Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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268
FXUS61 KPHI 270932
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
532 AM EDT Tue Jun 27 2017

.SYNOPSIS...
A weak cold front passes across the mid Atlantic states today.
High pressure will follow tonight and Wednesday. This high moves
offshore during the end of the week. Eventually, another weak cold
front will cross the mid Atlantic states Saturday night or Sunday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
An upper-level trough is forecast to sharpen eastward today as it
moves into the Northeast and Mid Atlantic. This will drive weak low
pressure well to our north, however a few weak cold fronts or
surface troughs will continue to push eastward. The first one early
this morning has teamed up with just enough instability and lift to
focus an area of showers and some thunder primarily up the I-95
corridor. It appears that the Chesapeake Bay played a role in this,
helping to boost the MLCAPE just enough to get lightning going. This
activity is expected to move northeast of our area through about
12z/8 AM. We continue with an area of likely PoPs in this
corridor. We then turn our attention to the west.

A more pronounced area of short wave energy is forecast to arrive
from west to east late this morning and through the afternoon. This
looks to be accompanied by either a weak cold front or surface
trough. While the trough aloft is forecast to be sharpening some as
it shifts eastward along with gradual cooling aloft, the bulk of the
lift is more focused to our north. The majority of the model
guidance is therefore showing much of the convection just to our
north. There should be however enough moisture and at least some
instability through the day to pop a few showers or thunderstorms.
Since the overall coverage is less certain still, we kept PoPs in
the slight chance to low chance range.

High temperatures are a blend of continuity, MOS and some high-res
guidance. Given lower heights and thickness values, the afternoon
high temperatures are forecast to be a bit cooler than
yesterday.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
The axis of an upper-level trough is forecast to move across our
area during the overnight. Any shower or thunder is expected to end
early as the main short wave energy is offshore, then some increase
in the subsidence should begin to overspread the area as surface
high pressure starts to arrive toward daybreak.

The sky is anticipated to become mostly clear as a result with any
clouds due to daytime heating flattening and then dissipating in the
evening. Low temperatures are mostly a blend of continuity and
MOS.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Highlights:

1. Heat index values near 100 possible for the urban corridor
   Saturday.

2. Highest chance of thunderstorms centered on later Saturday
   into Sunday when isolated svr may occur.


500 MB: A cold trough over the northeast USA tonight weakens to
the Maritimes by Thursday night as heights rise-warm over the
mid-Atlantic states. The warm ridge aloft, almost 2SD above
normal, on Saturday over the northeast gives way to a broad
trough over our area early next week.

Temperatures: Calendar day averages will be about 5 degrees below
normal Wednesday average near normal Thursday then 5 to 8 degrees
above normal Friday through Sunday, cooling slightly on Monday.  For
June,  the result should be an average temperatures generally 1 to 2
degrees above normal at our NWS primary climate sites except
around 1 to 1.5 degrees below normal at Mount Pocono.

Forecast basis: a 50 50 blend of 00z/27 GFS/NAM MOS Wednesday-
Thursday, 00z/27 GFSMEX MOS Thursday night-Friday and then only
dewpoints/max/min temps from the 0601z/27 WPC grids for D4-8.
Other guidance including POPS/Sky/Wind was continuity from the
330 PM Monday forecast.


The dailies...

Wednesday...Mostly sunny. Westerly wind.

Wednesday night...Clear, then high clouds toward dawn.  Light
southwest wind.

Thursday...Partly to mostly sunny (high clouds early, cu and ac
fields afternoon-night in waa. Southwest wind gusty to 25 mph
during the afternoon diminishes somewhat at night but still a
stirring mild wind. Small chance of a shower/showers late or
night except a thunderstorm north of I-78. SPC D3 consideration,
especially with bulk shear. MLC is rather paltry Thursday so
from my view, this a leftover gusty thunderstorm Thu night.

Friday...Partly to mostly sunny.  Southwest with gusts probably 20
mph or less. Heat Index should rise to 95 to 97 I-95 corridor.

Saturday...After predawn patchy fog or stratus,  partly to mostly
sunny. Southwest wind. Thunderstorm potential ahead of cold
front later in the day or at night. MLC 1800J. Bulk shear is
currently modeled low. Widespread heat index of 95 to 100 I95
with a WPC around 30 percent probability of HI in PHL.

Sunday...Partly to mostly sunny. Depending on psn of the cold
front there could be leftover showers and tstms. West southwest
wind.

Monday...Chance of a shower in the fcst but for now odds favor a
decent Monday. Low confidence on this day:

&&

.AVIATION /09Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Through today...VFR overall. An area of showers and embedded
thunderstorms will move across the KILG, KPHL/KPNE to KTTN areas
through about 09z. The visibility can be significantly reduced for a
short period of time in any heavy shower/storm. Isolated to perhaps
scattered showers/storms are possible again this afternoon, but
confidence regarding timing is low and therefore a mention is not
included in the TAFs. Mostly light and variable winds, becoming
westerly and increasing to around 10 knots late this morning and
afternoon. Some local gusts up to 20 knots this afternoon are
possible.

Tonight...Any shower/storm ends early, otherwise VFR with any
lingering clouds tending to dissipate. West to northwest winds
mainly 5 knots or less.

OUTLOOK...
Wednesday...VFR.  Mainly a west wind.

Wednesday night...VFR. light southwest wind.

Thursday...VFR with occasional variable cigs aoa 5000 ft.
Southwest wind gusts to around 25 kt in the afternoon. Chance of
a shower.

Friday...VFR.  Southwest wind may gust to near 20 kt in the
afternoon.

Saturday...Patchy IFR conditions possible predawn in fog or stratus.
Then VFR most of the day. Southwest wind. Thunderstorms ahead of
a cold front are expected but timing uncertain.

&&

.MARINE...
The conditions are expected to remain below Small Craft Advisory
through tonight. A southwesterly wind will become westerly late in
the day and tonight as a weak cold front moves offshore. This
offshore flow will increase at times tonight with some gusts to near
20 knots. A few isolated thunderstorms will be possible mainly this
afternoon.

OUTLOOK...
Wednesday...no marine headline. Rather quiet sea state.

Thursday...SCA possible for the near shore waters in the afternoon
then a possible Atlantic waters SCA for hazardous seas during Thursday
night.

Friday and Saturday...Just below SCA threshold.


RIP CURRENTS...
The forecast conditions for today should result once again in a
low risk for the development of dangerous rip currents.

&&

.CLIMATE...
June average temperature: temperatures are expected to generally
average 1 to 2 degrees above normal at our NWS primary climate
sites except around 1 to 1.5 degrees below normal at Mount
Pocono.

&&

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

&&

$$
Synopsis...Drag
Near Term...Gorse
Short Term...Gorse
Long Term...Drag 532
Aviation...Drag/Gorse
Marine...Drag/Gorse
Climate...



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