Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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719
FXUS61 KPHI 220213
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
913 PM EST Sat Jan 21 2017

.SYNOPSIS...
A warm front lifts north through the region tonight as high
pressure over the Western Atlantic continues to drift offshore.
A strong and complex area of low pressure will develop over the
Lower Mississippi Valley on Sunday, and that will lift towards
and pass through the Mid-Atlantic and Northeast for the start of
the new work week. Several weak disturbances will impact the
region late this week. A deep upper trough will build east by
next weekend.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
The earlier-issued Dense fog advisory seems to be working out
with many sites dropping below 1Sm with occasional 1/4 - 1/2 SM
vsbys at times. Lots of low- level moisture, light winds, and a
strong inversion above the surface will deep low clouds and fog
across the area through tonight.

A disturbance aloft is approaching the area from the SW and
rains are slowly spreading towards the area. We have raised pops
across thre s/e areas from the earlier fcsts.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/...
Low clouds and areas of dense fog will begin the day across
much of the area Sunday, with the possible exception across the
southern areas of the CWA where rainfall could be ongoing and
mixing out any fog. The dense fog should begin mixing out some
by 10-11 am, but low clouds and light fog will once again
continue through the day. The aforementioned low pressure
located along the front to our south will weaken and slide
offshore through the day, but the front will remain to our
south. Another disturbance aloft is expected to move across the
area during the day, which could help create some light rain
across the area later during the day.

&&

.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
Low pressure over the Mississippi Valley will slowly build to
the east, through the TN Valley and towards the Mid-Atlantic
Sunday night. Periods of rain will slide to the north and east
throughout the night, some of which could be heavy at times,
especially across the Delmarva and southern NJ.

Conditions get complicated on Monday.

NAM continues to carry a colder solution on Monday compared to
the GFS/ECMWF, and this results in a period of wintry precip,
mainly to the north of I-80, across the Poconos. Since the
column of colder air is a bit deeper compared to previous runs,
will carry a forecast of rain and sleet and will take out the
mention of freezing rain.

Another aspect of not for Monday is that both the 12Z GFS and
the 12Z ECMWF almost have a secondary low forming over the
western Atlantic, and that tracks to the north and east and well
offshore. What this appears to be doing is taking the heavy
rain with it, and as a result, the heaviest of the rain is
falling over the Atlantic waters. NAM continues to have this
area of heavy rain along the coast. For now, expecting the
heaviest of the rain to fall from after midnight Sunday night
through early Monday afternoon. By this time, the surface low
will be over the Mid-Atlantic, and then will begin to move
offshore by Monday evening. The overall big picture of the storm
remains in place, but there are some timing issues that have to
be watched.

65-75 KT LLJ will pass through the region from after midnight
Sunday night through Monday afternoon. This will coincide with
the heaviest rain and the strongest winds. Easterly pressure
gradient tightens, and a 12-18 hour window of strong and
potentially damaging winds are expected during this time. The
strongest winds will be along the NJ coast, with sustained winds
of 25-35 MPH with gusts 50-60 MPH. Along and to the south and
east of I-95, east winds will range from 20-30 MPH with gusts up
to 50 MPH. Back across the Lehigh Valley and into northern NJ,
winds could gust up to 40-45 MPH. High Wind Watches remain in
effect for coastal NJ, and Wind Advisories will likely be needed
for much of central and southern NJ, parts of SE PA, and into
northeast MD and DE.

Persistent onshore flow will also result in widespread minor
tidal flooding and beach erosion. Please refer to the
Tides/Coastal Flooding section of the AFD for more details.

Storm Total Rainfall amounts have trended downward with this
rain based on the tracks of the GFS/ECMWF, with around 0.75-1.5"
likely. But still cannot rule out some areas picking up close
to 3".

Conditions improve late Monday night as the low departs, but
showers could linger into Tuesday as upper low passes through
the region. Conditions finally dry out by Wednesday as high
pressure builds east.

After that, temps remain above normal through at least
Thursday. Cold front comes through on Friday, and temps drop
off, but still could be a few degrees above normal.

Pattern changes comes late in the week and over the weekend as
a deep upper trough builds east, bringing with it a return to
near and possibly below, seasonal temperatures.

&&

.AVIATION /02Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Lots of low-level moisture, light winds, and a strong inversion
above the surface area creating low clouds and fog which has
lead to widespread IFR conditions. The low clouds will likely
continue to lower this evening and tonight, and lead to areas of
dense fog tonight with VSBYS lowering to around 1/2sm or less
at times. The low VSBY will improve through the morning Sunday,
but the low CIGS will remain through the day.

An area of light rain could impact ACY/MIV during the morning,
with additional rainfall possible during the afternoon Sunday
for all areas.

South-southwest winds will become light and variable tonight.
Winds will increase out of the east-northeast by Sunday morning
and continue through the day.

OUTLOOK...
Sunday night through Monday night...IFR/LIFR with periods of
rain. Also, E winds continue to strengthen to 15-20 KT with
25-30 KT gusts at ABE/RDG, 25-30 KT with 35-40 KT along I-95
terminals, and the potential for 40-50 KT gusts near ACY late
Sunday night through Monday afternoon.

Tuesday...Lingering MVFR/IFR conditions with showers for at
least the morning. Gradually improving conditions from SW to NE
later in the day.

Tuesday night and Wednesday...VFR. NW winds 10-15 kt with gusts
to 20 kt.

Thursday...Isolated showers, possibly briefly lowering
conditions to sub-VFR.

&&

.MARINE...
Sub-Small Craft Advisory conditions will continue tonight into
Sunday across the waters. However, areas of dense fog will
continue, and the Dense Fog Advisory remains in place through
Sunday morning.

OUTLOOK...

Sunday night through Monday night...Easterly flow quickly ramps
up Sunday night. Expect gale force gusts to develop overnight
Sunday night and storm-force gusts along the northern and
central coastal waters by early Monday morning. Storm Watches
were converted to Storm Warnings for ANZ450-453, and Gale
Watches were converted to Gale Warnings for ANZ454-455, and DE
Bay. Conditions improve from S to N through Monday evening.

Tuesday...Lingering SCA conditions possible.

Wednesday through Thursday...Brief lull in SCA conditions
possible early Wednesday, and then conditions ramp up late
Wednesday through Thursday.

&&

.HYDROLOGY...
There is a potential for hydro impacts from the upcoming storm
system Sunday through Tuesday. In short, we don`t think there
will be any river flooding unless basin-wide rainfall averages
climb above three inches. The most sensitive responding rivers
still appear to be the Millstone and the Rancocas.

The entire region has some positive things working in its
favor...precipitation has been below normal the last 30 days,
there`s really no consequential snow cover to speak of, there`s
no ice on area waterways, streamflows are either running at or
below normal as is soil moisture, and last but not least, the
ground isn`t frozen. We think all the above will help mitigate
the flood threat.

We feel the latest MMEFS runs are running a bit hot. They show
some impacts with about 2.50 inches of rainfall. But taking into
account the above mentioned items and the fact that the rains
will fall over a 24 to 36 period, we feel more rain will be
needed for river flooding. MARFC in-house contingency runs agree
with this.

As of now, it appears poor drainage or nuisance flooding is
more likely, not flash flooding or river flooding with storm-
total rainfall amounts will generally between 1-3. There
would be more concern for flooding if we start seeing amounts
greater than 3 inches. With some of the wetter outlier model
solutions showing localized amounts near 4, it is still too
soon to rule out more significant flooding.

&&

.TIDES/COASTAL FLOODING...
An onshore flow is forecast to develop along the coasts of
Delaware and New Jersey on Sunday as an initial low passes off
the Middle Atlantic coast. Tide levels should begin to respond
to the onshore flow on Sunday but they will likely remain below
the coastal flooding thresholds at that time.

The onshore flow is expected to strengthen on Sunday night as
another low begins to approach from the southwest and the
pressure gradient tightens. A storm surge along the coasts of
Delaware and New Jersey is anticipated to build to 2 to 3 feet
by the early morning high tide on Monday. Increasing wave action
and the resulting beach erosion will likely compound the
effects of the surge. Waves breaking along the shore may build
to 5 to 8 feet. Widespread minor flooding is forecast to develop
along the coasts of Delaware and New Jersey, on the back bays,
along Delaware Bay and along Raritan Bay. Thankfully, the
astronomical tides will be relatively low on Monday since we are
coming off the last quarter phase of the moon. Nevertheless, a
number of roads will likely flood and become impassable for a
time around high tide.

The strongest of the onshore flow is forecast to occur on
Monday morning. The maximum surge of 3.5 to 4.0 feet is
anticipated to occur around the low tide late on Monday morning.
However, water will likely remain trapped in the back bays of
Delaware and New Jersey at that time.

The onshore flow is expected to begin weakening from south to
north on Monday afternoon as low pressure begins to arrive from
the southwest and the tight pressure gradient ahead of the
system lifts into New England. However, the storm surge is
forecast to remain in the 2.5 to 3.5 foot range along our
coastline around the high tide late on Monday afternoon. As a
result, another round of widespread minor coastal flooding is
expected at that time. There could be pockets of moderate
flooding from Long Beach Island up to Raritan Bay depending upon
the rate that the onshore flow actually begins to lessen.

Lingering minor coastal flooding is again possible for the high
tide early on Tuesday morning being that the flow is not
expected to become fully offshore until the daylight hours on
Tuesday.

Based on the forecast guidance and the expected pattern we are
not anticipating any coastal flooding on the upper eastern shore
of Maryland. Also, the potential for anything more than spotty
minor tidal flooding appears low for the tidal Delaware River
above the Commodore Barry Bridge. There may be some areas of
minor flooding on the portion of the tidal Delaware River that
runs between New Castle County, Delaware and Salem County, New
Jersey especially around Monday morning`s high tide.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Dense Fog Advisory until 10 AM EST Sunday for PAZ054-055-
     060>062-070-071-101>106.
NJ...Dense Fog Advisory until 10 AM EST Sunday for NJZ001-007>010-
     012>019.
     Dense Fog Advisory until 5 AM EST Sunday for NJZ020>027.
     High Wind Watch from late Sunday night through Monday evening
     for NJZ012>014-020-026.
     High Wind Watch from late Sunday night through Monday
     afternoon for NJZ022>025-027.
DE...Dense Fog Advisory until 10 AM EST Sunday for DEZ001.
     Dense Fog Advisory until 5 AM EST Sunday for DEZ002>004.
MD...Dense Fog Advisory until 10 AM EST Sunday for MDZ008.
     Dense Fog Advisory until 5 AM EST Sunday for MDZ012-015-019-
     020.
MARINE...Dense Fog Advisory until 10 AM EST Sunday for ANZ430-431-
     450>455.
     Storm Warning from 1 AM to 4 PM EST Monday for ANZ452-453.
     Storm Warning from 1 AM to 10 PM EST Monday for ANZ450-451.
     Gale Warning from 1 AM to 4 PM EST Monday for ANZ430-431-454-
     455.

&&

$$
Synopsis...MPS
Near Term...Robertson/PO
Short Term...Robertson
Long Term...MPS
Aviation...Robertson/MPS
Marine...Robertson/MPS
Hydrology...
Tides/Coastal Flooding...



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