Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 231837

Area Forecast Discussion
National Weather Service Mount Holly NJ
237 PM EDT Sun Oct 23 2016

An intense low pressure system will continue to move northward
across eastern Canada into tonight. A cold front will slide
across our region late tonight and early Monday morning. High
pressure from central Canada will then build toward our region
through mid week. A warm front, followed quickly by another cold
front, is expected in our region at the end of the week.


An intense surface low will continue to roll northward across
eastern Canada through tonight. As this occurs the upper-level
troughing relaxes over our area for a time, however a potent short
wave diving southeastward from the Midwest this afternoon will
arrive in our area late tonight.

The combination of thermal advection reversing, an inversion and
lingering cyclonic flow has resulted in a decent amount of
stratocumulus that expanded across much of our area this afternoon.
This is expected to dissipate from south to north into this evening
as some additional warming aloft occurs and the flow aloft starts to
back. The pressure gradient will lessen as well and with the onset
of less mixing, the gusty winds will diminish fairly quickly late
this afternoon and early this evening.

The short wave for the overnight time frame looks to be tracking a
bit farther north this afternoon as seen on the water vapor imagery.
Some of the guidance has shifted a bit farther northward with the
west to east precipitation axis. While there is a weak surface low
forecast to develop and slide across the northern half of our area
toward morning, much of the forcing appears to be tied to warm air
advection on the nose of a strong 850 MB jet. This will tighten the
thermal gradient and where this sets up will be the placement of a
ribbon of frontogenetic forcing. Our northern zones look to be on
the southern end of the precipitation axis, however are expected to
see at least some showers as this system zips east-southeast. As a
result, we continued the idea of quickly increasing POPs after
Midnight but also tightened the gradient southward. Looks like much
of the measurable QPF is north of I-80 northward. Farther southward,
just some increase in cloud cover is anticipated overnight.

As a low-level jet arrives overnight along with accompanying warm
air advection, temperatures are not expected to drop off as much and
may even rise a bit toward morning. Low temperatures were mostly a
blend of continuity and MOS.


A quick moving short wave and weak surface low will exit to our
northeast early in the morning. This feature will pull a cold front
offshore to start the day and will also amplify the trough aloft
across the region. As a result, any showers up north end very early.
As cold air advection increases during the day combined with
cyclonic flow and a tightening pressure gradient, there will be a
gusty wind once again. It looks like gusts should peak at around 30
mph especially from late morning on.

The increasing cold air advection combined with just enough moisture
should result in some stratocumulus lingering or developing. This
could be fairly extensive across the northern areas especially as
another strong short wave is forecast to arrive during the
afternoon. We therefore kept more clouds across the northern areas
through the day. Since the cold air advection is forecast to be more
notable in the afternoon, many areas should have temperatures get
into the 60s before the cooling starts. We used mostly a MOS blend
for the high temperatures.


Monday night and Tuesday...Monday night begins what will likely
be the first of several nights with frost/freeze potential for
much of the area where the growing season continues. With another
day of cold air advection, and even better radiational cooling
conditions, Tuesday night looks to be even colder, with a freeze
likely for much of southeastern PA (with the exception of the
Philly metro area), and the remainder of NW NJ. the high moves closer, flow shifts more northerly,
but still quite cool with highs about 10 degrees below normal.
There will be another risk for freeze on Wedensday night. However,
unlike Tuesday night, increasing clouds late may somewhat limit
radiational cooling. If the clouds come in early enough, this
could inhibit frost development. For now, kept a mention of patchy
frost for areas with a forecast min of 36 degrees or less, but may
have to adjust both the min temp forecast and mention of frost if
the trend continues with low and mid level clouds moving in before

Thursday and Friday...Models came into much better agreement with
the track of the extra tropical low lifting across the Great Lakes
region and into Quebec through this time. However, there remain
some timing differences, with the GFS being the more progressive
solution, while the ECMWF and CMC are slower. Still, with this
track, it looks like a warm front will lift through our region
sometime on Thursday. Within the warm sector, we should see
increasing chances for precipitation across the region. Then, a
cold front should sweep through the region on Friday.

Those are the factors that the models agree on. However, the GFS
is depicting a mid level short wave trough lifting through the
region ahead of the warm front on Thursday morning, providing
enough lift for some pre frontal precip. This is significant
because GFS model soundings across the Poconos indicate that if
this does happen, it could be a rain/snow mix. However, it is a
very small chance especially considering that neither the CMC and
ECMWF are depicting this and there would be limited opportunity
for moisture advection ahead of the front. For now have only
mentioned a chance of rain and snow across the Poconos through
Thursday morning, before switching precip to all rain.

Saturday and Sunday...models are depicting the cold front stalling
near or just south of our Delmarva counties. However, this seems
unlikely given the upper level northwesterly flow pattern.
Thus...have kept the forecast dry beyond Friday.


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

Rest of This Afternoon...VFR scattered to broken cloud bases around
5000 feet. West-northwest winds 10-20 knots with gusts to around 25
knots, then diminishing toward evening.

Tonight...VFR with an increase in clouds, especially north of KPHL.
Lower VFR ceilings should be confined from about KABE to KTTN on
northward. The main shower activity after 04z should stay north of
KABE and KTTN. West to southwest winds diminishing to 10 knots or
less, although some increase may occur toward daybreak Monday with a
wind shift to the northwest.

Monday...VFR ceilings around at or above 5000 feet. Northwesterly
winds 10-15 knots with gusts up to 25 knots.

Monday night through Wednesday...mostly VFR conditions expected.

Wednesday night and Thursday...ceilings should lower through the
overnight hours to MVFR (with possible localized IFR) by mid day
Thursday. Visibility restrictions possible with rain showers later
in the day.


Intense low pressure will continue to move well north of New England
through tonight, while an upper-level disturbance arrives from the
northwest toward morning. This will allow the winds to decrease
through this evening from south to north, however a low-level jet is
forecast to arrive toward morning before shifting offshore during
Monday morning. The strongest low-level flow is forecast to occur
during warm air advection, therefore it is questionable how much
mixing can occur down to the surface. Cold air advection then
increases during Monday especially in the afternoon. The combination
of both of these is expected to contribute to advisory level gusts
through Monday.

Since the stronger winds aloft are forecast to move through during
a less favorable mixing signature, gusts were kept below gale
force. While there should be a lull for awhile this evening,
especially on Delaware Bay, it should be short enough and
therefore a Small Craft Advisory is in effect for all zones
through Monday.

Monday night and Tuesday...SCA conditions are expected to
continue as winds should continue gusting above 25 kt (though seas
may drop off below 5 ft).

Wednesday and Thursday...winds and seas should stay below SCA

Thursday night...winds could increase above SCA criteria on the
coastal waters late.


MARINE...Small Craft Advisory until 6 PM EDT this evening for ANZ430-


Near Term...Gorse
Short Term...Gorse
Long Term...Johnson
Marine...Gorse/Johnson is the U.S. government's official web portal to all federal, state and local government web resources and services.