Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 200118
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
918 PM EDT Sun Mar 19 2017

.SYNOPSIS...
Low pressure southeast of Cape Cod will move farther out to sea
tonight and Monday while high pressure builds over the eastern
seaboard. A weak disturbance will pass through the region Monday
night. A cold front from this system will stall to our south on
Tuesday. A wave of low pressure will track along the boundary. High
pressure builds in from the northwest Wednesday before the center of
the high retreats off the Mid-Atlantic coast by Friday. A warm front
is expected to move into our region Friday into Saturday. The
boundary may start to return back southward as a cold front late in
the weekend.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
Objective surface analysis valid at 00Z indicates low pressure
continuing to pull away east of Cape Cod this evening with
northwesterly flow continuing across the Mid-Atlantic region.
Surface high in the eastern Ohio Valley is making steady
progress eastward in the low`s wake. Weak cold air advection
continues in between the two systems, though this was not
enough to keep temperatures from rising to levels we have not
seen here in quite some time. Guidance was generally too cold
today, and in some cases, way too cold. Most guidance was still
too cold this evening, likely in part owing to overdoing the
effects of the dwindling snowpack. Nevertheless, as the surface
ridge approaches the region tonight, winds will continue to
decrease underneath mostly clear skies, enabling excellent
nocturnal radiational cooling. Temperatures have responded in
some locations, with sharp decreases noted in northwest New
Jersey in particular in the past couple hours. Hourly
temperatures are notoriously tricky to forecast in such a
regime, but earlier adjustments appear to be working out for the
most part. One exception is in more urban areas, but even these
locations are likely to catch up to progged temperatures late
this evening.

Current forecast is in pretty good shape at this point with
forecast lows generally in the low 20s in the southern Poconos
and northwest New Jersey, around 30 in the I-95 corridor, and in
the low to mid 30s in southern Delmarva and the southern New
Jersey coast.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
Spring begins Monday morning at 6:29 AM.

Monday will be a pleasant day across the Delaware Valley with high
pressure in control. Sunny skies early will slowly cloud over late
as the next disturbance approaches late. Overall, a nice day
however. It will start the day chilly, but readings will warm to
close to normal with low 50s south and mid/upper 40s north. Winds
will generally be from the NW or W at 8 to 12 mph during the
day.

&&

.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
A rather flat large-scale pattern over the CONUS to start the period
Monday evening will quickly amplify Midweek with ridge building over
the Rockies and Great Plains as a downstream trough over eastern
Canada digs southward into Great Lakes and Northeast states. This
pattern change will be preceded by a weak disturbance (currently
located over the Intermountain West) passing through the northern
Mid-Atlantic region Monday night. The synoptic pattern then looks
to transition to ridging over the eastern CONUS late in the week
once the upper trough progresses off the east coast. A storm system
over the Southwest U.S. is then forecast to track eastward and
eventually help break down the East Coast ridge by next weekend.

Monday night...The aforementioned shortwave disturbance that passes
through our region is expected to be accompanied by some
precipitation. Yesterday`s model runs were in agreement that the
showers to our west would dissipate as they moved eastward into the
forecast area Monday night. While timing of these showers haven`t
changed much over the past 24 hours, today`s 12Z model runs maintain
these showers a bit longer into the night as they pass through the
region. Accordingly, PoPs have trended upward a bit. QPF ranges from
less than a tenth inch across NE PA/NW NJ to around a quarter inch
in Delmarva. Thermal profiles indicate rain for most of the area
except there is a brief potential for sleet to mix in at the onset
north of I-80.

Tuesday...Subsidence in wake of the Monday night system will provide
us with the opportunity for some sunshine and if that happens, it
could be the first day with above normal highs since March 9th. Our
forecast highs reflect this happening. However, NAM forecast
soundings introduce some skepticism with the potential for stubborn
stratocu with moisture trapped underneath the subsidence inversion.

Tuesday night and Wednesday morning...A strong 1040 mb surface high
over southern Manitoba builds southeastward toward our region. NW
flow around the high signifies a return to winter cold as we
technically transition to astronomical spring. A wave of low
pressure is expected to develop along a west-to-east oriented cold
front to our south. Some of the details still need to be resolved,
including how far north does the precipitation shield expand and how
quickly does the colder air arrive. Our deterministic forecast calls
for rain mixing with or perhaps briefly changing to snow as colder
air progress southward before it ends early Wednesday morning across
our far southern zones (south of Philadelphia). However, it`s much
more practical to think in a probabilistic framework this far out.
Our experimental probabilistic snowfall guidance shows very low
probabilities (about 10 percent) for over an inch of
accumulation in our southern zones.

Wednesday afternoon through Thursday night...Cold and dry conditions
with high pressure building over the region. Temperatures will
struggle to rise during the day on Wednesday as heating if being
offset by CAA. Afternoon highs both days in the 30s N/W of the Fall
Line and 40s S/E are 10-20 degrees below normal.

Friday and Saturday...A warming trend will be in store as high
pressure moves offshore and broad southerly return flow ensues up
the eastern seaboard. The Mid-Atlantic region will be positioned
between a building ridge over the Southeast states and a disturbance
moving around the northern periphery of the ridge. The track of this
shortwave trough will depend on how amplified the ridge becomes, but
it could track far enough south to bring a period of shower to the
region on Friday. Saturday is trending drier with models showing the
next storm system still organizing over the central CONUS. Saturday
also looks to be the warmest day of the period when the region will
be situated well within the warm sector ahead of the next system.
Highs in the 60s will make be reminiscent of February 2017!

Saturday night and Sunday...Models try to develop a rex-type
blocking pattern (to varying degrees of strength) over the Eastern
U.S. as the ridge over eastern North America tries to rebuild while
a closed low gets caught in the weak steering flow residing
underneath. This could lead to an unsettled weather pattern heading
into the second part of the weekend with the upper low nearby. There
is also a potential for wintry precip north of I-78 as strong high
pressure centered over eastern Canada is in a favorable position to
draw colder air southward. Forecast confidence is very low with the
details unclear this far out.

&&

.AVIATION /01Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

For the 00Z TAFs...VFR through Monday afternoon. Mostly clear
skies tonight followed by increasing upper level clouds
tomorrow, with developing CIGs near 10 kft by late afternoon.
NNW winds should decrease below 10 kts tonight slowly becoming
more westerly during the day tomorrow. Winds should generally
remain around 10 kts Monday.

OUTLOOK...

Monday night...A period of MVFR conditions possible with SHRA
likely.

Tuesday...Moisture trapped underneath subsidence inversion may
result in stratocu and MVFR CIGs. NW winds increasing to 10-15 kt by
the afternoon.

Tuesday night and Wednesday...A disturbance passes to our south.
Predominately VFR with the bulk of the precip likely staying south
of the terminals. If forecasts happens to trend northward by about
50 miles, then a period of RA or RA/SN mix would be possible mainly
from PHL southward. NW winds will become breezy on Wednesday with
gusts near 30 kt possible.

Wednesday night through Thursday night...VFR and light winds.

Friday...Mainly VFR but MVFR could develop if showers reach the
area. S-SW winds increase to 10-15 kt with gusts to 25 kt.

&&

.MARINE...
Winds have really dropped off on the coastal waters (well below
small craft advisory criteria), but the seas remain quite
elevated (generally 6-9 feet at this time). Needed to adjust the
wind forecast accordingly, but the advisory continues overnight owing
to the high seas.

OUTLOOK...

Monday night through Tuesday night...No marine hazards anticipated.

Monday and Tuesday...The conditions are expected to be dropping
below Small Craft Advisory criteria Monday, and then remaining below
for Tuesday.

Wednesday...N-NW winds increase as high pressure builds
southeastward. Widespread SCA likely though there is a brief
potential for gale force gusts in concert with rapid
pressure rises and deeper mixing..

Wednesday night through Thursday night....Winds and seas below SCA
criteria.

Friday...Sly winds strengthen to near 20 kt. Marginal mixing
potential with warmer water above the cooler waters but cannot rule
out the potential need for a SCA.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until noon EDT Monday for ANZ450>455.

&&

$$
Synopsis...Klein
Near Term...CMS
Short Term...O`Hara
Long Term...Klein
Aviation...CMS/Klein/O`Hara
Marine...CMS/Klein/O`Hara



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