Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 180141
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
941 PM EDT Thu Aug 17 2017

.SYNOPSIS...
A warm front will lift north of our region later tonight. As low
pressure tracks well to our north Friday, a cold front moves through
Friday night into early Saturday. High pressure then builds in later
Saturday and remains in place into Monday before shifting offshore.
The next cold front is scheduled to arrive during Wednesday.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
930 PM ESTF: Based on downstream trends, PoPs were trimmed back
until 03Z, then expanded from west east across the region. As a
warm front lifts north across the area overnight, we have likely
PoPs toward dawn in the northwest with chance PoPs elsewhere.
Given the moist and unstable air mass in place, isolated thunder
possible with localized heavy downpours.

Previous Discussion...
High pressure centered over New England continues to drift north and
east tonight. Meanwhile, low pressure over the western Great Lakes
will also continue to lift to the north and east tonight, and this
will drag a warm front into the region tonight. This front should
make it through the Delmarva and Delaware Valley late tonight, but
then gets hung up. Warm fronts are notorious for having a hard time
lifting too far north during the overnight hours, so do not think it
will make it too much past a line from around KABE to KTTN to KMJX.

There are several aspects to watch for this evening and tonight. For
the Delmarva, low level moisture continues to pool around a weak
stationary boundary that runs through northeast MD and DE. Some weak
shortwave energy is sliding along that boundary, and touching off
isolated to scattered showers and perhaps a few thunderstorms during
the early evening hours.

The second aspect to watch is the approaching warm front. Scattered
showers and thunderstorms will develop out ahead of this front, and
look to make it into far western zones this evening. However, looks
as if more widespread showers and thunderstorms will develop in the
pre-dawn hours on Friday as a strong H5 shortwave moves into the
Lehigh Valley and northwest NJ at that time. Will carry the highest
PoPs (likely) in association with that shortwave to the north, and
not until well after midnight.

A warm and muggy night on tap. Lows will be in the low to mid 70s,
and surface dewpoints will creep up into the lower 70s. Given the
widespread cloud cover that will spread into the region associated
with that warm front, not expecting much fog to develop, but patchy
fog is possible in some areas.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
Friday looks to be an active weather day.

Warm front continue to lift to the northeast Friday morning, and
showers and thunderstorms will continue to be associated with it.
Most activity will taper off from south to north in the morning, and
it looks as if there should be a period of dry weather from late
morning through early afternoon.

With the warm front north of the region, southerly flow users an
unusually warm and humid airmass into the region. Surface dewpoints
will soar into the mid and upper 70s, along with highs in the upper
80s to around 90 in the Delmarva, and in the mid to upper 80s for
most of NJ and southeast PA, and in the low 80s along the coasts and
in the mountains.

Going through the afternoon and into the evening, surface low
pressure over the western Great Lakes lifts north and east towards
Ontario. This drags a surface cold front and H5 trough with several
strong shortwaves east. 12Z NAM indicating low pressure forming over
the Delmarva early in the evening.

For the forecast area, 12Z NAM showing SB CAPE of over 3500 J/kg and
MU CAPE almost 5000 J/kg. This is in addition to a Lifted Index as
low as -8C, and 0-6 km Bulk Shear of 30-40 KT. PWATs also range from
2-2.5 inches.

All of this lines up for a period of strong to severe thunderstorms.
SPS has the area in a Marginal Risk for severe weather, and WPC has
the area in a Slight Risk for excessive rain.

There is a threat for hail and damaging winds, but with all of this
moisture, and based on model guidance, looks as if heavy rain and
flooding will be of primary concern. NAM showing a bullseye right
over the Delmarva and southern NJ, while the GFS is a bit farther
north with that bullseye. It all depends on how training of
thunderstorms develops, but it is looking as if somebody will pick
up a significant amount of rain Friday afternoon/evening.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
Summary...Convection ends Friday night followed by some lowering of
the dew points Saturday and Sunday. Some convection returns next
week especially Wednesday with the next cold front, then more
noticeable cooling should take place in the wake of that front.

Synoptic Overview...An upper-level trough swings across the Great
Lakes Friday into Saturday, then into the Northeast later Saturday.
The flow aloft should then turn more zonal as we start next week,
however another trough is forecast to amplify across eastern Canada
and eventually into the Great Lakes to the Northeast. We used a
blend of continuity/guidance for Friday night through Saturday
night, then blended in the 12z WPC Guidance.

For Friday night...A convectively active evening should be in
store as a cold front arrives along with an attendant short
wave. The guidance indicates either mixed modes or a line of
convection moving offshore toward later in the evening, with
perhaps some leftover convection lingering into the overnight as
the cold front arrives. There is the potential for some severe
thunderstorms in the evening prior to the boundary layer cooling
and/or the airmass becoming worked over. Given the potential
for at least some convective organization within a rather moist
airmass, any robust convection could be capable of producing
locally strong winds (wet microbursts). This is especially if
cold pool development becomes efficient. Otherwise, an axis of
precipitable water in the 2.0-2.5 inch range will lead to
torrential downpours. While the convection should be moving,
efficient rainfall rates could lead to a localized flash flood
risk especially in urban areas. The convection should be
weakening as it reaches the coast and beyond overnight. If some
thinning occurs in the clouds with light winds, some fog may
develop.

For Saturday and Sunday...Low pressure is forecast to lift well
north of our area with an associated cold front shifting off our
coast early in the morning. The main upper-level trough however lags
behind, with it arriving across the Northeast and Mid Atlantic late
in the day Saturday. Given the passage of the cold front, the
precipitable water decreases through the day. As a result the dew
points should lower at least some during the course of Saturday into
Sunday, although the dry air push looks to be most robust to our
north through Sunday despite high pressure building in. As the main
trough axis aloft arrives later Saturday, some guidance indicates
enough lift and instability to develop some convection to our west.
This may weaken by the time it reaches our area, plus the column
should be drying some. As a result, we kept Saturday post-frontal
dry for now. Sunday is looking warm with some humidity along with
plenty of sunshine anticipated.

For Monday and Tuesday...The flow looks to be more zonal aloft
during this timeframe. This drives high pressure eventually offshore
as it weakens some later Monday. The next upper-level trough
amplifies eastward from the Midwest and Upper Great Lakes Tuesday.
This will start to back the flow more southwesterly, and there may
be a lee-side trough present on Tuesday. As the moisture begins to
increase once again combined with the possibility for some
convergence and enough instability, some convective development is
possible Tuesday afternoon. We will continue with slight chance to
low chance PoPs as a result for Tuesday. Any convection should tend
to diminish during the night.

For Wednesday and Thursday...The prospects for convection will
depend on the timing of an incoming cold front. This will be due to
an upper-level trough arriving from the west, however a pre-frontal
trough may serve as the primary focus for showers and thunderstorms.
We will continue with chance PoPs with the idea for at least
scattered convection possible Wednesday. As of now, the cold front
should be offshore to start Thursday and high pressure centered just
north of the upper Great Lakes starts to build southeastward. This
should result in a cooler and drier airmass overspreading our region
for Thursday.

&&

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

As of 9:30 PM, some MVFR ceilings are developing at RDG. TAF
sites with VFR now will gradually lower to MVFR late tonight.

Showers and scattered thunderstorms will develop late tonight as a
warm front slowly lifts north through the terminals. Best chances
will be at KABE/KRDG, but then chances spread east early Friday
morning. MVFR/IFR conditions look to persist after rain ends for a
period late Friday morning, and then conditions lift to VFR by early
afternoon.

A cold front moves into the region from the west Friday afternoon.
Showers and thunderstorms with heavy rain will impact the terminals
with IFR conditions, as well as the potential for small hail and
gusty winds.

SSE winds 5-10 KT becoming light and variable at times overnight,
then S 5-10 KT Friday afternoon.

OUTLOOK...
Friday night...Some showers and thunderstorms move through mostly in
the evening with areas of MVFR/IFR conditions for a time, then if
clouds thin enough some local fog may develop. Southwest winds 5
knots or less, becoming west-northwest late.

Saturday...Some local fog possible very early, otherwise VFR.
Northwest to west winds around 10 knots, diminishing at night.

Sunday and Monday...VFR overall with light winds.

Tuesday...Mainly VFR. Winds becoming south-southwest 5-10 knots.

&&

.MARINE...
Winds and seas below SCA criteria tonight through Friday morning.
Friday afternoon, a southerly pressure gradient tightens, resulting
in 15-20 KT winds. With an inversion over the waters, not expecting
25 KT wind gusts. However, it does appear as if seas could build
close to 5 feet for a few hours in the afternoon. Will hold off on a
SCA for Friday afternoon and evening given low confidence in
widespread SCA conditions developing.

Scattered showers and thunderstorms will impact the waters tonight,
but then widespread showers and thunderstorms with heavy rain, small
hail, and gusty winds will impact the waters Friday afternoon and
evening.

OUTLOOK...
Friday night...A southwesterly flow increases some ahead of a cold
front, however forecast soundings overall indicate gusts should be
below 25 knots. If the increase in winds remain long enough, seas on
the ocean zones could build to 5 feet through the evening. Since it
looks marginal, held off on a Small Craft Advisory. Some
thunderstorms in the evening could produce locally strong wind gusts.

Saturday and Sunday...The conditions are anticipated to be below
Small Craft Advisory criteria as high pressure gradually builds in.

Monday and Tuesday...Light winds to start Monday should become
southerly, then increase some on Tuesday. While some gusts to about
20 knots are possible Tuesday, the conditions during this timeframe
are anticipated to be below Small Craft Advisory criteria.

RIP CURRENTS...

We are forecasting a solid moderate risk for the development of
dangerous rip currents on Friday for the DE and NJ shore, given
increasing southerly winds and seas. In fact, NJ shore will be
on the upper end of moderate.

&&

.TIDES/COASTAL FLOODING...
Based on current trends and all available guidance, we do not
anticipate minor flooding with the high tide tonight along the
eastern shore of MD.

&&

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

&&

$$
Synopsis...Gorse
Near Term...Franck/MPS
Short Term...Franck/MPS
Long Term...Gorse
Aviation...Franck/Gorse/MPS
Marine...Franck/Gorse/MPS
Tides/Coastal Flooding...Franck



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