Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 281057

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

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.



Today...Issued SPS around 530 AM 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) and near record warmth 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. Officially we dont reach
the early season heat index advisory criteria of 96 for the ILG-
PHL-TTN metro corridor or 100 HI elsewhere. However, its the
first heat wave of the season for parts of our area and D3, which
probably results in a daily cumulative toll on health for those
elderly or children with respiratory ailments and no air conditioned

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 easily defined lift mechanism except possibly a weak
trough from vicinity ALB to FWN. 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 5700m 1000-500mb
thickness is not excessive), therefore it seems difficult to
develop afternoon convection. Have a small probability up north
but convection is not our 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.


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 3 am Sunday?, 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


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
teleconnects 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

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

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

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


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

Today...any sub VFR vsby improves to VFR by 13z or 14z 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 vicinity

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

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


No marine headlines anticipated through tonight.

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.

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 cautiously forecast a LOW risk today but LOW
does not mean NO risk!!! For safety, please 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 interior heat
encouraging swimmers to swim 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 normal beach hours, at unguarded locations. Its just not worth
the risk.

We have received and posted our first of the season NJ LBI dawn
surf report.


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.


The monthly temperature at Philadelphia continues to project to
drop down to only 1 degree 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.




Near Term...Drag 658a
Short Term...Drag
Long Term...Franck
Aviation...Drag/Franck 658a
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