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FXUS61 KPHI 260850 AAA
AFDPHI

Area Forecast Discussion...UPDATED
National Weather Service Mount Holly NJ
450 AM EDT Wed Apr 26 2017

.SYNOPSIS...
The coastal low that has affected the East Coast the last
several days will begin pulling away from the region today, but
will be slow to exit. A cold front will move across the area
Thursday night, with a second Saturday night. A warm front is
expected to lift across the area Sunday night, followed by
another cold front on Monday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
07Z surface analysis shows a 1003 mb low pressure centered over
the lower Delmarva peninsula. A clear counter-clockwise
circulation is evident on a loop water vapor satellite imagery
and even radar imagery with the center of the nearly vertically-
stacked low about 80 miles to the E-SE of Wallops Island.

Overall coverage of showers have decreased area wide last night
but there is still are few areas of showers over S NJ, SE PA
and northern Delmarva that seems to be associated with a mid-
level shortwave trough/jet streak that is pivoting westward on
the northern side of the cyclone. Enhanced convergence near the
system's bent back warm front and inflow of modestly unstable
air over the Gulf stream could be responsible for focusing a
line of heavier showers that is moving downstream toward the
Phila. PoPs were updated for today to reflect a steady decrease
in showers from SW to NE today with generally scattered coverage
this morning, becoming isolated early this afternoon. It should
be noted that guidance, even hi-res CAM models, will likely
have a hard time capturing these small-scale features, so
monitoring real-time observations had a huge influence in the
forecast but continual refinements to the PoPs/weather grids
will likely be needed today beyond a few hours out in time.

Model guidance continues to show considerable spread regarding
temperatures today. MOS guidance from the MAV (highs in the 70s)
is 5-10F warmer than the MET (highs in the 60s) due to
differences in how much sunshine develops this afternoon. There
does not appear to be a notable mechanism to mix out the marine
stratus today with a strong low-level inversion remaining in
place, light N-NEly winds and no notable source of dry/cold air
advection. Accordingly, forecast weighted more toward the cooler
and cloudier NAM-based guidance though not quite as extreme. If
breaks in the clouds were to develop this afternoon, forecast
temperatures would need to be adjusted upward by several degrees
(which would be more likely to occur farther inland toward the
eastern shore of MD and our far western zones in eastern PA).

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
The coastal low will continue to weaken tonight as it moves out
to sea, passing south of Long Island/southern New England.
Marine moisture will remain trapped underneath the subsidence
inversion. Any stratus that mixes out this afternoon should
redevelop quickly after dark tonight. At least patchy fog is
anticipated late this evening and overnight, but the extent of
the fog should be limited by the presence of stratus beforehand.
It should be noted that some of the guidance shows dense fog
forming late tonight but we have not quite bit on that due to
the previously mentioned reasons. Cloud cover should curb how
much cooling takes place tonight. Forecast lows are in the 50s.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
On Thursday, our area will be under the influence of return
flow ahead of an approaching cold front. It is possible that an
isolated shower may develop across northeast Pennsylvania and
northern New Jersey as a weak short wave/vorticity impulse
approaches the area. However, the best chances for rain are
expected to be during the overnight hours as the front moves
across the area and a stronger short wave/vorticity impulse
moves across the area. Although much of the guidance dissipates
the more widespread showers as they approach, they do keep some
scattered showers across our area.

The front moves offshore Friday morning and any showers will
dissipate and/or move offshore through the morning as well. The
remainder of Friday will remain dry as weak high pressure
briefly affects the area.

By Friday night, another frontal boundary is forecast to begin
approaching the area from the north as it crosses Pennsylvania
and New York. The front will cross through the area during the
day Saturday, then stall to our south Saturday night into
Sunday. There is some slight timing differences in the model
guidance on when the front moves through. But as a couple of
short wave/vorticity impulses move across the area Friday night
through Saturday night, there will be a chance for
isolated/scattered showers.

The front will stall to our south during the day Sunday, which
should allow Sunday to be dry. Bring the front is a back-door
front coming from the north, it will bring cooler conditions on
Sunday. But the front will not stay to our south very long as it
is expected to lift back northward as a warm front Sunday
night/Monday morning. While there is a chance of showers Sunday
night into Monday morning, the more likely time period for any
precipitation would be later Monday into Monday night as a cold
front approaches from the west. Being we should enter into the
warm sector during the day, we should warm quite nicely, and in
turn some instability should build ahead of the front. Therefore
there will be a chance of thunderstorms Monday afternoon and
Monday night.

The front will move offshore by Tuesday morning, but a
secondary surface trough may move across the area during the day
Tuesday. With a short wave/vorticity moving across the area
during the day, there could be some isolated showers across
northeastern Pennsylvania and northern New Jersey.

&&

.AVIATION /09Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

IFR/LIFR CIGs with stratus will persist this morning. Some
gradual improvement in the CIGs to MVFR are expected, possibly
as early as as 13-16Z at RDG/ABE and 16-19Z for Phila terminals.
ACY/MIV/TTN are the most susceptible out of all the TAF sites
to remain IFR for most of the day. A return to IFR is expected
with cloud bases lowering below 1 kft AGL sometime this evening.
Current LAMP and MET guidance shows further lowering to LIFR
between 06-12Z.

NE winds 10-15 kt early this morning will gradually weaken over
time to 5-10 kt by afternoon. Light and variable winds are
expected tonight.

OUTLOOK...

Thursday...MVFR CIGS could continue into Thursday morning,
before improving to VFR during the day.

Thursday night...Generally VFR. A chance of showers overnight
which could temporarily reduce conditions to MVFR.

Friday...VFR conditions expected.

Friday night...VFR early. Patchy fog and low CIGS may develop
overnight.

Saturday...Becoming VFR during the morning, then a chance of
showers during the afternoon. Conditions may temporarily be
reduced with any showers.

Saturday night-Sunday...Low clouds and fog may develop
overnight Saturday and continue into Sunday leading to reduced
conditions. There will also be a chance of showers.

&&

.MARINE...
The Small Craft Advisory has been converted to a hazardous seas
with winds falling below 25 kt last night. Waves are currently
around 7-10 ft and will gradually subside to 4-7 ft late this
afternoon and tonight.

OUTLOOK...

Thursday-Thursday night...Small Craft Advisory for hazardous
seas extended through Thursday night.

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

Friday night-Sunday...Winds are expected to remain below
advisory levels. Seas may approach 5 feet at times, but mostly
remain around 4 feet.

&&

.TIDES/COASTAL FLOODING...
Tidal departures early this morning are 1 to 1.25 above
astronomical prediction. These departures should decrease
further today owing to much lighter N-NE winds today compared to
yesterday. The upcoming high tide this morning, which is the
lower astronomical tide will not be accompanied by another
round of minor flooding.

We will continue to monitor the subsequent high tide this
evening since it will be the higher one that occurs with a new
moon. If the water does not drain away from the coast fast
enough, we could see some localized minor flooding at that time.
ETSS and a few of the higher NYHOPS ensemble members predict
levels reaching minor threshold (but below local advisory
criteria) at Lewes, Cape May and Reedy Point while the majority
of the remaining guidance (including ESTOFS) keeps it short of
minor.

&&

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

&&

$$
Synopsis...Robertson
Near Term...Klein
Short Term...Klein
Long Term...Robertson
Aviation...Klein/Robertson
Marine...Klein/Robertson
Tides/Coastal Flooding...Klein
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