Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 151447

Area Forecast Discussion
National Weather Service Mount Holly NJ
1047 AM EDT Sun Oct 15 2017

A cold front will progress to the east and move through our
region later tonight. High pressure is then expected to settle
over the eastern part of the country for the period from Monday
through Saturday.


Updated public products to extend the period of time with the
low clouds across the area. Breaks have been few and far between
so far, but they should become more numerous after the noon
hour. Reduced the high temperature across the region a few
degrees also.

Changes are expected today as much warm air surges in ahead of
a strong cold front which arrives tonight.

First, we deal with the low clouds and fog, as low-level
moisture is trapped beneath an inversion within a zone of light
winds. This is also producing some drizzle this morning, and
the radar imagery indicates very weak returns across parts of
the area. The forecast challenge is the timing of this
dissipating. The forecast soundings continue to show increasing
vertical mixing occuring later this morning and especially this
afternoon from south to north. Given a southwest to west flow up
through the atmosphere today along with a strengthening low-
level wind field, the trapped low-level moisture should get
mixed out. However some forecast soundings, especially the
NAM/3km NAM, indicate a lower level inversion holding or evening
strengthening some this afternoon within a zone of warm air
advection. The issue is that these same soundings do show better
mixing with time beneath the inversion. Given the chance of
drier air entrainment from above the mixed layer, we believe
there will be increasing sunshine through the afternoon. If not,
then temperatures will need to be probably lowered.

Assuming that we effectively mix this afternoon, a gusty
southwest wind up to 30 mph can occur. This should be more
likely in the coastal plain and also the higher elevations.
Given the expected mixing and also the anticipated improving sky
conditions, afternoon temperatures are expected to surge into
the upper 70s to lower 80s for many places. As mentioned above
though, the temperature forecast today is dependent on the
extent/timing of the clearing. The dew points should favor the
60s for much of the area therefore it will feel a bit on the
humid side. This will all be short-lived though as a strong cold
front arrives later tonight. Showers associated with this front
are expected to remain well to our west during the day.


An upper-level trough moving across the Great Lakes and Ohio
Valley will drive surface low pressure well to our north
tonight. This will sweep a strong cold front across our area
later tonight, with a sharp wind shift to the northwest with its

The guidance shows that the bulk of the large scale forcing
shifts to our north, however enough convergence with the cold
front along with some lingering instability should allow for a
broken band of showers to arrive from the west later this
evening. The question is how much does this weaken with an
eastward extent. The organization of the showers should be on
the decline with an eastward extent, at least initially, given
the timing and little instability in place. As a result, held
off on adding thunder and continued with mainly chance PoPs
(scattered showers) from west to east. Much of the model
guidance indicates that the showers occur behind the cold
front/wind shift, and this characteristic could allow for some
more organized showers to continue farther east. The showers may
reorganize however toward morning across portions of Delmarva
to southern New Jersey as the right entrance of a 500 mb jet
moves across. Some increase in the winds could occur for a time
right behind the cold frontal passage due to pressure rises and
the onset of cold air advection. Low temperatures are mostly a
continuity/MOS blend.


A cold front is expected to be well off the coast and on its
way out to sea on Monday morning. Showers may linger near the
coast through the mid-morning hours. Otherwise, cool dry air
will build into our region on a northwest wind on Monday.

Surface high pressure is forecast to build eastward, with its
center settling over the Virginias on Tuesday before sliding off
the Middle Atlantic coast on Wednesday. The center of the high
is anticipated to move back to West Virginia and vicinity for
Thursday through Saturday. Our region will remain under the
influence of the high through the period. As a result, we are
expecting dry weather conditions.

A mid level short wave trough and an associated cold front may
arrive from the northwest on Thursday night or Friday. There
will be little moisture associated with these features and they
should have limited impact on our weather.

Temperatures are forecast to be near normal for mid-October on
Monday and Tuesday. However, readings will trend above normal
again during the period from Wednesday through Saturday.

Temperatures are forecast to drop into the middle and upper 30s
late on Monday night outside of urban areas and away from the
immediate coast, Delaware Bay and Chesapeake Bay. We are
anticipating patchy frost in the Poconos, the Lehigh Valley,
upper Bucks County, northwestern New Jersey and the Pine


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

Today...MVFR/IFR with areas of LIFR ceilings this morning are
expected to improve in a few hours, then to mostly VFR this
afternoon. Visibilities have improved somewhat already, but will
continue to do so by early afternoon. The timing of the improvements
is still of lower confidence. Light winds, becoming southwest
and increasing to 10-15 knots with local gusts to about 20
knots. While confidence is high regarding the increase in winds,
especially this afternoon, there is lower confidence regarding
the extent of the gusts.

Tonight...VFR overall, although some showers move through later
this evening and overnight (local MVFR possible) as a strong
cold front arrives. Southwest winds around 10 knots, however
gusts up to 25 knots are possible at times especially with a
wind shift to the northwest behind the cold front.

Monday through Thursday...Mainly VFR.


A minor adjustment to winds to account for a quicker increase,
but other than that all is running as expected attm.

Low clouds, some fog and drizzle will give way to improving
conditions later this morning and afternoon. Southwesterly flow
increases mainly this afternoon and tonight ahead of a strong
cold front. A low-level jet arriving mainly late this afternoon
and tonight should result in wind gusts to around 25 knots. It
is possible some gusts to 25 knots occur this afternoon as
mixing develops, then a greater chance of gusty winds tonight
just ahead of and behind the cold front. As a result, a Small
Craft Advisory starts at 18z/2 PM today for all zones and this
goes through at least tonight. Some showers are expected with
the cold front late tonight.

Monday and Monday night...A Small Craft Advisory is in effect
for a northwest wind gusting around 25 to 30 knots.

Tuesday through Thursday...No marine headlines are anticipated.


A northwest wind 10 to 20 MPH with gusts around 25 MPH is
expected for Monday. Relative humidity values are forecast to
drop into the 30s in much of our region on Monday afternoon.
Rainfall amounts associated with tonight`s cold frontal passage
should be light. As a result, there is a heightened fire weather
concern for Monday.


Much warmer air arrives during the day today, and depending on
the extent of clearing and mixing some places could approach
record high temperatures. Below are the record high temperatures
for October 15th, for the official climate locations that could
approach their record this afternoon. Again, this will depend
on how much clearing actually occurs.

Allentown, PA     82 in 1947
Reading, PA       83 in 1989
Trenton, NJ       84 in 1975
Georgetown, DE    84 in 1975


MARINE...Small Craft Advisory from 2 PM this afternoon to 6 AM EDT
     Tuesday for ANZ430-431-450>455.


Near Term...Gorse
Short Term...Gorse
Long Term...Iovino
Fire Weather...
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