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000 
FXUS61 KPHI 252247
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
547 PM EST Sat Feb 25 2017

.SYNOPSIS...
In the wake of the strong cold front moving through our region 
Saturday afternoon, cooler air will settle in over the region. High 
pressure then builds south of the region on Sunday before moving 
offshore on Monday. Low pressure approaching from the west will lift 
a warm front through the region on Tuesday. The aforementioned low 
will propagate from the Great Lakes region into eastern Canada.
The associated cold front will move through Wednesday night or 
Thursday. A quick moving low pressure system will slide over the
Mid Atlantic at the end of the week.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
545 pm update: Line of strong to marginally severe storms is
moving east through New Jersey and Delmarva at this time, with a
general downward trend noted in the past hour. Isolated severe
wind gusts remain possible as the storms continue toward the
Atlantic early this evening. Watch is being cleared gradually on
the west side, and has been done for areas generally northwest
of the Fall Line.

500 pm update: Expanded watches to include all of our New Jersey
counties and adjacent coastal waters.

Cold pools of individual cells have congealed as expected, and a
more contiguous line of storms has developed from Mount Pocono
to Wilmington to Easton. Several reports of hail up to quarter
size and damaging winds. Storms should begin a weakening trend
shortly as nocturnal boundary layer cooling begins and near-
surface wind trajectories become more influenced by the cooler
ocean waters. Nevertheless, present storm organization suggests
isolated severe weather remains a concern through mid-evening.

345 pm update: Severe thunderstorm watch 39 expanded to include
Sussex County, DE, and the lower portion of Delaware Bay.

Previous discussion below...

Negatively-tilted trough is approaching the Northeast this
afternoon with considerable large-scale ascent present from the
Carolinas northward to New England, thanks in large part to
strong differential cyclonic vorticity advection downstream of
the trough axis and enhanced upper-level divergence via jet
coupling. The warm sector has destabilized more than forecast 
this afternoon, with SBCAPE exceeding 1000 J/kg in much of the 
CWA south of I-76. With very strong shear profiles present 
(effective bulk shear 50+ knots; 0-1 km SRH ~100-150 J/kg), 
environment is quite favorable for severe storms generally along
and west of the Delaware River this afternoon pushing toward 
the coast this evening. The orientation of the deep-layer shear 
vectors is generally parallel to the cold front approaching the 
East Coast this afternoon. However, the strength of the low- 
level shear and the increased CAPE from afternoon surface 
heating has enabled more of a mixture of line segments and 
discrete cells as the storms have progressed eastward. This has 
increased the hail and tornado threat with the storms somewhat, 
particularly south of the I-76 corridor. As storm interactions 
continue and large- scale ascent continues to increase, cold 
pool interactions should become more pronounced, and convection 
is expected to congeal into a larger, more contiguous line, 
which should make damaging winds the primary threat as the event
wears on. This process already appears to be occurring with 
storms along the PA/MD border, for example.

Severe threat is generally lower for the counties immediately
adjacent to the Atlantic, as trajectories from the cooler ocean
waters have limited surface heating considerably. High-
resolution models continue to show a rapid diminishing trend in
convection as storms move east of the Delaware River this
evening.

Cold front will move through the CWA this evening, with storms
exiting the area by 03Z or shortly thereafter. Winds will
quickly veer to westerly and become gusty after frontal passage.
Temperatures will plummet behind the front, with temperatures
expected to bottom out in the middle 20s in the southern Poconos
to the middle 30s along the coast.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/...
Surface high will move through the Southeast on Sunday, but the
pressure gradient on the northeast side of the high will permit
a rather breezy day across the area, especially during the
morning hours. Cold air advection on the upstream side of the
potent vort max moving through the area today will make for a
sharply colder day. High temperatures are forecast to be near
seasonal averages, or about 30 degrees colder than what we saw
today.

&&

.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
Few changes made in the long term so as to focus on current hazards 
this evening.

Unsettled weather pattern is on tap through the week.

Monday and Tuesday are very low confidence especially with respect 
to precip chances. As long as the high isn't more progressive in 
shifting off shore, during the day Monday should stay dry. Monday 
night into Tuesday there is low confidence as to the extent of rain 
ahead of the arrival of the warm front and with the timing of the 
warm front. Models have continued their run-to-run jumpiness with 
the timing of this front.

Wednesday and Wednesday night looks to be the highest chance of 
precip with the region solidly in the warm sector. Thunderstorms may 
be possible as well if we can warm up enough to realize some modest 
surface based instability.

Cold front still on track to move through late Wedensday night or 
Thursday. Once it moves through, we should see a lull in the precip, 
albeit a brief one.

The next question will be the timing and track of a quick moving low 
which could move across the region Friday or Friday night. Moisture 
may be limited as there will be strong dry air advection behind the 
cold front, but there is some chance for precip around the area.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, 
KILG, KMIV, KACY and surrounding areas.

21Z amendments: Tweaked the timing of storm passage and
increased wind gusts expected with the storms given recent obs.
Isolated severe wind gusts (50+ kts) possible.

For the 18Z TAFs...Mostly VFR before storms move in this
afternoon. Timing looks to be 19-23Z for KRDG/KABE, 20-00Z for
KPHL/KPNE/KTTN/KILG, and 22-02Z for KMIV and KACY. Sub-VFR
VSBYs/CIGs expected as the precipitation moves through, with
gusty/erratic winds 30-45 kts possible in proximity to the stronger
storms. Winds will quickly veer to westerly or northwesterly 
after shower/storm passage late this afternoon/evening, with 
winds gusting 20-30 kts through tomorrow afternoon before 
diminishing.

Outlook...
Sunday night and Monday...VFR conditions expected.

Monday night through Thursday...periods of rain and showers are 
possible. When/if this does occur, MVFR ceilings or lower are 
possible. Highest chance is Wednesday and Wednesday night. There is 
considerable forecast uncertainty in this period.

&&

.MARINE...
Solid small craft advisory conditions expected through Sunday
afternoon, with potential for gale force gusts both with the 
passing line of storms this evening and for about a six-hour 
period after the cold front moves through the area this evening.
Threat continues to look too brief/marginal to upgrade to a 
gale warning, but will continue to monitor. 

Current obs continue to indicate patchy fog along the New Jersey
coastal waters, but this should rapidly diminish as storms
approach the area by 6 pm. Currently expect the dense fog
advisory to be allowed to expire as scheduled.

Outlook... 
Sunday night and Monday...wind gusts to 25 kt may 
linger for a few hours on Sunday evening, but will be below SCA 
criteria for the remainder of this period.

Tuesday through Wednesday...winds and seas should generally stay 
below SCA criteria, though wind gusts above 20kt will be possible on 
Wednesday. Periods of rain/showers possible, especially on Wednesday.

Wednesday night and Thursday...Northwesterly wind gusts around 25 kt 
are possible

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Dense Fog Advisory until 6 PM EST this evening for ANZ450>453.
     Small Craft Advisory until 6 PM EST Sunday for ANZ450>455.
     Small Craft Advisory until 6 PM EST Sunday for ANZ430-431.

&&

$$
Synopsis...Johnson

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