednesday. A second area of low
pressure will approach from the west and move across the mid-
Atlantic region on Thursday. As the low moves away to the
northeast on Friday, high pressure will begin to build in from
the west, continuing through the weekend and into early next
week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
A large broad mid/upper low, and surface low pressure will
continue to drift slowly northward across New England the rest
of this afternoon through tonight. As they do so, a short
wave/vorticity impulse will move across the area later this
afternoon into this evening. Meanwhile, an area of enhanced low-
mid level lapse rates will move across the area at the same
time, and combine with enhanced moisture in the low-mid levels.
This has created an enhanced area of, mostly, snow showers to
our west, which is expected to continue tracking eastward
through the remainder of the afternoon and into this evening.
Temperatures are fairly warm across much of the area, so for
most places, the precipitation will likely melt and fall as
rain, although there could be some snow flurries mixed in at
times with the rain across portions of eastern Pennsylvania and
northern New Jersey. The higher elevations of the Poconos and
northwest New Jersey may actually remain all snow as the
precipitation falls. However, with the temperatures being fairly
warm and this late in the season, we do not expect much if any
accumulation, maybe a light dusting for some higher elevations.

This activity should end quickly after sunset as the lapse
rates decrease and the short wave passes to the east. Once the
showers end, there could be some clearing from south to north
during the overnight hours. As this happens, we expect
temperatures to cool across the area. Some areas will likely
fall below freezing across portions of eastern Pennsylvania,
southern New Jersey and northeast Maryland where the growing
season has started. Therefore we have issued a Freeze warning
for these areas. The rest of the area that is forecast to fall
below freezing across other portions of northeast Pennsylvania
and northern New Jersey have not started their growing season.
There is also some areas across southeast Pennsylvania and
central/southern New Jersey and much of the Delmarva where
temperatures are forecast to remain above freezing. We did not
issue a Frost Advisory as we expect there to be some wind
overnight around 5-10 mph, and RH values may not be high enough
for widespread frost to develop. Some areas could continue
seeing winds gust 15-20 mph at times overnight.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
Quiet weather is expected on Wednesday as the low pressure to
our north continues to drift farther north away from the area,
while very brief and weak high pressure moves across the area.
No precipitation is expected during the day, although skies will
likely be partly to mostly cloudy. Highs will be warmer than
Tuesday, but will still likely be a couple of degrees below
normal. Winds will be gusty at times out of the west to
southwest, with gusts 15-20 mph.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...

For the extended period, a somewhat amplified flow pattern aloft is
forecast to continue over the CONUS through this week. A large scale
upper trof over the eastern U.S. through the weekend may give way to
some ridging early next week. At the surface, low pressure is
forecast to move across the northern mid-Atlantic region Wednesday
night and Thursday, accompanied by some fairly light precipitation.
This will be followed by a large area of high pressure building over
the area starting on Friday and continuing through the weekend and
into next week. Temperatures through this period are expected to be
somewhat below normal, although a moderating trend towards normal is
forecast for early next week. After Thursday, little if any precip
is expected for the area.

On Wed night and Thu a shortwave trof will approach from the
west and move through the area. Warm advection and precip will
spread east across the area overnight ahead of it and continue
on Thu. The best chance for precip looks to be Wed overnight
into Thu morning and generally north of PHL where the forecast
has likely PoPs.  It looks like the lower part of the
atmosphere should remain warm enough for all the precip to reach
the surface as rain. A warm front will try to move north into
our area ahead of the surface low, but the warm sector is
expected to affect only Delmarva and south NJ.

The surface low and trailing cold front will move offshore by
Friday afternoon with lingering but dimishing precip behind it.
Total rain amounts should be no more than about 1/3 inch at
most. Although cold advection sets in behind the cold fropa,
precip should remain rain until perhaps late afternoon in the
higher elevation of the Poconos and vcnty. The forecast has a
chance or slight chance for snow/rain showers there in the far
NW through Thu night and Fri.

The rest of the forecast through the weekend into early next
week has high pressure building in from the NW and remainomg
over the NE US. No precip is expected for this time, and the
cold air will gradually moderate so below normal temps over the
weekend should return to near normal for Mon/Tue.

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Rest of today...VFR conditions expected. Broken to overcast
skies around 4,000-5,000 feet will continue. Showers will move
into the area from the west, but are not expected to lead to
significant restrictions at this time. If snow ends up being the
predominant precipitation type, mainly for ABE/RDG, VSBYS may
temporarily lower to 5 or 6 miles. Will not include in the TAF
at this time until we get some ground truth upstream, closer to
the area since temperatures are still fairly warm. The rest of
the area will likely remain all rain.

Tonight...VFR with showers early this evening. Clouds may
diminish overnight from south to north. Winds diminish to
around 10 knots in the evening.

Wednesday...VFR conditions expected. Winds expected to increase
and gust 15-20 knots during the morning and into the afternoon.

Outlook...

Wednesday night and Thursday...Periods of MVFR to IFR conditions are
likely in rain and fog. Confidence high for at least MVFR. Light
winds, becoming W to NW and gusting to 30 kt Thursday afternoon.

Thursday night and Friday...Mainly VFR, but with NW winds gusting to
30kt. Confidence moderately high.

Friday night through Sunday...VFR conditions expected, with
diminishing NW winds.

&&

.MARINE...
Small Craft Advisory remain in place across the Atlantic coastal
waters and Delaware Bay tonight through Wednesday morning as
winds continue to gust 25 to 30 knots at times. Winds will
likely drop below 25 knots for a couple of hours late this
afternoon through this evening for some areas, but but a second
surge of wind is expected overnight and into early Wednesday
morning.

Wind gusts are then expected to diminish Wednesday afternoon as
they shift to the southwest and an inversion develops, plus by
this time, the pressure gradient may lower some as well.

Outlook...

Wednesday night through Thursday morning...No marine headlines are
anticipated.

Thursday afternoon through Friday...SCA conditions are likely
through much of this period. Gales are also possible Thursday night.

Friday afternoon through Sunday...No marine headlines are
anticipated.

&&

.FIRE WEATHER...
Although the relative humidity may drop to around 30 percent or
lower on Wednesday and winds could be gusting in the low 20s,
fuels are expected to remain moist with the rainfall on Monday
and the cloudy conditions Tuesday.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Freeze Warning from 2 AM to 8 AM EDT Wednesday for PAZ060-
     101>104-106.
NJ...Freeze Warning from 2 AM to 8 AM EDT Wednesday for NJZ016>022-
     027.
DE...None.
MD...Freeze Warning from 2 AM to 8 AM EDT Wednesday for MDZ008.
MARINE...Small Craft Advisory until noon EDT Wednesday for ANZ430-431-
     450>455.

&&

$$
Synopsis...AMC
Near Term...Robertson
Short Term...Robertson
Long Term...AMC
Aviation...AMC/Robertson
Marine...AMC/Robertson
Fire Weather...Robertson



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