Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 290605

Area Forecast Discussion
National Weather Service Mount Holly NJ
205 AM EDT Thu Jun 29 2017

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.


Early this morning...broken deck near 15000 feet will somewhat
limit radiational cooling. Light southwest wind.

Today...Mostly sunny with cu/sc forming 9am to Noon and then
dissipating late ion the day with some cirrus above. a gusty
southwest wind to 25 mph this afternoon. Dewpoints 10 to1 5
warmer than ydy. Max temp this afternoon 1 to 5f warmer than

fcst basis was generally a 50 50 blend of the 00z/29 GFS/NAM MOS
except the GFS MOS temps look too cool today so used a combo of
ydys max temp (continuity) with the warmer 00z/29 METMOS.

Of interest this shows better cooling along the coasts late
this afternoon as an increasingly gusty south wind cools the
beaches and increases the risk for the formation of dangerous
rip currents, especially heading toward the 7PM low tide (after
beach patrols go home) - a more risky time to be in the water
without the immediately swift and sure rescue opportunity
provided by on-duty lifeguards.


Fair (there may be a deck of sct-bkn clouds near 7000 ft,
especially late. Warm air advection with a stirring southwest
wind with scattered gusts 15-20 mph. Small chance of a shower
far north tonight where a little greater elevated instability

Forecast basis was a 50 50 blend of the 00z/29 GFS/NAM MOS with
the pops tempered by the morning conservative SREF.


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

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.


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

Through today...VFR sct-bkn aoa 15000 ft through 12z, then a
sct deck of 7000 ft develops late this morning with 3000-4000 ft
sct developing in NJ this afternoon where a little more low lvl
moisture is modeled. Southwest wind becomes gusty 20-25 kt this

Tonight...VFR sct to possibly bkn clouds near 6000 or 7000 ft.
South to southwest wind with scattered gusts to 15 kt.

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.


SCA headlines posted.

Winds and seas will begin to build mid day. Small craft advisory
conditions are expected to develop by mid afternoon along the
Atlantic coastal waters. 30 kt gusts should develop north of
Little Egg inlet this afternoon (ACY).

On the Delaware Bay, we probably will start the SCA sooner and
expand its reach to upper DE Bay.

Friday...Advisory conditions likely.

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.


We start low this morning and remain low on DE this afternoon
but will forecast mdt on the NJ coast as southerly winds
increase with gusts to 25 or 30 mph.

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 (45 pct of the injuries are to
the 10-20 year olds and another 40 percent of injuries are to
the 40-60 year old bracket). 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.


Will recheck and update this section at 5AM.

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 were based on our 330 am Wednesday forecast.


MARINE...Small Craft Advisory from 2 PM this afternoon to 6 AM EDT
     Friday for ANZ450>455.
     Small Craft Advisory from 6 PM this evening to midnight EDT
     tonight for ANZ431.


Near Term...Drag 205
Short Term...Drag 205
Long Term...CMS
Aviation...CMS/Drag/Johnson 205
Marine...CMS/Drag 205
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