Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 220543
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
143 AM EDT Tue Aug 22 2017

.SYNOPSIS...
High pressure will be centered in the western Atlantic through
Tuesday. A warm front resides in eastern Pennsylvania. A cold front
will approach from the west late Tuesday and slowly exit off the
coast on Wednesday. Canadian high pressure builds into the region on
Thursday and should to be the primary influence on our weather into
next Monday.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
Concerns last night regarding the low-amplitude vort max moving
through the region tonight seem to have been realized, as
isolated showers and even a couple of storms continue to develop
in an arc from northern to eastern Pennsylvania, in close
proximity to the perturbation in the primarily zonal midlevel
flow. High-resolution guidance simply is of little value with
these perturbations, it seems, at least for this warm season.
With last night`s simulations basically precip-free as the
perturbation moves through, each successive run tonight either seeks
to diminish whatever convection develops after an hour or two
with little similarity to reality or depicts a completely
alternative precip evolution to the previous simulation,
either of which basically provides zero confidence in the
subsequent evolution of the convection.

Fortunately, the showers are sparse and weak, but
unfortunately, they are strong enough to tip the buckets at most
sites they pass, which means PoPs are not negligible, despite
what the guidance would have a forecaster think. In this manner,
coarser guidance has been more insightful in at least
suggesting the occurrence of these showers (though overdoing
coverage/intensity, a common problem with parameterizations and
coarse grid spacing). With this in mind, the latest NAM/GFS
suggest the possibility of these sparse/weak showers the rest of
the night, particularly north of the Mason-Dixon Line. As a
result, boosted PoPs to mentionable the rest of the night across
the area, with subtly higher values in eastern PA and NJ.

Other big question mark is fog/stratus development. So far,
stratus has been slow to materialize, though patchy fog is
beginning to occur in the absence of low clouds in the usual
spots. Crunch time is in the 09Z-12Z time frame, where
statistical/operational model guidance both suggest a period of
low stratus/fog may occur in a larger area. With isentropic
ascent increasing late tonight, cannot ignore the guidance,
though it has been overdone so far. Did not change inherited
forecast much in this regard, as confidence is low. However,
made adjustments to sky cover and winds based on latest trends.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TODAY/...
Made some additional changes to the grids for this period.
Biggest change was to fine-tune the progression of PoPs from
northwest to southeast through the area starting mid-afternoon
and to lower PoPs a little bit. The reasoning is there remains
some discrepancy on convective coverage this afternoon with the
latest high-resolution guidance. Strongest larger-scale ascent
associated with a pre-frontal trough should remain generally
west of the region through most of the afternoon, and smaller-
scale ascent in advance of this trough appears rather
weak/nebulous. The latest WRF simulations offer little in the
way of terrain-induced convection in advance of the pre-frontal
trough.

Environment remains favorable for severe storms in northwest
portions of the CWA tomorrow afternoon, so kept or included
wording indicating the potential for frequent lightning, heavy
rain, and gusty winds with storms during the afternoon in areas
generally northwest of Philadelphia.

Otherwise, initially potentially more cloudiness and some fog may
start the day which would then scatter out as the low levels begin
to mix. Southerly flow will strengthen ahead of an approaching upper
trough and its attendant cold front, leading to warm/moist advection
and heat indices to around 100 degrees for the urban corridor from
Wilmington to Philadelphia and Trend then up to the NYC metro. A
heat advisory is valid from 1 PM through 8 PM Tuesday. The other
areas look to fall short of the advisory criteria.

Apart from the heat, there will be increasing chances of
thunderstorms toward evening as the upper trough approaches the
area, though the primary forcing for ascent and best chance for any
development will largely be to the north and west. At the moment,
some of the deterministic models seem a bit overdone with overall
convective coverage. Have sided more with higher resolution data
showing the best chance of thunderstorms over higher terrain in the
afternoon. Modest flow with veering profiles will once again support
the potential for strong to severe storms capable of damaging winds
and heavy rainfall toward late day.

&&

.LONG TERM /TONIGHT THROUGH MONDAY/...
500 MB: Troughing in the Great Lakes region during midweek...will
shift into the northeast Friday through this weekend.

Temperatures: The month of August so far has averaged within a degree
of normal except Mount Pocono where the monthly average was 2.5
degrees below. So... calendar day averages should still be a few
degrees above normal Wednesday, then from from Thursday through next
Monday should average 2 to 6 degrees below normal on a daily basis.

Forecast basis: a 50 50 blend of 12z/21 GFS/NAM MOS was used
Tuesday night-Wednesday night, then, unless otherwise noted,
the 12z/21 GFS MEXMOS was applied Thursday and thereafter the
15z/21 WPC D4-8 gridded elements of max/min T, 12 hr POP 6 hrly
dew/wind/sky.

Just tucking this in here... since I didn`t think this
warranted a climate section on its own...

ABE 7.94" of rain so far in August ranks #12 when compared to
complete month of August totals at Allentown,  13.47 in 2011 is
the record. Records date back to 1912 except we are missing
1920 and 1921.

The dailies...

Tuesday night...SVR potential early in decent MLC of over
1000J/more than adequate 0-6KM bulk shear (near 45KT), PWAT air
of 1.8 inches with residual showers overnight. Low temps about
9 to 12F above normal. The eastward sweep of SVR should be
primarily I-78 north. Strong westerly flow at mid levels
(40-45kt 700-500MB) should drive a decent lines of showers and
tstms across the area as per the 15z HRRRX. Least likely area
for any tstms is probably southeast of PHL across far s NJ and
DE. The 1703z D2 SPC outlook didn`t change from the 06z vsn.

Wednesday...The cold front will likely slow down when it reaches
Delmarva and perhaps eventually stall just south of the area
across southern VA Wednesday afternoon. Accordingly, it is
looking more probable that the majority of the forecast area
dries out on Wednesday, especially north of the Mason-Dixon
line. The UK is pretty wet on Wednesday in the se 1/2 of our
fcst area and...the EC looks pretty unstable. so...opted for a
SREF wetter blend to the 12z/21 ops pops...which means continuity
with the mid shift fcst.

Wednesday night-Monday...High pressure over Canada and the
Great Lakes region then starts to build southeastward toward the
area on Thursday. This expansive high should remain in control
through at least this weekend. The end result will be an
extended stretch of pleasant weather Thursday through Monday.
Highs in the upper 70s/lower 80s and and lows ranging from the
50s to lower 60s are forecast each day and night (possibly some
upper 40s), respectively. While the forecast remains dry for
these four days, there looks to be increasing cloud cover
heading into the weekend and potentially a risk for an isolated
shower with 1) the approach of the upper trough and 2) light
onshore flow helping to moisten low-levels over time.

&&

.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

For the 06Z TAFs...Big question marks tonight with VSBYs/CIGs,
as guidance is still strongly suggestive of low stratus and/or
fog developing at several of the terminals through 12Z. So far,
this has been overdone, but latest surface obs have trended in
the direction of at least temporary VSBY reductions at KRDG and
KABE as well as temporary CIGs at KPHL and KILG. Will continue
and/or add TEMPO groups to indicate this possibility at most
sites through 12Z. Thereafter, VFR is likely through the day,
with chances for storms gradually increasing north/west of KPHL
during the afternoon. Not at all clear if storms will affect the
urban corridor tomorrow night, but the chances are high enough
for at least a PROB30 mention after 00Z.

Outlook...
Tuesday night...Potential for MVFR or IFR restrictions with
showers and isolated storms. This activity should progress to
the east- southeast through the night.

Wednesday...Scattered showers and isolated thunderstorms mainly
for terminals S/E of PHL and earlier in the day. May start off
MVFR with lower CIGs but improvement to VFR is likely from NW to
SE during the day. A wind shift from SW to NW can be expected
early in the day w/ fropa.

Wednesday night through Saturday...VFR and generally light
winds. Cannot rule out an isolated sprinkle each afternoon or
evening.

&&

.MARINE...
Winds and seas remain tranquil across the waters tonight in weak
southerly flow. Patchy fog development is possible overnight.
An isolated shower or thunderstorm is also possible overnight.

Tuesday...SCA for the Atlantic coastal waters of NJ and DE
starting mid afternoon. S-SW winds are expected to strengthen
late in the day to 15-25 kt. Some gusts to 30 kt are possible
mainly late in the day when the winds will be strongest. Seas
will build to 4-6 feet in response to the wind field.

Outlook...
Tuesday night...SCA continues for the Atlantic coastal waters
of NJ and DE. S-SW winds with isolated gusts to 30 kt possible
mainly during the evening when the winds will be strongest. Seas
build to 4-6 feet in response to the wind field.

Wednesday...A wind shift from SW to W-NW should occur on Wednesday
behind a cold front. Winds will also decrease as well. There is
still a possibility that the SCA may need to be extended into
Wednesday morning if seas take a bit longer to subside below 5
ft.

Thursday through Saturday...Winds and seas below SCA criteria.

RIP CURRENTS...
A south to southwest wind will increase during the day Tuesday,
which will also allow the surf to increase some. Given a more
parallel wind to the coast overall, this should keep the rip current
risk reduced some. However, if a longer period swell becomes more
dominant then this may elevate the rip current risk more. Based
on this and in-house guidance, we went with a moderate risk of
rip currents for Tuesday. This will be re-evaluated early Tuesday
morning.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Heat Advisory from 1 PM this afternoon to 8 PM EDT this
     evening for PAZ070-071-102-104-106.
NJ...Heat Advisory from 1 PM this afternoon to 8 PM EDT this
     evening for NJZ010-012-015-017>019.
DE...Heat Advisory from 1 PM this afternoon to 8 PM EDT this
     evening for DEZ001.
MD...None.
MARINE...Small Craft Advisory from 3 PM this afternoon to 6 AM EDT
     Wednesday for ANZ450>455.

&&

$$
Synopsis...Drag
Near Term...CMS/Gorse/MMD
Short Term...CMS/Gorse/MMD
Long Term...Drag
Aviation...CMS/Drag
Marine...Drag/Gorse/MMD



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