Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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900
FXUS61 KPHI 230512
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
112 AM EDT Sun Apr 23 2017

.SYNOPSIS...
Several waves of low pressure will move west to east along a stalled
frontal boundary to our south from the Tennessee Valley into the
Carolinas through Sunday. High pressure will also be anchored to our
north. By Monday, another low pressure system will organize over the
Carolinas and move up the East Coast through the middle of next
week. Another frontal boundary will attempt to approach from
the north as high pressure becomes more dominant across the
region later next week.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
1230 am update: The region is now precip-free with skies
becoming partially to almost fully clear northwest of I-95.
Lowered PoPs the rest of the night across Delmarva and the
adjacent coastal waters and made minor adjustments to
temps/dews/sky cover based on recent trends. Otherwise, forecast
is in good shape.

Previous discussion...

Most of the showers associated with the front and weak low
pressure system have moved east of the area. One more batch
over Delmarva will push offshore over the next few hours. Clouds
will slowly decrease over the area tonight. There could still
be enough high clouds overnight to keep temperatures from
dropping too low. We have not seen the need to make any changes
to overnight lows. A bit of patchy fog is possible overnight,
but the drier air arriving should inhibit any widespread fog
from developing.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM MONDAY/...
1230 am update: Latest high-resolution guidance is keeping most
precip south of the area during the day, with only an advancement
northward to far southern Delmarva during late afternoon or
early evening. Reduced PoPs in the southern CWA, keeping them
unmentionable until around 4 pm.

Previous discussion...

Weak high pressure briefly builds across the area early Sunday,
before moving offshore Sunday. The frontal boundary to our south
remains well south through the day, but may begin advancing
northward late. Sunday may actually be a pretty nice day for much of
the area as dry air above the surface will be in place. There could
be some cumulus build up during the day along with some cirrus
overhead, but there should be a fair amount of sunshine across the
area, unless the cirrus gets too dense. Any precipitation that would
occur during the day would be confined once again to southern
portions of the Delmarva.

&&

.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
Sunday night: The stalled frontal boundary should be far enough
south to get a break in the light overrunning rainfall for most of
the region with light rain beginning to move northward in Delmarva
as moisture intersects the boundary and is condensed in the cooler
airmass. Low temperatures will generally be in the 40`s and stayed
close to the MET/MAV blend in this period for most weather elements.


Monday through Tuesday night:

A strong ensemble signal is present that low pressure will organize
over the Carolina coastline. However, some spread is present on
exactly where this will occur. Two different parts will be
interacting to form this low. The first part being the persistent
stalled frontal boundary and second is a low pressure system coming
from the Caribbean, bringing tropical moisture northward.

Model and ensemble guidance to some extent are struggling a bit on
the interaction of these two features, fairly typical for a few days
out. This will have a large impact on just how much rain falls as
the low moves northward throughout this timeframe. For now, the
forecast gives a lean toward the drier GFS and ECMWF. On the other
hand, the Canadian and the UKMET are are further west and wetter.
Incorporating the wetter CMC and UKMET has out a QPF forecast a bit
higher than WPC along with the GEFS and some other ensembles. A
concern is that the GFS and ECMWF are having issues resolving the
convection as the low moves northward. In some aspects this system
QPF wise is a reminder of the March storm with model tendencies. So
would not be surprised to see this system trend further west based
on potential convection issues. The good news in terms of excessive
rainfall is that we will still be in a cooler airmass with lower PW
values than ideal (max around an inch) for very heavy rainfall and
flooding concerns. Still over the course of 36-48 hours, 1 to
locally 2 inches of rainfall could fall with the highest amounts
over Delmarva with totals under an inch north and west of
Philadelphia.

Low clouds, rain and an easterly onshore flow will keep temperature
ranges small and rather chilly with highs in most spots only in the
50`s. This forecast trends several degrees cooler than MET/MAV.
Winds should pick up as well from the east with some gusts around 20
mph at times, not a favorable environment for mixing but with the low
pressure nearby a tighter pressure gradient will be present.

Wednesday and Wednesday Night:

Fairly good agreement is present to linger a few showers into early
Wednesday, with the CMC and ECMWF on the slower end. Temperatures
should start a rebounding trend with perhaps a few breaks in the
persistent gloomy overcast late Wednesday. Stayed close to ensemble
guidance on most elements.

Thursday through Saturday:

A busy late week and weekend in Philadelphia. unfortunately, the
forecast is riddled with more uncertainty by late week with
temperatures and further rain chances. A good piece of news is that
high pressure will become more dominant resulting more sunshine and a
break from the gloom most of next week will bring. Current thinking
is that precipitation from any fronts will stay northwest of the
region but concede rain chances may have to be added as more
information comes in. The modeling is bringing another front a
little further south with each run for the later half of the week
with a cooler/wetter trend. This could still reverse given 5-7 days
out. Temperatures will likely warm-up with a good shot for a more
persistent southerly flow on Thursday and Friday. The movement of
the approaching front will determine if the region sees a more
persistent flow from the north and east with more clouds and highs
in the 60`s or if the front stays north of the region allowing for
highs to push well into the 80`s or warmer (00z EC temps at 18z +5 F
even yield 90 in a few spots by Saturday. Will keep consistency from
the previous forecast and not swing one or way or another given the
4 standard deviation spread with the 00z ensemble guidance by next
weekend with temperatures.

&&

.AVIATION /05Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

For the 06Z TAFs...VFR conditions through the period. Partial to
almost full clearing will occur north of the Mason-Dixon Line
with more considerable mid and high cloudiness to the south
through the day. CIGs will lower gradually by late
afternoon/early evening to the south of KPHL but should remain
VFR through 06Z Monday at the terminals. Winds will veer from
northerly to easterly or southeasterly during the day, remaining
generally below 10 kts. Rain is expected to progress slowly
northward across the Delmarva Peninsula late Sunday night.
Overall confidence in the TAFs is slightly above average.

OUTLOOK...

Sunday night: VFR.

Monday and Monday night: MVFR and IFR spreading from south to north
for KMIV, KACY and KILG with rain. VFR KPHL and north. East
northeast winds 10-15 knots.

Tuesday through Wednesday: MVFR and IFR with several periods of
rain. Perhaps some intervals of VFR for KABE and KRDG where more
breaks in the rain are likely. Rain will end on Wednesday from west
to east with a slow improvement to VFR. Easterly winds becoming
northerly, gusts near 25 knots on Tuesday from the east decreasing
to under 10 knots from the north on Wednesday.

Wednesday night and Thursday: VFR. Winds under 10 knots.

&&

.MARINE...
Winds and waves are expected to be below Small Craft Advisory levels
tonight through Sunday. Although seas will be around 4 feet and
winds could gust around 20 knots at times.

OUTLOOK...

Sunday night: Seas building to around 5 feet late in response to low
pressure formation south of the region. Easterly winds increasing
from south to north as well but staying under 10 knots northern
ocean waters and 20 knots southern ocean waters.

Monday through Wednesday night: SCA seas and gusts throughout most
of the period. Wind gusts peaking from the east around 35 knots
Tuesday afternoon and evening on the ocean waters. High confidence
for SCA conditions with a low to moderate confidence in Gale
gusts. Both the GFS and ECMWF so a surge of 40 knot wind at 925
mb moving into the region at that time but conditions for mixing
aren`t expected to ideal late Tuesday. Winds decreasing and
turning northerly on Wednesday. Seas will also decrease starting
on Wednesday.

Thursday; Seas likely still over 5 feet with a slow decrease.

&&

.TIDES/COASTAL FLOODING...
A look at ensemble guidance shows the potential for minor coastal
flooding centered around the high tides on Tuesday for the Delaware
Bay and oceanfront. A persistent onshore flow will be in favor of
the tidal flooding along with the proximity to a new moon.

However, the low pressure system is not expected to strengthen much
below 1000 mb at this time. Both of those factors support the ensemble
guidance with keeping any tidal flooding in the minor category.

&&

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

&&

$$
Synopsis...Gaines
Near Term...CMS/Robertson/O`Hara
Short Term...CMS/Robertson/O`Hara
Long Term...Gaines
Aviation...CMS/Gaines
Marine...Gaines/Robertson
Tides/Coastal Flooding...Staff



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