Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 091338

Area Forecast Discussion
National Weather Service Mount Holly NJ
838 AM EST Sat Dec 9 2017

Low pressure near the North Carolina coast will intensify as passes
east of New England tonight and becomes a large storm in the
Maritimes on Sunday. A cold front settles down into Pennsylvania and
New Jersey on Monday. That front moves northeast as a warm front
Tuesday morning in association with low pressure crossing New York
state. A very strong cold front follows in its wake Tuesday night.
Another low pressure system will move east from the Ohio Valley
Thursday night and pass south of Long Island Friday.


Mesoscale Discussion #1 8:30 am:

Well, we are having quite the fight with dry air across most of
the region now. Dewpoints are coming in much lower to support
the majority of modeling which is already verifying on the wet
side for locations away from the coast. Banding may help the
snow start sooner at some locations with one just west of the
Philadelphia metro now and another which brought visibilities
down fairly quickly at Atlantic City last hour with a dusting
on the ground.

The snow/rain(sleet) line is currently from Georgetown DE into
Cape May NJ.

Given the trends, QPF and snowfall have both been lowered
through the early afternoon and pops trimmed back quite a bit
through the morning.

A look to why this is occuring is a decent amount of FGEN
forcing offshore in association with convective development. In
essence, the thunderstorms offshore are robbing our region of
more moisture coming into play.

I don`t want to take products down too hastily this morning,
but that is definitely a possibility as we progress through the
morning given the continued drying trend. Many areas look dry
for much of the morning now.

630 am update: The dry air near the surface continues to be a
hindrance for snow making the move northward, but it looks like
the process is beginning now thanks to substantial large-scale
lift approaching the area. As a potent vort max approaches the
Mid-Atlantic today, jet dynamics combined with considerable
differential cyclonic vorticity advection will permit widespread
precipitation to develop north into the area. Timing of the vort
max appears to have slowed by 3-6 hours...another reason for the
slower onset of precipitation across the area. However, the dry
air near the surface will continue to pose problems early on in
the event for much if any snow to accumulate north of the
Mason-Dixon Line. The trend of the HRRR and HRRR-X has been
downward with QPF today, though not substantially so. The 06Z NAM
Nest continues rather bullish in comparison but has shifted the
snow banding somewhat eastward. The dry look of the GFS is
worrisome, particularly with considerable precipitation ongoing
in the western Atlantic (which may prevent as much northward
advection of more substantial moisture).

Using an updated consensus blend of the 06Z GFS, 06Z NAM Nest,
several HRRR cycles, the 00Z RGEM/WRF-ARW/WRF-NMM, the latest
WPC guidance, and some continuity, QPF went down a few
hundredths of an inch across the area, with most places
receiving up to an inch less of snow than the forecasts this
past evening. This warrants no change to any of the winter
headlines at this point, though the winter storm warning for the
counties bordering the Delaware River looks pretty borderline.

General thinking is aligned fairly similarly to the previous
forecast, with the ramp-up in snow during the next 6 hours and
the best period of accumulation for the CWA this afternoon (into
early this evening). Did slow the increase of PoPs north of the
Mason-Dixon Line a little bit more, but based on latest radar
trends, expecting to see some flakes in the urban corridor
within the next couple hours.

Given the rapidly evolving nature of the forecast given some
short-fused model uncertainty/discrepancies, expect frequent
updates/amendments to the forecast today. Sometimes, higher-
confidence forecasts can deteriorate as the event nears, and
this appears to be a classic case of that.


Vort max will be moving through the region tonight, and the
large-scale lift ahead of it will be shunted eastward during the
evening hours. This should mean a fairly quick west-to-east
progression of the western fringe of the snow this evening, with
the region likely drying out after midnight. End times for the
winter storm warnings and winter weather advisories look OK at
this time.

Snow totals will likely be under 3 inches in the Lehigh Valley
and Poconos, but given the fairly aggressive 00Z NAM output, may
see some marginal advisory-level totals here. Will keep an eye
on this tonight. Meanwhile, 2-4 inch totals look possible for
the urban corridor, with 5+ totals expected south of I-195 and
southeast of I-295 (with increased confidence the farther south
and east you go, at least until the beaches). Confidence in snow
totals is around average, but slightly less than that in the
urban corridor, where the sharpest gradient in snow totals

There will likely be some improvement in the sky cover late
tonight, but I suspect any influence on temperatures will be
minimal, especially with the winds on the upswing as this
occurs. Nevertheless, it will be cold, particularly with some
fresh snow on the ground. For this reason, went slightly below
consensus statistical guidance for lows tonight, though
confidence is not particularly high.


500 MB: A trough over the northeast USA Sunday, briefly recedes
north Monday and then redevelops southward and even stronger
(-4sd) into the northeast USA Tuesday night. It then dominates
our weather into next weekend with one more significant short
wave rolling eastward through the bottom of the trough into our
area Thursday night.

Temperatures: Presuming we have some snow cover by the end of
today, calendar day averages will mostly likely at or below
normal, daily, from yesterday-Friday through next Saturday. The
best chance for a normal max/min calendar day is Tuesday, but
the downside payoff wont be pleasant as calendar day averages
Wednesday and Thursday should be 9 to 13 degrees below normal
(max temps Wednes`day` up to 15F below normal with that days
high temperature possibly at 1201am). The cold should ease
toward next weekend.

Forecast basis:  Unless otherwise noted a 50 50 blend of the 00z/9
GFS/NAM MOS was applied Sunday-Monday,  the 00z/9 GFS MEX was used
Monday night and Tuesday and then the 05z WPC gridded elements of 12
hr pop/max-min temp, and 6 hourly sky/td/wind were used for D4-8
which takes us to next Saturday.

The dailies...

Sunday...Brisk with west northwest wind gusting to 30 mph. some
flurries possible in the Poconos. Overall high confidence.

Monday...Partly sunny and nice (except flurries still possible
over the Poconos) as a cold front approaches. West wind.
Moderate to high confidence.

Monday night...per WPC qpf, blended partial thickness and
blended snow ratios we have fcst a 1-3" snow event north 1/2 NJ
and much of E PA but less than 1 inch Philly. Its conceivable
there`d be spotty freezing rain as the pcpn event ends with the
warm frontal passage toward dawn. This event may require a
winter weather advisory. Uncertainty on QPF amount and so, average

Tuesday...Brisk with a shower or flurries developing e PA and
nw NJ, in assn with a cold frontal passage. Gusty west wind
25-35 mph in the afternoon. High confidence.

Tuesday night and Wednesday...Windy and turning much colder as
very cold trough aloft swings down into and through our area.
Temperatures Wednesday will be much below normal with sub-
freezing highs (teens Poconos) except lower 30s southeast
coasts. Flurries or scattered snow showers possible, mainly e PA
e MD and nw NJ. West to northwest wind gusts 30-40 mph,
especially Wednesday where isolated gust to 45 mph are possible.
Wind chill probably down into the single digits at daybreak
Wednesday. High confidence.

Thursday...Still quite cold. Snow potential, especially
afternoon and night as an arctic wave heads east from the upper
Ohio Valley. The question...does it track north of our area or
close to the Mason Dixon line. This needs to be monitored for a
snow advisory (fluff) event possibility. Thursday morning low
temps in Philly down into the upper teens too! Wind chill
forecast down to about -9 in the Poconos and single digits most
of the remainder of the area. Average confidence. Uncertainty is
regarding the low track north and milder non event for us, or
down across MD out to LI which would be a fluffy snow event,
especially I78 north.

Friday...Fair as per WPC guidance. Average confidence on the


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


Continued to delay precipitation onset with the latest TAF
amendments. Snow onset likely delayed till after 15z away from
KMIV and KACY where snow has already started.

VFR before the snow begins...sub-VFR once the snow
begins, with IFR VSBYs likely, especially this afternoon and
especially from the urban corridor southeastward. Exact timing
of the more substantial restrictions is somewhat low confidence;
however, midlevel precipitation is beginning to develop at this
time, so the onset of snow is probably within a couple of hours
at the city terminals. Light/variable winds today. Confidence:
High with sub-VFR conditions, though low on timing/severity;
very high with winds.

Tonight...Gradual improvement to VFR after snow moves northeast
of the area by late evening. Residual sub-VFR CIGs may continue
through/after midnight, especially along/east of the urban
corridor. Light/variable winds becoming west or northwest 5 to
15 kts late. Overall confidence in evolution is high but with
exact timing is low.


Sunday...VFR. West wind wind gusty to near 30 kt. A few snow
showers possible Poconos.  High confidence.

Monday...VFR. West wind.  High confidence.

Monday night...Restrictions psbl with snow except possibly rain
south. Average confidence.

Tuesday...VFR with a possible flurry or sprinkle north. West
wind gusty near 30 kt. High confidence.

Tuesday night and Wednesday...VFR WIND with flurries or snow showers
possible e PA E MD and nw NJ. West-northwest gusts 35 kt with
isolated 40 kt possible, especially Wednesday. High confidence.


North winds are picking up on the waters early this morning,
and this trend should continue through the day. A small craft
advisory goes into effect at 9 am for the Atlantic zones of New
Jersey and at 5 am for the Atlantic zones of Delaware. This
timing looks quite good at the moment, so made no changes to
these products. There may be a lull in winds this afternoon and
this evening before they pick up again tonight (with a more
westerly flow by this time).

Expect a few visibility restrictions in rain and/or snow
through the day with a gradual tapering trend tonight (generally
south to north).

High confidence on all forecasts for the days listed below.

Sunday...SCA flag with psbl short fuse upgrade to Gale in a few
zones with WNW winds.

Sunday night...SCA possible. Winds and seas decreasing.

Monday...Sub-SCA. Fair weather. West wind.

Mon night..rain snow likely and sub sca.

Tuesday...Westerly SCA probable.

Tuesday night and Wednesday...Gale northwest wind 35-40 kt gusts
as the coldest air of the season arrives.


Record daily snowfalls for the date are listed below for Saturday
December 9. Note...compaction and melting on contact may limit
amounts below our forecast in some of our forecast area.

ACY 3.5-1933
PHL 2.9-1942
ILG 5.0-1928
ABE 9.1-2005


PA...Winter Weather Advisory until 1 AM EST Sunday for PAZ070-071-
NJ...Winter Weather Advisory until 1 AM EST Sunday for NJZ008>010-
     Winter Storm Warning until 1 AM EST Sunday for NJZ016>024-027.
     Winter Weather Advisory until 1 AM EST Sunday for NJZ001-007.
DE...Winter Weather Advisory until 7 PM EST this evening for DEZ001-
     Winter Storm Warning until 7 PM EST this evening for DEZ002-
MD...Winter Weather Advisory until 7 PM EST this evening for MDZ008-
     Winter Storm Warning until 7 PM EST this evening for MDZ015-
MARINE...Small Craft Advisory until 5 PM EST Sunday for ANZ450>453.
     Small Craft Advisory until 5 PM EST Sunday for ANZ454-455.
     Small Craft Advisory from 6 AM to 5 PM EST Sunday for ANZ430-


Near Term...CMS/Gaines
Short Term...CMS
Long Term...Drag
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