Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 222146
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
446 PM EST Sun Jan 22 2017

.SYNOPSIS...
Low pressure over the Gulf Coast will drift north and east
tonight and will be over the Mid-Atlantic by Monday afternoon.
This low will then meander off the New Jersey coastline Monday
night before departing late Tuesday. Weak high pressure builds
across the region on Wednesday. Low pressure passes north of the
area Wednesday night, and then a deep upper trough builds into
the Eastern United States for the weekend.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
Several vorticity impulses will move across the area tonight
within the southwest flow aloft as a large closed low begins to
move northeastward out of the southern gulf states and into the
southeast states. Low clouds and fog continue this afternoon and
evening, and may continue into the night. In addition, periods
of light rain will move across the area through this evening and
overnight as well. Temperatures are forecast to stay steady, or
slowly fall through the night. However, any wintry
precipitation is not expected across the far northern areas
until during the day Monday.

Winds are generally out of the east to northeast around 5 to 10
mph this afternoon and evening. However, as the surface low to
our south begins to lift northward overnight, the pressure
gradient begins to increase across the area, and winds will
begin gusting 25-35 overnight and into early Monday morning.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
The strong low pressure system to our south will continue to
move northward toward the Mid-Atlantic through Monday. As it
moves into the Mid-Atlantic, a secondary low is forecast to
develop near the Virginia/North Carolina coasts, before
intensifying and taking over as the main low late in the day. A
strong pressure gradient will remain across the area, and winds
are expected to increase across our entire forecast area. Winds
of 40-50 mph are expected everywhere except coastal New Jersey,
where 55 to 60 mph winds could develop. A Wind Advisory is in
effect for all areas except coastal New Jersey where a High Wind
Warning is in effect.

Periods of rain will continue across the area as well during
the day Monday as several vorticity impulses will move across
the area as they swing around the closed low that will be
located to our south. Periods of moderate to even heavy rainfall
could develop during the day, especially across portions of
eastern New Jersey. A Flood Watch is in effect for a portion of
eastern New Jersey starting late in the afternoon.

Another concern during the day Monday will be wintry
precipitation across Carbon and Monroe, Pennsylvania and Sussex,
New Jersey. Temperatures are expected to continue to lower
through the day, and fall below freezing across portions of
these counties, especially the higher elevations. Therefore, a
wintry mix of snow, sleet, and freezing rain could develop
during the day Monday. A Winter Storm Watch is in effect
starting midday Monday and continuing through the day.

&&

.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
Surface low pressure will move into the western Atlantic waters
Monday evening, where it will become nearly stationary due to a
blocking high over eastern Canada and northern New England.
Meanwhile, 65-75 KT LLJ will slide along the coast Monday
evening, departing by daybreak Tuesday.

This complex system will continue to have significant impacts
for the region.

Heavy rain: Persistent and deep onshore flow will usher
abundant low level moisture into the region with PWATs over an
inch. Meanwhile, approach of the aforementioned LLJ will result
in moderate to heavy rain developing across eastern NJ. Both the
NAM and the GFS indicating an area of frontogenesis along the
NJ shore, corresponding to where the heaviest rain is expected.
Between 1 and 3 inches of rain is possible Monday afternoon and
Monday night, with locally higher amounts possibly up to 4
inches. With the low slow to depart, there could be some
lingering bands of precip into Tuesday morning, but the
departure of the LLJ, that is not likely. Flood watch has been
issued for much of northeast NJ, but think greatest effects will
be right along the shores.

Winds: Tight easterly pressure gradient forms between the two
aforementioned systems. Combined with the LLJ, the strongest of
the winds will linger across parts of northeast NJ Monday
evening, before diminishing Monday night. Most Wind Advisories
and High Wind Warnings should be over by midnight Monday night,
but the pressure gradient should be strong enough to result in
20-30 MPH wind gusts into much of NJ, parts of southeast PA, and
possibly into DE. Winds diminish by daybreak Tuesday.

Coastal Flooding: Please refer to Coastal Flooding section of
the AFD below.

Wintry Precip: NAM continues to be the coldest solution, but
there are some signals from the GFS that enough cold air will
wrap around the back side of the low to allow for a wintry mix
Monday night. Enough warm air aloft will mix into the region
with surface temps falling below freezing across Carbon, Monroe,
and Sussex counties to allow for freezing rain, snow, and sleet
Monday evening, and then that will change to freezing rain and
sleet going through Monday night. For those areas, a Winter
Storm Watch has been issued for the wintry weather that will
impact not only Monday, but Monday night as well. Going back
into the Lehigh Valley and into northern NJ, rain should mix
with sleet. There is the potential for sleet to spread back
farther into southern NJ and into southeast PA, but for now,
will keep the frozen precip confined to northern areas. An
additional 1-3 inches of snow is possible in the Watch area, as
well as between 1/10 and 2/10 inch of additional ice.

From Tuesday on:

Although surface low departs Tuesday morning, upper level low
will lag behind. With several strong shortwaves wrapping around
this low, can expect another shot of precip to impact the region
through Tuesday afternoon. precip should be rain for the bulk
of the area, but a wintry mix is possible for Carbon and Monroe
counties.

Conditions dry out Tuesday night as high pressure builds into
the region and moves offshore during the day Wednesday.

Temps will run a few degrees above normal on Tuesday, generally
in the upper 30s to mid 40s, while a return to above normal
temperatures expected on Wednesday with highs in the 40s and
50s.

Low pressure will organize over the Great Lakes as a deep upper
trough builds into the western U.S. This portends a pattern
change for the end of the week and weekend. Cold front pushes
through the region Wednesday night. For now, expecting dry
conditions. However, GFS is indicating an area of precip
developing offshore with the passage of the front. It is
possible that more organized precip can develop, but models
currently keep it offshore, so will go with a mainly dry
forecast with its passage.

Colder air then builds into the region for the end of the week
and weekend with a return to more normal temperatures. Highs
will fall into the 20s and 30s over the weekend.

&&

.AVIATION /22Z SUNDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

IFR conditions continue through the period with low CIGS and
VSBYS. Periods of light rain, drizzle, and/or fog will continue
into this evening and overnight. More steady rainfall is
expected to move across the TAF sites starting Monday morning
and continuing through the day.

Winds generally out of the east to northeast around 5 to 10
knots will continue into this evening, before they begin to
increase overnight. Wind are expected to become gusty later in
the night and into daybreak Monday, then continue to increase
through the day Monday. Gusts 30-40 knots are possible for most
areas, with stronger gusts closer to the coast.

OUTLOOK...
Monday night...Strong winds and heavy rain with IFR/LIFR
conditions expected for NJ terminals E of I-95 Monday evening.
For the rest of the terminals, winds should diminish to 15-20 KT
with gusts up to 30 KT Monday evening as winds turn to the N.
Some wintry precip possible for KABE/KRDG.

Tuesday...Lingering IFR conditions with rain continuing through
the day. Sleet or snow may briefly mix in at ABE before ending.
Gradually improving to MVFR and then VFR from S to N late 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.

Friday...VFR. West winds 10-20 KT.

&&

.MARINE...
The Dense Fog Advisory was extended through the evening hours
as there is still expected to be fog across the waters. Near
term guidance indicates it should begin dissipating through the
evening hours.

Storm and Gale Warnings remain in place across the waters. A
strong low pressure system will move toward the area overnight
and through Monday, increasing the pressure gradient across the
area. Strong winds are expected to develop overnight tonight and
continue through the day Monday.

OUTLOOK...
Monday night...East flow becomes NE. Sustained winds 20-30 KT
with 35-45 KT gusts will linger over northern ocean waters, and
then winds diminish a bit by daybreak Tuesday. For all other
ocean waters and DE Bay, winds diminish to 15-20 KT with 20-30
KT gusts.

Tuesday...Strong SCA conditions expected, with another round of
Gales possible on the ocean waters Tuesday afternoon.

Wednesday...Sub-SCA conditions expected.

Thursday through Friday...Wind gusts 25-30 KT possible.

&&

.HYDROLOGY...
Flood Watch has been issued for most of NE NJ. Impacts will be
due to excessive rainfall, as between 1-3 inches of rain with
locally higher amounts, possibly up to 4 inches, is expected
Monday afternoon and Monday evening.

River flooding should be limited 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.

&&

.TIDES/COASTAL FLOODING...
The Coastal Flood Advisory has been extended to cover both high
tides on Monday. It is in effect for the coastal areas of New
Jersey and Delaware, Delaware Bay and the tidal Delaware River
below the Commodore Barry Bridge area.

The onshore flow is expected to strengthen tonight 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 2.0 to 2.5 feet by the
early morning high tide on Monday. Areas of minor flooding are
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 may flood and become
impassable for a time around high tide.

The strongest of the onshore flow is forecast to occur on
Monday. The maximum surge of 3.5 to 4.0 feet is anticipated to
occur during the afternoon and perhaps into the early evening
hours. 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. The expected surge
along with the wave action and the forecast rainfall will likely
result in additional and more widespread minor flooding around
the high tide from late Monday afternoon into Monday evening.
The impacts are anticipated to be greater than those on Monday
morning. Also, there could be pockets of moderate flooding from
Long Beach Island up to Raritan Bay.

The onshore flow is expected to begin weakening from south to
north on Monday night as low pressure passes off the Middle
Atlantic coast and the tight pressure gradient ahead of the
system lifts into New England. However, 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 Tuesday afternoon.

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.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Wind Advisory from 5 AM to 4 PM EST Monday for PAZ060>062-070-
     071-101>106.
     Winter Storm Watch from Monday afternoon through Tuesday
     morning for PAZ054-055.
NJ...Flood Watch from Monday afternoon through late Monday night
     for NJZ008-010-012>015-020-026.
     Wind Advisory from 5 AM to 7 PM EST Monday for NJZ007>010-015.
     Wind Advisory from 3 AM to 4 PM EST Monday for NJZ016-021.
     Coastal Flood Advisory from 3 AM to 9 PM EST Monday for
     NJZ012>014-020>027.
     High Wind Warning from 3 AM to 9 PM EST Monday for NJZ012>014-
     020-026.
     High Wind Warning from 3 AM to 4 PM EST Monday for NJZ022>025-
     027.
     Coastal Flood Advisory from 5 AM to 10 PM EST Monday for
     NJZ016.
     Wind Advisory from 5 AM to 4 PM EST Monday for NJZ017>019.
     Winter Storm Watch from Monday afternoon through Tuesday
     morning for NJZ001.
DE...Wind Advisory from 3 AM to 4 PM EST Monday for DEZ001>004.
     Coastal Flood Advisory from 3 AM to 9 PM EST Monday for
     DEZ002>004.
     Coastal Flood Advisory from 5 AM to 10 PM EST Monday for
     DEZ001.
MD...Wind Advisory from 3 AM to 4 PM EST Monday for MDZ008-012-015-
     019-020.
MARINE...Dense Fog Advisory until 1 AM EST Monday 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 7 PM EST Monday for ANZ430-431-454-
     455.

&&

$$

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



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