Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 240148
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
948 PM EDT Mon Apr 23 2018

.SYNOPSIS...
A large area of high pressure will shift slowly east of the
region overnight. A low in the lower Mississippi Valley will
lift into the Mid Atlantic states by Wednesday, with widespread
rain arriving across the region in advance of the system.
Another low pressure system passing to the south of our region,
could bring another chance of rain late this week. By late in
the weekend, high pressure begins to build south towards the
region.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
Quiet conditions will continue tonight across the area as high
pressure remains offshore of the New England coast. Our area
will remain under the influence of southeast return flow, which
will allow moisture to increase some overnight. Overnight lows
will drop through the night as winds diminish and mostly clear
skies will begin the night. However, with the flow being more
from the southeast, and the fact some clouds may begin moving in
late in the night, overnight lows are not expected to be as
cool as Sunday night. Most places should drop close to normal,
except portions of the Pine Barrens which might drop a few
degrees lower and have some patchy frost develop. We will
include patchy frost in the forecast, but not issue a Frost
Advisory yet as there is still uncertainty as to how low the
temperatures will get.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...
Dry conditions will begin the day Tuesday, before rain chances
increase through the day. An occluding low pressure system will
continue moving across the southern Appalachians early in the day,
before slowing moving into the Carolinas by the afternoon. Our area
will be well north of the low pressure system and the slowly lifting
warm front through the daytime hours. However, south-southwest flow
aloft will lead to increasing moisture through the day. A couple of
short wave/vorticity impulses will lift across the area through the
day as well. Rain chances increase, especially during the afternoon
as the second of the short waves moves through and the best
moisture/lift combination takes place. There is little to no
instability forecast, so no thunderstorms are expected at this time.

Winds will become gusty through the morning and into the afternoon
with gusts reaching 20 to 25 mph at times.

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
Overview: The main focus through this period remains on the
Tuesday night and Wednesday time frame and the potential for
heavy rain (though this threat appears to be decreasing).
After that, the next chance for rain looks to be primarily on
Saturday as our region will be in between a low off the coast
and a cold front approaching from the west.

Details: The low continues to very slowly progress northeast,
reaching the Mid Atlantic by Wednesday. The window for heavy
rain has narrowed a bit, now looks to be confined mainly late
Tuesday night, though lingering light rain is possible into
Wednesday evening.

As for the heavy rain threat, precipitable water values on
Tuesday night are still expected to be well above normal,
possibly in the 90th percentile for this time of year. However,
there are a few factors working against the heavy rain threat.
The warm cloud layer is relatively shallow (generally 8000 to
9000 ft), thanks to continued below normal temperatures.
Additionally, storm motions aren`t expected to be particularly
slow, around 20 mph, although training storms could make this
point moot. In general, it looks like the highest threat for
heavy rain will be over the coastal plains of Delmarva and far
southern NJ.

There is some very limited elevated instability Tuesday night,
but it is very elevated (generally for parcels at or above 850
mb). Thus, have not mentioned thunderstorms in the forecast for
now, but will continue to monitor this potential as we get
closer.

For the late week period, there remains some uncertainty in how
two low pressure systems, one lifting out of the southeast and
becoming a coastal low and one digging out of the north central
U.S., will interact and what the impact will be for our region.
However, the models today are in better agreement than what we
have seen the last few days, with a brief chance for rain on the
back side of the coastal low on Friday night or Saturday, and a
limited impact from the cold front moving in from the west.

&&

.AVIATION /02Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Tonight...VFR conditions will continue overnight. South-
southeast winds around 5-10 knots will become light and
variable most places overnight.

Tuesday...VFR conditions will begin the day across the TAF sites
through at least midday Tuesday. However, clouds will thicken and
lower through the afternoon, and could eventually reach MVFR by late
in the afternoon or early evening. Rain will also be moving from
the south during the afternoon hours.

Outlook...

Tuesday night and Wednesday...MVFR and even localized IFR
conditions will be possible through Wednesday morning as
periods of rain are likely across the region. Easterly wind
gusts up to 20 kt are possible. Moderate confidence on flight
categories, but low confidence on the timing of any flight
category changes.

Thursday...VFR conditions expected with light westerly winds.
High confidence.

Friday and Saturday...Mostly VFR conditions are likely, though
there is a chance that rain and low clouds could lower
conditions to MVFR at times. Wind is forecast to be westerly or
southwesterly near or below 10 kt. Moderate confidence on
flight categories and wind.

&&

.MARINE...
Conditions will remain below advisory levels tonight and into the
first half of Tuesday. However, winds could begin gusting around 25
knots by the afternoon, while seas are expected to build to 5 feet
or higher by the afternoon as well. Therefore the Small Craft
Advisory remain in place starting noon Tuesday.

Outlook...

Tuesday night and Wednesday...Easterly and southeasterly winds
increase to 15-20 kt with gusts up to 30 kt, and seas build up
to 10 feet on the Atlantic coastal waters. Wind gusts near 25 kt
are also expected on the Delaware Bay primarily Tuesday night.

Wednesday night and Thursday...Winds shift to northwesterly and
should decrease. The main question will be how long seas remain
above 5 feet on the coastal waters. Current forecast is for
elevated seas into late Thursday, but there is still
considerable uncertainty with this.

Friday and Saturday...Winds and seas are currently forecast to
stay below SCA criteria on all waters. However, there remains
some uncertainty with if a coastal low develops and intensifies.
If this happens, there is a slight chance for SCA criteria,
primarily on Saturday.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory from noon Tuesday to 6 PM EDT Wednesday
     for ANZ450>455.
     Small Craft Advisory from noon Tuesday to 2 AM EDT Wednesday
     for ANZ430-431.

&&

$$
Synopsis...Johnson/99
Near Term...99
Short Term...Robertson
Long Term...Johnson
Aviation...Johnson/Robertson/99
Marine...Johnson/Robertson



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