Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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712
FXUS61 KPHI 270825
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
425 AM EDT Thu Jul 27 2017

.SYNOPSIS...
A weak front will move across the area later today and tonight. Low
pressure will develop along the front across the Ohio Valley and
then move towards our area Friday through Saturday. The low will
strengthen as it slowly moves away later Saturday and Sunday. High
pressure will begin to build over the area next week.


&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
A cold front that is currently over the upper Midwest will approach
from the northwest late in the day while a warm front approaches
from the southern Mid-Atlantic region. Models show a weak wave of
low pressure (area of enhanced pressure falls) forming along the
intersection of a lee surface trough to east of the Appalachians and
the 850 mb warm front late this afternoon.

There appears to a couple of features to support convective
development this afternoon. The first source of lift will be ahead
of an upper shortwave trough that will be digging in from the Upper
Midwest this afternoon. The bulk of the lift from this disturbance
will pass through our northern zones (north of I-78) during the
early to mid afternoon. Expect this first round of showers to be
light across the north since morning cloud cover will limit
destabilization in the boundary layer beforehand. Farther south,
scattered showers and storms may develop late in the day (after 4
PM) owing to isentropic lift near the low-level warm front.
Consensus among hi-res CAMs points to convective initiation occuring
in E PA and S/C NJ between I-78 and the Mason-Dixon line). The
official forecast reflects this scenario though we extended the axis
of higher PoPs southward toward the axis of higher instability near
the Chesapeake Bay.

D1 Convective Outlook from SPC includes the entire region in a
marginal risk for severe storms. The environment could become
supportive of marginally severe wet-microbursts, especially across
the southern half of the forecast area, with steepening low-level
lapse rates and MLCAPE increasing to around 1500 J/kg late this
afternoon.

Forecast high temperatures range from the mid to upper 70s north of
I-80 to the upper 80s in eastern MD.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/...
Showers and storms will likely continue into tonight and possibly
expand in coverage this evening as another shortwave disturbance/
remnant MCS (currently over the Midwest early this morning) arrives.
Even though the majority of hi-res guidance (NAM Nest, HRRR, NCEP
WRF ARW/NMM, and NCAR ensembles) focus the bulk of the convection
over E PA and S/C NJ (between the I-78 corridor and the PA
Turnpike), there is still some skepticism that it may be too far
north. Accordingly, kept chance PoPs in the forecast for Delmarva as
well.

A somewhat slow W-NW storm motion (15-20 kt) that is oriented
parallel to the H8 warm front could potentially provide a training
setup of storms. Accordingly, there is a concern for locally heavy
rainfall and even localized flash flooding tonight. After
coordination with surrounding offices, held off on a Flash Flood
Watch since heavy rainfall will likely be very isolated. However, we
will mention this conditional threat in the HWO.

Drier air behind the cold front should reach NE PA and NW NJ late
tonight.

Forecast low temperatures range from the low 60s in the Poconos to
the mid 70s in the downtown Philadelphia, at the shore and in
Delmarva.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
A prolonged period of wet weather will continue Friday and into
the weekend as a front and low pressure moving along it will be
across the Middle Atlantic region. The models seem to be slower
in moving the low away Saturday, so pops have been increased for
the weekend. Overall, total rainfall when all is done will be
around 1-2 inches over nrn NJ and the Lehigh Valley, 2-3 inches
over the Delaware Valley and srn NJ and 3 - 3.5 inches over Delmarva.
Thunderstorms may bring locally higher totals to some areas. Eventually,
flood watches may be issued for parts of the region, so pay attention
to future forecasts. Temperatures will be mostly below normal Friday
and into the weekend.

Improved conditions are expected Monday which should last into
the middle of next week. It`s possible that future fcsts may
begin to show some possibility for some showers Monday, with the
trend of the low pulling away slower beginning to show up.
Confid in this is still limited, so we will keep the dry fcst
for now. Temperatures will be near normal Mon/Tue and then rise
to a little above normal for Wed.

&&

.AVIATION /08Z THURSDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

MVFR CIGs in low clouds have been persistent over RDG and ABE for
most of the night. These low clouds may expand into the I-95
terminals mid to late morning. CIGs should eventually increase to
above 3 kft AGL between 18Z and 21Z this afternoon.

A PROB30 group for TSRA was added for most TAF sites after 21Z
today. The threat of storms will continue through the evening and
possibly overnight. Uncertainty in location and coverage of storms
precludes any mention of showers or storms in a prevailing group.

VFR CIGs are favored for tonight (MVFR in storms) though there is
some potential for low clouds to redevelop overnight. Fog could
develop if there are breaks in the cloud cover.

A S-SW wind under 10 kt can be expected today and tonight.

OUTLOOK...

Friday thru Saturday...Widespread lower CIGS/VSBYS with frequent
    showers and tstms.
Sat night thru Monday...Mostly VFR. Scattered showers Sat night
    and near the shore Sunday with lower conditions possible.

&&

.MARINE...
A Wly wind around 5 kt early this morning will become SWly 5-10 kt
after sunrise, then Sly 10-20 kt this afternoon and tonight. Wind
gusts could briefly approach 25 kt this evening but most areas
should remain below SCA owing to weak mixing profiles. Seas on the
ocean will be in the 2-3 ft range.

OUTLOOK...
Fri...Mostly sub-SCA conditions. Showers and tstms.
Fri night...Winds and seas building to near SCA. Showers and
 tstms.
Sat thru Monday...SCA conditions expected. Showers Saturday.

RIP CURRENTS... There risk for the development of dangerous rip
currents will be lower than in recent days as the previous
onshore winds veers out of the southwest this morning and south
this afternoon. There will be an underlying 10-sec period swell
which could enhance the rip current potential a bit but overall
a low risk day is anticipated.

&&

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

&&

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



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