Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 151759
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1259 PM EST Wed Nov 15 2017

.SYNOPSIS...
High pressure over our area weakens and retreats toward the
Canadian Maritimes later today and tonight. A strong cold front
moves across our area during Thursday, then high pressure
builds in during Friday. A warm front lifts to our north
Saturday, followed by a cold front Saturday night. High pressure
then builds to our south Monday into Tuesday.

&&

.NEAR TERM /THROUGH 6 PM/...
Clouds are finally starting to develop and move north across
the region, so allowed increasing sky cover for remainder of
today. Temps have been a little warmer than progged so nudged up
highs just a smidge. Northeast wind will veer southeast as the
day wears on.

&&

.SHORT TERM /TONIGHT/...
Cloudy and relatively mild with nearly steady temps, possibly
rising a bit during the night along and east of I-95. Showers
ahead of a cold front are expected to develop, mainly after
06z/16 as per HRRRX NAM/GFS...mostly likely toward 09z/16. That
means a band of showers-rain moving from W to E across PA/MD
into our western zones around 06z. Meanwhile showers sprouting
ahead of the main band in DE/NJ sometime between 06z-09z.
Amounts .05 to 0.25". Less than average confidence on rainfall
ILG/MIV southward but the 00z/EC has .10 or greater over
virtually all of our area 6z- 12z Thu. The drivers for the
rainfall area are 100m 500mb height falls over our area ahead of
a modeled 160M 12 hr HFC vt max sliding sewd through sw NYS at
12z Thu, also a nice 850 trough with considerable absolute
vorticity in its axis as it crosses our area by 12z. That and
some pretty good jet cores at 850mb-40 kt 700mb-55kt in PA-OH
aiding the forcing.

This forecast was a 50 50 blend of the 00z/15 GFS/NAM MOS.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Summary...Strong cold fronts move through Thursday and again
Saturday night with milder air ahead of them, followed by colder
air and gusty winds.

Synoptic Overview...A strong upper-level trough moves through
the Northeast Thursday before lifting out on Friday. Some weak
ridging then arrives, but another upper-level trough quickly
arrives Saturday night into Sunday. As more pronounced ridging
moves across west-central Canada, a downstream trough is
forecast to begin amplifying across the Great Lakes and
Northeast Tuesday. The flow aloft overall looks progressive for
awhile, however there are signals that next week may feature
more amplification with a trough settling into the East.

For Thursday...A strong upper-level trough moves across the
Northeast during Thursday. A surface low tracks well to our
north, however a secondary low should develop offshore of Long
Island early then track northeastward. Showers are expected to
end early, with perhaps some lingering across the far north
through about early afternoon. This will depend on the extent of
wrap around moisture. Cold air advection sets in during the
course of the day, however this is initially stronger at 850 mb
and above. The result will be a milder day for much of the area.
The more notable cold air advection closer to the surface is
forecast to arrive later in the day. The forecast soundings
indicate deeper mixing occurs during the day especially in the
afternoon. Based on the soundings, there is about 25-30 knots of
wind available to mix down and therefore the wind gusts in the
forecast were increased. The winds will diminish some at night,
however given the continuation of cold air advection in the
lower levels and the pressure gradient remaining tight this
process may be slowed.

For Friday and Saturday...The strong upper-level trough moves
away during Friday with the flow becoming more zonal. High
pressure at the surface builds across our region Friday which
will result in less wind through the day. It is noted that the
stat guidance especially is warmer for Friday, however the
modeled 2 meter temperatures are colder. The latter is rather
close to our going forecast, and therefore we opted to make
little in the way of changes. Clouds increase Friday night ahead
of the next incoming upper-level trough. There is fairly strong
warm air advection in advance of this, allowing a warm front to
become better defined as it lifts northeastward. This should
produce a zone of showers or rain along and to its north,
however this appears to be slower in the guidance overall as we
go through Saturday. As a result, we slowed down the PoP
increase from west to east Saturday. Low pressure is forecast to
strengthen as it lifts across the eastern Great Lakes region
Saturday morning and then into Canada. The associated cold front
sweeps through our area Saturday night. Increasing southerly
flow ahead of the cold front Saturday will drive milder air into
the region, then be replaced by west to northwest winds behind
the frontal passage. We continue to bring in likely to
categorical PoPs through Saturday night. Given the timing, held
off in adding thunder.

For Sunday and Monday...While some showers may linger early
Sunday as the cold front shifts offshore, drying is expected.
Low pressure tracks near northern New England Sunday, with a
tight pressure gradient in place along with cold air advection.
Sunday is looking like another breezy/windy day as mixing
deepens during the day. The air turns colder and therefore lake
effect snow should develop off the eastern Great Lakes. It is
possible that a streamer attempts to approach the Poconos later
Sunday afternoon and evening. The winds should diminish at least
some Sunday night. High pressure starts to build in during
Monday, however its center is to our south- southwest. Overall,
a cold day is expected Monday.

For Tuesday...Renewed upper-level trough development is
forecast to take place into the Northeast. This drives low
pressure once again near and north of the Great Lakes with a
trailing cold front approaching from the west. As the flow backs
in response to this system, some warming will be draw
northward. The main forcing with this system may end up lifting
to our north, therefore continued with a dry forecast at this
time.

&&

.AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

This afternoon...VFR conditions expected for rest of today. Mid
level clouds will advect north across the region this afternoon
in advance of an approaching trough. There is a small chance
ceilings will lower to MVFR very late in the day. Light
northeast wind to start becoming southeast this afternoon.
Confidence in ceiling forecast height is average, and for winds
is above average.

Tonight...VFR or MVFR CIGS to start, then degrading to probable
IFR conds toward 09z/16 in showers stratus fog with stratus fog
contg even after showers end vcnty Philly westward near 11z (no
mixing). Confidence above average on sub MVFR conditions late
at night. Light southeast wind becomes light southwest or west
toward 11z/16.

OUTLOOK...
Thursday...Areas of MVFR conditions possible early with
showers, otherwise VFR with clouds thinning with the passage of
a cold front. Southwest winds 5-10 knots, become northwest and
increasing to around 15 knots with gusts to 25-30 knots in the
afternoon. The winds gradually diminish some at night. Moderate
confidence.

Friday...VFR overall, then clouds increase and lower some at
night. High confidence.

Saturday...A period of MVFR/IFR conditions with showers,
especially in the afternoon and at night. Some improvement in
the conditions should occur late at night as a cold front moves
through. Moderate confidence.

Sunday...VFR overall. Some flurries or snow showers possible in
the afternoon mainly north and west of ABE. Northwesterly winds
15-20 knots with gusts up to 30 knots. Moderate confidence.

&&

.MARINE...
SCA remains for parts of the waters through tonight and may
need a final extension northward to the Sandy Hook area (for
tonight) in an afternoon update. The NAM is pretty strong up
there where as the GFS is sub SCA. Still to be determined.


OUTLOOK...
Thursday...The winds shift to the west and northwest as a
strong cold front moves through. This will result in deeper
mixing with time and gusts 25-30 knots especially in the
afternoon. Northwesterly gale force gusts are probable Thursday
night, and therefore a Gale Watch has been issued for all zones.
The extent of the winds will depend on how quick the colder air
arrives, enhancing the vertical mixing.

Friday...Gale force gusts within a northwesterly flow may
linger Friday morning, however the winds are expected to be
diminishing.

Saturday and Sunday...Gale force gusts from the southwest are
possible Saturday afternoon and evening, then gale force gusts
from the west and northwest are possible later Saturday night
and Sunday.

&&

.CLIMATE...
November will probably average below normal throughout our
forecast area. It is expected that the last 16 days will average
at best normal, more likely...below normal...though how much
below normal we`re unsure. I cannot at this time see the
departures listed at the end of this section, warming...and
suspect they`ll run colder by a degree or 3.

The pattern as signaled for many recent days by the
NAEFS/GEFS/ECEFS is basically a trough constantly reloading in
the northeast USA through the end of the month.

So the positive departure excesses from September 14 through
November 6 are history. What we`re experiencing now is what fall
is supposed to be like...chillier. And its probably going to be
a big difference when compared to last November which averaged
1 to 3 degrees above normal.

The numbers below represent the departure from average for the
first 14 days of the month and those departures will probably go
more negative by time the 30th arrives.

ACY -0.2
55N -1.2 (ACY Marina)

ABE -1.0

ILG -0.9

MPO -3.4

PHL -1.5

RDG -2.7

TTN -1.5

GED +1.0


Basically that says the coldest departures were across
the northwest part of our forecast area and the mildest
departures from normal, along our southeast edge.


&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Watch from Thursday evening through late Thursday night
     for ANZ430-431-450>455.
     Small Craft Advisory until 6 PM EST this evening for
     ANZ451>453.
     Small Craft Advisory until 6 PM EST Thursday for ANZ454-455.

&&

$$
Synopsis...Gorse
Near Term...Drag/Johnson/RCM
Short Term...Drag
Long Term...Gorse
Aviation...Drag/Gorse/Johnson/RCM
Marine...Drag/Gorse
Climate...



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