Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 230134
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
934 PM EDT Mon May 22 2017

.SYNOPSIS...
Weak low pressure in southeast Virginia will move east of New
Jersey later tonight while a weak cold front in central
Pennsylvania crosses our area. Low pressure in North Carolina
Tuesday will pass east of the mid Atlantic coast Tuesday night.
Weak high pressure is left behind over our area on Wednesday.
Stronger low pressure organizes in the Ohio Valley Wednesday
night sending its associated fronts through the mid Atlantic
states early Friday. Then high pressure follows on Saturday.
However another storm system should develop over the nations
midsection Sunday, threatening to mar a portion of Sunday or
Memorial Day with a period or two of showers.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
Showers have moved offshore. A few lingering pop up showers are
possible going through tonight as a weak cold front over central
Pennsylvania tracks east and passes through the region late
tonight. Otherwise, cloudy and humid with abundant low level
moisture due to the rainfall that fell today.

Some fog has developed, mainly in the valleys, and in southern
and eastern NJ where the heaviest rain fell. Despite 1/4SM VSBY
at ACY and MPO, as well as at BLM earlier this evening, do not
expect widespread dense fog to form. Therefore, will not issue a
Dense Fog Advisory. A Special Weather Statement may be issued,
however, if more areas of fog develop. For now, will generally
cap VSBYs as low as 1SM.

Depending on when that weak front passes through, ceilings and
fog would lift prior to daybreak, as opposed to after daybreak
with heating of the day.

Overnight lows will generally range from the mid/upper 40s north
of I-80 to the mid 50s for most of NJ and SE PA, and near 60s
for the Delmarva.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...
The frontal boundary that moves across the area overnight is
expected to stall to our south. An area of low pressure is
forecast to move along this boundary from the south, as the
first low pressure lifts to the northeast. The low pressure, nor
the frontal boundary, is expected to make its way back into our
area during the day, however there will be a chance of showers
to lift across portions of the area later during the day on the
north side of the low. The greatest chance for showers will be
across southern Delmarva and southern New Jersey. With no
instability forecast, we do not expect any thunderstorms across
our area.

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
12Z NAM was not applied for this discussion. It was not off to
a good start today.

Hazards: it seems almost inevitable that some sort of hydro
related products will eventually be needed by next Monday in
this pattern, but whether its more than nuisance flooding FLSs
is debatable. All we know is that widespread 1.5 to 2.5"
occurred in less than 12 hours over parts of the Delmarva and
southeast NJ today with a fairly large area of 3+ in southeast
NJ. Suspect isolated max amt of 4" today with small streams and
rivers still rising late today. Legacy DIX STP was best with
considerable low bias in DOX STA/STP. MRMS was also low.

500mb: A sharpening and unusually strong high amplitude trough
in the nations midsection will close off and weaken as it moves
to New England Friday, with ridging to follow this weekend. The
next trough will be organizing in the Great Lakes region early
next week.

Temperatures: Calendar day averages Wednesday-Saturday near
normal warming somewhat Sunday and Monday, ahead of the next
trough.

Forecast basis: a 50 50 blend of the 12z/22 GFS/NAM MOS for
Tuesday night through Wednesday night, then the 12z/22 GFS MEX
MOS for Thursday with the 15z WPC guidance for Thursday night-
next Monday. We did modify guidance temps downward Thursday and
Sunday in response to GGEM and ECMWF cooler pattern.

The dailies...

Forecaster confidence is below average on details the entire
period.

Tuesday night through Wednesday...Depending on the path of the
next coastal...a pretty good chance of rain far southeast and
probably dry north where a nice Wednesday is expected. SREF PWAT
is down.

Wednesday night and Thursday...Looks wet in warm air advection.
Questionable how much warming can occur with what looks to be a
negative tilt southern extension of the trough moving toward
our area and causing a triple point low to approach through PA.
That also may permit a better chance for isolated thunderstorms.
SREF PWAT back up to 1.75".

Friday...Partly sunny and a breezy west wind should develop
with afternoon warming and only a small chance of a residual
shower in the north.

Memorial Day weekend...A good start is anticipated Saturday.
However, Sunday and Monday should see a period or two of
showers and possible thunderstorms. Still plenty of uncertainty
but its not looking to be a picture perfect weekend.

&&

.AVIATION /01Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Below average forecaster confidence for tonight through Tuesday
morning.

Generally speaking, an IFR forecast for low CIGs on tap for at
least most of tonight. Some terminals are already reporting LIFR
conditions, and KACY has 1/4SM VSBY in FG.

Fog will continue to develop, but despite IFR/LIFR CIGs, do not
expect much in the way of IFR or lower VSBYs. VSBYs should stay
at MVFR, for the most part.

Depending on when that front comes through, conditions may begin
to lift prior to daybreak. The LAV guidance has the low CIGs, so
will trend with that forecast. But there may be AMDs needed
should conditions lift earlier than expected.

Light and variable winds through the overnight. As the occluded
frontal boundary moves across the area, winds will become
north-northwest to north-northeast. Speeds through the next 24
hours are expected to be 5-10 knots or less.

OUTLOOK...

Forecaster confidence on details is below average through Friday
morning.

Tuesday night...VFR conditions north and possibly the entire area
but a risk of MVFR/IFR conditions in showers vicinity
KMIV/KACY. East to northeast wind.

Wednesday...VFR except possible MVFR/IFR conditions during the
morning KMIV and KACY. East to northeast wind gusts to 15 mph.

Thursday...VFR CIGS with probable periods of MVFR/IFR conditions in
showers and isolated thunderstorms. Easterly winds probably
becoming southeast to south at night.

Friday...Improving conditions after any early morning showers end
with mainly VFR expected. Westerly wind gusts to 25 kt possible
in the afternoon.

Saturday...VFR. Westerly wind.

&&

.MARINE...
Winds and waves will remain below Small Craft Advisory levels
tonight through Tuesday. Wind directions will begin out of the
southeast this evening and overnight, before becoming north-
northwest to north-northeast late tonight into Tuesday as an
occluded front moves across the area.

Areas of fog expected on the waters, but think for the most
part, VSBYs should be greater than 1NM. There is the potential
for fog to reduce VSBYs to locally less than 1 NM through
Tuesday morning, but for now, confidence remains low, and will
not issue a Marine Dense Fog Advisory.

OUTLOOK...

Forecaster confidence on this outlook section is below average
Tuesday night and Wednesday then above average thereafter.

Tuesday night and Wednesday...may need an SCA for hazardous seas
for the DE Atlantic coastal waters. Otherwise northeast to east
wind with gusts generally under 20 kt.

Thursday...SCA probable for the well organized Ohio Valley low.
East to southeast winds build the seas to between 5 and 8 feet
on the Atlantic waters.

Friday...SCA may continue for leftover hazardous seas on the
Atlantic portion of our coastal waters. Westerly wind develops
with nearshore gusts to 20-25 kt.

Saturday...Marine headlines not likely.

&&

.HYDROLOGY...
Hazards: it seems almost inevitable that some sort of hydro
related products will eventually be needed by next Monday in
this pattern, but whether its more than nuisance flooding FLSs
is debatable. All we know is that widespread 1.5 to 2.5"
occurred in less than 12 hours over parts of the Delmarva and
southeast NJ today with a fairly large area of 3+ in southeast
NJ. Suspect isolated max amt of 4" today with small streams and
rivers still rising late today. Legacy DIX STP was best with
considerable low bias in DOX STA/STP. MRMS was also low.

&&

.TIDES/COASTAL FLOODING...
Astronomical tides are at some of their highest of the year
this Wednesday and Thursday evenings. Some onshore flow will
contribute positive surge departures and its probable that a
few locations along the Atlantic coasts of DE and NJ will exceed
the minor coastal flood advisory threshold. It only takes about
.6 to .7 feet surge above high tide to reach the threshold.
It`s all in the details, which are yet to be confidently
determined. The SIT and ETSS modeling, forecasts minor with a
small chance of a moderate episode if, prior to high tide,
onshore flow is strong enough and for enough duration. Right
now...no action needed.

&&

.CLIMATE...
As of 5:25 PM EDT, a record rainfall of 1.85" was recorded at
ACY. This breaks the old record of 1.19" set in 1909.

&&

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

&&

$$
Synopsis...Drag
Near Term...Robertson/MPS
Short Term...Robertson
Long Term...Drag
Aviation...Drag/Robertson
Marine...Drag/Robertson
Hydrology...
Tides/Coastal Flooding...
Climate...


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