Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 162023
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
423 PM EDT Mon Apr 16 2018

.SYNOPSIS...
Low pressure was located over northeastern Pennsylvania this
afternoon. A cold front will move off the Mid-Atlantic coast
this evening as the low lifts into upstate New York. The low
will stall over southern Quebec late tonight through Tuesday
night. Another area of low pressure organizing over the Midwest
will track eastward into the Northeast U.S. Wednesday night and
Thursday. Meanwhile, a warm front with this system will try to
move northward into our region before the trailing cold front
moves through late Thursday. High pressure gradually builds into
the region late this week and into the weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
An area of low pressure moving across northeastern Pennsylvania will
continue lifting northward tonight, before combining with another
low pressure moving out of the Great Lakes region well to our north.
There will continue to be some scattered showers on the backside of
this low into the late afternoon and evening hours as the mid-level
trough and associated short waves/vorticity impulses move into the
area. The showers will not occur everywhere. However, most shower
activity will dissipate later this evening as the sun sets. The two
areas that have the better chance of showers this evening will be
northeastern Pennsylvania and northern New Jersey which will be
closer to the departing low, as well as southern portions of
Delmarva which will be closer to the base of the short wave/vorticity
impulse. Temperatures will be cooling off through the night, so
if any showers to continue across the higher elevations, some
snow could mix in at times later this evening and overnight.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...
The aforementioned low to our north will continue to spin across
southeastern Canada through the day Tuesday, which will keep our
area under west to northwest flow. With this northwest flow, we
expect cloud cover to remain across much of the area through much of
the day. The weather should start out dry, and likely remain dry for
most places. However, as the day progresses, low-mid level lapse
rates increase, while there remains some enhanced low-mid level
moisture and a short wave/vorticity impulse moves across the area as
well. This is forecast to lead to a chance of showers by the
afternoon. Temperatures are forecast to warm to make all
precipitation rain, except possibly the highest elevations across
the Poconos where some snow showers may occur.

Otherwise, a partly to mostly cloudy day, with winds becoming gusty
later in the morning into the afternoon can be expected.

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
Tuesday night...A upper low will remain cutoff over the Saint
Lawrence Valley with the northern Mid-Atlantic region positioned
near the base of the trough. Westerly flow will maintain an
seasonably cold airmass across our region. Isolated rain/snow
showers (snow for the higher elevations) possible during the
evening with the passage of an upper shortwave disturbance,
especially along and north of I-80. Temperatures will generally fall
into the mid 30s overnight (slightly cooler in the southern Poconos
and warmer in the cities and near the coast). Do not anticipate the
need for Frost/Freeze headlines at the moment with lows forecast to
be just above freezing in areas where the growing season has begun
in SE PA, C/S NJ and Delmarva. Additionally, a 5-15 mph westerly
wind overnight will not be favorable for frost formation.

Wednesday...The cutoff low to our north will finally weaken and
eject downstream as a kicker system digs around the backside of the
trough. This upstream low will track eastward across the Midwest on
Wednesday. Strengthening southwesterly flow ahead of the low will
help advect draw a warm front northeastward toward our region during
the afternoon. There is still considerable uncertainty regarding if
the warm front makes it into our region. Given a lack of thick
stratus and some type of CAD wedge north of the boundary, the
forecast reflects the warm front reaching the Delmarva. Highs will
likely be in the mid 60s south of the front and only low to mid 50s
to the north.

Wednesday night and Thursday...Low pressure tracks eastward into
the Northeast states. Models are still in disagreement regarding the
track of the low with one camp of solutions favoring a track to our
north across upstate NY and southern New England while others take
the low right across the middle of the forecast area. Temperatures
will depend on the exact track of the low with temperatures
generally in the 50s north of the low and attendant warm front and
60s to the south. Regardless, temperatures should start to fall
during the afternoon on Thursday with strong occurring CAA behind a
cold front. PoPs were limited to likely north of I-80, to slight
chance in the Delaware Bay region and to chance everywhere in
between due to uncertainty in the low track.

Thursday night...Low pressure is expected to move offshore. An
isolated shower or high-elevation snow shower is possible during the
evening. Northwesterly flow will advect colder/drier air into the
region, aiding the keeping the boundary layer mixed during the
night. Breezy winds gusting to 25-35 mph are expected. Forecast lows
range from around freezing at Mount Pocono to around 40 degrees in
the I-95 corridor and coastal plain.

Friday: Surface low will lift into the Canadian Maritimes, but the
cold/deep trough aloft will remain over the Northeast. Additionally,
with a surface high building into the Midwest/Ohio Valley during the
day, the surface pressure gradient across the Mid-Atlantic should
remain elevated, allowing for breezy northwest winds to continue
through the day. Kept highs well below seasonal averages, and
increased winds/gusts given the expected surface/upper-level
pattern. Finally, there are some indications that weak perturbations
in the northwest flow will move through the northern Mid-Atlantic
and Northeast, and this may be enough to generate some sprinkles or
light showers in eastern PA and northern NJ. Added some slight-
chance PoPs to the grids for this period.

Friday night and Saturday: CMC/ECMWF seem to indicate that the
trough axis will move to the east of the region during this period,
but the 12Z GFS indicates an upstream perturbation that is
considerably stronger and slower. So far, this solution looks
somewhat anomalous, so I was not inclined to be more pessimistic in
the northern CWA on Saturday (when the GFS-simulated vort max
glances the area). However, will keep an eye on this, as it may
require another day of slight-chance PoPs for light showers. With
northwest midlevel flow remaining (and resultant relatively low
heights), Friday night and Saturday will likely remain below
seasonal averages temp-wise. Winds will be diminishing Friday night,
so expect temperatures to be a little bit colder than the previous
night, but Saturday may be a little warmer than Friday as the trough
is anticipated to begin its move out of the area.

Saturday night through Sunday night: Models show another weak wave
in the northwest flow moving through the region during this period,
but are in quite a bit of disagreement regarding the precipitation
this perturbation generates. ECMWF appears to be the most
aggressive, but the GFS is shunted well south (in closer proximity
to an approaching upper-level low). The CMC generates little
precipitation with the northwest-flow vort max, suggesting a more
moisture-starved system. That seems a little suspect given the
fairly potent look to the vort max and the implications on synoptic-
scale lift. Do think the best chances for showers are near/south of
the Mason-Dixon Line, where slight-chance to chance PoPs reside
Sunday and Sunday night. Expecting a slight warmup during this
period given the development of light return flow and subtle warm
advection during this period; however, kept max temperatures on the
low side of consensus given potential for increased cloud cover and
precipitation.

Monday: Models begin to diverge with the upper low moving into the
Southeast, but all seem to indicate the Mid-Atlantic will be on the
upstream side of a surface high. Even so, I am not sold on the dry
look of the models with the approach of a system into the eastern
U.S., particularly given the large discrepancies in the simulations.
Kept some mention of slight-chance PoPs this period (though my
suspicion is that Monday will be dry). Whatever pattern ends up
occurring, it seems that heights/thicknesses will be somewhat higher
than previous days, so nudged temperatures a little bit upward from
Sunday.

&&

.AVIATION /20Z MONDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Tonight...CIGS vary between MVFR and VFR this afternoon. All sites
are expected to lift to VFR by this evening and remain VFR through
the overnight hours. There is a chance for some scattered showers
across the area, but none are expected to significantly reduce
visibilities at this time. Winds have shifted at all TAF locations to
southwest or west now that the low has lifted to our north, and
remain gusty this afternoon. The gusts will diminish later this
afternoon into this evening, but a steady breeze is expected
through the night.

Monday...After a VFR start, CIGS are expected to lower to MVFR for
at least ABE and RDG. It is possible the remainder of the TAFS may
approach MVFR, but we have kept them VFR at this time. Winds will be
gusty again Tuesday with gusts reaching 20-25 knots out of the west
again, occasionally gusts up to 30 knots will be possible.

Outlook...

Tuesday night and Wednesday...VFR. W winds 5-15 kt gusting 20-25
kt.F Forecast confidence:  High

Wednesday night and Thursday...There is high uncertainty
regarding how far north the warm front makes it which is
important for aviation as the greater chance for MVFR or even
briefly IFR conditions and rain will reside north of the
boundary. Forecast confidence:  Low

Thursday night...Improving to VFR everywhere. NW winds will
become gusty. Forecast confidence:  High

Friday: Predominantly VFR, though there may be some sprinkles or
light showers northwest of PHL. Northwest winds 10 to 15 kts with
gusts to 25 kts. Moderate confidence.

Friday night and Saturday: VFR with northwest winds diminishing.
Moderate confidence.


&&

.MARINE...
We have cancelled the Gale Warning for the southern New Jersey and
Delaware Atlantic coastal waters as well as Delaware Bay, and
replaced them with a Small Craft Advisory. There will likely be a
lull in winds later this evening, but the winds are expected to
increase again later in the overnight and continue through the day
Tuesday.

For the northern New Jersey Atlantic coastal waters, we have
continued the Gale warning through 7 pm at this time. We expect
there could be a surge of winds just before sunset, even through
winds are currently below gale force. Once the Gale Warnings is
cancelled, we will need a Small Craft Advisory through Tuesday here
as well.

Even without the wind gusts, seas will remain elevated on the ocean
through tonight and into Tuesday.

Outlook...

Tuesday night...SCA was issued for the DE Bay and the S NJ and DE
coastal waters with W winds 15-20 kt gusting 25-30 kt. This SCA will
eventually need to be expanded up the coast to include the rest of
the NJ coastal waters once the current Gale Warning expires.

Wednesday and Wednesday night...SCA conditions may linger into the
first part of the morning with sub-SCA conditions to follow.

Thursday and Thursday night...SCA conditions likely, first with SWly
flow ahead of a cold front on Thursday, then with W-NW winds behind
the fropa Thursday night. There is a brief potential for gales
immediately behind the front Thursday night.

Friday: Advisory-level northwest winds are likely, but conditions
should be dry.

Friday night and Saturday: Sub-advisory winds/seas expected.


&&

.TIDES/COASTAL FLOODING...
245 PM Update...Upgraded the tidal portion of the Delaware
River (including Philadelphia and Trenton) to a Coastal Flood
Warning as positive tidal departures that were initially in the
3-4 ft range this morning have been slow to decrease this
afternoon likely due to a slight delay in the cold fropa and
associated wind shift from southerly to westerly. The
Philadelphia tidal gauge had already reached moderate criteria
just after 2 PM. High tide at Philadelphia will occur in the
next few minutes but will take about 60-80 minutes later to
occur farther upstream toward Trenton.

130 PM Update: Coastal Flood Warnings have been allowed to
expire for coastal NJ and DE. Most tidal gauges captured minor
tidal flooding. However, the severity of flooding was
exacerbated by freshwater runoff from the heavy rain that
occurred with the morning high tide.

The Coastal Flood Warning for the upper Delaware Bay (New
Castle, DE and Salem, NJ counties) continues through 2 PM. Reedy
Point peaked at only one-tenth inch below moderate threshold
with the midday high tide.

A Coastal Flood Advisory was issued for the eastern and
northeastern shore of the Maryland Chesapeake Bay for this
afternoon and evening. Positive tidal anomalies about both
Tolchester and Cambridge tidal gauges have been increasing
steadily over the past few hours and has not been particularly
well forecast by models. Expect these surge values to remain
steady or even start to decrease heading toward the next high
tide (just after 5 PM at Cambridge and just after 8 PM at
Tolchester Beach) in conjunction with a wind shift from Sly to
Wly.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Coastal Flood Warning until 6 PM EDT this evening for PAZ070-
     071-106.
NJ...Coastal Flood Warning until 6 PM EDT this evening for NJZ015-
     017>019.
DE...None.
MD...Coastal Flood Advisory until 11 PM EDT this evening for MDZ008-
     012.
     Coastal Flood Advisory until 9 PM EDT this evening for MDZ015-
     019-020.
MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ430-431-
     452>455.
     Gale Warning until 7 PM EDT this evening for ANZ450-451.

&&

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


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