Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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931
FXUS61 KPHI 211634
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1234 PM EDT Fri Apr 21 2017

.SYNOPSIS...
A front will remain across our area today. Low pressure tracks
across southeastern Canada later today and its associated cold
front crosses our area late this afternoon into this evening.
This front then stalls to our south over the weekend as low
pressure tracks along it. This low will remain off the Mid
Atlantic coast through early next week, then a cold front slowly
moves across the eastern Great Lakes region Wednesday and
Thursday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
The low cloud deck will be slow to erode from the west and
southwest as warm air overrides it. Today`s temperature forecast
remains one of low confidence in parts of our region due to the
uncertainty surrounding the cloud cover.

We expect the low clouds to linger all of the day in northern
and central New Jersey with highs there expected to be in the
upper 50s. Much of northeastern Maryland and Delaware will
likely continue to break out of the low clouds with maximum
readings there anticipated to be in the 75 to 85 degree range.
The most uncertain is the temperature forecast for eastern
Pennsylvania, northern Delaware and southern New Jersey where
highs should range anywhere from the lower 60s to the lower 70s
depending upon how much clearing takes place.

Weak low pressure is forecast to develop in the Chesapeake Bay
region late this afternoon as a cold front approaches our region
from the northwest. Conditions will destabilize in northeastern
Maryland and Delaware due to the increasing sunshine and
surface heating. As a result, we will keep the mention of a
chance of showers and thunderstorms in that area.

The wind should remain onshore in northern and central New
Jersey. Where the clouds break, the wind direction is forecast
to become southwest.

Some fog will linger in the elevated terrain of eastern
Pennsylvania and northern New Jersey, and along the coast into
the afternoon.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/...
Patchy fog, otherwise considerable cloudiness. Leftover early
evening showers/iso tstms vcnty s NJ and the Delmarva. At least
partial clearing late.

This part of the fcst was a 50 50 blend of the 00z/21 GFS/NAM
MOS.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Summary...Some wet weather for at least parts of the area over
the weekend; watching an offshore storm Monday through Tuesday;
much warmer air should start to arrive at the end of the week.

Synoptic Setup...A weakening upper-level trough is forecast to
be sliding across the Northeast Saturday. A trailing strong
short wave in the Central Plains shifts eastward Saturday with a
surface low developing just ahead of it. This feature looks to
strengthen and close off as it moves into the Southeastern
States Sunday. This system will then slowly move eastward to off
of the coasts of the Carolinas Monday. This closed low should
then gradually open up and shift northeastward through
Wednesday. The track of this feature will be key in the amount
of rain our area may get Sunday and especially early next week.
The overall trend is for a farther south track along a stalled
baroclinic zone. As we go into the end of next week, significant
short wave energy is forecast to sharpen a trough as it ejects
into the Plains. This will start to allow for downstream
ridging. Despite this, during this time of the year the
potential for backdoor fronts is present. A frontal boundary is
forecast to be to our northwest while high pressure builds east
of James Bay Canada. While currently this front is forecast to
remain out of our area, this evolution will be monitored in the
event stronger high pressure allows for the boundary to slip
farther south. We used a model/continuity blend for Saturday
through Sunday night, then blended in the 00z WPC Guidance.
Several adjustments were then made following additional
collaboration with our neighboring offices.

For Saturday and Sunday...What appeared to be initially a wet
period of time for the region especially Sunday has lessened
overall. The trend in the guidance is for much of the rain to be
farther south, focused mainly in Delmarva for our area. This
depends on an upper- level trough that starts to close off
Sunday as it gets to the Southeastern States. Meanwhile, an
upper-level trough sliding across the Northeast Saturday is
forecast to gradually weaken. There will be a frontal boundary
stalled to our south and a lead short wave Saturday may result
in showers overspreading our southern areas. There is
uncertainty with this, given some timing differences.

Additional energy with some more organized lift may then slide
across the southern half of the area Saturday night with more
showers or a period of rain. Thereafter, surface low pressure
associated with the closing off low in the Southeast States
tracks rather far south. This appears to place much of the
convergence and associated lift south of nearly our entire area
Sunday. Some overrunning may occur though with showers for
portions of Maryland, Delaware and far southern New Jersey,
however farther north Sunday may end up being dry as high
pressure builds into southeastern New New England or just
offshore. We note that some guidance lowers the surface dew
points over the weekend from north to south, indicating a
stronger push from northern high pressure.

As a result of the above including collaboration with WPC and
our neighboring offices, we made significant changes to the PoPs
for the weekend. There is a tighter gradient from south to
north with any period of high PoPs now across parts of the
southern zones. However, did keep a swath of chance PoPs for a
time Saturday as a lead impulse may produce a corridor of
showers. Kept these PoPs no higher than chance given low model
agreement. We also changed the weather grids to showers from
rain. There may be enough subsidence Sunday afternoon for some
breaks of sunshine especially up north, then we await to see how
the surface low well to our south tracks for Sunday night and
thereafter. The onshore flow influence looks to be delayed until
mostly later Sunday however the airmass looks to be cooler
overall.

For Monday and Tuesday...The closed low in the Southeast States
is forecast to gradually lift east and even northeastward. This
will drive surface low pressure across the far western
Atlantic, however some guidance now brings this closer to the
coast. The trend in the model guidance is for wetter conditions
now for much of this time frame. Additionally, a strengthening
onshore flow may lead to minor coastal flooding concerns during
mainly Tuesday`s high tide (new moon occurs on Tuesday). Given
the uncertainty, will leave a mention out of the hazardous
weather outlook. This is a dynamic and fluid setup especially
when dealing with closed lows, and for now did increase PoPs for
much of this time frame (especially Monday) with a chance PoPs
everywhere and even a period of low likely closer to the coast.
This more unsettled weather with an onshore flow will result in
cooler conditions.

For Wednesday and Thursday...An upper-level trough is forecast
to amplify as it ejects into the Plains. This will start to
build downstream ridging. Meanwhile, the closed low near the
East Coast should be opening up some as it moves away Wednesday.
This results in a loose pressure gradient along with a warming
airmass. While it should be turning much warmer, it still looks
to be cooler along the coast as there should be a sea/bay breeze
influence especially Wednesday. A frontal boundary is forecast
to be stretched from the Plains to the eastern Great Lakes then
just north of New England. This front should remain there as a
ridge builds, however given this time of year one must watch
surface high pressure building across eastern Canada. This could
push the surface front farther south if strong enough opening
the door for a cooling affect from the ocean. As of now, looks
like a warmer airmass will dominate overall. After some
lingering slight chance PoPs Wednesday, we went dry thereafter.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...IFR and MVFR conds in st/fog. Conditions improving
only slightly this afternoon. Right now for this afternoon we
think cigs 1000-2000 ft but there will be some cigs below 1000
ft. Mostly east wind 5 to 15 kt.

Tonight...MVFR or IFR cigs for a time during the early night
should improve late at night. Patchy IFR fog possible. Wind
becoming light north.

OUTLOOK...
Saturday and Sunday...Low confidence with timing and details.
VFR may dominate much of this time frame, however an area of
showers Saturday probably from about RDG to PNE to MJX southward
could bring times of MVFR/IFR conditions. An area of additional
showers Saturday night into Sunday mostly south of a ILG to ACY
line could also result in periods of lower conditions.

Monday and Tuesday...Potential for widespread MVFR/IFR
conditions with showers or periods of rain as a storm system
parallels the East Coast. Confidence on the low side given
uncertainty with the track of the storm system.

&&

.MARINE...
No headlines at this time. Winds easterly with a more southerly
component for a time Cape May southward during early afternoon
before winds everywhere turn to northeast late today or this
evening and then northerly tonight

Gusts near or above 25 kt may be possible in the vicinity of
any thunderstorms late this afternoon in the De Waters. Wave
heights on our ocean waters are forecast to favor the 3 to 4
foot range. Waves on Delaware Bay are expected to be 1 to 2
feet.

OUTLOOK...
Saturday and Sunday...The conditions are anticipated to be
below Small Craft Advisory criteria. However, an onshore flow
should start to increase mainly later Sunday night as low
pressure moves off the coast of the Southeast States.

Monday and Tuesday...Confidence is on the low side with the
timing and details. Low pressure may parallel the East Coast
producing rounds of showers or periods of rain. A strengthening
northeasterly flow, which may peak on Tuesday, looks to gust to
30 knots at times. This will also significantly build the seas.
Small Craft Advisory conditions look to occur, and as of now
there is lower confidence if wind gusts can reach gale force
sometime during this time frame.

&&

.CLIMATE...
Top 10 warmest April is virtually assured for parts of our
area. This despite some chilly rains early this coming week and
associated potentially significantly colder than normal maximum
temps. The reason is the eventual bounce back with very warm
temps, probably developing at the end of next week.

The numbers this morning have Philly #2, 0.3"F from a record
and Allentown at record (0.1F) above.

Lets for now say that we`re too optimistic about overall warmth
and lose a degree on the average... that will shave the
positive departure from 5 and 6 degrees to 4 or 5 above normal.

&&

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

&&

$$

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



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