Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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639
FXUS61 KPHI 210026
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
826 PM EDT Thu Apr 20 2017

.SYNOPSIS...
A backdoor cool front is expected to stall across the Mid
Atlantic tonight into Friday. As an area of low pressure moves
out of the Great Lakes region and into southeast Canada, it will
pull a cold front across the area late Friday into Friday
night. This front will stall to our south over the weekend. An
area of low pressure is forecast to strengthen as it moves off
the southeast coast late Sunday, then move north offshore of the
east coast through early next week. A cold front is expected
either next Wednesday or Thursday as an area of low pressure
moves out of the Great Lakes region into southeast Canada.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
A line of showers and thunderstorms remains just to our
southwest in MD and VA. This should continue east and could
bring showers, and maybe a thunderstorm to areas from Philly
southward. However, a southeasterly and easterly surface flow
has been persistent for the last few hours leading to a stable
boundary layer for much of the region. Even though this initial
line will not reach locations north of Philly, kept the highest
PoPs across the north for later tonight when a second round of
showers and isolated thunderstorms will be possible as the
surface low and a mid level shortwave trough approaches the
region.

We are not anticipating any severe weather with tonight`s
system. However, there could be localized brief downpours and
perhaps some small hail.

The southeast flow will likely result in the development of
areas of fog in the elevated terrain of eastern Pennsylvania and
northern New Jersey due to upsloping.

Tonight`s minimum temperatures will favor the 50s in our region.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
Low clouds are forecast to linger into Friday morning, along
with a chance of showers and thunderstorms. The fog may persist
in the elevated terrain, as well.

Some brief improvement is possible during the early afternoon.
However, there is another chance for showers and thunderstorms
as a cold front approaches from the northwest.

The wind may settle into the southwest around 8 to 10 MPH
during the late morning and early afternoon hours bringing warm
air into our region. High temperatures are anticipated to be in
the 60s in our far northern counties, in the 70s in the middle
part of our forecast area, and in the lower 80s in much of
northeastern Maryland and Delaware.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
There is the potential for some unsettled periods during the
long term, but not the entire time expected to be active. Some
uncertainty with some model differences at times during the
forecast.

On Friday night, the frontal boundary that affects our area
during the day pushes south of the area. With the boundary to
our south, there is also the possibility for some vorticity
impulses to move across the area withing the southwest flow
aloft. The NAM is the most aggressive bringing in precipitation
overnight, while the GFS, ECMWF, and Canadian aren`t as
aggressive. We will keep a slight chance of showers overnight
due to the proximity of the frontal boundary to the south and
potential short wave interaction.

As we move into Saturday and continuing into Sunday, the front
is expected to stall to the south, while an area of low pressure
moves along the boundary well to our south. With stronger short
wave/vorticity impulses expected to move across the area this
weekend north of the frontal boundary and low pressure system,
there will be a better chance of showers across the area
Saturday through Sunday. The most likely areas for showers is
expected to be from around the Philadelphia area southward.
There will remain a chance of showers Sunday night across
southern New Jersey and southern Delmarva, but chances decrease
through the night as the short wave/vorticity impulses begin to
move past the area.

By Sunday night into Monday the low pressure to our south is
expected to move offshore of the southeast coast, then begin
lifting northward offshore of the east coast into the middle of
the week. There is some timing differences in how fast the model
guidance lifts this low offshore of our area, but there is a
general consensus for a enhanced chance of showers around
Tuesday for much of the area. Depending on how fast this system
lifts north of the area, there could also be a small chance of
showers into Wednesday. However, if it`s far enough offshore,
most places should be precipitation free.

There is also the possibility of a cold front approaching the
area during the middle of next week as well. The ECMWF has it
approaching Wednesday into Wednesday night, while the GFS has it
approaching Thursday into Thursday night. This frontal boundary
could bring another chance of showers to the area Wednesday or
Thursday.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Ceilings should lower through the evening hours. After 06Z,
widespread MVFR conditions are expected. Several sources of
guidance show IFR and even LIFR conditions. However, have chosen
to include ceilings only as low as 1000 ft AGL in the TAFs for
now (guidance was also depicting IFR ceilings last night which
never developed, so there may be a low level moisture bias in
the guidance now). In addition, two rounds of shra are possible,
the first between 02 and 06Z, and the second between 06 and
12Z. Isolated tsra is possible as well, but confidence and
coverage are too low to include in the TAFs at this time.

Ceilings should slowly improve after 12Z. However, another
round of showers and scattered thunderstorms is possible Friday
afternoon. We have low confidence in the specific timing and
coverage of the precipitation after 18Z so it is not mentioned
in the TAFs at this time.

The wind is expected to settle into the southeast and east for
tonight around 6 to 10 knots. The wind should become light and
variable for a time on Friday morning before becoming southwest
around 5 to 10 knots for late Friday morning and Friday
afternoon.

OUTLOOK...

Friday night...VFR conditions are currently forecast for Friday
night, but there is a chance for lower conditions to develop
with a chance of showers.

Saturday-Sunday...Better chance for showers and lower
conditions over the weekend. The highest chances for showers and
lower conditions are expected from the Philadelphia area
southward.

Sunday night...A chance of showers remains for coastal New
Jersey and central/southern Delmarva with possible lower
conditions there. Elsewhere, improving conditions overnight.

Monday-Monday night...Mostly VFR, lowering clouds overnight.

Tuesday...MVFR conditions possible with showers, especially
eastern New Jersey.

&&

.MARINE...
The wind should favor the east and southeast tonight and the
south and southwest on Friday at speeds of 15 knots or less.
Wave heights on our ocean waters are forecast to favor the 3 to
4 foot range. Waves on Delaware Bay are expected to be 1 to 3
feet.

OUTLOOK...
Friday night-Saturday...Winds may gust around 20 knots at times
Friday night into Saturday, but overall should remain below
advisory levels.

Saturday night-Sunday...Conditions expected to remain below
advisory levels. Winds shift from a west-northwest direction to
a northeast direction overnight Saturday.

Sunday night...Winds and waves begin to increase to advisory
levels late Sunday night into early Monday morning.

Monday-Tuesday...Small Craft Advisory conditions likely.

&&

.CLIMATE...
Overall projections today were identical or slightly warmer
than yesterdays. Confidence is above average on the info below.

April will probably be a top 10 warmest in the period of record
for multiple long term climate stations in our forecast area.
Depending on the warmth of this afternoon and Friday and how
cold it is on Sunday or Monday when a chilly rain may occur, we
appear to be on our way for a top 2 warmest April on record.

The usual analysis process first 19 days done, the 20th-26th
per our 330 am fcst, the 27-30th from FTPRHA.

PHL projects #2, 4.9 above normal and 0.5degs from record. POR 1874
ABE projects #2, 6.1 above normal and 0.4degs from record. POR 1922
ACY projects rer 5.0 above normal,0.4F above prev 2010 56.3 POR 1874

So, there could be some slippage but its unlikely that any of
these locations can slip below #10.

&&

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

&&

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



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