fxus61 kphi 190135 

Area forecast discussion 
National Weather Service Mount Holly New Jersey 
935 PM EDT sun Mar 18 2018 

high pressure over the Midwest will gradually track to the 
south and east tonight and then will move off the mid-Atlantic 
coast by Monday evening. Canadian high pressure then re- 
establishes itself north of the Great Lakes. Meanwhile, a 
complex area of low pressure will move east emerging over the 
mid-Atlantic Monday night into Tuesday. This system may linger 
off the coast through Wednesday before moving off to the north 
and east by Thursday. High pressure returns from the north to 
close out the work week. Low pressure then approaches next 


Near term /until 6 am Monday morning/... 
quiet conditions expected overnight tonight across the area. 
High pressure builds to the south of the area through tonight, 
while a weak area of low pressure pushes a backdoor cold front 
across the area. This back door cold front will be a dry frontal 
passage, and not bring any precipitation. It likely will not 
bring much cloud cover with it either, except across the far 
northern portions of the area. Winds will become light and 
variable, and another night of below normal temperatures is 

The hourly temperatures and dew points were adjusted in a few 
spots to account for some local variations. Winds have become 
light in most spots. 


Short term /6 am Monday morning through 6 PM Monday/... 
quiet conditions continue through the daytime hours Monday as 
high pressure noses in from the north, then builds offshore 
during the day and keeps dry weather in the forecast. Clouds are 
expected to increase from south to north late in the day in 
advance of the next approaching system, but no precipitation is 
expected during the daylight hours. Highs on Monday are expected 
to remain below normal. 


Long term /Monday night through Sunday/... 
coastal storm on track to impact the region Monday night through 
Tuesday, and then a secondary coastal storm will impact the 
region Tuesday night and Wednesday. 

Models are coming into better agreement in terms of timing and 
placement of the upcoming storm system, but there remain 
differences which keep this a low confidence forecast. 

Low pressure emerges southwest of the Delmarva Peninsula 
Tuesday morning, and tracks just offshore by Tuesday evening 
before stalling off the mid-Atlantic coast. An upper trough then 
follows behind it Tuesday night and Wednesday, and then the low 
re-intensifies on Wednesday just offshore before lifting to the 
north and east and towards the Canadian Maritimes Wednesday 

Models continue to indicate a fairly sharp cutoff with how far 
north the precip gets. GFS keeps pretty much all of the area 
north of I-195 dry Tuesday and Wednesday with the passage of 
both systems. The European model (ecmwf) is a bit farther north with the extent 
of the heavier precip, but generally keeps the southern Poconos 
dry. The NAM is similar to the GFS in terms of the cutoff. The 
Canadian is similar to the European model (ecmwf) in terms of placement of the 
qpf, but is much wetter. As a result, capped pops at slight 
chance for far northern zones, and otherwise slight chance/low- 
end chance for areas generally north of I-195. 

For the rest of the region, pops ramp up to likely/categorical 
for the Delmarva and southern New Jersey Monday night and Tuesday, and 
likely pops for Tuesday night and Wednesday with the passage of 
the secondary low. 

Ptype remains an issue. For area north of I-195, where pops are 
the lowest, temperatures and thermal profiles will remain cold 
enough to support all snow. But with the bulk of the precip 
staying to the south, not expecting much more than an inch or 
two with the two systems. 

For southern portions of the Delmarva and into extreme southeast 
NJ, there will be enough of an onshore flow to keep temperatures 
warm enough to support at least a mix, and snow mainly at night. 
This keeps accumulations down to less than an inch for the 
coast, and maybe 1-2" inland. 

The big question lies for the I-95 corridor, from generally 
Trenton to Wilmington, and then south towards Georgetown. At 
night, temperatures will be cold enough for all snow. But the 
heaviest of the precip falls during the day. And during the day, 
there should be a wintry mix of snow, sleet, and rain. This 
keeps accumulation down, even with the passage of the secondary 
storm on Wednesday. For this area, generally going with 1-3", 
but there may be some spotty areas of up to 4". 

After running the snow probability graphics, the high-end has 
come down from this morning, and now there is generally the 
potential for 5-7". 

A Winter Storm Watch is not necessary at this time for this 
event, but there is the potential a Winter Weather Advisory may 
be needed if these accumulations hold. 

In addition to the precip, strong northeast winds develop on 
Tuesday, generally 15-20 mph with gusts up to 30 mph, and 
stronger right along the coast. These winds continue into 
Wednesday, and may even strengthen a bit. 

Behind the departing system, winds turn to the northwest on 
Thursday. High pressure builds in from the north and west. 
Temperatures will be well below normal levels through this week. 

Yet another storm system is gearing to make a run at US next 
weekend, but it is way too soon to determine impacts at this 


Aviation /02z Monday through Friday/... 
the following discussion is for kphl, kpne, kttn, kabe, krdg, 
kilg, kmiv, kacy and surrounding areas. 

Tonight...VFR conditions continue tonight. Northwest winds 
diminish after sunset and become light and variable for most 

Monday...VFR conditions continue, although clouds increase from 
south to north during the day. Light and variable winds early 
become northwest, then northeast during the day, with speeds 
around 5-10 knots. 


Monday night and Tuesday...onset of precip still looks to be 
late evening over the Delmarva as rain. By late Monday night and 
Tuesday morning, widespread MVFR/IFR conditions likely from 
kphl/kpne southward. There is high uncertainty as you go 
farther north toward Abe regarding if snow will make it that 
far north with a sharp cutoff in precip expected. For miv and 
Acy, a considerable amount of mixing of rain, snow and perhaps 
sleet is likely while snow is favored farther northward toward 
phl. NE winds 10-20 kt with gusts 25-35 kt (higher end of range 
near the coast). Moderate confidence. 

Tuesday night through Wednesday night...additional precip is 
possible, mainly snow (except along the coast). Confidence in 
precip occurring is moderate near the coast and decreases 
farther inland. MVFR or IFR conditions would be possible if snow 
reaches the terminals but confidence is low at this point. 
Gusty north-NE winds expected, especially near the coast. 

Thursday...VFR. Northwest winds gradually relax. High confidence. 

Friday...VFR. Northwest winds. Moderate confidence. 


conditions expected to remain below advisory levels for tonight 
through Monday, although winds may gust around 20 knots at 


Monday night through Tuesday night...a gale watch is now in 
effect for the northernmost New Jersey ocean waters Tuesday night, and a 
gale watch remains in effect for all other ocean waters for 
Tuesday and Tuesday night. The strongest winds are expected over 
southern ocean waters where gusts could reach 40-45 kt. Wave 
heights in our southern coastal waters are forecast to build to 
8-12 ft and to 6 to 10 ft for northern coastal ocean waters by 
late Tuesday. Conditions should remain below gale on Delaware Bay, but 
gusts to 35 kt are possible at the mouth of lower Delaware Bay. 

Wednesday...the potential for gales exists but it speeds will 
depend on how close a second coastal low tracks to our area and 
how fast it deepens which is still uncertain. Adjusted wave 
heights 2 to 3 feet above wavewatch guidance with the potential 
for northeasterly gales to continue. 

Wednesday night and Thursday...Small Craft Advisory conditions likely. 

Friday...sub-sca conditions expected. 


Tides/coastal flooding... 
an extended period of strong northeasterly winds are expected 
to occur from late Monday night through Wednesday with two 
coastal storms tracking south and east of the region. Positive 
tidal anomalies will increase with each successive high tide as 
water piles up along the coast. The threat for coastal flooding 
looks to develop as early as the Tuesday evening high tide along 
the New Jersey-Delaware coast and continue through the Wednesday night high 

The degree of coastal flooding will depend on the track and 
strength of both coastal lows, which is still uncertain 
especially with with the second one Tuesday night into 
Wednesday. Etss, estofs and other tidal prediction guidance from 
Stevens flood advisory system indicate minor to possibly low- 
end moderate coastal flooding is most likely. 


Phi watches/warnings/advisories... 
New Jersey...none. 
Marine...gale watch from Tuesday morning through late Tuesday night for 
Gale watch from Tuesday evening through late Tuesday night for 


near term...Robertson/po 
short term...Robertson 

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