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178
FXUS61 KPHI 261736
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
136 PM EDT Mon Jun 26 2017

.SYNOPSIS...
A large area of surface high pressure builds into the eastern United
States early in the new week before moving offshore on Wednesday. A
series of upper troughs will swing through the East Coast during
this time as well. A series of low pressure systems will pass north
of the region late this week and into the new weekend, and each low
will drag a weak boundary through the region during this time. A
more developed area of low pressure is possible Sunday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
900 am update: Forecast is in good shape, and no changes were
made for the morning update.

Previous discussion below...

A strong short wave dropping southeastward from the Midwest and
upper Great Lakes early this morning will amplify an upper-
level trough eastward through the day. This will result in
gradual height falls across our area by late in the day. At the
surface, a weak cold front across the area early should tend to
dissipate through the day or become a surface trough.

The model guidance shows several short waves within the flow
out ahead of the aforementioned amplifying trough. This along
with cooling aloft should generate enough lift and instability
to develop showers and thunder (probably low-topped) to our
northwest and west. Much of the guidance keeps this activity
just to our west/northwest by late in the day, however terrain
influence may help pop a few isolated showers in the
northwestern zones after 18z/2 PM. The forecast soundings
overall show a fair amount of dry air, however there is some
instability that develops this afternoon. The forecast amount of
instability within these soundings looks to be minimal and
given that convection will probably be low-topped, we only
carried a shower mention and kept PoPs across the northwest
zones no higher than slight chance. We also slowed down the
arrival of the mentionable PoPs. The forecast soundings show the
convective temperatures being reached more likely across the
interior, therefore placed the higher sky cover mainly near and
west of I-95.

Based on the incoming height falls through the day, the
thickness values will also lower some and therefore high
temperatures are expected to be a bit cooler than yesterday. The
flow should be light enough to allow the surface winds to turn
more southerly along the coast. The high temperatures are mostly
a blend of MOS and continuity.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/...
An upper-level trough will continue to sharpen across the
eastern Great Lakes and Ohio Valley through the night. This will
drive weak low pressure just north of the Great Lakes, with
either a weak cold front or surface trough extending southward
from it. There is plenty of short wave energy within this
trough, however much of this looks to hang back over the Great
Lakes and Ohio Valley. The main short wave is forecast to move
across the eastern Great Lakes late tonight, and this is where
much of the shower activity should be focused. For our area,
some guidance is a bit more robust with a lead short wave and
want to produce a few showers/thunder overnight near the
Delaware Valley. This cannot be ruled out as there may still be
a surface trough in place or weak incoming cold front, however
confidence is not all that high and therefore kept it dry for
now.

Otherwise, there should be more cloudiness around for awhile
tonight especially the mid level variety as some instability may
linger with cooling aloft. The lowering of the dew points
during the day today should recover some tonight as winds go
light, however despite more cloud cover for a time the
temperatures are expected to be cooler given the overall air
mass will be cooling with the incoming upper-level trough. Low
temperatures were mainly a blend of continuity and MOS.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
The long term period will start off relatively cool and dry, as
high pressure and an upper trough build in from the west. When
both the high and the trough move offshore during the mid-week
period, return flow will usher a hot and humid airmass back into
the region for the end of the week and into next weekend.

By Tuesday, the center of the surface high positions itself
over the Southeast U.S. and the base of the upper trough digs
into the Ohio Valley and Appalachians. A few strong shortwaves
will dive into the base of the trough, and this trough will
traverse the East Coast in the afternoon and evening. There is
not much moisture associated with this systems, as a drying west
winds result in surface dewpoints in the upper 40s to around
50.Although isolated to scattered showers and thunderstorms are
possible, do not think they will be long lived or widespread.
Highest PoPs will be to the north and south, where the strongest
shortwaves will be. Highs remain several degrees below normal,
generally in the mid to upper 70s, though the Poconos will be
about 10 degrees cooler.

Any lingering showers will taper off in the evening as the
trough moves offshore. By Wednesday morning, the center of the
high establishes itself over the Mid-Atlantic. Dry and pleasant
conditions on tap with low humidity values as highs will
approach 80 for most of the region, and dewpoints will be in the
low 50s.

Surface high pressure begins to move offshore Wednesday night.
This results in a pattern shift for the end of the week and
weekend, as heat and humidity gradually build.

With surface high pressure entrenched off the Southeast U.S.
coast, and an upper ridge gradually builds east through the end
of the week, return flow sets up and pushes temps through the
80s on Thursday, and gradually into the lower 90s Friday and
Saturday. In addition, surface dewpoints rise back up, first
into the low 60s Thursday, and then approaching 70 and low 70s
over the weekend.

Meanwhile, several strong shortwaves will dive into the
northern states, and several areas of low pressure will develop
to the west and will pass north of the region. As they track
north of the region, they will drag a weak frontal boundary
through the Northeast and Mid-Atlantic. Will carry slight
chance-chance PoPs from Thursday through Saturday.

Upper ridge moves offshore late Saturday, and then an upper
trough begins to dig towards the East Coast. Sunday could
feature a bit more precip across the region as low pressure
develops south of the region and lifts to the north. Will carry
high chance PoPs for Sunday.

&&

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

For the 18Z TAFs...VFR through the period. SCT-BKN cumulus deck
around 6000-8000 feet this afternoon, with winds west or
southwest 5 to 15 kts with occasional gusts to 20 kts. Tonight,
skies may clear for a time after sunset and then begin to
increase late, especially northwest of KPHL. Winds mostly light
and variable. VFR cloud deck likely tomorrow with isolated to
widely scattered storms possible. Coverage looks too sparse and
confidence in occurrence is not high enough for TAF inclusion at
this time. Winds should pick up tomorrow afternoon, with gusts
to 20 kts possible.

OUTLOOK...

Tuesday: Mainly VFR. Isolated to scattered SHRA/TSRA possible
with brief MVFR or lower conditions. W-SW winds 10-15 KT with
gusts to 20 KT in the afternoon.

Tuesday night...VFR. Any lingering SHRA/TSRA taper off in the
evening. Light W-NW winds.

Wednesday...VFR. W winds 5-10 KT. Gusts 15-18 KT possible in
the afternoon.

Wednesday night...VFR. S winds 5-10 KT.

Thursday...VFR. Isolated SHRA/TSRA possible at KABE/KRDG. S
winds 5- 10 KT north and west of I-95, and 10-15 KT with 20-25
KT gusts at terminals south and east of I-95.

Thursday night and Friday...Mainly VFR, but scattered SHRA/TSRA
possible, mainly north and west of I-95. SW winds 10-15 KT with
gusts up to 20 KT.

&&

.MARINE...
The conditions are expected to remain below Small Craft
Advisory criteria through tonight. A northwesterly wind will
turn southwest to south this afternoon and tonight (possibly
south-southeast in Delaware Bay for a time as a sea/bay breeze
occurs). While some gustiness will occur at times, gusts are
expected to be less than 25 knots.

OUTLOOK...

Tuesday...Sub-SCA conditions. Scattered showers and
thunderstorms. possible.

Tuesday night...Any showers and thunderstorms taper off in the
evening. Sub-SCA conditions.

Wednesday...Sub-SCA conditions.

Thursday through Friday: SCA winds/seas possible as stronger
southwesterly winds become established. A slight chance of
storms on Friday.

.RIP CURRENTS...
The forecast conditions through this evening should result in a
low risk for the development of dangerous rip currents.

&&

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

&&

$$
Synopsis...MPS
Near Term...CMS/Gorse
Short Term...Gorse
Long Term...MPS
Aviation...CMS/Gorse/MPS
Marine...Gorse/MPS