Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 280128
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
928 PM EDT Mon Mar 27 2017

.SYNOPSIS...
A warm front will move northward into Delmarva and southern New
Jersey, stalling as it pushes into southeastern Pennsylvania this
evening. Low pressure will track through the Ohio Valley tonight and
cross our region Tuesday evening. High pressure will build down from
Canada for Wednesday and Thursday. Another low pressure system will
move through our area on Friday and Saturday. Weak high
pressure returns for Sunday and Monday.

&&

.NEAR TERM /THROUGH TUESDAY/...
A frontal boundary will stall across central/southern Delmarva
and southern New Jersey overnight tonight. This will keep a
light easterly flow across the northern third of the area
overnight tonight. An area of low pressure will drift through
the Ohio River Valley and toward western Pennsylvania by
daybreak. Meanwhile, southwest flow aloft will develop across
the area as a mid-level trough approaches from the west as well.
Several short wave/vorticity impulses will move across the area
overnight tonight, which is expected to create scattered
showers overnight and through daybreak. It is possible that a
couple of rounds could develop. One around midnight, then
another alter in the overnight and through daybreak.

With this easterly flow, areas of fog will likely develop
across much of the region. This has already started in some
areas, with parts of the northern coast of New Jersey developing
dense fog. We`ve issued a Dense Fog Advisory for Monmouth and
Ocean counties where the dense fog has already started. The rest
of the area is a little tricky. This initial dense fog was
likely due to clear skies and the ability to radiate quickly. As
we go through the next few hours, some mid-high level clouds
are expected to move into the area. In addition, scattered
showers will are also expected to move across the area through
the night as well. These both could help keep fog from becoming
widespread dense across the area. However, we do expect areas of
fog to develop, some which could be dense at times. So it is
possible that the Dense Fog Advisory may need to be expanded
westward.

&&

.SHORT TERM / TUESDAY THROUGH 6 AM TUESDAY/...
Surface low in the Ohio Valley at 12Z Tuesday moves east along
or just south of the Mason-Dixon Line into Maryland/Virginia by
00Z Wednesday as the shortwave trough begins a curve toward the
southeast on the outer fringe of a subtropical ridge in the Gulf
of Mexico. Degree of cooling in the midlevels combined with
strong ascent downstream of the vort max will promote the
development of storms in much of the Mid-Atlantic on Tuesday.
Most model guidance suggests two rounds may occur. The first
would occur early in the day as elevated convection along the
nose of enhanced low-level isentropic ascent. After a brief lull
in the late morning and early afternoon, more convection is
expected to develop along/east of the low and southward-
extending cold front, aided by substantial cooling in the
midlevels and large-scale ascent via differential cyclonic
vorticity advection. Vertical shear looks relatively weak, and
MUCAPE values will generally remain at or below 1000 J/kg,
especially if early-day convection eradicates preexisting
instability. Prospects for severe convection look limited at
best, but scattered storms are still a good bet as the rounds of
precipitation move through the region, particularly in the
afternoon. Best chances appear to be along and south of the
Mason-Dixon Line given the track of the low and the associated
lifting mechanisms. However, overall uncertainty remains given
the multiple rounds of precipitation that may occur. Therefore,
a general broadbrush of chance to likely PoPs was placed in the
grids for now.

There is better potential for locally heavy rainfall given the
decent PWATs, multiple rounds of precipitation expected, and
potential for localized training storms, especially if storms
can align along the baroclinic zone. Though widespread flooding
is unlikely, think isolated instances of nuisance short-term
flooding is a possibility. This will continue to be monitored in
later forecasts.

Temperature forecast remains a concern with the placement of the
eastward-extending warm front again a complicating factor. Stuck
close to continuity and the cooler side of the guidance for now,
but large errors in these values are likely given the poor
handling of the frontal boundaries thus far and the increased
complications from precipitation.

&&

.LONG TERM /TUESDAY THROUGH MONDAY/...
Tuesday night will continue to have some showers as the low pressure
system makes its way offshore and to the east of our forecast area.
Showers will decrease in coverage from west to east and should
completely clear the area by late Tuesday night.

As far as convection goes for Tuesday night, instability looks
extremely limited and overall the best instability looks to be
mainly to the south of our area. Might be a rumble of thunder or two
but confidence is pretty low that we see much convection through the
night.

Canadian high pressure will start to push southward into our area on
Wednesday and persist across the region through Thursday. While the
system itself isn`t particularly strong, it should allow for us to
dry out as a decent northwest flow is across the area on Wednesday.
Winds will lighten up for Thursday with cooler air settling over the
Mid-Atlantic. Highs will be slightly above normal on Wednesday and
much closer to normal on Thursday.

The next system starts to advance towards our area on Thursday. Low
pressure will develop to our west and then move into the Tennessee
Valley by late Thursday. AS the low continues to move towards our
area, we will start to see cloud cover increase and showers will
once again start to move into the area. Expect some light rain to
develop in the vicinity of the approaching warm front

The center of the low will cross just to the south of the region on
Saturday, by around mid morning. The rain will end from west to east
and we should see a little bit of clearing occur. However, there is
not much of a flow behind this system and it may end up remaining
pretty cloudy, especially for areas inland.

Weak high pressure will again be around the region for Sunday
and Monday. Models indicate that a shortwave may move through later
on Sunday, which may trigger a few light showers. Confidence is low
at this time.

&&

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

Conditions improved to VFR everywhere late this afternoon into
this evening, and will continue for another couple of hours.
However, conditions will lower through this evening and into the
overnight hours. MVFR conditions will develop after 02z, then
IFR conditions will develop after 04z. It is possible that dense
fog may develop overnight, but confidence is still low in dense
fog for now, so we`ll keep the lowest VSBYS at 1sm for now.

We expect a couple of rounds of possible showers through tonight
into Tuesday. The first round is expected to develop by 06Z and
move eastward through daybreak. Second round of precipitation
likely after daybreak Tuesday into midday. Lightning strikes
are possible, but low confidence on timing/occurrence at this
time precludes their presence in TAFs. Third round of
precipitation likely tomorrow afternoon/evening. Storms are more
likely with this round. Winds should be generally light and
variable.

Conditions may improve to MVFR briefly during the day Tuesday,
but likely lower again by later afternoon/evening.

Winds overnight will be light, generally 5 knots or less. Any
direction is expected to be vary from northeast to southeast.

OUTLOOK...

Tuesday night...MVFR/IFR conditions possible in showers and
fog... becoming VFR late. Light winds will become north to
northwest overnight around 10 knots or less.

Wednesday...VFR conditions expected. Winds will be north around 10
to 15 knots, some gusts up to 25 knots possible.

Thursday...VFR conditions expected. Light north winds... becoming
east late afternoon.

Friday...IFR or MVFR conds in periods of rain. East to southeast
winds around 10 to 15 knots.

Saturday...MVFR/IFR conditions early in rain... becoming VFR late.
North to northeast winds around 10 knots or less.

&&

.MARINE...
Dense Fog Advisory has been issued for the New Jersey coastal
waters north of Liggle Egg. Areas south may also develop fog
overnight, but it is less certain at this time. This will be
monitored closely this evening. Winds/seas will be under
advisory criteria through Tuesday. Chances of showers/storms
late tonight through Tuesday, likely occurring in several
rounds.

OUTLOOK...

Tuesday night...Southerly winds will become west then northwest
overnight. Speeds around 10 to 15 knots. Seas 2 to 4 feet.

Wednesday...Northerly winds around 10 to 20 knots with gusts around
25 knots late. Seas increasing to around 5 feet . A SCA will likely
be needed as conditions near criteria by Wednesday evening.

Thursday...Decreasing seas through the day. North winds becoming
easterly late in the day, mainly around 10 to 15 knots.

Friday...East to southeast winds around 10 to 20 knots with gusts up
to 30 knots possible. Seas will build through the afternoon,
exceeding 5 feet by Friday evening. A SCA looks likely.

Saturday...SCA conditions continue. Northerly winds around 10 to 15
knots. Seas will remain above 5 feet through Saturday, starting to
subside a bit late.

&&

.CLIMATE...
PHL temperatures continue to project near 1 degree F below
normal for the month as a whole.

&&

.EQUIPMENT...
KNEL appears to be reading 10F too warm the past couple of
days.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...Dense Fog Advisory until 10 AM EDT Tuesday for NJZ013-014-020-
     026.
DE...None.
MD...None.
MARINE...Dense Fog Advisory until 10 AM EDT Tuesday for ANZ450-451.

&&

$$
Synopsis...Meola
Near Term...CMS/Robertson
Short Term...CMS
Long Term...Meola
Aviation...CMS/Robertson/Meola
Marine...CMS/Robertson/Meola
Climate...
Equipment...



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