Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 231529

Area Forecast Discussion
National Weather Service Mount Holly NJ
1129 AM EDT Sun Oct 23 2016

An intense low pressure system will continue to move northward
across eastern Canada today 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.


The water vapor imagery late this morning shows a well developed
midlevel cyclone just north of Maine. At the surface, deep low
pressure is also centered just north of Maine.

As we go through this afternoon, an upper-level trough weakens
across the area as it shifts more into the Canadian Maritimes. A
tight pressure gradient remains across our area for much of today,
which will result in a gusty westerly wind. However, the airmass is
warming aloft (not as steep low-level lapse rates overall) which
should temper the mixing at least some. The 12z Sterling, VA RAOB
shows the warming that has taken place since last evening below
about 700 MB. Overall though, breezy conditions are expected then
the gradient relaxes toward late afternoon with the expectation of
the winds diminishing at that time.

The combination of the cold air advection shifting to our north and
east with incoming warm air advection has helped to strengthen a low-
level inversion. This is evident on the 12z Sterling, VA RAOB just
below 700 MB. Some moisture is trapped beneath this inversion in
the form of stratocumulus and some of this is expanding eastward
across our western zones. The cloud cover was increased especially
across the northwestern areas. Given the warming aloft, a milder
afternoon is expected. The hourly temperature and dew point grids
were adjusted based on the latest obs, then the LAV/LAMP guidance
was blended in through early afternoon.


A weak disturbance will move acrs nrn areas tonight.  This feature
will bring some showers, but any rain will be over with before
daybreak.  The vast majority of the guid keeps precip confined to
nrn sections, while the ECMWF wants to bring it a bit further s. For
now will go with the more nrn soln.


Monday and Tuesday...models have sped up the development of the
northwesterly flow pattern, and thus the cold air advection.
Therefore, expect a bit of a cooling trend with highs tomorrow,
and then even more pronounced on 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.

Through This Afternoon...VFR. West-northwest winds (locally west-
southwest early) at 10-20 knots with gusts to around 25 knots,
then diminishing late in the afternoon.

Tonight...VFR with an increase in clouds. Lower VFR ceilings from
about KABE to KTTN on northward with some showers arriving after
04z. 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 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.


The Gale Warning has been replaced with a Small Craft Advisory.
This advisory goes through 22z today for now. Gusts should be
mostly up to 30 knots now as the best mixing has shifted to our
northeast. The winds are expected to diminish from south to north
late this afternoon and this evening, however another cold front
arrives late tonight. This should result in an increase in the
winds once again, and therefore the advisory will probably need to
be extended especially if there is not a long enough break in the
elevated conditions.

Monday 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...Nierenberg
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.