Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 180428

Area Forecast Discussion
National Weather Service Mount Holly NJ
1228 AM EDT WED MAY 18 2016

A weak low pressure system will track to our south and east
through Wednesday, then strengthen Thursday as it shifts farther
offshore. High pressure builds in later Thursday and Friday. Low
pressure is forecast to arrive on Saturday before shifting
offshore Sunday, however it may then meander near the Mid Atlantic
coast during Monday and Tuesday.


The last of the showers in our region were drifting off the
southern part of the New Jersey coast shortly after midnight. The
sky will remain mostly cloudy overnight and there is a chance that
some additional light rain showers may drift into southeastern
Pennsylvania from the west toward morning.

The weak frontal boundary that was dropping southward through our
region has become a bit difficult to find. The wind has gone calm
at many locations and where there is a light wind speed, the
direction is favoring the northwest.

The cloud cover should keep temperatures from dropping much
during the balance of the night. Lows should be mostly in the 45
to 50 degree range.


Overall, expect dry conditions for most of the CWA during the day
Wednesday. The exception to this will be over southern and
southwest parts of the CWA as a stronger shortwave approaches
while weakening and another weak low forms along the stalled front
to the south. The eastern/northern extent of precipitation remains
in question, and have kept chance pops for portions of the CWA.
Best chances for the lowest pops and breaks in the cloud cover
remain in the Poconos and northern NJ.


The synoptic setup is comprised of an upper-level trough that
swings across the Northeast and Mid Atlantic Wednesday night into
Thursday before shifting offshore. Some weak ridging then builds
in later Thursday and Friday before it also shifts offshore. As a
closed low drops down across the West Saturday, a ridge develops
in the Plains. Some energy sliding southeastward across the Great
Lakes is forecast to develop into a closed low on the East Coast
during the second half of the weekend and early next week. This
feature should then meander around the area but should be pushed
eastward later Tuesday as upstream energy amplifies in the Midwest
to Great Lakes. This pattern will tend to result in unsettled
weather at times, with Thursday and Friday being the better
weather overall. We used a model blend approach for Wednesday
night into Friday, then blended in the 12z WPC Guidance with
continuity thereafter. Some adjustments were then made following
additional collaboration with our neighboring offices.

For Wednesday night...An upper-level trough is forecast to be
near the area, however the attendant surface low shifts well
offshore. It appears the main lift and deeper moisture are also
offshore and with high pressure edging closer from the west, we
went with a dry forecast along with decreasing clouds.

For Thursday and Friday...The upper-level trough migrates across
the region Thursday then a ridge begins to arrive before shifting
offshore later Friday. This will drive a fairly expansive surface
high pressure system over our area, although it does shift
offshore during Friday. Some guidance is now sharpening the trough
up more as it moves across the region Thursday. Given enough
moisture and cooling aloft, there could be a few showers Thursday
afternoon. This looks mostly terrain-based as of now. This could
be overdone, however for now added a slight chc pop for Thursday
afternoon for the higher elevations of the northwestern areas. As
energy then amplifies across the Great Lakes and Ohio Valley,
downstream large scale ascent and warm air advection strengthens.
This allows for clouds to increase Friday night with the potential
for some rain to start overspreading the western and southern
zones toward early Saturday morning. Warmer air is expected during
this time frame.

For Saturday and Sunday...The incoming trough is forecast to
close off in the east during this time frame, especially as the
pattern resembles an omega block once again. The placement of the
actual closed low and associated surface low development will
determine the timing, areal extent and amount of rain the region
sees. It does appear that Saturday is wetter than Sunday at this
time. There should be ample lift and moisture available for this
system, which may result in the rain becoming heavy at times.
Since some model guidance is a bit farther to the south and there
is still plenty of time to to fine tune the details, we did not
raise pops a lot. The pressure gradient should tighten and
potentially lead to breezy conditions especially along the coast.
The main rain may shift offshore Sunday, however the surface low
may end up meandering given the closed nature of the trough aloft.

For Monday and Tuesday...A weak surface high is forecast to
extend around weak low pressure that may be near our coast. This
may be a secondary surface low that develops associated with
another short wave rotating around the closed low. If the low is
captured enough it may make a run inland later Monday or Tuesday.
Overall, the pattern is at least showery through this time frame
and if enough heating can take place given the colder air aloft
with the closed low, then perhaps some low-topped convection is
able to form. We did not make a lot of changes to this time frame
given more uncertainty with the details.


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

Overall conditions will probably remain mostly VFR or perhaps
upper-end MVFR with some light fog possible overnight. Winds very
light shifting to N then NE by dawn. Mostly VFR wed with bkn-ovc
cloud cover in most areas.

Wednesday night through Friday...VFR overall as high pressure
builds in later Thursday and Friday. An isolated shower is
possible Thursday afternoon well north and west KPHL. Clouds then
increase Friday night ahead of our next system.

Saturday and Sunday...MVFR/IFR conditions with periods of rain,
with the wettest time frame looking to be Saturday into Saturday
night when the rain could be heavy at times.


Very quiet marine weather through Wednesday. No marine headlines
expected. Wind gusts will be generally less than 10 kts and seas
1-3 feet on the Atlantic coastal waters and 2 feet or less on
Delaware Bay.

Wednesday night through Friday...High pressure gradually builds
over the area and therefore the conditions are expected to be
below Small Craft Advisory criteria.

Saturday and Sunday...Low pressure moves across the region then
slows as it gets offshore Sunday. Increasing east to northeast
winds should gust to Small Craft Advisory criteria and seas should
build. It is possible that a period of low-end gale force gusts
occur, however this is dependent on the strength of the low
pressure system.





Near Term...Iovino
Short Term...99
Long Term...Gorse
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