Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 272032

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

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.


Warm front has plagued the forecast today, with a shortwave
trough moving through the St. Lawrence Valley and an attendant
surface low weakening in southeastern Ontario unable to force
the boundary northward through the CWA today. Instead, the front
has reached a line from Belmar to Philadelphia, with cool/cloudy
conditions to the north and mostly cloudy and warm conditions to
the south. As usual, models are struggling to realize the
temperature gradient across the boundary, with temps generally
about 3-8 degrees too warm to the north of the boundary and up
to 5 degrees to cold to the south. Expect the temperature
uncertainty to continue into the evening as the evolution of the
front remains one of the primary forecast concerns overnight.
Forecast lows are generally mid 40s north to mid/upper 50s
south, with much below average confidence.

Cyclogenesis in the Mid-South is working to counter any progress
the warm front can make this afternoon/evening, as pressure
falls along the baroclinic zone in the Ohio Valley will impede
the warm front`s poleward movement. A small but potent vorticity
maximum in southern Missouri will move eastward to Ohio/Kentucky
by 12Z Tuesday. Large-scale ascent will increase downstream into
the Mid-Atlantic tonight. Precipitation will begin to develop
later tonight as isentropic lift increases. Midlevel cooling
will also allow for elevated instability to increase, so a
couple rumbles of thunder may occur with any showers that
develop. Latest high-resolution guidance brings in a first round
of precipitation from around midnight to around daybreak,
primarily confined to the present location of the warm front.
The last few runs of the HRRR have suggested fairly decent
precipitation rates may develop with the strongest storms, and
have converged in a general corridor where the convection tracks
(roughly along and up to 50 miles southeast of I-95). However,
there is less than stellar support from other convection-
allowing models, so uncertainty remains high. As such, kept PoPs
in the chance category for now, but did make the highest PoPs
in the aforementioned corridor during the overnight hours.

Another concern overnight is the fog. With plenty of low-level
moisture in place and the seemingly ever-present warm front
straddling the CWA, patchy fog seems like a good bet. Winds are
expected to be light tonight, which will only aid in its
development. However, the presence of precipitation complicates
the forecast a bit, so not completely sold on the prospect of
widespread/dense fog. Nevertheless, this remains a distinct
possibility, and its development will be monitored closely this


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.


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

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.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Low confidence forecast tonight. MVFR/VFR conditions south of
warm front and IFR conditions north of warm front will
deteriorate to IFR/potentially LIFR after sunset tonight.
Showers possible 06Z to 11Z, with best chances along the
Delaware River. Second round of precipitation likely after
daybreak Tuesday. 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.


Tuesday night...MVFR 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.


Potential for more dense fog tonight; however, presence of
precipitation complicates issuance of an advisory at this point.
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.


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.


Georgetown 3/28 RER max is 80 set in 1960.

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


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




Near Term...CMS
Short Term...CMS
Long Term...Meola
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