Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 211955

Area Forecast Discussion
National Weather Service Mount Holly NJ
355 PM EDT Wed Mar 21 2018

A major Nor`easter located just east of the Mid-Atlantic coast today
will slowly track to the northeast. High pressure will attempt to
build southward to start the weekend, as cold air settles into the
region. A system will move through the eastern U.S. Saturday
night and Sunday but may stay south of the northern Mid-
Atlantic. High pressure should then build back in for early
next week as the main upper-level low finally lifts to the


This section will be updated by 4:30 pm.

1030AM update: Increased snow amts N central NJ through ptns of
e central and se PA down thru e MD and srn-central DE, and then
tonight coastal NJ Atlc nwd. WSW is out. 1030 AM STS is posted
and reminder...only 8A today to 8A Thursday. We know its struggling
to snow snow north of I80 and we haven`t changed amts there and
sweat it out as models insist it will snow pretty good there
this aftn/early tonight. We have not changed PHl amt at all
until we know the reality at 230 PM.

930 AM checking. STS snow grid as posted on PHI web site and in
briefing pkgs conts as is and is from 8AM today through 8 AM
Thursday and does NOT include what occurred between 8 AM ydy and
8AM this morning. No change yet but once we see the 12z NAM
trends we will probably make an adjustment higher to the w and n
of PHL. We need time to digest multiple sources of ongoing info
and hourly updated model guidance. Hopefully most folks I95
nwwd know the storm is ON! to the east, we hear wheres the snow
but we think its coming. Need more time to assess.

Max snowfall so far 4.5" in Lehigh County and ABE officially at
8AM 2.1" snowfall rates in the e PA NNJ band now mostly 1-almost

Cold sub freezing air pouring swwd into the snow area ... see
Sandy Hook 31F!

No change in the following pgh updates from 8 AM:

No further briefing pkgs today but we will update the STS as it
becomes apparent to us that amounts will differ more 2 inches
from our fcst. Will we get 10-12" in PHL? Possibly not but its
going to be quite a mess. We already have a widening areas of
MDT snow developing eastern PA and nw NJ just west of the I95
corridor and its only 12z with many reports coming in now of 2"
snowfall in the last several hours.

Also fwiw...temps rose into the upper 30s overnight in the I95
corridor when pcpn quit and still the ice/snow mix held strong.
Ground has been cooled.

We will be issuing LSR`s only for 1/hr snowfall accumulations
and glaze reports elsewhere that totaled 0.1 or greater. Lets
continue sending those reports to us via our skywarn or social
media accts. Both the glaze overnight and the storm total snow
and lets us know how the roads are doing and if any power

Collapsing thicknesses as pcpn now is changing to sleet and then
snow most of our I95 area except the coast this morning.

Synoptically: no chg from pattern recognition. A big event is
unfolding and we just need to be patient. Significant nearly
stationary banding now evident N central MD to eastern Berks
western Bucks County to near NYC. EVERYONE will share in snow
but the main axis of the S+ should be vcnty MQS-CKZ-SMQ. Higher
amts than forecast are possible in the NNJ part of our fcst,
particularly Northampton, Warren, Hunterdon, Morris, Somerset
but we`re not raising at this time until we know for sure what
we already have at 230 PM. If we will be STS only
and posted to our office winter wx web page and also SM.

Inverted trough developing from the developing primary coastal
low through through Ches Bay and that can aid lift to its
northeast. Monitoring.

Regardless of snow amounts, SUBSTANTIAL impacts, including
power outages and additional tree damage, are likely due to a
combination of heavy snow, strong winds, blowing and drifting

Mesoscale wise: please note we are aware of a typical Berks
county downslope shadow in western Berks. I can see storm totals
ranging 7" western Berks to 15" near Huffs Church in the high
terrain far eastern Berks. No changes yet but we are aware of
the potential.

Snow ratios: in 3 hr increments 12z tdy-07z tonight.
PHL 9,10,11,12,12,12 to 1 using snow ratio blender.

In the cldr ptn of storm e PA/NNJ vcnty I78 10,12,12,12,12,12
to 1 with Poconos slightly higher snow ratio. So even if qpf
were to decrease this aftn, snow ratios will permit greater
hourly accums.


This section will be updated by 4:30 pm.

The low will begin to move northeast away from the region,
bringing a gradual end to the snow but still expect an easy 1-3"
after 8 PM. For the most part, expect any significant
accumulations to be over with around 2 AM. Light snow showers
may linger into the early morning hours, especially across
northern NJ. For low temperatures, have gone on the lower side
of guidance in anticipation of a widespread snow pack by that


With the ongoing winter storm, very little time was spent on the
long-term forecast today. Main concerns include a chance of some
rain/snow showers with a northwest-flow vort max passage on Friday
and the ultimate track of a system moving through the eastern U.S.
this weekend. Few changes were made to the previous forecast overall.

The major coastal storm affecting our region today and tonight will
be pulling northeast away from the area tomorrow. The region should
be precip-free by 12Z, so a dry forecast is in place. Went below
guidance for highs given expected snow cover and albedo effects
tomorrow (and possibly somewhat persistent partial to considerable
cloud cover). Temps generally 10-15 degrees below seasonal averages.
Breezy conditions expected but nothing out of hand. Given the
dryness of the snow, may see a little bit of blowing snow in the
higher terrain, but winds look pretty marginal so did not feel the
need for inclusion in the grids anywhere. Skies may not completely
clear out Thursday night, so did not stray too far from guidance for
lows...only slightly below consensus at this point.

Friday`s forecast is tricky, as a potent northwest-flow vort max
moves into the Northeast during the day. Its passage will be during
the day, so timing is favorable for some instability showers,
especially in northern/eastern portions of the area (given the track
of the vort max). ECMWF is not enthused by the prospects, but
coarser models tend to underforecast this threat. Some indications
from GFS of this threat, and I think it is reasonable to keep slight
PoPs during the afternoon for areas northeast of Allentown to Philly
to Atlantic City. I did lower PoPs a little given insufficient
support from longer-range hi-res guidance. Temperatures again below
guidance given expected clouds and remaining snow cover (and once
again 10-15 degrees below climatology).

Another strong vort max moves southward from eastern Canada on
Saturday as a separate vort max moves west-to-east through the
central U.S. and interacts with the digging perturbation. Attendant
surface low in the central plains Saturday looks to be shunted
southward somewhat as it tracks into the Ohio/Tennessee Valleys.
Just how far south is a challenging question, with considerable
disagreement among the operational guidance. The GFS looks
especially suspicious, though, given its very progressive evolution
of the northern-stream vort max. The 12Z CMC/ECMWF are slower and
(as a result) have the track of the southern-stream system a little
farther north. This may give the Delmarva Peninsula a chance for
some precipitation Saturday night and Sunday. Additionally, there
are some indications that as the surface low intensifies off the
coast that wraparound (light) precipitation may affect the eastern
CWA on Sunday (aided by the passage of the main northern-stream vort
max). Cannot discount chances of rain and/or snow for much (if any)
of the area during this period, so kept slight-chance PoPs in the
grids. If the southward-deflection of the southern-stream system
occurs later than anticipated, this may bring somewhat heavier
precipitation into the area (especially Delmarva). A low-confidence
forecast exists for this period, to be sure.

Strong ridging develops early next week as the strong cyclonic
vortex meanders eastward off the Atlantic coast. This should bring a
prolonged dry period to the area along with a warming trend. A
system may affect the region by mid to late week, but models have
large timing differences. Generally included slight-chance to low-
chance PoPs on Wednesday for now, but temperatures fortunately look
more seasonal by then.


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

Today...IFR/LIFR developing in snow, some of it heavy at times
with 1/4S+ VV003. The chg has occurred now to MIV and ACY.

Plowing ops will be needed at all TAF sites this aftn and tonight.
Drifting snow will develop at most if not all TAF sites by late
this aftn. NE wind with occasional gusts 20-30 kt except gust
35-40 kt possible KACY. Winds trend N late today.

Tonight...IFR conds in snow 00z-03z/22 becoming VFR as nw winds
gust 20-30 with drifting snow and blowing snow at some of the
TAF sites.

Storm total accums 8 AM today through 5AM Thursday as of our
current fcst. Amounts should be accurate to within 3".

RDG 6-8"
ABE 10-13"
TTN 13
PNE 11
PHL 11
ILG 10
ACY 8 Probably hammers there late aftn/evening.
GED 4 Still 4 but might be under done there by 4"

Thursday: VFR with northwest winds 10 to 20 kts with
gusts 25-30 kts. High confidence.

Thursday night: VFR with northwest winds 5 to 15 kts. High

Friday: Potential for MVFR or low VFR CIGs and rain/snow showers,
especially northeast of ABE-PHL-MIV. Northwest winds 5 to 15 kts
with potential for gusts to 20 kts or so. Moderate confidence.

Friday night and Saturday: VFR with northwest winds 5 to 15 kts,
potentially lighter during the night. High confidence.

Saturday night and Sunday: Some potential for sub-VFR conditions
with light rain and/or snow. Winds north to northeast under 10 kts
Saturday night and 10 to 20 kts on Sunday. Very low confidence.


Winds haven`t gotten to storm force yet, but think that remains
a possibility through 3Pm. If not we downgrade to gale all
STW`s. Seas are already observed over 12-15` 44091 to 44009,
and those should continue to increase through at least the
afternoon. Winds to begin to diminish as the storm moves away
from the region tonight but still gale conditions for much of
the night.

Thursday: Remnant gale-force northwest gusts becoming small-craft
advisory criteria during the day. Seas slowly subsiding.

Thursday night: Small-craft advisory northwest winds continue.

Friday and Friday night: May see a lull in winds on Friday morning,
but they should increase again by afternoon evening to advisory

Saturday and Saturday night: Sub-advisory winds/seas expected. A
slight chance of rain and/or snow late, especially near/south of
Delaware Bay.

Sunday: Advisory-level northeast winds expected, with some potential
for gales (though this is low confidence). Some potential for rain
and/or snow.


Northeast flow continues along the coast in northern NJ, but farther
south, winds have already shifted to the north across southeast NJ
and DE. In addition, winds have diminished a bit and range from 15-
20 mph with gusts 25-30 mph. Going through this evening, winds will
continue to shift to the northwest.

Although tides remain elevated tonight, they should not be as high
as previously expected given that winds will not be quite as strong
as forecasted. In general, expecting departures around 2 feet for
northern and central NJ, 2 to 2.5 feet for southern NJ and DE zones,
including land adjacent to Lower DE Bay, 1.5 to 2 feet for northern
NJ, and around 1.5 feet for land adjacent to Upper DE Bay and the
Tidal DE River.

Coastal Flood Advisory remains in effect for northern NJ and parts
of NJ and northern DE adjacent to Upper DE Bay and DE River as minor
coastal flooding is expected with the upcoming high tides.

The Coastal Flood Warning remains in effect for all other areas,
though now anticipating widespread minor coastal flooding with areas
of moderate coastal flooding, as opposed to widespread moderate
coastal flooding.

At this point, only spotty minor flooding is anticipated for the
tidal Delaware River between the Commodore Barry Bridge and Trenton.
No tidal flooding is expected for the upper eastern shore of
Chesapeake Bay.

Between this evening and early Thursday morning, these should be the
last rounds of coastal flooding for this storm.


This section will be updated later this evening.

NEAR record breaking March snowfall accumulation forecast at
PHL. The record is 12 inches on March 13-14, 1993. #9 is 8.8
inches March 3-4, March 4-5 in 1981. 1.1" so far at PHL mstly

Allentown record March two day storm total snow 19.9 inches
March 19-20 1958 (close to a 60 year anniversary). # 10 is the
10.2" event of 3/29-30/1970. We have a very good chance of top
10 at ABE, barring any last minute model surprises. 2.1" as of
12z and all this morning.

Wilmington record March two day storm total snow is 17.9 inches
3/19-20/1958. # 10 is 7.9" 3/2-3/1960. We have a good chance of
cracking top 10 list. 1.4 so far at ILG, mostly ydy.

Atlantic City record March two day storm total is 11.6 inches
3/1-2/69 and #7 is 7 inches 3/4-5/15 and 3/6-7/53. There is a
chance of cracking top 10.

We`ll update again at 5 PM today we know whats occurred and
whats coming.

Also from Rutgers-Dave Robinson and Mat Gerbish at our request

Since we think northern NJ per the Rutgers areal definition will
exceed 5 inches of snow (areal average), probably end up closer
to 15 inches or more... this would put northern NJ by Thursday
morning 8 AM as the highest ever in our recorded history. Right
now the average is 18.9 there and we expect it to rise to ~25 or
30" by 8AM Thursday (this has yet to happen), which would
propel us to the snowiest March on record in NNJ.

The monthly MARCH record for a single station in NJ to our
knowledge via Rutgers is 43.0 in March 1958 at Canistear
Reservoir. Oak Ridge had 42.0 and Greenwood Lake 36.8 also in

This storm may drive the current Jefferson Township and Rockaway
values which are near 33 inches, above those records...we just
dont know yet but by this time! We`ll let the reality determine
the outcome.

This is all stated to place some perspective on what we think
this upcoming event can produce for us.

Daily Record Snowfall

Site  3/20          3/21          3/22
----  ----          ----          ----
PHL   9.6"  (1958)  4.7" (1932)   3.0" (1914)
ACY   5.0"  (1914)  5.9" (1889)   2.4" (1964)
ILG   10.3" (1958)  5.4" (1964)   3.0" (1943)
ABE   16.5" (1958)  4.3" (1964)   2.6" (1992)

Snowfall as of (3/18/18)

Site  March `18  Rank   Since 7/1  Rank
----  ---------  ----   ---------  ----
PHL   7.6"       26     22.2"      56
ACY   3.5"       17     28.0"      13
ILG   6.1"       24     19.9"      48
ABE   7.5"       26     32.8"      39

Daily Record Rainfall

Site  3/20          3/21          3/22
----  ----          ----          ----
PHL   1.76" (1958)  2.24" (2000)  1.90" (1977)
ACY   2.56" (1958)  1.98" (2000)  1.54" (1903)
ILG   1.99" (1913)  3.21" (2000)  2.22" (1977)
ABE   2.12" (1958)  1.42" (1983)  2.49" (1977)
RDG   3.03" (1958)  1.57" (1890)  2.70" (2000)
TTN   1.74" (1958)  2.02" (1980)  2.25" (1977)
GED   2.12" (1975)  1.94" (2001)  1.20" (1964)
MPO   2.13" (1975)  1.28" (1950)  2.74" (1980)


PA...Winter Storm Warning until 2 AM EDT Thursday for PAZ070-071-
     Winter Storm Warning until 11 PM EDT this evening for PAZ054-
NJ...Winter Storm Warning until 2 AM EDT Thursday for NJZ001-
     Coastal Flood Warning until 5 AM EDT Thursday for NJZ020>027.
     Coastal Flood Advisory until 5 AM EDT Thursday for NJZ012>014-
DE...Winter Storm Warning until 2 AM EDT Thursday for DEZ001>004.
     Coastal Flood Warning until 5 AM EDT Thursday for DEZ002>004.
     Coastal Flood Advisory until 5 AM EDT Thursday for DEZ001.
MD...Winter Storm Warning until 2 AM EDT Thursday for MDZ008-012-
MARINE...Gale Warning until noon EDT Thursday for ANZ450>455.
     Gale Warning until 4 AM EDT Thursday for ANZ430-431.


Near Term...Drag/Johnson
Short Term...Drag/Johnson
Long Term...CMS
Tides/Coastal Flooding...MPS
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