Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 170521
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
121 AM EDT Mon Jul 17 2017

.SYNOPSIS...
Unsettled weather is on tap for much of this week as a trough
approaches from the west and stalls west of the Appalachians.
That trough moves across the region through Tuesday night. High
pressure briefly returns Wednesday, before a pair of cold fronts
approach the region late this week.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
Some cloud cover will likely build back in overnight. Starting
with higher clouds, then lowering some. Temperatures will be a
couple of degrees warmer than last night as southwest flow aloft
helps to bring in slightly warmer and more moist air. No
precipitation is expected overnight.

Winds will likely become light and variable. There is a small
possibility for some light fog overnight through daybreak, but
visibilities are not expected to lower very significantly.

&&

.SHORT TERM /MONDAY/...
Fair weather is expected during the morning Monday, but an
approaching disturbance will bring increasing clouds and the chc
for showers and a few tstms by afternoon. The SPC has mentioned
a marginal possibility for severe storms across the N/Wrn
areas. The Slight risk area touches the Southern Poconos region
however. Gusty winds and perhaps would be the hazards if/when
tstms develop. Temperatures will rise into the mid/upper 80s
across the area, a bit cooler near the shore. Winds will be
mostly S or SE at 5 to 10 mph.

&&

.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
The main features to watch in the long term will be a mid and
upper level trough which will be very slowly progressing east
through the early week, and a pair of cold fronts late in the
week.

Monday night through Wednesday...The mid and upper level
shortwave trough may be stalled Monday night just to our west,
but is then expected to very slowly slide east. Interestingly,
despite the trough in the area, mid and upper level flow will be
relatively weak through this time, resulting in meager shear
values. Therefore, expecting mostly diurnally driven convection
to develop near the trough axis each afternoon, but do not
expect much in the way of organized convective complexes.

With relatively low precipitable water values (at least for this
time of year) expect the flooding threat to be limited Monday
and Tuesday. However, by Wednesday, if the surface low develops
far enough south, we could see precipitable water values climb
above 1.5 inches (still not unusual for late July, but on the
increase). That, combined with slow storm motions could mean
localized heavy rain. There are still too many contingencies to
include in the HWO for now though.

Thursday through Sunday...Still considerable uncertainty with
the timing and evolution of features in this period. The
previously mentioned mid and upper level trough should be well
off shore through this period, leaving us in a northwesterly
flow pattern. Consequently, a pair of cold fronts are expected
to dig far enough south to propagate through and well south of
our region. The first one looks on track to move through
Thursday or Thursday night. There should then be a period of
tranquil weather as a low and mid level short wave ridge briefly
propagates over the region. Then a second, and perhaps stronger
(at least in terms of temperature gradient) front is expected
very late in the period (Saturday or Sunday). Favored area and
timing for precipitation will be with and just ahead of each of
these fronts.

&&

.AVIATION /05Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Through today...VFR overall with cloud bases mainly between 3500-
8000 feet. There is a chance for light fog locally near sunrise,
however this is low confidence and therefore not included. Some
showers and thunderstorms are expected to develop especially this
afternoon, with the greatest coverage near and north and west of
KABE and KRDG. Included a PROB30 for thunder at these two terminals,
then included a VCSH at KTTN to KPHL/KPNE to KILG with less
certainty regarding thunder. Any shower or thunderstorm will be
capable of producing locally significant but mostly brief
restrictions, mostly visibility. Light and variable winds overall,
becoming south or southeast and increasing to 6-10 knots.

Tonight...Mostly VFR. Any showers and thunderstorms should dissipate
eastward during the evening. Some low clouds and/or fog may develop
toward daybreak Tuesday, especially where rain occurred. The low
cloud/fog potential is of low confidence at this time. South-
southeast winds 4-8 knots becoming mostly light and variable.

Outlook...
Tuesday through Friday...mostly VFR conditions expected. Chances
for showers and thunderstorms, primarily in the afternoon and
evening, could lead to brief MVFR or lower conditions.

&&

.MARINE...
Weak high pressure across the waters tonight will further weaken and
low pressure will form to the west. Winds will generally be S
to SE around 5 to 10 knots through Monday, although by Monday
afternoon some gusts around 20 kts are possible. Fair weather
expected through early afternoon Monday, then isolated showers
or thunderstorms may occur late in the day.

Outlook...
Tuesday through Friday...Winds and seas should generally stay
below SCA criteria. However, gusty winds may be possible in the
vicinity of any thunderstorms (primarily in the afternoon and
evening hours).

Rip Currents...
There is a low risk into this evening for the development of
dangerous rip currents. A south to southeast wind Monday will
increase in the afternoon, however light enough along with seas
around 2 feet results in an overall low risk.

&&

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

&&

$$
Synopsis...Johnson
Near Term...Robertson/O`Hara
Short Term...O`Hara
Long Term...Johnson
Aviation...Gorse/Johnson
Marine...Johnson/O`Hara



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