Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 280730

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

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


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.


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


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 high falls across the
eastern conus to eject the closed-mid level low off 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 increasingly 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, 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.

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

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


Predominantly 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.

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

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.


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.


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 deeptropical 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 24to 36 hours, and have trended
precipitation estimates upward accordingly. Thankfully, this
system will be progressive, with a 12to 18 hour period of
rainfall. Given the tropical moisture(Precipitable Water values
near 2.00" ~ 2-3 Standard Deviationsabove normal) and a southeast
low-level jet impinging upon theterrain, 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. This
could result in urban, poor drainage, and minor stream flooding.


We`ll be adding considerable information to this section between
4 and 5 am Saturday.

The KMPO temperature sensor was fixed yesterday.




Near Term...Drag
Short Term...Drag
Long Term...Franck
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