Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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599
FXUS61 KPHI 280147
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
947 PM EDT WED JUL 27 2016

.SYNOPSIS...
Periods of showers and thunderstorms are expected later Thursday
into Friday with heavy rainfall and potential for localized flash
flooding.  Unsettled weather may continue at times into the weekend
before drier weather moves in sometime early next week.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
Only minor changes were needed for the update. A few of the
temps/dew points across the north were running high. Sky cover was
also knocked down a bit. HRRR indicates some psbl showers south
later tonight. Decided to keep the slgt chc in for the overnight.
Everything else looks fine.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
The frontal boundary is expected to remain draped across the
Delmarva during the morning hours...but shower activity may be
limited until daytime heating kicks in a little later in the day,
and as an impulse moves east along the front later in the morning or
in the afternoon. the best chances for shower and thunderstorm
activity on Thursday will be from Philadelphia south...especially
from late morning through the afternoon. SPC has placed a good
portion of this area under a Slight risk for severe weather on
Thursday, but if any severe weather does develop, it appears the
most likely time may be late afternoon and evening. The main threats
will be strong winds and heavy rain. Temperatures will once again
peak over 90 degrees in most locations (expect in the higher
elevations up north),and with dewpoint temperatures in the 70s south
and 60s north, the air will once again feel very muggy.
However...even though it will feel hot and uncomfortable...threshold
values for excessive heat advisories and warnings are not expected
to be met.

&&

.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
Highlights...

* Main Concern: Heavy Rain/Flash Flood Potential Thu night/Fri
* Secondary concern: Strong to Severe Thunderstorm Risk Thu night/Fri

Details...

Thursday night and Friday...

1) Heavy Rain/Flash Flood Potential Thu night and Fri:

Interesting setup Thu night into Fri as there appears to be several
ingredients coming together that have us concerned for localized
flash flooding.  A couple of shortwaves will combined with
anomalously high Pwats, perhaps near record levels of 2.5+ inches.
In addition, a mid level trough axis along with a low level surface
boundary will provide a focus for convection.  This combination will
yield periods of showers and thunderstorms with pockets of heavy
rainfall and a localized flash flooding potential.  The biggest
uncertainty is where the axis of heaviest rain sets up as that will
be determined on the mesoscale level. However, given the above
ingredients felt it warranted a Flash Flood Watch for the southern
2/3rds of our region Thu night into Fri.

Timing also remains uncertain, but highest risk might be late thu
night into Friday morning when main shortwave moves across the
region.  However, more showers and thunderstorms are possible Friday
afternoon.

2) Strong to Severe Weather Potential Thur night and Fri:

The other concern will be the potential for strong to severe
thunderstorms, with the greatest concern across the southern half of
our region.  This is where SPC SREF and other guidance shows the
greatest risk of 1000+ J/KG of MLCape along with a period of 40+
knots of 0 to 6 km shear.  Tough to pin down the timing, but late
Thu night/Fri am may be when the greatest risk exists with the
shortwave.  Biggest concern would be strong to damaging straight
line wind gusts, but will have to watch for the low risk of a spinup
near the triple point.

Saturday through Wednesday...

Unsettled weather will continue at times this weekend, given
boundary in the vicinity.  Timing uncertain this far out, but some
guidance suggesting the greater risk for scattered
showers/thunderstorms will be on Sunday with the next shortwave.
Given clouds and risk of precip will keep highs mainly in the 80s
to near 90 this weekend.

Drier and somewhat less humid may finally arrive early next week.
However, afternoons will still be quite warm with highs well up into
the 80s to near 90.

&&

.AVIATION /02Z THURSDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Generally VFR conditions expected overnight and thru the day Thursday.
We have mentioned some fog across KMIV/KACY for late tonight,
this will be the only exception. Winds will be light generally
from the S or SW. High and mid level clouds will increase over the
area Thursday. Low clouds (still VFR) will arrive during the late
afternoon. Winds Thu will be S/SW at 5 to 10 knots.

Outlook...
Thursday night through Sunday...Scattered showers and tstms will
lower cigs/vsbys at times.

&&

.MARINE...
Mainly sub-sca conditions are expected through the short-term
period. however...gusty winds and rough seas are always possible in
the vicinity of convective activity.

Outlook...

Thursday night through Monday...

Relatively weak pressure gradient will keep winds and seas below
small craft advisory thresholds through the period.  Main concern
for mariners is the threat for strong thunderstorms Thursday night
and Friday.

Rip currents...
There is a low risk for the development of dangerous rip currents
for today due to the relatively light wind and tranquil seas.

The probably risk for the development of dangerous rip currents
on Thursday is also low.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Flash Flood Watch from Thursday evening through Friday evening
     for PAZ070-071-101>106.
NJ...Flash Flood Watch from Thursday evening through Friday evening
     for NJZ009-010-012>027.
DE...Flash Flood Watch from Thursday evening through Friday evening
     for DEZ001>004.
MD...Flash Flood Watch from Thursday evening through Friday evening
     for MDZ008-012-015-019-020.
MARINE...None.

&&

$$
Synopsis...99
Near Term...O`Hara
Short Term...Miketta
Long Term...99
Aviation...O`Hara/99
Marine...Miketta/99



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