Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 222051

Area Forecast Discussion
National Weather Service Mount Holly NJ
451 PM EDT Sat Oct 22 2016

Strong low pressure over Maine will move north through eastern
Quebec Sunday. A cold frontal passage occurs late Sunday night, then
strong high pressure over central Canada gradually builds to our
north by Wednesday. A frontal system will approach from the west on
Thursday and probably move east of the mid Atlantic coast on Friday.


A dramatic change in our weather compared to the last several days.
This is courtesy of a negatively tilted upper-level trough in the
Northeast. This feature will continue to pivot to the north-
northeast through tonight, driving deepening surface low pressure
into eastern Canada. The light showers and sprinkles associated with
the axis of the strong trough aloft will continue to shift eastward
for the remainder of the afternoon. However, an axis of deformation
to our north may allow for some showers to continue to wrap back
into our northern zones into this evening. The airmass is cooling
and therefore cannot rule out a few wet snowflakes mixing in over
the highest elevations of the Poconos late this afternoon.

Otherwise, cyclonic flow and cold air advection will maintain an
area of clouds although this is expected to thin out from south to
north. Any clouds that linger through the night should be across the
northwestern zones given some contribution by the eastern Great
Lakes (i.e. lake effect). The winds will continue to be gusty into
this evening especially given a tight pressure gradient, cold air
advection and good low-level mixing. As we go through tonight
however, the mixing should diminish especially as the thermal
advection weakens or even turns into very weak warm air advection
toward daybreak. As this occurs, the winds are expected to diminish
some with less gustiness. If the winds drop off enough, temperatures
will drop more however we are expecting at least some wind to
continue through the overnight.

As for the low temperatures, we used a MOS/continuity blend however
did not undercut these given the expectation that a breeze remains
through the night.


The upper-level trough weakens across the area during Sunday as it
shifts toward the Canadian Maritimes. Strong surface low pressure
however well to our north-northeast combined with high pressure to
our southwest will keep a tight pressure gradient across our region.
This will result in a gusty west-northwest wind through the day,
which should increase quite a bit by mid to late morning as the
boundary layer warms. The warming aloft though may temper the true
mixing a bit. The gradient does relax toward late afternoon and
therefore we are expecting the winds to start diminishing.

The airmass overall looks rather dry through the day, however as
some warming occurs around 850 MB and with residual cyclonic flow
areas of stratocumulus may result especially across the northwestern
areas. Given the low-level warming expected to occur, a milder
afternoon is expected. High temperatures were mostly a blend of MOS
and continuity.


500 MB: Cold closed low in Quebec weakens over the Maritimes
Wednesday as a short wave amplifies over the western Great Lakes
region. That short wave moves off the mid Atlantic coast Friday
followed by ridging Saturday.

Temperatures...October through the 21st has averaged around 4 degrees
above normal. The chill during midweek will make a significant dent
in that above normal departure. Calendar day averages Monday should
be around 3 to 5 degrees above normal, cooling to between 5 and 10
degrees below normal Wednesday and Thursday, then warming at or above
normal Friday and Saturday.

Forecast basis: Unless otherwise noted, a 50 50 blend of the 12z/22
GFS/NAM MOS guidance Sunday night-Monday night, thereafter the
12z/22 GFS MEXMOS Tuesday and then the 15z/22 WPC guidance Tuesday
night - Saturday, at times blended with the 12z/22 GFS MEXMOS guidance.

The dailies...

Sunday night...A secondary surface cold front slides through the
region late (after midnight). As mentioned by previous shifts, a
tight band of frontogenetic forcing along and just ahead of this
front should lead to some showers. However, limited synoptic
scale lift means that coverage should be limited across our
region, mainly north of I-78, possibly focused along I-80. PWAT
does increase to three quarter inch from the 0.3 inches 12 hours
earlier. lows several degrees above normal. Confidence: above

Monday and Tuesday...A gusty northwest flow pattern sets up to start
the work week, resulting in cold air advection taking hold Monday
night and Tuesday.  Gusty northwest winds each afternoon to 25
mph. Diurnal cloud cover, with greatest coverage during each
afternoon.  Confidence: well above average.

Frost or freeze will probably occur early Wednesday morning across
much of eastern PA and NJ late Tuesday night as winds decrease-
decouple with the approaching ridge of the Canadian centered and
originating airmass. The growing season has ended in the Poconos
and Warren-Sussex counties of nw NJ. GFS 2m temps, combined with
the WPC guidance and 12z/22 GFS mex, suggests that we may be able
to post a freeze watch followed by a freeze warning for the I-78
region northward. It appears likely that if this forecast scenario
is close to reality that the growing season will likely end for
much of the area are along and north of I-78.

Wednesday...Sunny and rather cool with a light northerly flow.
Confidence: above average.

Wednesday night...If its clear through 08z/27, there would be a
repeat of frost or freezing conditions, similar to that of 24
hours earlier. However, there is uncertainty regarding skycover,
associated with a developing warm front to our west. Confidence:
average or just below average with the primary uncertainty being

Thursday...Modestly interesting for late October. Chilly high to the
northeast and an approaching frontal system from the west with the
associated short wave in the Great Lakes heading east southeast.
thickening clouds with a cold rain possible by late in the day.
Confidence: average.

Rain looks quite likely for Thursday night.

This appears to be a possible developing occlusion with low
pressure forming on the occlusion by Friday morning along the mid
Atlantic coast.

Friday...Quite a bit of uncertainty. For now, have forecast the
progressive solution...any morning showers should end allowing a
rainfree afternoon with a solution of a surface low moving east
from the mid Atlantic coast, as per WPC and 00z/22 NAEFS guidance.
However, there is uncertainty and so our skycover forecast is a
little heavier than what might normally be associated a
progressive drying out solution.

Saturday...Looks nice with high pressure dominating.


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

Rest of this Afternoon...VFR ceilings, with some thinning especially
from KRDG to KPHL to KACY on southward. A few light showers and
sprinkles will continue to shift east and northeast, and no
visibility restrictions are expected. Northwesterly winds 15-25
knots with gusts up to 35 knots.

Tonight...VFR. West-northwest winds 15-20 knots with gusts up to 35
knots diminish some through the night. The wind gusts should become
less frequent through the night.

Sunday...VFR. West-northwest winds increase to 15 knots with gusts
up to 30 knots by mid to late morning, then diminish late in the


Sunday night...VFR cigs developing during the night but there is
a small chance of MVFR conditions in showers after midnight,
primarily for TAF sites north of KPHL. Confidence: above average.

Monday through Wednesday...Mostly VFR conditions expected.  Gusty
northwest winds 20 to 30 kt both Monday and Tuesday afternoons.
Confidence: well above average.

Thursday...VFR cigs developing by midday, probably deteriorating
to IFR or MVFR conds later Thursday night in showery rains.
easterly wind becoming southeast to south late. Confidence:


A Gale Warning remains in effect for all zones into Sunday.

Deepening low pressure tracking into eastern Canada will continue to
circulate a colder airmass across our area. This combined with a
tight pressure gradient and stronger flow is working with efficient
low-level mixing. As a result, wind gusts of 35-45 knots are
expected although these will diminish from south to north overnight
and during Sunday. No changes were made to the ending times and
trends will be monitored. A Small Craft Advisory will be needed for
a time following the end of the Gale Warning. Given the strong
offshore wind, seas will be elevated however the highest will be in
lower Delaware Bay and then farther from the Atlantic coast.

Sunday night...SCA conditions may continue into Sunday night,
especially outer waters. Confidence: average

Monday and Tuesday...SCA conditions with northwest winds occasionally
gusting above 25 kt, though seas may drop off below 5 ft.
Confidence: well above average.

Wednesday...No marine headlines anticipated. northerly winds gusting
under 20 kt expected to turn northeast at night. Confidence: above

Thursday...An SCA may be needed for the Atlantic waters Thursday
night. Confidence: Below average.


Have quickly rereviewed the monthly departures. Will focus much
more closely in the sunday afternoon AFD. For now...still expecting
around a #15 warmest October in PHL/ABE, that said with average
confidence. Will probably have a much better handle on the monthly
ranking in late Sundays AFD.


MARINE...Gale Warning until noon EDT Sunday for ANZ452>455.
     Gale Warning until 6 PM EDT Sunday for ANZ450-451.
     Gale Warning until 2 AM EDT Sunday for ANZ430-431.


Near Term...Gorse
Short Term...Gorse
Long Term...Drag 451
Aviation...Drag/Gorse 451
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