Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 191021

Area Forecast Discussion
National Weather Service Mount Holly NJ
621 AM EDT Wed Apr 19 2017

High pressure from near the Canadian Maritimes and extending down
the East Coast shifts eastward today and tonight. Low pressure
tracks from the Great Lakes to New England today and tonight and
weakens, however its frontal boundary settles into our area. Another
low moves east across the Great Lakes into the Northeast Thursday
and Friday, with the main cold front moving through our area during
Friday. A storm system approaching from the Ohio Valley Saturday
night will track to our south Sunday into Monday resulting in
unsettled weather. High pressure builds in from the north for



Today...A partly sunny start after considerable cloudiness at
sunrise, then probably gives way to overcast sky during the
afternoon. There may be a late afternoon sprinkle in the
northwestern part of our forecast area as a weak cold front
approaches from the north. Max temps a few degrees cooler than
what occurred Tuesday, near normal (except a few degrees below
normal south along De Bay), this due to cloud cover. We do note
the 00z/19 NAM cycle has warmed its modeled temps as has the EC.
This part of the forecast is a 50 50 blend of the 00z/19 GFS/NAM
MOS, blended higher with the RGEM except blended cooler along
De Bay due to onshore flow. Todays high in PHL should be around


Mostly cloudy. Stratus should develop by midnight over parts of
eastern PA and develop sewd to near I95. Instability aloft is
increasing and there should be a few showers near and N of I-80
in the proximity of a slowing southward developing cool front.
The far south the shower risk tonight...for now
the NSSL WRF and SPC WRF both suggest cutting off the srn extent
of the showers near I-78. The 00z/19 EC showers extend a little
further south of I-78.

Lows will be milder...8 to 13 degrees above normal. Light south
to southwest wind.

This part of the forecast was a 50 50 blend of the 00z/19


Summary...Much warmer for most of the area Thursday and especially
Friday; unsettled weather mainly Sunday into early next week along
with some cooling.

Synoptic Setup...A ridge slides across the area Thursday with a
trough moving across the Great Lakes into Friday then the Northeast
Saturday. A secondary short wave sharpens a trough in the Central
Plains Friday night into Saturday, which then moves eastward to the
East Coast Sunday night. This feature may be slow to exit eastward
early next as it closes off to our south. The next short wave trough
quickly moves from the Midwest and Great Lakes region Tuesday to
near northern New England Wednesday however this may weaken as more
energy ejects into the Plains. This will all translate to changeable
weather across our area, featuring a significant warmup for much of
the area along with the potential for a widespread rainfall during
the second half of the weekend. We used a model/continuity blend for
Thursday through Friday night, then blended in the 00z WPC Guidance.
Some adjustments were then made following additional collaboration
with our neighboring offices.

For Thursday and Friday...Low pressure is forecast to be exiting New
England and weakening Thursday morning, however a cold front will be
trailing to its southwest and down into a portion of our area. There
should be plenty of low-level moisture in place and with incoming
warm air advection, low clouds should be around for at least
Thursday morning. Some fog is also possible. A short wave trough
moving across the Great Lakes region will drive surface low pressure
eastward, and the aforementioned front ties into this low as a warm
front. Some showers and perhaps some thunder moves through mostly
the northern half of the area Thursday morning then there may be a
break. Some additional showers/thunder should develop Thursday
afternoon to our west with an incoming warm front and associated
lift. This activity at least in a weakened state should slide across
the region later Thursday afternoon and especially Thursday night.

The main forcing associated with a stronger wind field and warm air
advection is forecast to be sliding by to our north Thursday night
into Friday morning, therefore the heavier and more organized
showers and thunder should be north of us. The cold front looks to
move through prior to midday Friday, and with most of the forcing
weakening there should be limited convection outside of early
morning activity. Given the track of the surface low and timing of
the front, looks like any convection should be the more typical
variety. Clearing should take place during Friday afternoon as
the cold front shifts east and the winds shift to the west and
northwest along with drier air advecting in. The main cold air
advection looks to be delayed especially below 850 mb therefore
Friday is expected to be rather warm.

For Saturday and Sunday...The cold front from Friday looks to stall
to our south. As energy slides eastward, a surface low tracks into
the Ohio Valley Saturday afternoon. This will result in increasing
clouds and warm air advection up and over the low-level boundary.
Some showers may develop across the southern areas Saturday
afternoon, however stronger and more organized lift arrives later
Saturday night and especially Sunday. While the surface low is
forecast to track to our south Sunday and Sunday night, plenty of
ascent should drive an area of rain. There is the potential for a
widespread rainfall on Sunday, some of which can be moderate/heavy.
There will be a low-level onshore flow (breezy at the coast),
allowing for much cooler conditions on Sunday. This will also keep
any instability elevated, however as of now thunder was not added to
the forecast. PoPs were increased later Saturday night and Sunday
for much of the area. Some guidance hinting at convection to our
south Sunday, which may keep the deeper moisture inflow from getting
as far north as our area. The system should shift offshore later
Sunday night with some improving conditions.

For Monday and Tuesday...A closed low may develop Monday across the
Southeastern States, which would slow the departure of low pressure
along a frontal zone. This appears to occur far enough south and
east to keep the associated main rain shield offshore. However,
there still could be some showers especially Monday. Meanwhile, high
pressure is forecast to build down from eastern Canada and combined
with low pressure to our south will produce an onshore flow. This
may keep clouds in place especially if enough drying/subsidence with
the surface high remains farther to our north, along with cooler
temperatures particularly along the coast.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Today...After early cigs 700 to 1700 feet dissipate...VFR with
cigs near 10000 ft developing this afternoon and lowering late
in the day. Southeast wind should gust 15-20 kt this afternoon.

Tonight...VFR cigs with probable cigs 500-1500 ft developing
late over eastern PA (KRDG/KABE)...possibly into KPHL toward
10z Thursday. scattered showers in vcnty of KABE/KRDG/KTTN.
Light south wind.

Thursday...MVFR or IFR ceilings (some fog possible) should improve
to VFR ceilings in the afternoon. Some showers or a thunderstorm are
anticipated especially in the morning then again late day and at
night, which can result in brief IFR conditions. Southwesterly winds
mainly 5-10 knots.

Friday...Mainly VFR ceilings which should clear out in the afternoon
and at night following the passage of a cold front. A few showers or
a thunderstorm possible especially through midday. Southwest winds 5-
10 knots in the morning becoming west-northwest 10-15 knots and
potentially gusty in the afternoon.

Saturday...Mainly VFR during the day as clouds increase, then sub-
VFR ceilings may develop at night with some rain arriving late.
Northwest to north winds less than 10 knots.

Sunday...MVFR/IFR conditions probable with periods of rain. Some fog
possible. East-northeast winds around 10 knots with some gustiness
especially closer to the coast.


No headlines at this time but an SCA may be needed later today.
A general east southeast flow prevails today...gusting around
20 kt. Isolated gusts possible to near 25 kt. Winds turn south
tonight and then southwest by dawn Thursday.

Seas on the Atlantic waters with an easterly short period swell
are steep and just below the 5 ft threshold now and should
remain there into tonight.

Thursday and Friday...The conditions are expected to be below Small
Craft Advisory criteria. Some fog possible especially each overnight
and morning.

Saturday...The conditions are anticipated to be below advisory
criteria. Winds become light for a time, then turn northeast and
increase some at night.

Sunday...Small Craft Advisory conditions should develop as a
northeasterly wind increases and allows seas to build into the 4-6
foot range on the ocean zones. Periods of rain are forecast to


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.




Near Term...Drag 622
Short Term...Drag
Long Term...Gorse
Aviation...Drag/Gorse 622
Marine...Drag/Gorse 622
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