Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 290003
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
803 PM EDT Wed Jun 28 2017

.SYNOPSIS...
A broad area of high pressure will slowly drift eastward into the
western Atlantic through the end of the week. A cold front will
gradually sag southeastward from the Great Lakes and southeast
Canada this weekend to the Ohio Valley and southern New England
early next week.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
No big changes needed. The daytime CU has dissipated and the
only cloud cover tonight will be the CI/CS shield overhead.

It looks like radiational cooling will be rather efficient over
night. Therefore, stayed close to the previous forecast which
is slightly below the NAM and GFS MOS guidance especially north
and west of the fall line and over the Pine Barrens.

A few adjustments to hourly temps in the next few hours (mostly
upward).

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
With the surface high to our east, and increasing pressure gradient
in response to the surface low over the Great Lakes region moving
further east, expect low level southwesterly flow to develop and
increase. In response, we should see a considerable warming trend
tomorrow, with highs about 5 to 10 degrees higher than today.

Also, late in the day as the low continues to slowly propagate
eastward, we could see isolated showers and thunderstorms generally
from the Lehigh Valley to NW NJ and further northwest. I have kept
it at a slight chance though as the low should stay well to our west
through this time and the upper level jet will be too far north to
provide much in the way of lift across our region.

&&

.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
A long duration of hot and humid weather is in store for the
region, with chances of precipitation increasing this weekend
with occasional chances continuing next week.

Operational models are in reasonably good agreement with the
synoptic-scale details through the long-term period. Smaller-
scale vorticity maxima within the southwest upper-level flow
developing this weekend and large-scale trough passage next week
have typical strength/timing differences among the model suite,
with no obvious model biases noted. As such, pretty much loaded
the grids with a model blend through the long term with minor
modifications for better office collaboration, somewhat lower
weighting for the 12Z CMC late in the period owing to its
increased discrepancies from consensus, and dampened PoPs Monday
onward owing to larger uncertainties associated with vort max
timing.

At 00Z Friday, a potent vort max will be moving through southern
Ontario and the adjacent Great Lakes with downstream ridge
amplification fully commenced in the Northeast. After a rapid
warmup on Thursday, this will be followed by a rapid increase in
surface dew points on Friday and Saturday. With highs near or
above 90 in the urban corridor, this leads to a rather
uncomfortable period for the area. I was a little hesitant to
increase dew points to the values of MAV/MEX MOS, which have
been a little on the high side in similar patterns so far this
spring/early summer. As such, I dropped heat indices somewhat
both Friday and Saturday, especially since model soundings show
well-mixed soundings during the afternoons. With a rather deep
mixed layer, I strongly suspect MOS is overdoing surface
moisture, especially on Friday. Much stronger weighting was
given to MET guidance Friday and manual adjustment Saturday
given NAM BUFKIT soundings.

Regarding the aforementioned vort max, as it slides eastward
into far northern New England, an upstream vort max will move
into the Upper Midwest by Friday afternoon. This will be the
start of prolonged southwesterly midlevel flow in the eastern
U.S. Several perturbations in the faster flow will progress
northeastward from the central U.S. to the Northeast for the
following several days. With a strong Bermuda high remaining
influential into the southeastern U.S., this will force the
associated vorticity maxima to elongate in a positively-tilted
orientation as they progress northeastward. Upstream
perturbations will then amplify the vorticity with decreasing
latitude (farther southward) with time.

The result of this above-surface pattern is a frontal boundary
moving at a snail`s pace southeastward as midlevel flow becomes
parallel to its orientation, broadly from New England to the
Great Lakes and southwestward this weekend. Several embedded
surface waves of lower pressure will progress northeastward
along this boundary, likely enhancing convection in surrounding
regions. Precipitation chances increase gradually Friday through
Sunday in this regime from northwest to southeast. Models are
in reasonably good consensus showing a particularly strong wave
moving through the Mid-Atlantic region Saturday afternoon
through Sunday morning.

This brings about the most challenging aspects of the weekend
forecast. The first is highs on Saturday, with
models/statistical guidance trending noticeably downward with
temperatures on Saturday as potential sky cover/precipitation
influences may inhibit maximum warming. The second is figuring
out timing/location of highest PoPs. Decided to keep a
conservative approach for this forecast, both by lowering max
temperatures slightly on Saturday and by capping PoPs to chance
Saturday through Sunday. Sunday may end up being fairly dry if
the vort max projected to move through Saturday night is
followed by fairly strong transient ridging (as depicted to
some degree by the GFS and ECMWF). The 12Z CMC, meanwhile,
brings a quick-to-follow perturbation through the area on
Sunday, generating more convection in much of the region. With
so much uncertainty with these small-scale details, it is
formidably challenging to get too specific with PoPs at this
point. There are decent indications of a push of drier surface
air moving into the area by Sunday night as the cold front makes
its strongest progress into the region.

For early next week, models are showing a somewhat drier day
Monday upstream of the perturbations moving through during the
weekend followed by a potent trough moving through the Northeast
Tuesday and Wednesday. Kept slight chance to chance PoPs for
much of the area (highest north) Tuesday and Wednesday, but kept
Monday mostly dry given the reasonably strong consensus at this
point. However, with the front failing to make progress
southeastward through the region, temperatures will remain warm,
and humidity will remain high.

&&

.AVIATION /00Z THURSDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Mostly VFR conditions are expected through the TAF period. After 18Z
Thursday, there is a slight chance of showers and thunderstorms
across portions of southeastern PA including KRDG and KABE. However,
coverage is too limited to include in the TAFs at this time.

Winds will become light from the southwest tonight. By 12Z tomorrow
though, we should see winds across the region shift to south-southwesterly
and increase to 10 to 15 kt with gusts above 20kt.

OUTLOOK...
Thursday night and Friday...VFR conditions expected. Slight
chance of storms northwest of KPHL, which may locally lower
CIGs/VSBYs. Southwest winds 5 to 15 kts (stronger during the
day), with gusts to 20 kts possible.

Friday night through Sunday...Mostly VFR, but chances of storms
through the period, especially Saturday afternoon through
Sunday. Locally lower visibility and ceilings likely. West to
southwest winds 5 to 15 kts, with gusts to 20 kts possible
during the day (more so Saturday).

Sunday night and Monday...A potential dry period, with mostly
VFR conditions. Light winds, primarily westerly.

&&

.MARINE...
Winds and seas will begin to build mid day Thursday. Small craft
advisory conditions are expected to develop by mid afternoon along
the Atlantic coastal waters.

On the Delaware Bay, it is unlikely that winds will get above 25 kt
through the day time hours. However, southerly wind gusts above 20kt
are possible on the lower bay.

OUTLOOK...
Thursday night and Friday...Advisory conditions likely, with
gusts near gale force possible Thursday evening.

Friday night through Sunday...Advisory conditions may linger
Friday night, with generally sub-advisory conditions thereafter.
However, a period of stronger winds may occur Saturday night as
well. Frequent chances of storms through the period.

Sunday night and Monday...A drier period may occur. Sub-advisory
conditions are expected.

RIP CURRENTS...

The outlook for tomorrow is that the low risk for the formation of
dangerous rip currents will continue into at least tomorrow morning.
Depending on how quickly the winds increase tomorrow afternoon, the
risk may increase to moderate by late in the day, but that remains
uncertain at this time.

Low risk does not mean NO risk and it is always advised for ultimate
safety, swim in the presence of the lifeguards. They have the
flotation devices that can more easily save a life.

Where the surf zone waters are steeper, there may be fewer rip
currents, but there, the danger of wave related injury increases.

An ongoing DE 6+ year study shows the 10-20 year and 40-60 year
old age groups most vulnerable. The 10-20 year old surf zone
injuries are associated with body boarding and body surfing,
while the 40-60 year old waders are knocked down by wave action,
especially with back turned to the ocean.

Males are statistically far more likely to be injured or lose
their life in the surf zone but as June 15 reminds us...ANYONE
is vulnerable.

&&

.CLIMATE...
Monthly avg temp for June

PHL projecting 74.5 or 1.2 degrees above the average of 73.3

ABE projecting 70.8 or 1.7 degrees above the average of 69.1

Seven of our eight long term climate sites will average above
normal probably from about 8 tenths of a degree for TTN/ILG to
as much as 2 degrees for GED.

Mount Pocono is our only below normal average and projecting
1-1.5 below).

These projections are based on our 330 am forecast.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory from 2 PM Thursday to 6 AM EDT Friday for
     ANZ450>455.
     Small Craft Advisory from 6 PM Thursday to midnight EDT
     Thursday night for ANZ431.

&&

$$
Synopsis...CMS/99
Near Term...Johnson/O`Hara
Short Term...Johnson
Long Term...CMS
Aviation...CMS/Johnson/O`Hara
Marine...CMS/Johnson
Climate...



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