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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KPHI 191330
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
930 AM EDT Wed Jul 19 2017

.SYNOPSIS...
A cold front will approach the region slowly today and Thursday
before stalling in the Mid-Atlantic Friday and Saturday. A wave
of low pressure along the front may lift it northward on Sunday
before dragging a cold front through the area early next week.
High pressure will build in behind the cold front by the middle
of next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
-- Changed Discussion --
Abundant sunshine as high pressure becomes entrenched over the
Mid-Atlantic. Hot and humid conditions in place as highs top off
in the low 90s for most areas, and in the mid to upper 80s
along the coasts and up in the Pocono Mountains. Surface
dewpoints start out in the upper 60s across the Poconos and NW
NJ, in the lower 70s for most of the forecast area, and into the
mid 70s in the Delmarva. Going through the afternoon, SW winds
increase to 5-10 MPH, and this may be enough to allow for some
afternoon mixing which would bring dewpoints down in the upper
60s to around 70. As a result, max heat index should fall just
short of 100 (topping off at 97-99) across the urban corridor
from around Somerville to Trenton to Philadelphia, and from
100-102 from Wilmington to Georgetown, DE. This should be just
short of Heat Advisory criteria.

Closed H5 low centered over the Carolinas opens up this
afternoon and begins to drift offshore. This allows several
strong H5 shortwaves associated with it to lift to the north and
west from the ocean towards NJ. An axis of isolated to
scattered showers and thunderstorms should develop across the
region this afternoon, but highest chances will be mainly south
and east of a line from Somerville, NJ to Doylestown, PA to
Wilmington, DE. Afternoon SB CAPE values will be 1000-1500 J/kg,
a Lifted Index as low as -2C, and 0-6 km Bulk Shear of 10-20
KT. PWATs will be from 1.5-1.75", which are not that high for
this time of the year, but any storms that develop will be slow
moving, and may result in localized poor drainage flooding.
Given the scattered nature of these storms, will cap PoPs at low
chance.
-- End Changed Discussion --

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
-- Changed Discussion --
Any lingering showers and thunderstorms taper off this evening
with loss of diurnal heating. Low level moisture increases once
again as surface dewpoints rise back up into the lower 70s
across much of the forecast area. Another warm and muggy night
on tap with lows in the upper 60s to low 70s, and in the mid and
upper 70s in Philadelphia.

Patchy fog may develop once again late tonight.
-- End Changed Discussion --

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
-- Changed Discussion --
Main forecast challenges in the long term will be the heat
through this weekend and frequent chances for storms Thursday
night through at least early next week.

The general synoptic pattern includes amplification of a
longwave trough in eastern North America during the period,
aided by the progression of a fairly potent shortwave trough
along the U.S./Canada border late this week, and the
retrogression of subtropical ridging from the southeastern U.S.
to the southwestern U.S. Main forecast uncertainties this period
include the speed and rate of amplification of the shortwave
trough as it approaches the Great Lakes this weekend and the
subsequent impacts on the larger-scale trough early next week.

The 00Z operational models are in fairly poor agreement
regarding the speed of the shortwave trough, with about a
24-hour spread in timing between the fast ECMWF and slow CMC by
Sunday in the Great Lakes region. These discrepancies only
worsen as the upscale impacts occur with the longwave trough in
eastern North America early next week, with the CMC anomalously
deep/slow with the large-scale troughing along/east of the
Appalachians and the ECMWF on the fast fringes of consensus in
moving the trough offshore by next Tuesday. Though the GFS is in
the middle of the pack with timing, the depth of the longwave
trough is on the flat side of the model consensus, which does
not gel with the amplification of upstream ridging particularly
well. This is especially suspicious given its noticeably deeper
upstream trough in the Pacific Northwest.

Forecast rationale was to use mostly a blend of the ECMWF and
CMC late this week and this weekend, given the suspiciously flat
look of the GFS with large-scale troughing during this period.
However, increased the weighting of the GFS and decreased the
weighting of the CMC with time as the deep/slow progression of
the longwave trough becomes more and more anomalous with the CMC
solution.

Specific forecast details: There appear to be three periods of
elevated chances of convection through Monday. The first is
Thursday night, as a convectively-enhanced perturbation rapidly
progresses east-southeastward through the Great Lakes into the
Northeast. The behavior of the perturbation will be strongly
tied to the development of a mesoscale convective system in the
Upper Midwest and its progression in the northern tier of the
country thereafter. Forecasts of such convective evolution are
low-skill, in general, and the fairly jarring shifts in
track/speed with subsequent model runs and poor agreement among
the models for a particular model start time are bearing this
out. Nevertheless, the simulated convection in the GFS looks
particularly suspicious, as it appears to occur too far upstream
of the midlevel perturbation (unlikely). The northward shift of
the ECMWF with QPF is plausible, however, given models`
tendencies this year to underestimate the strength of
subtropical ridging. As such, kept highest PoPs Thursday night
generally north of I-78, with the highest chances likely to be
to the north of the CWA. Transient ridging upstream of the
perturbation will likely keep much of Friday and Friday night
dry, so the lowest PoPs of the extended occur during this
period.

The second period of more elevated chances of convection occurs
Saturday and Saturday night, as another perturbation moves
rapidly eastward from the Midwest to the northern Mid- Atlantic
region. With a stalling front across the area by this point,
smaller-scale lift will likely aid in the
development/maintenance of a progressive convective cluster
moving along the quasi-zonally oriented boundary during this
time. Though the CMC looks overdone with upscale growth,
including the development of a compact surface low, suspect the
prolonged southerly fetch of warm/moist air will promote
considerable instability and moist convergence for potentially
strong convection associated with the convective cluster
(potential MCS). The ECMWF has a similar look to the overall
evolution, which lends some confidence that this scenario will
pan out. Main question is timing, as the perturbation looks to
move through the region generally somewhat after peak heating.
This suggests best chances for convection will be in the
northwest CWA with gradually diminished chances southeastward
(with convection generally waning as it moves through the area).
For now, kept chance PoPs for most of the area during this time
frame.

The parent shortwave trough approaches the region late this
weekend into early next week, with timing becoming more and more
uncertain by this point. Nevertheless, an associated cold front
will likely approach the area, and synoptic-scale
lift/dynamical support improve for the development of fairly
widespread convection in the Sunday-Monday period. Given
uncertainty of timing, kept PoPs elevated (though capped at
chance) through this period. Importantly, however, convection is
not expected to occur throughout this time frame...it is just
not possible to pinpoint the timing more so than this at this
point.

Tuesday and Wednesday, PoPs slowly decrease, but did not remove
them entirely given the potential for the large-scale trough to
stick around.

Regarding temperatures...Thursday and Friday look hot. Forecast
highs may be a little conservative, particularly Friday when
shortwave ridging may provide extra subsidence/compressional
warming. However, some mixing should also occur, so dew points
may lower into the upper 60s to around 70 during max heating.
With forecast highs in the mid 90s in the urban corridor, this
would keep heat indices just shy of advisory criteria. Notably,
the Delmarva region will likely have somewhat higher dew points,
with heat indices easily exceeding 100 and approaching 105
during peak heating. An advisory certainly cannot be ruled out
here.

Convection or its debris suggest Saturday and Sunday may be
somewhat cooler. After cold frontal passage early next week,
temperatures may actually be somewhat less than average by the
end of the period.
-- End Changed Discussion --

&&

.AVIATION /14Z WEDNESDAY THROUGH SUNDAY/...
-- Changed Discussion --
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Patchy stratus until about 14Z, then predominantly VFR.

Isolated to scattered SHRA/TSRA possible again this afternoon.
Best chances will be from KTTN-KPNE-KPHL-KILG, but confidence of
a storm developing over any given terminal is low, so will not
mention in the TAFs at this time.

Generally VFR conditions continue late tonight into Thursday
morning. Patchy fog possible once again.

LGT/VRB to calm winds through mid-morning, then W-SW winds 5-10
KT. Winds become nearly calm after 00Z Thursday.

OUTLOOK...
Thursday through Friday night...Generally VFR. Chance of storms
generally north of KPHL Thursday night. Patchy fog possible
near dawn in favored rural/valley locations. Winds generally
west at or below 10 kts. Confidence slightly above average.

Saturday through Sunday...Increased chances of thunderstorms,
with sub-VFR conditions likely in their proximity. Winds
generally light and somewhat variable during the period, except
stronger/erratic near any storms. Confidence somewhat below
average.
-- End Changed Discussion --

&&

.MARINE...
-- Changed Discussion --
SW flow becomes S this afternoon and increases to 10-15 KT on
the ocean waters, and remains 5-10 KT on DE Bay. Ocean seas will
average 2-3 feet, and waves on DE Bay will be 1-2 feet.

Winds diminish tonight.

OUTLOOK...
Sub-advisory conditions are forecast through the period.
Chances for storms exist through the period, especially Thursday
night in the New Jersey coastal waters and Saturday-Sunday
everywhere.

RIP CURRENTS...
Due to an an underlying 10 to 12 second southeasterly swell
that has shown up on the ocean waters the last couple of days,
will go ahead and factor that into the rip current forecast. As
a result, for this afternoon, will expect a moderate risk for
the development of dangerous rip currents at NJ ocean beaches.
Since the wind flow will be a bit more offshore at DE ocean
beaches, will continue to forecast a low risk for the
development of dangerous rip currents at DE ocean beaches.

Remember! Low risk does not mean no risk, so swim in guarded
beaches and take precautions when going into the water.
-- End Changed Discussion --

&&

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

&&

$$

Synopsis...CMS
Near Term...MPS/Iovino
Short Term...MPS
Long Term...CMS
Aviation...CMS/MPS
Marine...CMS/MPS