Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 200730
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
330 AM EDT Thu Apr 20 2017

.SYNOPSIS...
A front settles southward into portions of our area today and
tonight, and may linger across our northern areas on Friday. Low
pressure in the Great Lakes today into tonight will track across
southeastern Canada during Friday. An associated cold front will
move through our region later Friday. Another low will track from
the Southern Plains to the Southeastern States through the weekend.
High pressure then builds into our region from the north Tuesday and
Wednesday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Early this morning...showers developing nicely esewd along and
north of I-78 where widespread measurable occurs prior to
sunrise. The southern extent of the measurable by 12z is less
certain. Temps have started rising since midnight in the weak
waa light southerly sfc flow.

Today...scattered morning showers, probably most active near
and south of I78 to vcnty ILG-ACY line which includes much of
se PA and s central NJ. Showers end by midday with breaks in
the clouds during the afternoon and warmer temps accompany the
brightening sky. Highs 5 to 10 degrees above normal. Light wind
should remain less than 12 MPH. The direction is forecast to be
variable in the morning with an onshore component developing in
the afternoon.

Marginal instability is expected to develop from the Susquehanna
River Valley of Pennsylvania down into the Chesapeake Bay region.
As a result, there is a chance that showers and thunderstorms
will develop out there late this afternoon, possibly advancing
into the furthest western and southwestern edge of our forecast
area around 6 PM.

FCST basis a 50 50 blend of the 00z/20 GFS/NAM MOS with mesoscale
coastal waters cooling blended in using the latest rgem nam
hrrr 2m temps.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/...
Any convective elements early should weaken ewd across e PA
this even. A more substantial batch of showers and isolated
tstms should advance ewd from the Ohio Valley into our area late
at night...mainly PA/NJ. With PWAT near 1.4"...thunder related
elements could produce heavy rain. Light wind. Abnormally mild
tonight with lows 13 to 17 degrees above normal.

FCST basis was a 50 50 blend of the 00z/20 GFS/NAM MOS.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Summary...A surge of much warmer air for much of the area Friday;
turning cooler over the weekend and unsettled especially Sunday;
warming then during the first half of next week.

Synoptic Setup...An upper-level trough moves from the Great Lakes to
New England Friday and Saturday, although it weakens. A secondary
short wave sharpens a trough in the Central Plains Friday night into
Saturday, which then moves eastward to the Southeastern States
Sunday night and Monday. This feature exits into the western
Atlantic early next. Some ridging tries to build into the East
during the first half of next week, as a broad upper-level trough
with several embedded short waves is from the Northern Plains and
Midwest to the Great Lakes. This results a couple of systems moving
through, the first in the form of a cold front later Friday. The
second feature is stronger with plenty of moisture and lift with it
Sunday , however trends are for this to be farther south. The
pattern may then favor calmer conditions early next week. We used a
model/continuity blend for Friday through Saturday night, then
blended in the 00z WPC Guidance. Some adjustments were then made
following additional collaboration with our neighboring offices.

For Friday...A sharp upper-level trough moving across the Great
Lakes and toward the Northeast is forecast to gradually weaken.
Surface low pressure however will continue to translate eastward,
sliding by to our north through the day and into the night. Surface
high pressure up across the Canadian Maritimes may still be extended
southward toward our coast with a frontal boundary draped in the
vicinity of our northern areas. If this boundary remains in place,
cooler air will be found across our northern areas. A cold front is
then forecast to move through in the afternoon or evening, and much
of the guidance has slowed this down. The slower passage of the cold
front would usually mean a greater chance for convection as it
arrives during peak heating, however much of the forcing still looks
to slide by to our north with perhaps a split. Much of the guidance
however does show enough convergence with the front and some
increase in the instability to initiate new convection in the
afternoon and early evening mainly from I-95 on south and east. It
will be a race though between developing instability and drying
arriving from the northwest. Looks like any convection would be
scattered at this point and then it shifts offshore during the
evening. Therefore, kept PoPs a bit higher to start with leftover
Thursday night activity, then again in the afternoon and evening
with potential for renewed development. The instability is not
forecast to be all that robust and shear is on the weaker side (a
bit stronger north compared to the south). As a result, any
thunderstorms should be sub-severe.

Some clearing should take place during Friday afternoon as some
drying aloft works in. The main cold air advection is delayed
especially at and below 850 mb and many areas will be into 70s to
lower 80s. Given the prospects of the aforementioned boundary across
the northern areas though, trended high temperatures a bit cooler
here.

For Saturday and Sunday...The cold front from later Friday looks to
stall to our south. As energy slides eastward, a surface low tracks
into the Tennessee Valley Saturday afternoon then to the Carolinas
Sunday morning. A surface trough should extend north-northeast from
this low. There still remains some differences among the guidance
with the northern extent of the associated rain shield. Some of the
farther north models have shifted southward, and this looks to be
due to surface high pressure building southward quicker and also a
weaker upper-level trough axis. Overall, Saturday should feature
more clouds than sunshine however the rain should hold off until
later at night for the southern areas.

The surface low is forecast to track well to our south Sunday and
Sunday night, however a ribbon of enhanced lift should result in an
areas of rain especially across the southern half of our area. The
PoPs were adjusted to show a slower increase Saturday into Sunday,
and were lowered to chance across the far north. Given the model
differences, did not want to make significant changes at this time.
The prospects of lots of clouds and areas of rain with an onshore
flow will result in a much cooler day Sunday. The system should
shift offshore Sunday night with some gradual improvement.

For Monday and Tuesday...A trough that may become closed off for a
time across the Southeastern States should slowly shift into the
western Atlantic during this time frame. This will take surface low
pressure with it, however we will have to monitor if any
precipitation can back in from the ocean. We favored some low PoPs
as a result mainly for the eastern zones. Meanwhile, high pressure
is forecast to build down from eastern Canada and combined with low
pressure to our south and east will produce a northeasterly low-
level flow. This will increase at times with it most noticeable
closer to the coast, which is where it will be cooler and more
clouds should be around. High temperatures each day may be very
similar.

For Wednesday...A broad upper-level trough in the Northern Plains,
Midwest to the Great Lakes will result in low pressures tracking
well to our west. The first one lifts into Canada north of the Great
Lakes Wednesday with a trailing cold front. Given the idea of the
trough hanging back to the west longer, this cold front is currently
forecast to remain well to our west with high pressure building down
the East Coast. This will result in warming, although again cooler
closer to the coast with an onshore wind.

&&

.AVIATION /08Z THURSDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Through 12z Today: Widespread MVFR cigs may lower to IFR cigs
by daybreak. scattered showers vcnty KRDG- KABE- KTTN.

After 12z Today...Cigs slowly lifting to VFR by 17z. scattered
showers this morning. please see TAF for shower timing details.
Light wind.

Tonight...VFR cigs to start could become IFR in stratus/fog
after 08z/21. scattered showers late with a possible
thunderstorm with heavy rain. Light wind.

OUTLOOK...
Friday...MVFR/IFR ceilings (possible visibilities) in the morning
should improve to VFR ceilings around 5000 feet from about Midday
on. Some showers or a thunderstorm should move through early, then
some isolated to scattered showers/thunderstorms in the afternoon
and evening especially from TTN, PNE/PHL to ILG on eastward. Brief
MVFR/IFR conditions with any heavier showers or a thunderstorm.
Mainly southwest winds 5-10 knots, becoming northwest 10-15 knots in
the afternoon or early evening as a cold front moves through.
Improving conditions Friday night in the wake of the cold front.

Saturday and Sunday...VFR during the day Saturday with ceilings
gradually lowering. The conditions then lower to MVFR/IFR with
periods of rain and potentially fog late Saturday night and
especially Sunday. The greatest chance for widespread rain appears
to be from near the PHL metro area on southward. Northwest to north
winds 10 knots or less Saturday, becoming northeast 10-15 knots
Saturday night and especially Sunday (some gustiness near the
coast). The conditions should improve later Sunday night.

Monday...VFR overall, however a few showers are possible mainly in
the area of MIV/ACY. Northeasterly winds 5-15 knots with some gusts
near the coast.

&&

.MARINE...
No marine headlines anticipated through Tonight. Wind speeds
are forecast to remain below 20 knots and wave heights will
generally remain less than 5 feet.

The wind direction over our waters will favor the southeast to
southwest quadrant.

OUTLOOK...
Friday...The conditions are anticipated to be below Small Craft
Advisory criteria. The winds will be shifting around as a cold front
moves through in the afternoon or evening. Some showers and
thunderstorms are possible especially in the afternoon and early
evening.

Saturday and Sunday. The conditions should be below Small Craft
Advisory criteria Saturday, then winds and seas increase Sunday as a
northeasterly flow strengthens.

Monday...While wind gusts may lower to below 25 knots, seas may
build some more as a northeasterly flow continues.

&&

.CLIMATE...
This section will update around 5am with a quick check on the
overall monthly expectations. No change from the early
Wednesday discussion below.

April will probably be a top 10 warmest in the period of record
for multiple long term climate stations and depending on the
warmth of this Thursday-Friday and how cold it is on Sunday when
there is a potential chilly washout for at least part of the
day...we could be heading for a top 2 warmest april on record.

The usual analysis process first 18 days done, the 19th-25th
per our 330 am fcst, the 26-29th from FTPRHA and the last day,
normal max/min for the date.

PHL projects #2, 4.9 above normal and 0.5degs from record. POR 1874
ABE projects #2, 5.9 above normal and 0.6degs from record. POR 1922
ACY projects rer 5.0 above normal,0.4F above prev 2010 56.3 POR 1874

So, there could be some slippage but its unlikely that any of
these locations can slip below #10.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...Gorse
Near Term...Drag
Short Term...Drag
Long Term...Gorse
Aviation...Drag/Gorse
Marine...Drag/Gorse
Climate...Drag



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