Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 022248
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
648 PM EDT Tue Jun 2 2020

.SYNOPSIS...
A warm front lifts north and east of our area later tonight. A cold
front approaches our region Wednesday as low pressure moves out of
the Great Lakes. That front is expected to become nearly stationary
and remain over the Mid-Atlantic region through the end of the week.
Another cold front will then approach Friday or Saturday as 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 AM WEDNESDAY MORNING/...
As of 630 PM, a warm front continues to become better organized to
our west and southwest. This front will lift across our region later
tonight. Ongoing low-level warm air advection and the presence of
northwesterly flow aloft is resulting in mostly mid level cloudiness
across the area. This is thinning some to the west. A weak short
wave embedded in the northwest flow aloft may result in some
sprinkles this evening, otherwise we wait for a stronger short wave
overnight. Just made some tweaks with this update to keep the
forecast current with the latest observations and trends.

Otherwise, a better chance of heavier showers and some thunderstorms
looks to occur late tonight into the pre-dawn hours of early
Wednesday morning. The warm front will lift across the area
overnight, while a stronger short wave moves across the area. At the
same time, instability (mostly elevated) is on the increase with
values rising to and above 1000 J/kg. This is expected to lead to an
area of showers and some thunderstorms, especially across the
northern half of the area. There will be enough shear of 40-50+
knots given an increase in the flow around 850 mb to potentially
lead to some stronger storms overnight. The CAPE profile on the
forecast soundings are thin but fairly tall therefore updraft
strength may be on the weaker side. There is the potential for some
small hail with a stronger core, and if a stable layer near the
surface is weak or non-existent then locally gusty winds will be
possible. PW values increase to 1.25-1.5 inches, so there will be a
possibility of brief heavy rainfall with the stronger cores.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
On Wednesday, there will be a couple of rounds of showers and
thunderstorms, with the possibility of some storms becoming severe.
After the overnight showers and thunderstorms move offshore by
daybreak, a few hours of rain free conditions are expected as the
warm front will have lifted to our north and we will be in between
short waves/vorticity impulses. There may actually even be some
partial clearing through the morning hours. However, this will be
temporary as the first of a couple of short waves/vorticity impulses
move into the area by midday, while a pre-frontal trough develops
and moves across the area as well. With CAPE values remaining in the
1000-2000 J/kg range, an area of showers and thunderstorms is
expected to move across the area, especially northeast Pennsylvania
and northern/central New Jersey by late morning into early
afternoon. As the afternoon progresses and we approach the evening
hours, the cold front will be slowly moving into the area while
slowing down. Showers and thunderstorms are then expected to develop
along the cold front as it sags southward and a couple of more short
waves/vorticity impulses move across the area. With the instability
forecast, plus shear values of 35-45+ knots, some storms will have
the potential to become severe. The flow is fairly unidirectional,
and with 700-500 mb winds of 40-50 knots, damaging winds will likely
be the main threat. However, with mid-level lapse rates of 7.0-7.5
C/km, there will also be the chance for some large hail for the
tallest storms even though the freezing level/wet bulb 0 heights are
fairly high.

The thunderstorms will continue into the evening hours as the front
begins to stall across the area. Instability will begin to wane
during the evening hours progress, therefore the threat for more
widespread thunderstorms and severe weather will diminish as the
evening moves into the overnight hours, although there will be a
chance of showers through the night as multiple vorticity impulses
move across the area.

Also, the heat and humidity builds. Temperatures will warm well into
the 80s, even lower 90s for some areas. However, dewpoints will be
low enough to keep heat index values from reaching Heat Advisory
criteria, although it will be a hot and humid day.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Heading into Thursday the region will be dominated by a stalled
frontal boundary. The question is where will that boundary
eventually stall and then the dominos will start to fall as a
result. Based on a consensus of the guidance there is still too
much spread to feel confident in exactly where the front stalls,
however we are increasing in confidence in the overall threat
of heavy rainfall.

From a synoptic standpoint, we`ll be under zonal flow aloft with a
modest west to east jet axis to our north. By Thursday morning,
the right rear entrance region will be generally over western
to west central PA. We`ll see upper level divergence moving over
northern VA into DelMarVa through the day which should promote
modest lift locally. While this happens, we`ll see moisture
increase from the south and west and becomes maximized in a
narrow ribbon somewhere along the Maryland/PA state boundary.
The strongest axis of 850mb- 300mb moisture transport pushes
right over the PHL metro area and we`ll see PWATS in the area of
1.7" to 1.8". The other fly in the ointment is that the mean
flow aloft will become parallel to the surface boundary meaning
training storms will be possible. Projecting out the potential
speed of storms, BUFKit soundings suggest MBE vectors will drop
below 15 kts for portions of the region. This suggests that we
could have slow moving training storms capable of maximizing
efficient rain. While basin wide averages could be quite small,
we could certainly see an axis of convective rainfall on the
order of 1-2 inches and perhaps up to 2-3 of a highly localized
region. Based on coordination with WPC and surrounding offices,
WPC has bumped up the D3 marginal risk to include the PHL metro
and areas south. We`ll certainly continue to monitor and
evaluate where the threat for highest rainfall will end up being
maximized, or hopefully instead the threat doesn`t materialize!

Heading into the weekend, the pattern remains unsettled and active.
We`ll see several rounds of scattered showers and diurnally driven
thunderstorms. Forcing remains around the stalled frontal boundary
and the boundary eventually slides offshore as a weak upper level
trough pushes a stronger cold front through the region Friday night
into Saturday.

We should see a reprieve in the active weather starting the new week
as high pressure and quiet sensible weather returns. Temps should be
a tad on the chilly side to start next week but it will be under
light winds and clear skies.

&&

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

Tonight...VFR overall. Some showers and thunderstorms should arrive
from the northwest mainly after 06z and potentially impact briefly
the ABE area to just north of PHL. Southwest winds mainly 10 knots
or less. Low confidence regarding thunder given uncertainty with
coverage.

AMD NOT SKED for RDG due to a comms issue.

Wednesday...VFR overall. Some showers or a thunderstorm may be
around very early, then more widespread showers and thunderstorms
should occur later in the afternoon. Any showers or thunderstorms
could lead to a brief period of MVFR or IFR conditions, and a
stronger storm may produce wind gusts to or above 40 knots. West-
southwest or west winds 12-15 knots, with later morning and
afternoon gusts to 20-25 knots. Low confidence on thunderstorm
details including specific timing.

Wednesday night...Mostly VFR conditions are expected. However, there
will be a chance of showers and thunderstorms continuing into the
evening hours. Wind gusts will diminish by sunset.

Outlook...
Thursday...Prevailing VFR, with a period of MVFR/IFR Thursday
afternoon and evening with showers and storms. Highest chances
for prevailing reduced visibilities will be from PNE/PHL south.
Light northwesterly winds becoming light southwesterly in the
afternoon then light and variable overnight. Low to moderate
confidence.

Friday and Saturday...Generally VFR, but additional rounds of
showers and thunderstorms are possible and could impact the TAF
sites especially Friday. Light southwesterly winds Friday
becoming light northwesterly on Saturday. Moderate confidence.

&&

.MARINE...
Tonight...Conditions expected to remain below advisory levels
tonight, although winds could gust around 20 knots at times.

Wednesday...Conditions expected to remain below advisory levels,
although winds will gust around 20 knots at times, and may approach
advisory levels. However, a low level inversion is expected to keep
the stronger winds from mixing down. There is a chance of showers
and thunderstorms, some which could be strong to severe during the
afternoon.

Wednesday night...Conditions are expected to remain below
advisory levels, although winds could continue to gust around 20
knots at times. A chance of showers and thunderstorms continues
into the evening hours, before chances diminish overnight.

Outlook...
Thursday through Saturday...Winds and seas should stay mostly
below SCA criteria. Several rounds of widely scattered showers
are expected to track through the region and 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...Deal
Near Term...Gorse/Robertson
Short Term...Robertson
Long Term...Deal
Aviation...Deal/Gorse/Robertson
Marine...Deal/Robertson


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