Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 262234

Area Forecast Discussion
National Weather Service Mount Holly NJ
634 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 front 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 our region from the west on Sunday.


For the 630 PM update, lowered temperatures a little for many areas
based on observations. These should remain nearly steady for awhile
tonight as the onshore low-level flow continues. Some drizzle is
occurring closer to the coast and a check of some web cams indicates
wet roads, therefore drizzle was added. PoPs were lowered overall
for awhile as we await the arrival of a short wave and an increase
in warm air advection/lift above the shallow cool layer. This should
allow for shower development later tonight especially north and west
of I-95. No other major changes were made at this time.

Otherwise, as a vorticity maximum moves northeastward tonight the
backdoor front in the southern Mid Atlantic should lose its
southward progress and move poleward as a warm front. However,
models often overdo the progress of such fronts, especially at
night, and suspect this bias does exist to some degree with today`s
simulations. We generally undercut guidance temperatures tonight by
a few degrees, and this may not be enough, particularly in the
notoriously stubborn southern Poconos, where temperatures have been
running under guidance today by about 3-5 degrees. With that in
mind, temperatures will remain near freezing early this evening at
elevations around/above 1000 feet, so light glazing may continue on
elevated surfaces via a thermal/moisture profile suggestive of
freezing drizzle. No societal impacts have been noted with the
frozen precipitation, and with temperatures gradually warming
overnight, am not expecting much if any impact going forward. By
late evening/overnight, precipitation is expected to be all liquid
(including after contact with surfaces), and the forecast features
all rain tonight.

High-resolution models have been consistent in showing large-scale
ascent increasing in the northern Mid Atlantic downstream of the
Great Lakes vort max late this evening and overnight, with a batch
of rain moving into NY/PA/MD/WV this evening. Given the
northeastward motion of the vort max, our CWA will be on the
southern fringe of the strongest ascent, with good agreement among
hi-res guidance that chances for precipitation drop off considerably
south of the Mason-Dixon Line. As such, have the highest PoPs
(categorical) in portions of eastern Pennsylvania to northern New
Jersey with these then lowering to the south and east.

One other question mark tonight is the extent/severity of fog.
Currently, thinking that winds will be high enough to preclude
more substantial/widespread fog formation (especially with the
approach of the shortwave trough, which tends to also mitigate
widespread fog). However, fog formation may be more substantial
near and off the coast, where dew points will exceed sea surface
temperatures late tonight. Thicker fog, should it develop over
the ocean, may advect onto the New Jersey coast and create local
visibilities below a half mile.


By tomorrow morning, the shortwave trough in the Great Lakes is
expected to continue northeastward into the St. Lawrence Valley,
with the organized large-scale ascent moving into New England
and adjacent southeast Canada. Precipitation will likely become
more showery by late morning, with perhaps even hints of blue
sky in portions of Delmarva during the afternoon. The warm
front will sweep northward well into New England by this point,
and with the warm southerly fetch, temperatures will warm
substantially above today`s values. Forecast highs are 15-25
degrees above today`s values. Forecast temperatures may be on
the low side if partial sunshine occurs, especially considering
the general negative bias of guidance in warm sector regimes
this winter.

There is some question if localized lift can generate more
convective showers during the afternoon tomorrow. Forecast
soundings are at least marginally supportive of this,
particularly northwest of I-95, where residual colder air at
midlevels combined with a well-mixed boundary layer may permit
development of isolated/scattered showers. Felt compelled to
include a mention of isolated thunder during the afternoon
given the indications of positive buoyancy during peak heating.
This is conditional, however, as transient ridging upstream of
the departing shortwave trough may preclude sufficient lift
necessary for the development of any convection.


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.

Tonight...MVFR/IFR ceilings continue, along with areas of MVFR
visibility due to fog. The extent of the fog is less certain at this
time. Some showers are expected to develop between 03Z and 06Z at
KRDG and KABE and toward 09Z at KTTN, KPNE, KPHL and KILG. East-
northeast winds mainly 10 knots or less, becoming southeast late.

Monday...MVFR/IFR conditions should improve to VFR during the
afternoon, however some showers should continue through the day
(most coverage in the morning north and west of KPHL). A
thunderstorm may occur during the afternoon mainly north and west of
KPHL, however coverage should be isolated. The chance of showers is
lower at KACY amd KMIV, although there may be some drizzle in the
morning. Southeast winds up to 10 knots, turning south during the
morning then southwest in the afternoon.

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.


Marginal but persistent small craft advisory conditions will
continue through Monday. East to southeast winds 10-20 kts with
higher gusts will occur, especially this evening and again
tomorrow afternoon. Seas will likely remain elevated (above 5
feet) offshore the New Jersey coast through at least tomorrow
afternoon. As a result, extended the small craft advisory
through 7 pm Monday.

Another concern is fog tonight and Monday, with dew points
expected to surge to values above sea surface temperatures late
tonight and tomorrow. Visibilities will likely lower to around a
mile at times late tonight and tomorrow and may become less than
a mile locally. At this time, not confident enough in issuing an
advisory, but will monitor closely tonight for potential need
for statements or advisories.

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/Gorse
Short Term...CMS
Long Term...Franck
Tides/Coastal Flooding...
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