Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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777
FXUS61 KPHI 280822
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
422 AM EDT SAT MAY 28 2016

.SYNOPSIS...
High pressure anchored near Bermuda will slowly lose its grip on our
region over the weekend. Tropical Depression Two is forecast to
become Tropical Storm Bonnie, and make landfall near South Carolina
Sunday night, then interact with a cold front moving through our
area Monday night. Bonnie is anticipated to remain near the
Carolinas through the middle of the week, as high pressure over
Atlantic Canada extends southward over our region.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Prior to sunrise: patchy fog and haze near sunrise...thickest fog
probably nw NJ to the lehigh valley. otherwise patchy cu and
cirrus early this morning. of interest...dewpoints are several
degrees cooler all along the mid atlantic coast when compared to
24 hours ago! light south to southwest wind.

Today...Issuing an SPS for first heat wave and associated modest
heat related risk for early season warmth.

Expecting hottest day of this late May warm spell today and 1-3F
warmer than ydy in e PA and most of NJ...even a degree or so warmer
in De and MD E shore than ydy.

Early season heat wave (Day 3 of the minimum three consecutive
90f or greater required) is at hand for PHL/PNE/ABE/WRI while
other locations nearby in our CWA have been 89-90F the past two
days and today will be day 3, but cannot officially note them as
heat wave potential inclusive (GED/REG/TTN). So, after any early
morning low clouds/fog/haze...a sunny or mostly sunny day with
the forecast likely verifying above any of the warmest available
guidance be it bias corrected, or individual NCEP op MOS, super
and national blends (both blends 3-4F cooler than our official
fcst). Best of the automated guidance is the adjust MET but have
fcst warmer than that as well.

16c at 850mb with a south-southwest wind gusty to 20 mph this
afternoon should allow temps to rise to 1-3F above yesterdays
values (especially with slightly lower dewpoints), at least for e
PA and interior NJ. Heat index 90-95 for NJ and e PA and maybe
only near 90 DE/E MD. No advy but issuing an SPS. Synoptic Health
Warning Network has been highlighting early season concerns for
the I95 corridor the past several days. so while officially we
dont reach the HI early season criteria of 96 for the ILG-=PHL-TTN
metro corridor or 100 HI elsewhere, we think its the first heat
wave of the season for parts of this area and day3 and probably
a daily cumulative toll on health for those elderly or children
with respiratory ailments and no air conditioned relief.

Not anticipating convection this afternoon even in the pwat axis
of 1.5" near and north of I-78...northern portion of our CWA.
There is no known lift mechanism. Suppose something isolated can
develop but with suppressant ridging at mid levels (500mb) today
and lack of excessive moisture (PWAT of 1.5" for a 570 1000-500mb
thickness is not excessive), therefore it seems difficult to
develop afternoon convection. Have a small pop up north but
convection i do not think is weather story for today.

Max temps again 10-16F above normal with a general 50 50 blend of
the 00z/28 GFS/NAM MOS guidance except heavy reliance on the
warmer NAM BL and 2M temps, along with the overzealous RGEM and
HRRR warmth but corroborated by the GGEM and ECMWF 2m temps.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/...
Fair and not quite as warm as this morning because the PWAT lowers
northward from 1.5 to 1 inch tonight (also means lower dewpoints)
with minimum temps probably 6 to 12F above normal and a light
south to southwest wind. It may be hazy with patchy fog possible
after midnight?, primarily in the countryside. Lower dewpoints
reduce my concern for fog late tonight. In fact clouds from the
leading edge of nwwd moving tropical moisture may be arriving
over the Delmarva early Sunday morning.

Meanwhile, Any evening showers along and north of I-80 in NNJ/ne
PA quickly end during the evening.

This part of the fcst was a 50 50 blend of the 00z/28 GFS/NAM MOS
guidance.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
The mid-level flow on Sunday features the northern stream along
the US/Canadian border, with a rex block off the southeast US
coast. The cutoff upper low associated with this block
telleconnects well with another cutoff low over the desert
southwest. This block will serve as the key steering mechanism for
what is forecast to become Tropical Storm Bonnie. A couple of
shortwaves in the northern stream will then be paramount in
determining what becomes of the blocking pattern off the southeast
coast and the remnants of Bonnie. At this time, a lead shortwave
moving through the Great Lakes Sunday night and Monday is
expected to draw a substantial amount of moisture northward along
the lee of the Appalachians, with the synoptic setup favorable for
a PRE (Predecessor Rain Event). A second, more vigorous shortwave
digging in the upper Mississippi Valley by mid-week should induce
strong enough height falls across the eastern conus to eject the
closed-mid level low and the remnants of Bonnie gradually
northeastward.

On Sunday, the mid-level ridge axis and associated subsidence moves
over the region early in the day, with increasing cloudiness in its
wake. Expect a warm and increasingly humid day with high temps in
the middle 80s to near 90. With cloudiness increasing from the
south, as well as a southeast flow in place, the warmest readings
will be in northern NJ and the Lehigh Valley.

Sunday night into Monday...Confidence is increasing for a heavy
rainfall event, particularly near and northwest of the I-95
corridor, as Tropical Storm Bonnie interacts with an approaching
front (see the hydrology section below for more details). Also,
there will be the potential for thunder as well, with sufficient
instability present. Gradual improvement is expected by Monday
afternoon.

Tuesday thru Thursday...Fair weather is anticipated at this time.

Friday...A cold front may approach the region with a chance of
showers/thundershowers.

&&

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Through 12z...VFR with MVFR vsby possible toward 10z in haze or
patchy fog. Small chc IFR stratus fog vcnty KRDG/KABE toward 10z.
Light south to southwest wind.

After 12z...any sub VFR vsby improves to VFR by 15z with only sct
clouds aoa 5000 ft and a south to southwest wind gusty to 18 kt
during mid afternoon. no convection expected at the TAF sites
though not impossible to see a brief mid afternoon shower vcnty
KABE/KRDG.

Tonight...VFR with possible patchy MVFR vsby in haze or patchy
fog toward 10z sunday. Light south to southwest wind.

OUTLOOK...
VFR, with the exception of Sunday night thru Monday night, when
MVFR is possible in low clouds, fog, and +SHRA.

&&

.MARINE...
No marine headlines anticipated through tonight.

Winds were gusty 20-30 kt for a time this past Friday evening
(44065 and Weather Flow sensors) along the coasts and this drove
seas upward to near 5 ft at 44065. That was stronger than we expected
24 hours ago. The southerly flow and associated wind wave have
subsided.

So in essence, a south to southwest wind occasionally gusty to
between 20-22 kt into tonight with Atlantic coastal waters seas
2-4ft and De Bay 1-2 Ft.

OUTLOOK...
Winds and seas are expected to remain below SCA criteria during
the Sunday thru Wednesday time frame. Some maritime fog is
possible Sunday night.

RIP CURRENTS: We will probably cautiously forecast a LOW risk
today but LOW does not mean NO risk!!! For safety swim in the
presence of lifeguards. there will be an incoming 2-3 foot mixed s
to se swell today of varying periodicity 4 to 9 seconds. A cooling
southerly wind will develop this afternoon with gustiness possibly
enhancing water buildup behind the sandbars which should mean an
enhanced low risk. Am concerned about early season heat and about
swimmers in unguarded areas. We do not want any news headlines
about rip current fatalities. These fatalities are all
preventable. They tend to occur in the younger male population,
after hours unguarded locations. Its not worth the risk.

&&

.HYDROLOGY...
As tropical storm Bonnie nears the South Carolina coast and a cold
front approaches the Ohio Valley, the synoptic ingredients will
become favorable for a PRE (Predecessor Rain Event): the lee of the
Appalachians will be within the RRQ of a 100 knot upper level jet
just downstream of the mid-level trough axis. This area will also be
on the northwest fringe of a deep tropical moisture plume,
coincident with a low-level theta-e axis, which will be supported by
a surface trough/weak low pressure. The models have increasingly
advertised this scenario over the past 24 to 36 hours, and have
trended precipitation estimates upward accordingly. Thankfully, this
system will be progressive, with a 12 to 18 hour period of rainfall.
Given the tropical moisture (Precipitable Water values near 2.00" ~
2-3 Standard Deviations above normal) and a southeast low-level jet
impinging upon the terrain, some heavy rainfall rates are likely.
The heaviest rainfall should center on Sunday night into at least
Monday morning, with 1 to 2 inches possible. There is the potential
for minor urban and poor drainage flooding, and perhaps some small
streams as well.

&&

.CLIMATE...
The monthly temperature at Philadelphia continues to project to
drop down to 1 below normal from the 4 below normal of several
days prior to this spell of warm weather.

near record warmth anticipated this afternoon at

MPO record high 86 in 2012
ABE Record high 93 IN 1941
RDG Record high 92 IN 1977, 1941, 1914

The KMPO temperature sensor was fixed yesterday.

&&

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

&&

$$
Synopsis...Franck
Near Term...Drag
Short Term...Drag
Long Term...Franck
Aviation...Drag/Franck
Marine...Drag/Franck
Hydrology...
Climate...422a



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