Area Forecast Discussion
Issued by NWS Mt. Holly, NJ
FXUS61 KPHI 210008
Area Forecast Discussion
National Weather Service Mount Holly NJ
708 PM EST Fri Jan 20 2017
Weak low pressure passes north of the region this evening, and
then weak high pressure builds east late tonight through
Saturday. A strong and complex area of low pressure will impact
the Mid-Atlantic from Sunday through Tuesday. Conditions briefly
dry out for the mid-week period, and then unsettled weather
returns to close out the work week.
.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
The last of the steady rain was being drawn out to sea early
this evening with a few showers in its wake.
We should remain between two weather systems tonight. A mid
level short wave trough is forecast to continue lifting
northeastward over the Great Lakes while another short wave
moves eastward off the Middle Atlantic coast. Low pressure at
the surface is expected to weaken near Lake Huron while another
weak low is anticipated to move from the North Carolina coast
eastward and out to sea. Weak ridging should impact our region
with a light southeast to southwest surface flow.
Abundant low level moisture is forecast to remain in
northeastern Maryland, Delaware, New Jersey and eastern
Pennsylvania tonight with drying expected aloft. The resulting
subsidence should result in lowering clouds along with the
development of patchy fog and drizzle.
Temperatures are anticipated to remain nearly steady or they
may rise a few degrees during the night due to the southerly
flow and the cloud cover.
.SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
Weak high pressure will build along the coast on Saturday and
we should see mainly dry conditions across the region.
The southerly flow will keep moisture up across the area and
its possible some light drizzle may remain into early Saturday.
Unfortunately, skies will remain pretty clouded over through the
day. However, we will still see temperatures rebound a bit and
rise into the upper 40s across the north to mid to upper 50s
across much of the area as the warm southerly flow continues.
.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
High pressure along the East Coast moves offshore Saturday
night. During this time, closed H5 low will strengthen and
develop over the Southern Plains and Gulf Coast. In addition,
high pressure north of the Hudson Bay will begin to nose its way
to the south and east. Dry conditions expected Saturday night,
but clouds associated with the developing low will stream
northward into the Mid-Atlantic and Northeast.
The first wave of rain lifts to the north and east and into the
Mid- Atlantic by Sunday afternoon and Sunday night.
There is still uncertainty with regards to the forecast from
there. 12Z NAM is the coldest solution, with the high over the
Hudson Bay dropping farther to the south and east compared to
the ECMWF and the GFS. This allows a shallow layer of below
freezing temperatures to filter into far NW zones, generally to
the north and west of I-80, and results in a prolonged period of
freezing rain. Since the GFS and ECMWF have a similar position
with the high and have the surface low farther north than the
NAM, will side with the warmer solution. That does not rule out
at least a brief period of sub-freezing temperatures across
parts of the Poconos, so will mention a chance of freezing rain
on Monday. Latest trends lower the potential of freezing rain,
but it cannot be ruled out at this time.
Waves of rain slide along the coast Monday and Monday night as
that low approaches from the south and west. A tight easterly
gradient develops between the high to the north and the
approaching low. Combined with a 65-75 KT LLJ, can expect strong
easterly winds to develop on Monday, with winds potentially
gusting to 50 MPH for much of NJ to the east of I-95, and gusts
potentially reaching 60 MPH along the NJ coast. Wind Advisories
will likely be needed, and there is the possibility for High
Wind Watches/Warnings for sometime on Monday.
With high PWATs upwards of 1.5", can expect periods of moderate
to locally heavy rainfall during this time. Please refer to the
Hydrology section of the AFD for flooding information.
Low pressure lifts through the region Monday night, and precip
tapers off fairly quickly behind the departing low. Winds also
diminish fairly quickly as the gradient relaxes.
Few lingering showers possible on Tuesday as H5
trough/shortwaves pass through the Northeast.
Dry conditions expected on Wednesday as high pressure builds
through the region.
A deep upper trough then approaches for the late week period.
Unsettled weather is possible through the end of the work week,
but at this time, does not appear to be anything widespread and
organized. A return to more seasonal temperatures is expected by
.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Abundant low level moisture will remain in our region for
tonight and Saturday.
We are anticipating lowering conditions for tonight with all
eight of our TAF sites dropping into the IFR range and perhaps
the LIFR or VLIFR category for a time. There should be lingering
light rain showers this evening, then areas of fog and drizzle
overnight into Saturday morning.
We will maintain the low conditions into Saturday morning with
some improvement possible for Saturday afternoon. We have kept
the forecast somewhat pessimistic due to the presence of a
temperature inversion aloft that should keep the low level
moisture from lifting and mixing significant. The TAFs indicate
an improvement to MVFR for the afternoon. However, this remains
a low confidence forecast.
A light and variable wind tonight may settle into the southwest
for Saturday around 5 knots.
Saturday night...VFR. High confidence. Nearly calm winds.
Sunday...Conditions deteriorating from S to N in the afternoon
with IFR conditions in RA. Moderate confidence. East winds
increasing to 10-15 KT.
Sunday night through Monday night...IFR and lower conditions in
RA. East winds 15-20 KT with 20-30 KT gusts at terminals north
and west of I-95, and 20-30 KT with 30-50 KT south and east of
I-95 likely on Monday. Conditions gradually improve Monday
night. Moderate confidence.
Tue...Scattered showers with brief sub-VFR conditions.
No hazards are anticipated on the waters through Saturday. Seas
remain around 4 feet along the coast with a 10 to 12 second
period. Seas are gradually subsiding and should drop to around 2
to 3 feet tonight. East to southeast winds around 10 knots or
less tonight will become more southerly on Saturday.
Saturday night through Sunday morning...No marine headlines
anticipated during this time. Seas will genly be around 2 to 3
ft with wind 10 kts or less.
Sunday afternoon through Monday night...Easterly flow increases
to 15-20 KT with 25-30 KT gusts by Sunday afternoon. Sunday
night, east winds increase to 20-30 KT with 35-45 KT gusts by
Monday morning. For northern and central ocean waters, a Storm
Watch was issued for Monday for the potential for 50-60 KT
gusts. Meanwhile, a Gale Watch was issued for Monday for
southern ocean waters and DE Bay for the 35-45 KT gusts. Periods
of moderate to heavy rain expected during this time with low
VSBYs. Conditions should improve late Monday night.
Tuesday...Lingering SCA conditions possible.
Wednesday...Sub-SCA conditions expected.
Before heading into the weekend, I wanted to address the
potential hydro impacts for the Sunday through Tuesday period.
In short, we don`t think there will be any river flooding unless
basin-wide rainfall averages climb above three inches. The most
sensitive responding rivers right now appear to be the
Millstone and the Rancocas.
The entire region has some positive things working in its
favor...precipitation has been below normal the last 30 days,
there`s really no consequential snow cover to speak of, there`s
no ice on area waterways, streamflows are either running at or
below normal as is soil moisture, and last but not least, the
ground isn`t frozen. We think all the above will help mitigate
the flood threat.
We feel the latest MMEFS runs are running a bit hot. They show
some impacts with about 2.50 inches of rainfall. But taking into
account the above mentioned items and the fact that the rains
will fall over a 24 to 36 period, we feel more rain will be
needed for river flooding. MARFC in-house contingency runs agree
As of now, it appears poor drainage or nuisance flooding is
more likely, not flash flooding or river flooding. But keep an
eye in things if more than 3.00 inches of rain come to fruition
and you reside in a flood prone river basin.
It continues to appear as though an onshore flow will develop
on Sunday, then it should strengthen on Sunday night. The
onshore flow is forecast to be strong on Monday morning and it
may begin to weaken gradually from south to north along the
coasts of Delaware and New Jersey on Monday afternoon and Monday
evening as low pressure approaches from the southwest and the
pressure gradient relaxes.
We will continue to monitor the two high tide cycles on Monday.
However, the astronomical tides are rather low at that time so
we are anticipating mostly minor flooding. Any flooding may be
enhanced by the wave action. Breaking waves are forecast to be
in the 5 to 8 foot range. There is the potential for localized
moderate flooding, especially from Long Beach Island up to
Raritan Bay, based on the latest guidance.
The first high tide on Monday occurs in the early morning hours
along the oceanfront and it would require a surge of about 2.0
to 2.5 feet to reach the minor flooding threshold. The second
high tide which occurs in the late afternoon along the
oceanfront would require a surge of about 3.0 feet to reach the
minor flooding threshold.
MARINE...Gale Watch from late Sunday night through Monday afternoon for
Storm Watch from late Sunday night through Monday afternoon