Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 262000

Area Forecast Discussion
National Weather Service Mount Holly NJ
400 PM EDT Sun Mar 26 2017

Low pressure moving from the Mississippi Valley into the Great
Lakes will lift a warm front northward through our region overnight
into Monday morning, followed by a cold frontal passage Tuesday night.
Canadian high pressure will build into the area Wednesday through Friday.
Low pressure and its associated fronts in the Ohio Valley on Friday will
move through our region on Saturday. A weak area of high pressure is
expected to move into the area from the west on Sunday.


930 am update: No changes to the forecast this morning. Onshore
flow to the north of a backdoor cold front is inundating the
region this morning with low clouds and occasional drizzle.
Mount Pocono remains below freezing, so a light glaze may occur
on trees, but no societal impacts have been observed or reported to
this point. Conditions will change little through the day, with
temperatures unable to rise in the low clouds and maritime
origins of the low-level flow. Expect little to no measurable
precipitation through the daytime hours as the midlevels should
remain relatively dry.

Previous discussion...

High pressure across Quebec/Maine will move offshore today while
weak low pressure slowly moves to near Chicago by evening. A
steady onshore flow will continue across our area thru the day.
This will promote an abundance of low clouds and fog along with
some light rain and drizzle at times. Pops will be greatest
across the northern areas where we have hi chc pops and the pops
taper off s/e to slgt chc across srn DE. Unlike Saturday,
temperatures will be below normal, with highs only in the 40s in
most areas, and a few low 50s over Delmarva.


The overall pattern will not change much tonight with high
pressure north/east and weak low pressure moving closer across
the Great Lakes. The onshore flow will continue and the chances
for more measurable rains will increase tonight. The best
chances will again be across the southern Poconos, Lehigh valley
and north NJ where we will have categorical pops. The pops
decrease back to likely over the Del Valley and cntrl NJ and the
chc pops over south NJ and srn DE. Patchy fog and drizzle will
occur between the bouts of rain. Overall qpf will not be that
great, a few hundredths across the south and up to 1/4 inch up


A split flow regime will continue across the conus during this period.
While the northern stream remains displaced to the north, our primary
weather makers will be in the southern stream. A closed low along the
lee of the Rockies will gradually open and eject northeastward. Its
associated weak surface reflection and cold front will traverse the
middle Atlantic, impacting our sensible weather Monday night into
Tuesday night. A cyclonic flow aloft on Wednesday will gradually give
way to ridging on Thursday. The next southern stream system in the
pipeline will impact our region Friday into Saturday.

With good run to run model consistency in terms of the timing of the
cold frontal passage Tuesday night, the next challenge will be how the
next southern stream system is handled during the Friday and Saturday
period, including the extent of phasing with the northern stream. In
particular, the track of this system and the extent of the cold air to
the north will have implications on any potential p-type issues.

Temperature-wise, around 10 to 15 degrees above normal on Tuesday, then
generally normal to several degrees above normal from Wednesday through
next weekend.

Another round of showers is expected mainly after midnight Monday night
into Tuesday night across the entire region. With the cold frontal passage
Tuesday night, showers should move offshore prior to Wednesday morning. We
do not anticipate any p-type issues during this period. We expect patchy fog
Monday night, which may linger into Tuesday afternoon north of a PA Turnpike
to I-195 line. Within the warm sector in advance of the cold front, we expect
sunshine to break out, especially south of the aforementioned line with
temperatures reaching the upper 60s to lower 70s. A low-level moist
tongue will also be in place across this area, with Dew Points well
into the 50s. Models indicate some weak ML Cape, along with negative
Lifted and Showalter Indices, within an environment characterized by
poor lapse rates and weak shear. We have included a chance of thunder
in the forecast, and there may be some localized heavier downpours as
well. Given low Precipitable Water values around one inch, urban and
small stream flooding is not a concern.

In the wake of the cold front, expect a return to fair weather for
Wednesday and Thursday.

Beyond day four, the models have come into better agreement on the
Friday-Saturday system, but there still remains uncertainty regarding
the degree of phasing between the northern and southern stream,
including p-type implications. The GFS, ECMWF, and Canadian take low
pressure from the MS Valley into the Great Lakes, with varying degrees
of secondary development over the middle Atlantic. The UKMET is an
outlier, maintaining a purely southern stream system, with the primary
low moving through the middle Atlantic. In fact, the UKMET represents
yesterdays GFS solution, as it was previously in the former camp. The
forecast brings in chance PoPs Friday into Saturday. Also, given the
orientation of the surface high, cold air damming to some extent is
likely. Thickness values indicate the cold air is right on our door
step, so p-type could be an issue, especially across the northern half
of our CWA.


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

For the 18Z TAFs...General MVFR conditions will continue through
early this evening, likely deteriorating to IFR or possibly LIFR
during the overnight hours. Moderate confidence that light rain
will move in between 03Z and 06Z at KRDG/KABE and by 09Z at
KTTN/KPNE/KPHL/KILG. There may be slow improvement to MVFR after
sunrise, but scattered showers may continue through the morning
hours. Less confidence of precip at KACY/KMIV. Winds generally
east 10-15 kts through early evening with a trend toward S or
SSW late tonight through the morning hours tomorrow.


Monday night...Periods of MVFR/IFR likely in low clouds and fog across
all TAF sites.

Tuesday...MVFR likely Tuesday morning at all TAF sites, with improvement
to VFR by afternoon at all but ABE, RDG, and TTN.

Tuesday night...MVFR possible at ABE, RDG, and TTN early, otherwise,

Wednesday thru Thursday...Predominantly VFR conditions expected.

Friday...VFR conditions may deteriorate to MVFR with the arrival of
the next weather system.

There is the potential for northwest wind gusts up to around 25 knots
Tuesday night into Wednesday.


We will keep the SCA flags as they are on the waters and just
raise the next segment with the 330 am issuance, instead of
waiting until 6 a.m. The onshore flow will continue today and a
gradual building of seas will continue. Scattered showers and
patchy fog expected.


Seas may remain elevated into Monday night, and the SCA may need to extended
into this period. A period of northwesterly wind gusts may reach SCA criteria
on Wednesday. Sub-SCA conditions are expected on Thursday. The approach of the
next system may lead to a return to SCA conditions by Friday.


Onshore flow is expected through Monday morning. Astronomical tides are
also increasing, coincident with the New Moon this Monday, March 27.
The ESTOFS remains most aggressive of the guidance suite for the
sunrise Monday high tide along the DE and NJ oceanfront. Although the
ESTOFS has outperformed the other guidance in recent past events, its
trends for positive departures (surge) appear to be lessening. Both the
SIT multi model review and GFS ETSS are more conservative, and
verifying better as of Sunday afternoon. The wind will also be trending
more parallel (southerly) to the shore by sunrise Monday, so any minor
tidal inundation flood risk remains a low potential with no action at
this time.


March as a whole for PHL, is still on track to average one half
to 1 degree below normal, despite the warmth of ydy through


MARINE...Small Craft Advisory until 7 PM EDT Monday for ANZ450>453.


Near Term...CMS/O`Hara
Short Term...O`Hara
Long Term...Franck
Tides/Coastal Flooding...
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