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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KPHI 020748
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
348 AM EDT Tue Jun 2 2020

.SYNOPSIS...
A warm front is expected to lift through the region later today.
It will be followed by a cold front approaching our region
tomorrow as low pressure moves out of the Great Lakes. That
front is expected to stall as a stationary front and remain
over the Mid Atlantic region through the end of the week.
Another cold front will then approach Friday or Saturday as
another area of low pressure moves out of the Great Lakes. High
pressure should finally start to build back in from the west by
later in the weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
One last comfortable day with seasonable temperatures and reasonable
humidity levels across the area. An approaching warm front will be
close enough to bring occasional high and mid level clouds to the
area today. A couple showers are possible for the wrn areas this
afternoon (mostly) but only Berks County will have any pops higher
than slight chc. High temperatures will be mostly in the low to mid
70s for most areas with some 60s up on the higher elevations across
the far NW areas. Winds will turn SW and be 5 to 10 mph this morning
and 10 to 15 mph this afternoon.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
Clouds will continue to increase and lower across the area
tonight as a warm front approaches from the west. Scattered
showers are expected later tonight, especially across the
northern areas where chc pops are in the fcst. We have included
a slight chc for thunder across the N areas towards dawn as
upper CAPE increases. It will be a mild night as the humidity
levels become more noticeable. Lows will only drop into the
mid/upper 60s for Delmarva and srn NJ while up north lows will
be in the upper 50s to low 60s. Winds will remain mostly from
the S at 5 to 10 mph.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Overview: The focus in the long term period remains on the front
which is expected to approach our region tomorrow as a cold
front before stalling over the Mid Atlantic through the
remainder of the week. As a result, expect several rounds of
showers and storms from Wednesday through Saturday. Tranquil
weather conditions should return at the beginning of next week
as high pressure builds south.

Details:

Wednesday...On Wednesday, the primary focus is on the potential
for severe storms, specifically with damaging winds as the
primary hazard. There could be two main rounds of showers and
storms on Wednesday. The first could come Wednesday
morning/early afternoon as a short wave trough digs southeast
and off shore. This will be well ahead of the main front. As
mentioned by the previous shift, not all models depict this
round developing. However, if storms do develop this early, it
would primarily affect E Central PA and northern NJ, and should
serve to decrease the instability those areas ahead of the
arrival of the front, thus limiting the severe storm risk in
these areas. The second round ahead of and with the arrival of
the cold front is expected late Wednesday afternoon and
Wednesday evening. For areas that don`t see earlier convection,
staying in the warm sector most, if not all, day will allow
temperatures to be in the upper 80s or even lower 90s by the
time we see storms propagating into the region. Consequently, we
should see surface based instability. It is interesting to note
though that many of the operational models depicted a much more
robust inversion between 800 and 700 mb (as compared to model
soundings from yesterday`s runs). Consequently, instability
profiles are skinnier, and most models are now depicting CAPE
values in the 1000 - 1500 J/Kg range - still more than
sufficient for strong updrafts but a far cry from the 3000 J/kg
that the NAM was depicting at one point. 0-6 km bulk shear
values near 40 kt should promote sustained updrafts.

As for hazard types, as mentioned above the primary concern is
damaging winds as 50kt + winds from 700 or even 850 mb may be
able to mix down with a strong enough updraft. With a rather
skinny instability profile withing the hail growth region and
the melting layer above 10K ft AGL, the hail threat should be
limited, though not completely gone. As for the tornado threat,
model hodographs depict nearly unidirectional flow through the
tropopause until well after 00Z, which would significantly limit
the threat. Having said that, will have to watch how quickly
the front sinks down as some directional shear will be possible
right within the vicinity of the front.

Thursday...By Thursday, attention shifts from severe winds to
heavy rain potential. While the best synoptic scale lift will
likely be west of our region, mesoscale lift along the stalled
front could be enough for convective initiation. The big
question then becomes where that front will be. At this point,
it looks like it will be somewhere south of the PA turnpike/195
corridor. While the best lift will be west of us, and model QPF
values aren`t especially impressive, there are a few big factors
which could increase the heavy rain threat. First, mean flow
looks like it will be nearly parallel to the stalled front,
increasing the risk for training storms. Secondly, expect a deep
warm cloud layer (as deep as 12K ft) by late Thursday.
Precipitable water values also look to be well above normal. At
this point, the highest risk for heavy rain in our area should
be over northern Delmarva, where flash flood guidance is higher.
However, if we get training storms just 30 to 50 miles further
north, it could affect portions of the urban corridor where it
will take much less heavy rain to cause flash flooding.

Friday and Saturday...A few more rounds of showers and storms
are possible Friday and Saturday, though there is considerably
more uncertainty through this period. The stalled front weakens
as frontolysis takes over, and the highest potential for
additional showers look to come with a mid level short wave
trough on Friday, followed by another cold front approaching the
region on Saturday.

Sunday and Monday...High pressure should build southward over
our region leading to a return to tranquil, albeit cooler,
conditions.

&&

.AVIATION /07Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Today...VFR conditions are expected through the day with slowly
increasing clouds. A couple showers NW are possible this afternoon,
but not enough of a chc to include in the TAFS attm. Winds will be
light from the W this morning and then become SW at 8 to 12 knots
this afternoon. Moderate confid.

Tonight...VFR conditions are expected to continue as clouds bases
continue to lower. Some OP models are showing some showers arriving
across the N areas after midnight, so some lower conditions are
possible there. The rains seem to stay north/east of KABE/KTTN so we
will not include any lower CIGS or VSBYS in the present tafs. Winds
will be S to SW around 10 knots. Moderate confid.

Outlook...

Wednesday...Prevailing VFR, though a period of MVFR or even IFR
conditions will be possible primarily late Wednesday into
Wednesday night as showers and storms cross the region. Variable
and gusty winds will be possible with the strongest storms.
Outside of the storms, winds should be westerly around 10 to 15
kt becoming light northwesterly overnight. Moderate confidence
in the overall pattern, but low confidence in the timing and
extent of storms.

Thursday...Prevailing VFR, with a period of MVFR/IFR Thursday
afternoon and evening with showers and storms. Visibility
restrictions in heavy rain will be possible. Light northwesterly
winds becoming light southwesterly in the afternoon then light
and variable overnight. Low confidence.

Friday and Saturday... Mostly VFR, but additional rounds of
showers and thunderstorms are possible especially Friday, which
could result in brief MVFR conditions. Light southwesterly
winds Friday becoming light northwesterly on Saturday. Moderate
confidence.

&&

.MARINE...
High pressure will continue to move away from the waters while a
warm front approaches from the west. Sub-SCA conditions are expected
today and tonight. Winds will slowly increase tonight though with
some gusts around 20 knots expected by dawn. Overall, fair weather
is expected today and much of tonight with only a small chc for a
shower tonight.

Outlook...

Wednesday through Saturday...winds and seas should stay mostly
below SCA criteria through the period with the following
exceptions: First, on the coastal waters, wave heights could
approach 5 feet especially Wednesday into Thursday. Secondly,
several rounds of showers and storms will be possible through
this period, locally higher winds and waves will be possible in
the vicinity of stronger storms, especially on Wednesday.

Rip currents...

A LOW risk for rip currents is forecast for today and Wednesday.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...Johnson
Near Term...O`Hara
Short Term...O`Hara
Long Term...Johnson
Aviation...Johnson/O`Hara
Marine...Johnson/O`Hara