Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 240723

Area Forecast Discussion
National Weather Service Mount Holly NJ
323 AM EDT Tue Apr 24 2018

A low in the Southeast will progress slowly northeastward through
the Mid-Atlantic region, bringing widespread rain to the area
Tuesday night and Wednesday. Another low to the south may move
northeast near or off the East Coast late this week. Broad high
pressure will build into the eastern United States this weekend into
early next week.


1035 mb high pressure remains anchored well south and east of Cape
Cod. Meanwhile, low pressure lies over the Southeast U.S., and will
lift to the north and east throughout the day today.

The offshore high will be slow to depart today, and based on latest
model trends, seems to have an influence on how far north and east
the low will be able to make today. As a result, the latest models
seem to have slowed down the onset of precip associated with the

00Z NAM/RGEM/GFS/ECMWF and even the 00Z HRRRX all seem to keep much
of NJ dry until 00Z, and only light precip through the Delmarva and
southeast PA. As a result, went ahead and lowered PoPs across the
region for most of the day, and will hold off on bringing likely
PoPs into southwest portions of the CWA until towards the end of the
first forecast period.

Onshore flow will keep coastal areas much cooler than inland areas,
and highs will only be in the 50s for coastal NJ/DE, and in the
lower 60s for much of the Coastal Plain of NJ and interior portions
of the Delmarva.


High pressure over the western Atlantic drifts out to sea this
evening, and then low pressure over the Southeast lifts into the Mid-
Atlantic by midnight tonight, then continues to lift through the
Delmarva by Wednesday morning, then continues to track to the north
and east and will be north of NYC by early Wednesday evening. As an
upper trough digs into the Northeast, models indicating a weak
secondary low possibly forming over the Mid-Atlantic, and this low
lifts through the region during the day Wednesday.

Onshore flow ushers abundant low level moisture into the region
mainly from after midnight tonight through around daybreak
Wednesday, and this will be the window for potentially heavy rain.
PWATs will range from 1.25-1.5".

Showers generally taper off from south to north Wednesday
morning/afternoon, but another round of showers is possible as that
weak secondary low lifts through the region.

Main forecast concern in the long-term period is the large model
disagreement remaining with a system`s track from the Southeast to
the western Atlantic late this week and potential impacts, if any,
in the northern Mid-Atlantic.

The system affecting the area on Wednesday will be on its way out
Wednesday night. At 00Z Thursday, the surface low will be in the
vicinity of New York City, with lingering showers possible as
the main vort max pushes east through the northern Mid-Atlantic
and adjacent New England. There is actually quite a bit of
disagreement regarding the track of the vort max, with the GFS
considerably farther north than most of the other deterministic
models/solutions. Generally excluded it for sensible weather
Wednesday night, with the result being higher PoPs most of the
night (though any rainfall would be light). There may be some
isolated showers in the Poconos that linger through Thursday,
but generally have low to unmentionable PoPs after 12Z Thursday
for now. Nevertheless, a farther south track of the vort max
would necessitate higher PoPs in this region through the day
(and increased cloudiness).

Not a lot of cold air upstream of the system, and the cold advection
generally ceases after 12Z Thursday, so max temperatures on Thursday
look seasonable. Agreement among statistical guidance is below
average (and remains so through the long term). Used a bias-
corrected blend of raw operational guidance for temperatures
Wednesday night through Saturday, given the rather variable output
among guidance in general and the fairly changeable conditions
during this period.

Difficult forecast Thursday night, as models are all over the place
with a surface low moving through the southeastern U.S. This low
will be tied to a vort max in the Mid-South on Thursday lifting
northeastward Thursday night as an upstream positively-tilted trough
digs into the Mississippi Valley. The track of the downstream vort
max is highly sensitive to the speed of the upstream trough, with
the GFS slower, allowing for a sharper turn to the northeast (and a
closer track to our region). The ECMWF is stronger but much faster
with the southern-stream portion of the Mississippi Valley trough,
and this kicks its simulated surface low farther offshore. The ECMWF
keeps any precipitation tied to the low in southern/eastern portions
of the CWA late Thursday night and Friday. Notably, the ICON and CMC
also have drier, more progressive, and somewhat shallower solutions.
The GFS indicates rainfall of a quarter to three quarters of an inch
would be possible across most of the area with a direct hit.

The discrepancies in solutions necessitate keeping PoPs rather low
(generally below 50 percent) across the area during this period
(given the tip of the scales to a drier solution), but I think
maintaining higher than climatology PoPs is warranted, especially
near the coast. Implications on temperatures are profound, with a
closer track of the system meaning cooler/cloudier conditions. Large
errors with temps/sky cover/winds are possible this period, owing to
the array of solutions apparently feasible.

Thereafter, the northern-stream vort max digs into the Great Lakes,
New England, and southeast Canada. An attendant cold front will move
slowly through the Northeast on Saturday. Think the northern CWA
will be close enough to the vort max (spatiotemporally) to see some
instability showers as surface heating allows for increased lapse
rates during the day and enhanced convergence (via orographic
effects in addition to the larger-scale front) provides the
necessary source of lift to generate precipitation. Kept PoPs for
most of the area Friday night and Saturday, with a diurnal maximum
Saturday afternoon, particularly north of the I-76 corridor.
Rainfall amounts should be light, and showers should diminish
rapidly after sunset Saturday night. Again, all of this hinges on
the proximity of the vort max to the area, so there is a decent
chance of nothing precip-wise during this period.

High pressure builds into the eastern U.S. Sunday through Tuesday
with dry conditions through this period. Some cooling will occur
Sunday given the origins of the high (and trough passage in the
Northeast), but a quick warmup will occur thereafter as midlevel
ridging amplifies rapidly as it becomes anchored over the eastern
half of the country. Temperatures should return to seasonal by
Monday and likely much warmer than that the following days.


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

Today...VFR conditions will begin the day across the TAF sites
through this afternoon. MVFR CIGs lift from south to north late in
the day. Rain will also be moving in from the south late in the
afternoon. East winds 10-15 kt throughout, with 20-25 kt gusts

Tonight...MVFR/IFR conditions in rain and fog. East winds 10-15 kt,
diminishing to less than 10 kt late.

Wednesday...Rain ends from south to north in the morning. Scattered
showers persist through the afternoon. Flight categories improve to
VFR late in the day. East winds less than 10 kt become LGT/VRB in
the middle of the day, then shift west in the afternoon.


Wednesday night: Lingering/local sub-VFR conditions possible in
the evening with scattered showers, but improvement to VFR will
gradually occur with time. West to northwest winds 5 to 15 kts.
Low confidence.

Thursday: Probable VFR, but SCT-BKN CIGs around 4000-5000 feet
possible during the day. West to northwest winds 10 to 15 kts with
occasional gusts 20-25 kts. Moderate confidence.

Thursday night and Friday: Increasing cloudiness, with a slight
chance of showers, especially south of PHL. Some potential for sub-
VFR, but confidence is low as the track of the associated system is
unclear at this time. Light and variable winds Thursday night are
anticipated to become southerly on Friday.

Friday night and Saturday: Predominantly VFR, with a slight chance
of showers north of PHL during the day. Winds changing from south to
west or even northwest on Saturday, potentially with a few gusts
during the afternoon. Moderate confidence.


Small Craft Advisory conditions develop this afternoon on
all waters as a tight pressure gradient forms between departing high
pressure over the western Atlantic waters and low pressure
approaching from the Southeast. Winds gusting to 25-30 kt expected
on all waters from late this afternoon through around midnight
tonight, then winds diminish on DE Bay late. For the ocean, strong
winds continue through tonight, and although winds diminish to 15-20
kt by Wednesday morning, seas remain elevated and above SCA levels
through the day Wednesday. VSBY restrictions in moderate to locally
heavy rain and fog expected from this evening through Wednesday
morning, then scattered showers possible through the day Wednesday.


Wednesday night and Thursday: Seas over the Atlantic should remain
above advisory criteria, but winds will likely be below criteria
everywhere. Some lingering showers Wednesday night, but any
visibility restrictions should improve Wednesday night.

Thursday night and Friday: A chance for rain and visibility
restrictions, though confidence is low. Winds/seas likely below
advisory criteria.

Friday night and Saturday: Winds/seas below criteria, with fair
weather on Saturday.


MARINE...Small Craft Advisory from noon today to 6 AM EDT Thursday for
     Small Craft Advisory from noon today to 5 AM EDT Wednesday for


Near Term...MPS
Short Term...MPS
Long Term...CMS
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