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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KPHI 152059
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
459 PM EDT Sun Oct 15 2017

.SYNOPSIS...
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
Sunday. There may be a weak cold frontal passage Thursday night.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
-- Changed Discussion --
500 pm update: I made several changes to the grids for this
evening. I lowered the winds considerably, especially early this
evening, based on guidance being 5+ kts too strong this
afternoon with mixing. I implemented heavy use of high-
resolution guidance in forecasting sky cover, based on coarser
models generally poor performance today (except the NAM). I
modified temperatures/dew points to exhibit less of a diurnal
trend from increased cloud cover and warm/moist advection in
advance of the cold front moving through the area tonight/early
tomorrow. Finally, I tweaked the timing of showers tonight based
on latest trends observed to our west this afternoon, though I
did keep the overall depiction of highest PoPs north/west and a
gradual decrease south/east as large-scale lift slackens in
combination with overall minimal instability.

I did also modify minimum temperatures a bit (generally warming
them a couple degrees), as I suspect cold air advection
upstream of the front will be offset somewhat by increased
mixing immediately after frontal passage.

Previous discussion below...

Rather cloudy conditions continue over the area this afternoon. Fcst
overall not as nice as expected with little mixing occurring today.
A slow improvement possible as synoptic winds slowly increase ahead
of approaching front. This front will cross the area from NW to SE
overnight and bring drier and colder air to the region. Gusty winds
up to 25 mph and showers will accompany the front. Overall rainfall
totals will probably only be a tenth of an inch or less. Temperatures
will drop behind the front reaching the upper 50s/low 60s north
and low/mid 60s south/east.
-- End Changed Discussion --

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
High pressure will begin to build over the area Monday. Clearing
will continue to progress NW to SE over the area. Winds will remain
gusty through the day as decent cold air advection continues.
Temperatures will mostly remain level or continue a slow fall thru
the day. Early highs in the low 60s north and low/mid 60s
south/east.

&&

.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
Hazards...A frost advisory was issued for a small portion of ne PA
and nw NJ early Tuesday and coverage may need to be expanded in
future forecasts. Additionally... the GFS is flagging potential
widespread freeze frost for eastern NJ early Wednesday.

Fire weather: please see that section for a brief note on elevated
fire danger potential midday and Monday afternoon.

500MB:  A short wave crosses New England Monday evening. Another
trough passes by, further north across Quebec Tuesday and yet
another on Thursday, then very strong ridging (several standard
deviations above normal) develops across the Great Lakes and
northeast USA late in the week through at least next weekend.

Temperatures...

October has averaged 8 to 9 degrees above normal for the first 14
days of the month, excluding the ACY Marina where departures from
normal are less due to the tempering influence of the nearby ocean
(both warm and cold). Calendar day averages Tuesday should be a
a couple of degrees below normal, warming Wednesday to nearly 5
degrees above normal, and eventually to daily averages 10 degrees
above normal Saturday and Sunday. The daily diurnals will be larger
than usual due to the dry airmass in place and light wind fields,
nighttime lows probably a little colder than statistically modeled.

Forecast basis...This forecast was a 50 50 blend of the 12z/15
GFS/NAM MOS Monday night-Tuesday night, the 12z/15 GFS MEXMOS
Wednesday and thereafter the D4-8 WPC 15z/15 12 hourly max/min T
and POP, and the 6 hourly td, wind and sky. The very warm GFS
D4-8 was not used.

The dailies...

Monday night...Gusty northwest winds to 20 mph early, otherwise
chilling with late night decoupling aiding radiational cooling and
the likelihood of scattered to widespread frost near dawn Tuesday,
north of I-78, especially near and north of I-80. A frost advisory
was issued where temperatures appears to give us the best chance
for frost or freezing occurrence. (Isolated frost and freezing
temps have already occurred in this advisory area).

The gusty north wind may continue all of Monday night along the
coasts and there is uncertainty whether there will be enough
decoupling s of I-78 to have issued any advisories there at
this time.

Tuesday-Sunday...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 Sunday. Our region will remain
under the influence of the high through the period. As a
result, we are expecting dry weather conditions. This flux of
the high pressure centroid may also be associated with a slight
temporary cooling Friday with the preceding possible weak cfp
Thu night.

&&

.AVIATION /21Z SUNDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Good mixing still hasn`t developed across the area, some eroding of
the ST around the edges, but not much in the way of any decent
improvement in the low clouds across our area. Several AMDS already
issued to prolong the low clouds in across the area. Conditions may
go VFR by evening and then remain that way until fropa. Limited
confid in fcst details overall. Scattered showers and lower
conditions probable with the front, then a switch to NW winds behind
it late tonight. Monday, decreasing clouds early then SCT clouds for
the afternoon. Winds gusting to 25 knots by late morning and lasting
into the afternoon.

OUTLOOK...
Monday night through Friday...VFR with mostly a light westerly
wind...possibly gusty 15-20 kt Thursday and Friday.

&&

.MARINE...
We will keep the SCA flag in place since winds will be increasing as
the front nears, and gusty winds will continue after fropa.
Scattered showers moving across the waters with the front after
midnight and probably mostly done shortly after dawn. Winds mostly
SW ahead of the front and turning NW behind it. Fair weather
expected Monday.


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

Tuesday..There may be a leftover northerly wind SCA lingering
into the early daylight hours on Tuesday.

Tuesday night through Friday...No marine headlines are anticipated
with generally a westerly flow and gusts under 22 kt.

&&

.FIRE WEATHER...
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.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Frost Advisory from 2 AM to 8 AM EDT Tuesday for PAZ054-055.
NJ...Frost Advisory from 2 AM to 8 AM EDT Tuesday for NJZ001-007-
     008.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 AM EDT Tuesday for ANZ430-431-
     450>455.

&&

$$
Synopsis...Drag
Near Term...CMS/O`Hara
Short Term...O`Hara
Long Term...Drag
Aviation...Drag/O`Hara
Marine...Drag/O`Hara
Fire Weather...Iovino/O`Hara