Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 260409
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1209 AM EDT Wed Apr 26 2017

.SYNOPSIS...
Low pressure will track northeast just off the Delmarva and New
Jersey coastline through Wednesday. Meanwhile, low pressure will
move from the upper Mississippi Valley into James Bay, and send a
weak cold front into our area Thursday night and Friday morning.
A backdoor front is expected to move through the region Saturday
evening. Another low pressure system moving from the lower Plains
into Quebec and its associated fronts will affect the area Monday
into Tuesday.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
Onshore winds will advect ocean fog inland into coastal
communities overnight. The fog may become locally dense in
spots during the overnight with visibility reduced to as low as
one-quarter mile at times. At least patchy dense fog could
persist into the morning and affect the morning commute.
A Special Weather Statement was issued and patchy dense fog
wording was added to the forecast grids to highlight the threat
to travelers.

Previous Discussion...
The water vapor imagery as of 01z shows the center of a well
defined closed low just exiting the northeast coast of North
Carolina. An area of heavier rain is moving across central to
northern New Jersey with precipitation more showery to the south
and west. Some lightning continues near and east of the closed
low center, and a mesoscale analysis shows the axis of
instability offshore although there is some MUCAPE across the
southern half of our area. The lightning has been maintained
offshore in the axis of higher instability, which is also more
surface-based there. Since this should mainly stay offshore,
removed thunder for our area. Made adjustments to the hourly
temperature, dew point and wind grids to keep them current.
Adjusted the PoPs based on radar trends and some assistance from
the HRRR guidance, with highest values across the northern
areas awhile longer. Transitioned the precipitation over to
showers as this looks to be the mode over much of the area.
However given the low celings and wealth of low-level moisture,
some drizzle can occur outside of the showers.

With abundant low-level moisture in place, stratus will be
maintained and some forecast soundings show the inversion lowering
some overnight. In addition, some fog is anticipated however how
widespread it becomes or dense is a forecast challenge. The stratus
may dominate, however we will continue with a fog mention although
not anticipating the need for a Dense Fog Advisory at this time.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
Low pressure off the NJ coast will continue to drift to the north on
Wednesday. Upper level low will be just to the west of it, and with
several strong shortwaves passing through the region, can expect
some showers throughout the day. Otherwise, fog gradually dissipates
through the morning, but clouds will remain across the region for
most of the day. Winds back a bit to the N/NW by afternoon, but the
gradient will be light, and winds will remain less than 10 MPH.
Highs generally top off in the 60s, but a few spots may come close
to 70.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
Overall - Warm pattern shaping up. The closed low that is
currently over the southeast US finally exits the area early
Wednesday night. With the polar jet displaced far to the north,
several disturbances traversing the southern stream jet will aid
in building a stout sub tropical ridge off the southeast US
coast from Friday into the early part of next week. As another
closed low ejects out of the midwest on Monday, a stronger and
more active cold frontal passage occurs in the late Monday into
early Tuesday period.

Wednesday night into Thursday should feature dry weather under brief
ridging aloft. With some partial clearing Wed night, light and
variable winds within the col between weather systems, and
abundant low-level moisture given recent rainfall, patchy fog is
likely. The fog will burn off early Thursday, with partly sunny
skies expected. A light southeast flow will maintain cooler
temperatures at coastal locals. Highs will be around 10 degrees
above normal.

A weak cold frontal passage Thursday night into Friday will lead to
considerable cloudiness, but moisture will be limited, with only a
slight chance of showers across the region. In fact, with the strong
subtropical high building over the southeast/orientation of the mid-
level flow, this front is expected to washout/stall in the
vicinity of the I-95 corridor. With deeper moisture and mid-
level support exiting the region, expect a return to partly to
mostly sunny skies by Friday afternoon. A weak synoptic flow
should encourage a robust sea breeze. Highs will be several
degrees warmer than Thursday, around 15 degrees above normal,
but far from daily record highs.

Clouds increase again Friday night into Saturday with another cold
front moving toward the region. There is considerable uncertainty
regarding the timing of this front, with model solutions varying
from Saturday morning to Saturday night, with a backdoor component.
In addition, there is also uncertainty regarding the precip
potential, with quite a bit of spread amongst the model
solutions. For now, we maintained chance PoPs, including thunder
on Saturday. Kept thunder out of the forecast for Sunday given
more stable maritime airmass, except Delmarva, with closer
proximity to the frontal boundary. Still some uncertainty with
high temperatures on Saturday, which will be dependent on the
location of the front, but expect them to be similar to Friday.

There is model agreement that the front will be just to our south-
west on Sunday, with the area under the influence of a northeast low-
level flow. This would favor a stratocumulus deck and perhaps some
sprinkles, mainly across the higher terrain. Kept thunder out of the
forecast for Sunday given stable maritime airmass, except Delmarva,
with closer proximity to the frontal boundary. High temps still be
several degrees above normal.

Moving into early next week, more showers and thunderstorms likely
Monday and Monday night with the next cold frontal passage. Expect a
return to drier weather on Tuesday. Continued above normal
temps.

&&

.AVIATION /04Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Overnight...IFR, locally LIFR, ceilings. Some fog/drizzle especially
late will result in times of IFR, perhaps LIFR, visibilities
although the extent and timing is less certain. Showers will
continue for awhile, but taper off from southwest to northeast
overnight. Northeast winds 10-15 knots with local gusts up to 20
knots, diminishing to 10 knots or less.

Wednesday...IFR/LIFR in the morning due to low clouds and some fog,
then conditions slowly improve to IFR/MVFR in generally the 15-18Z
timeframe. Isolated to scattered showers are possible especially
through midday. Northeast to north winds mainly 5-10 knots.

OUTLOOK...
Potential for fog leading to visibility restrictions Wed night
and MVFR ceilings persisting into Thursday. Predominantly VFR
Thursday night into Saturday. MVFR possible in low clouds Sat
night into Sunday.

&&

.MARINE...
A Dense Fog Advisory was issued for the coastal waters and lower
Delaware Bay for tonight and the morning. Area beach webcams
show dense fog while nearby AWOS (WWD, BLM) are reporting
visibility under 1 NM. Just about all available model guidance
has been indicating dense fog developing over the marine areas.
There is low confidence in the dissipation time of the dense
fog. Most of the guidance indicates improvement in visibility
before noon local time, but with minimal mixing, an absence of
dry air advection and moisture trapped underneath a very strong
thermal inversion, there is a potential that the fog could continue
into the afternoon in spots. Dense fog could even persist into
or redevelop Wednesday night.

Previous Discussion...
The Small Craft Advisory for Delaware Bay is cancelled as winds are
diminishing. The Small Craft Advisory for the ocean zones continues
through Wednesday night, as winds diminish overnight from south to
north however it should take awhile for the seas to subside.

OUTLOOK...
SCA-level seas likely persist through Friday with a return to
sub-SCA conditions thereafter.

&&

.TIDES/COASTAL FLOODING...
The Coastal Flood Advisory was expired for coastal areas of New
Jersey and Delaware, including Delaware Bay as high tide has
passed. Most locations received minor tidal flooding at high
tide which was earlier this evening.

The advisory was also cancelled for the zone that covered the
tidal Delaware River downstream from the Commodore Barry Bridge
and the extreme upper part of the Delaware Bay (including New
Castle, DE and Salem, NJ counties). High tide at Reedy Point
has just passed within the hour with the tidal gauge cresting
just below minor coastal flood threshold.

The diminishing wind overnight along with the lower astronomical
tides on Wednesday morning should preclude another round of minor
flooding with Wednesday morning`s high tide.  We will keep an eye on
the higher astronomical tides on Wednesday evening. If the water
does not drain away from the coast fast enough, we could see some
localized minor flooding at that time.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Dense Fog Advisory until noon EDT Wednesday for ANZ431-450>455.
     Small Craft Advisory until 6 AM EDT Thursday for ANZ450>455.

&&

$$
Synopsis...Franck
Near Term...Gorse/Klein
Short Term...MPS
Long Term...Franck/MPS
Aviation...Franck/Gorse
Marine...Franck/Gorse/Klein/MPS
Tides/Coastal Flooding...Klein



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