Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 211613

Area Forecast Discussion
National Weather Service Mount Holly NJ
1213 PM EDT Mon Aug 21 2017

High pressure centered over the coastal Mid Atlantic will begin
to move offshore today. The high will shift farther out to sea
on Tuesday while a cold front approaches from the Midwest. The
cold front is expected to move through the region early
Wednesday and then stall nearby to our south Wednesday afternoon
and evening. Canadian high pressure building into the region on
Thursday will continue to influence our weather through the
upcoming weekend.


Minor updates to reflect current sky cover as many are eagerly
anticipating today`s solar eclipse. Scattered to broken cumulus
are developing mainly across eastern PA and western NJ but may
slowly spread eastward through the afternoon. Occasionally a few
thicker patches of cirrus traverse overhead as well, all of
which make an uncertain forecast for those hoping to get a
glimpse of the eclipse.

In the larger-scale, a low-amplitude vort max will be progressing
into Pennsylvania today, and there will be attendant ascent
just downstream. With smaller-scale lift associated with
orographic effects and differential heating (via sky cover
gradients, e.g.), convection is expected to develop to our west
this afternoon. The timing of convective initiation looks to be
early afternoon, but the placement should be comfortably west of
the CWA (at least at onset). However, with large-scale ascent
present, this may further aid the development of a thicker low-
cloud deck, and the anvils from developing convection may advect
rapidly eastward into eastern Pennsylvania. General thinking is
that the greatest area of concern is in the Poconos and near
Reading. There are also some convection-allowing models (CAMs)
that develop a couple showers this morning in Delmarva, and
residual clouds from any showers that develop here may be a
hindrance to eclipse viewing.

With all of the above in mind, the current sky cover forecast is
fairly optimistic east of the Delaware River (generally mostly
sunny), somewhat more pessimistic from the Lehigh Valley west and
northwest (increasing cloudiness this afternoon) and in between in
Delmarva (partly to mostly sunny). Enjoy the show.

Other big concern today is storm chances. The environment is
decently favorable for organized storms capable of isolated strong
to severe wind gusts and hail. MUCAPE will approach/exceed 2000 J/kg
this afternoon in the far western CWA, and effective shear will be
increasing through the day as the aforementioned vort max
approaches. CAPE-shear parameter space will become favorable for
severe storms this afternoon and evening, and CAMs are suggestive of
scattered, semi-discrete storms developing in the higher terrain of
central Pennsylvania moving eastward into the area late this
afternoon. Included mention of gusty winds, hail, and heavy rain
with storms generally west of Philadelphia during this period. With
scattered coverage and questions regarding timing, have no more than
chance PoPs in the grids at the moment.

Forecast temps are a blend of GFS/ECMWF/NAM MOS with a bit of
continuity and CAM-based 2-m temperatures added to the blend. It
should feel noticeably more uncomfortable today with increasing
surface moisture.


Main forecast concerns tonight are precipitation chances and
fog/stratus development.

CAMs are not overly optimistic in maintaining convection after
sunset, and this makes sense given little in the way of surface
support for smaller-scale lift. Nevertheless, the presence of a
passing vort max leaves me concerned, especially since these have
been poorly simulated in general this summer. Notably, another vort
max should be moving through northern Pennsylvania late tonight, and
may provide glancing ascent to the northern portions of the CWA
during this time. Global/coarser models are keeping precipitation
chances around for much of the night in the area, with CAMs
generally showing little to nothing after 03Z. With such disparity,
I find it difficult to remove PoPs entirely or to elevate them
beyond slight to low-chance during the period. Notably, MET MOS PoPs
went way up with the 00Z package, and I have a hard time trending
the opposite direction of statistical guidance.

Meanwhile, some guidance (notably, the NAM, as per usual) develops
low clouds and/or fog in portions of the area tonight. I am somewhat
skeptical, given that a shortwave trough will be moving through
during the evening/early overnight hours. With attendant lift
associated with this and the aforementioned upstream vort max, cloud
cover may be too prevalent to get more than patchy fog in the usual
valley/rural spots. The chance for low stratus seems more pronounced
as warm/moist advection will be in full swing during the overnight
hours (downstream of a much more potent vort max moving into the
Great Lakes region). Combined with uncertainty regarding
precipitation forecast, only mentioned fog in the Lehigh
Valley/Poconos/NW NJ region for now, though patchy fog certainly
cannot be ruled out elsewhere.

Low temps were a combination of MOS guidance and should be a few
degrees above seasonal averages across the region. It will be muggy
thanks to increased dew points.


On Tuesday, a longwave trough associated with an upper low near
James Bay is forecast to amplify over the Great Lakes region as
shortwave energy digs around the backside of the trough. The
surface low will deepen in response to increasing height falls
as it tracks northeastward across Ontario and Quebec. The
attendant cold front will progress eastward through the Midwest.
Looking farther downstream and closer to our area, models show
development of a pre- frontal/lee trough over central PA and
Potomac Highlands during the afternoon hours.

Tuesday will clearly be the hottest of the next 7 days with
forecast highs in lower 90s (except cooler 80s near the coast
and at higher elevations in NE PA/NW NJ). The humidity will also
be a factor with dewpoints reaching the mid 70s across Delmarva
and southern NJ and lower 70s just about everywhere else. Peak
heat indices in the upper 90s to near 100F are likely for the
urban I-95 corridor and points south/east.

With the expectation that focused lift will remain to our west
near the pre-frontal trough on Tuesday, convection over the
forecast area will likely be disorganized and generally isolated
in coverage (perhaps widely scattered across NE PA where the
terrain could locally enhance lift) during the afternoon.
Slightly better chances for showers and storms arrive Tuesday
evening as convection along this trough propagates eastward into
our area. PoPs are highest in our far western zones (west of
the Delaware Valley and near the eastern shore of MD). Showers
and storm are then expected to move eastward into the Delaware
Valley and coastal plain at night as the cold front catches up
with the pre-frontal trough and moves into the area. Even though
the environment should be supportive of showers and storms
continuing into the night owing to the arrival of deep synoptic
lift, PoPs decrease slightly with eastward extent due to some
uncertainty concerning the intensity and coverage of convection
later with the nocturnal boundary layer tending to stabilize
further into the night.

The approach of a low- and mid-level jet ahead of the cold front
will yield increasing shear profiles (0-6 km bulk shear 35-45 kt) by
evening. Additionally, weak to moderate amount of instability
(MLCAPE 1500 to 2000+ J/KG) is forecast to be available to fuel
convective updrafts. Some storms could be strong to severe,
primarily during the afternoon and evening hours, with this
setup. The latest D2 Convective Outlook from SPC includes the
southern Poconos in a slight risk for severe storms with a
marginal risk extending southeastward into the I-95 corridor.

The latest 00Z models have trended faster with the cold front as it
moves through eastern PA and NJ early Wednesday morning. The front
will then 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. While PoPs were also lowered farther
south, the close proximity to the cold front means that there is
still a chance that showers and storms linger into the day.
Additionally, some of the models (most prominently, the 00Z NAM)
develop a wave of low pressure along the stationary front in
southeastern VA. More organized convection could shift back
northward toward Delmarva and possible far southern NJ by evening.

High pressure over Canada and the Great Lakes region then starts to
build southeastward toward the area on Thursday. This expansive high
will remain in control through the weekend. The end result will be
an extended stretch of pleasant weather Thursday through Sunday.
Highs in the upper 70s/lower 80s and and lows ranging from the mid
50s to lower 60s are forecast each day and night, 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


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Cumulus field will develop during the day, with bases generally
in the 4-6 kft range. Exception will be some brief MVFR ceilings
at RDG this afternoon. Retained TEMPO -TSRA to KRDG/KABE given
somewhat increased confidence that convection will move into
eastern PA late this afternoon into the evening hours. Generally
increasing cloud cover from this afternoon through tonight, and
latest guidance shows at least some potential for sub-VFR
CIGs/VSBYs Monday night. Winds generally light/variable through
15Z, then becoming mostly southerly 5-10 kts.


Tuesday...Early morning fog should dissipate by mid morning, leading
to VFR for most of the day. Showers and storms develop during the
afternoon but current thinking is coverage should initially be
isolated and mainly confined to western terminals (ABE/RDG).

Tuesday night...Potential for MVFR or IFR restrictions with
slightly higher chances for showers and 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 Friday...VFR and light winds.


Generally tranquil conditions on the waters today, though southerly
winds should pick up this afternoon/evening. Speeds should attain 5-
15 kts with gusts to 20 kts or so, especially off the New Jersey
coast. Seas will generally range from 2 to 3 feet. There may be some
patchy fog early this morning, but this should dissipate rapidly
after sunrise.


Tuesday...SCA was issued for the Atlantic coastal waters of NJ
and DE for late Tuesday afternoon and night. S-SW winds are
expected to strengthen late in the day to 15-25 kt. Isolated
gusts to 30 kt are possible mainly during the evening when the
winds will be strongest. Seas will quickly build in response to
the wind field to 4-6 ft.

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 and Friday...Winds and seas below SCA criteria.

The rip current risk has been upgraded to moderate for today,
mainly from late this afternoon through this evening, when a
longer-period swell develops (around 9-10 seconds), seas begin
to increase, and southerly winds increase to around 15 kts with
higher gusts. The combination of the above factors and the
presence of a new moon suggests the rip current risk will be
increasing through the day. As such, felt best to raise the risk
to moderate.


Based on observations yesterday evening and current projections
for the high tide this evening, will hold off on issuing a
coastal flood advisory this morning. Current model projections
continue to indicate levels reaching near minor flood thresholds
but consistently below advisory thresholds. Spotty minor
flooding is probable this evening, but confidence is too low at
this point to issue an advisory. Nonetheless, there is some
concern that a more onshore component of the wind (south to
possibly south-southeast) may contribute to somewhat higher
levels than guidance suggests this evening. Should this be
observed during the day, the threat for minor flooding would


MARINE...Small Craft Advisory from 3 PM Tuesday to 6 AM EDT Wednesday
     for ANZ450>455.


Near Term...CMS
Short Term...CMS
Long Term...Klein
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