Area Forecast Discussion
Issued by NWS Mt. Holly, NJ
FXUS61 KPHI 231336
Area Forecast Discussion
National Weather Service Mount Holly NJ
936 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.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
The water vapor imagery 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, some increase in the winds are
expected then the gradient relaxes toward late afternoon with the
expectation of the winds diminishing.
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. Just to our west, moisture is trapped beneath this
inversion in the form of stratocumulus and some of this should be
pulled eastward. May need to add more cloudiness to at least the
Poconos for awhile. 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. The winds were adjusted down
initially for many areas, then a continued increase was shown for
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
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.
.LONG TERM /MONDAY THROUGH SATURDAY/...
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.
Wednesday...as 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.
.AVIATION /14Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
Rest of Today...VFR. West (locally west-southwest winds early) at 10-
15 knots, becoming West-northwest with gusts to around 25 knots by
late morning, 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.
We held onto the Gale Warning for now (all areas except Upper
Delaware Bay) as a few gusts, based on the available obs, are to 35
knots. The trend is expected to be down through the day for much of
the area given the airmass is now warming (thus reducing the mixing
with time). Once the Gale Warning is removed it will be replaced
with a Small Craft Advisory for a time. Seas were lowered a bit
based on the available obs and a continuation of the strong offshore
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...Gale Warning until 6 PM EDT this evening for ANZ450>452.
Gale Warning until noon EDT today for ANZ431-453>455.
Small Craft Advisory until 4 PM EDT this afternoon for ANZ430.