Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 220333
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1133 PM EDT Fri Apr 21 2017

.SYNOPSIS...
An area of low pressure will move across southeast Canada tonight
and Saturday as a cold front pushes south of the area. This front
will stall to our south over the weekend while an area of low
pressure is forecast to move along the boundary to our south. This
low will 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 to move through the Great Lakes region
Wednesday and approach our area Thursday.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
1130 pm update: Made some changes to the PoPs/Wx grids tonight
based on latest hi-res solutions, radar/satellite trends, and
objective surface analyses. A band of storms continue from near
Seaside Heights to Atco in southern/eastern NJ at this time.
Storms appear to be rooted near 900 mb, where marginal but
sufficient instability exists for these storms. With the near-
surface layer stable, these storms pose little or no risk of
severe weather, but they are producing fairly decent rain rates
with several rain gauges recording between a quarter to three
quarters of an inch as the storms pass.

Hi-res guidance is moving these storms off the coast too quickly
and generally diminishing these storms too readily. There are
hints from the latest simulations that at least isolated storms
may occur for the next few hours in southern New Jersey, so
raised PoPs to at least slight chance here through the early
morning hours.

Additionally, latest mesoscale models are bringing light rain
showers into Delmarva in the 07Z-11Z time frame, with light
rain/showers overspreading much of the CWA near or after
daybreak this morning. Given the solid agreement among these
simulations, felt the need to raise PoPs substantially late
tonight in Delmarva and adjacent portions of far southern PA and
southern NJ. Will continue to monitor latest trends, with
further updates to the near-term forecast likely.

Previous discussion...

Isolated showers and thunderstorms continue generally in and
south of Philadelphia (where clearing occurred earlier today).
However, expect the coverage of these storms to wane over the
the next few hours as the surface low slides off shore.

A cold front approaching from the northwest is expected to pass
through our region late tonight. The arrival of some drying
with the front should help to break some of the low cloud cover
at that time.

The wind should remain from the east and northeast in much of far
eastern Pennsylvania and New Jersey into tonight. A varying wind
direction is anticipated elsewhere in our forecast area. Wind speeds
should remain less than 10 MPH except perhaps in the vicinity of any
thunderstorms that develop.

Minimum temperatures are forecast to favor the 40s in the north and
the 50s in the south.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
1130 pm update: Made some changes to the forecast for Saturday.
Latest mesoscale model simulations show light rain/showers
overspreading much of the CWA this morning near/after daybreak.
Increased PoPs across the area through the day. Suspect tomorrow
will not be an all-day soaker by any means, with one round of
steadier precipitation during the morning - followed by spottier
showers around midday - followed by another round of steadier
rain by late afternoon/early evening. However, latest WRF-ARW
/WRF-NMM and NAM Nest simulations suggest potential for fairly
steady rainfall for most of the CWA with the second batch, and
think PoPs were too low during this time frame especially. They
may still be too low, especially north of the Mason-Dixon Line.
Further updates to the forecast likely once the latest round of
simulations are assessed.

Previous discussion...

Some moisture is forecast to override the low level cold air in the
wake of the frontal passage as the mid level flow remains from the
west southwest. While there may be some break over our northern
counties, our central and southern counties are anticipated to
remain mostly cloudy on Saturday. Also, some light rain is possible
mainly across northeastern Maryland, Delaware and southern New
Jersey.

Maximum temperatures are expected to favor the lower 60s, except in
the elevated terrain up north where readings should get no higher
than the 50s.  The wind is forecast to be from the northwest at 5 to
10 MPH on Saturday.

&&

.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
For Saturday night into Sunday, the front is expected to remain
stalled to our south while an area of low pressure moves along the
boundary well to our south across the southeast states. Several
short wave/vorticity impulses are expected to move across the area
this weekend north of the frontal boundary and low pressure system.
This will lead to a chance of showers for portions of the area,
especially across southern New Jersey and the Delmarva areas. Sunday
looks like the shower activity may become less widespread as we lose
some of the enhanced moisture/lift combination. However, showers
chances increase Sunday night across southern New Jersey and
southern Delmarva again.

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 up the coast, but there is a general consensus for a
enhanced chance of showers for Monday through Tuesday for much of
the area. Depending on how fast this system lifts north of the area
and it`s placement off the coast will depend on where shower
activity will occur.

By Wednesday, the low should be lifting to our east then northeast
and bring shower activity to an end for Wednesday. This will then
start a warming trend for the end of next week. A cold front is
forecast to approach the area Thursday, with showers approaching as
well. These showers may dissipate as they approach. Even behind this
frontal boundary, return flow develops and thicknesses begin to
increase for the end of the week, so temperatures may warm Thursday
into Friday several degrees above normal.

&&

.AVIATION /03Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

MVFR ceilings, lowering to IFR at some sites, are expected to
continue through at least 06Z. After 06Z, confidence is low as
to how quickly we will see improving conditions. A wind shift
(from easterly now to northerly late tonight), may start the
improving conditions as early as 06Z. However, climatologically,
it is rare to see improving conditions between 06 and 09Z,
especially given how extensive the low cloud deck is across the
region.

Earlier thunderstorms should continue to wane through 03Z.
Showers are possible for KPHL/KPNE and areas south after 12Z,
but timing and coverage is uncertain at this time. Rain should
not be heavy enough to impact the visibility.

Winds are expected to back out of the northwest by 12Z.

OUTLOOK...

Saturday night-Sunday night...Chance for showers and lower
conditions over the weekend, mainly southern New Jersey and Delmarva
areas. Areas farther north may remain dry and VFR through the
weekend.

Monday-Tuesday night...MVFR and IFR conditions likely with periods
of showers.

Wednesday...Improving conditions. MVFR CIGS possible early, but
return to VFR during the day.

&&

.MARINE...
The wind should favor the east to north quadrant tonight and the
northwest on Saturday 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.

Areas of fog will persist on our ocean waters into tonight.

OUTLOOK...

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 night...Small Craft Advisory conditions likely.

Wednesday...Winds expected to drop below advisory levels, but seas
may remain elevated.

&&

.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...CMS/Iovino/Johnson
Short Term...CMS/Iovino
Long Term...Robertson
Aviation...Iovino/Johnson/Robertson
Marine...Iovino/Robertson
Climate...Drag



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