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000
FXUS61 KPHI 191920
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
320 PM EDT Thu Oct 19 2017

.SYNOPSIS...

High pressure located west of the region will gradually build
eastward through the end of the week. A cold front with several
waves of low pressure will approach the region on Tuesday and slowly
move east through the middle of next week.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...

Mid afternoon surface analysis shows a broad area of high pressure
positioned to our south across the southern mid-Atlantic region and
Southeast states. A cold front was located over the eastern Great
Lakes region, stretching southwestward along the Ohio Valley. There
was also a pre-frontal surface trough near the lee side of the
central Appalachians.

The cold front will approach from the west this evening and then
move through the forecast area late tonight into early Friday
morning. This front should not have a significant affect on our
weather except for a wind shift from the southwest to the northwest
in wake of fropa and high clouds streaming in (mainly across
northern sections).

There may be enough of a pressure gradient over the area to keep
most locations from decoupling aside from the typical sheltered
valleys in NE PA and NW NJ as well as the Pine Barrens. Forecast low
temperatures were blended from various models, then manually
adjusted to enhance the temperature differences between these
sheltered areas where the radiational cooling setup is more
favorable (low to mid 40s) vs the higher elevations, urban locales
and coastal communities (low to mid 50s).

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...

The cold front will move off the mid-Atlantic coast during the
morning. The center of the surface high will shift northward along
the Appalachian spine during the day.

Minimal cold-air advection behind the front will be offset by
warming from downsloping winds off the mountains and strong solar
heating. The net result will be high temperatures that are similar
to today- upper 60s across the higher terrain and low to mid 70s
elsewhere.

The pressure gradient between the departing cold front and high
pressure to our west will produce a temperate northwest breeze
around 10 mph with gusts 15-20 mph.

There are some fire weather concerns for tomorrow with drier air
arriving in wake of fropa. See fire weather section below for
details.


&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...

Friday night through Monday: High pressure will gradually build
eastward into the region later this week then move offshore by the
early part of next week. This will continue the already dry and warm
pattern in place. The blend of MET/MAV along with ensemble mean
guidance will likely be too low with afternoon high temperatures
each of these days. Highs have a decent shot to reach well into the
70`s with plenty of sunshine each day. Overnight lows will
generally bottom out in the 40`s and 50`s. Winds will also be
light till the high pressure system shifts offshore on Monday,
when winds become more southerly and increase a bit.

Monday night through Thursday:

A cold front with several waves of low pressure will slowly move
eastward across the Ohio Valley toward and then through our region
throughout most of next week. A fairly strong ridge in the mid-
levels is expected to be in place across the North Atlantic
which will have the potential to really slow the eastward
progression of the front.

Ensemble spread on timing is fairly high in this time period.
However, the general idea is for a strong LLJ and southerly
advection to usher in moisture and increase winds even further ahead
of the front. Gusts have the potential to reach 30 mph as well. Much
needed rain will then occur ahead of the frontal passage. If this
front checks up or stalls, additional areas of low pressure
could keep rain going through the end of the forecast period.
Right now, the GFS is likely falling into a typical progressive
bias while the ECMWF/CMC may still be slow with the frontal
passage. Rain shower chances extend through Tuesday and
Wednesday given the uncertainty.

Temperatures are also uncertain as well. Current thinking is to be
slightly slower than the ensemble mean with the frontal timing, we
trended a little warmer through the middle of week. High
temperatures in the 60`s and 70`s are still expected with warmer
lows in the 50`s and 60`s. Winds will likely become westerly and
decrease after the frontal passage.

&&

.AVIATION /19Z THURSDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

VFR through Friday. A SW wind around 10 kt this afternoon will
subside to 5 kt or less after sunset this evening. A wind shift from
SW to NW is expected to occur late tonight as a cold front moves
through. Winds increase again Friday morning to around 10 kt with
some gusts of 15-20 kt.

OUTLOOK...

Friday night through Monday night: VFR. Winds variable and under 10
knots till Monday then southerly with gusts reaching 15 knots on
Monday.

Tuesday and Tuesday night: Sub-VFR with shower chances increasing
throughout the day. Southerly winds 10-20 knots with gusts up to 30
knots. A windy period is likely but uncertainty exists with the
timing.

&&

.MARINE...

SW winds generally between 10-15 kt this afternoon. The development
of an Ambrose jet late in the day will locally enhance wind speeds
to near 20 kt off the coast of NJ through this evening.

A wind shift out of the NW is expected late tonight/early Friday
morning will accompany a fropa. NW winds of 10-15 kt with gusts to
20 kt are expected during the morning, gradually decreasing during
the afternoon. Seas of 3-4 ft can be expected in the coastal waters
and 2 ft or less in the Delaware Bay with a longer period (9-
10 sec) SEly swell.

OUTLOOK...

Friday night through Monday night: Seas and winds will stay well
under SCA criteria with seas of only a few feet. Winds will generally
be under 10 knots till Monday then increase from the south.

Tuesday and Tuesday night: Southerly SCA gusts with seas building
over five feet by Tuesday night. Current forecast may be a little
aggressive in building seas given southerly flow, but higher than
normal timing uncertainty.

&&

.FIRE WEATHER...

Northwest flow will advect dry air into the region behind a cold
front on Friday. These winds will be somewhat breezy (10-15 mph with
peak gusts to 20 mph), especially across NE PA and N NJ. RH values
may bottom out in the lower to middle 30s by the afternoon.
Additionally, fine fuels are dry (below 10 percent) across the
region. These conditions could potentially support an enhanced risk
for the spread of wildfires by the late morning or afternoon. A
Special Weather Statement may need to be issued on the overnight
shift for locations north of the Mason-Dixon line.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...Gaines
Near Term...Klein
Short Term...Klein
Long Term...Gaines
Aviation...Gaines/Klein
Marine...Gaines/Klein
Fire Weather...Klein