Area Forecast Discussion

FXUS61 KOKX 172056
Area Forecast Discussion
National Weather Service New York NY
356 PM EST Sat Feb 17 2018
Low pressure passes to our south tonight, followed by high
pressure for Sunday. The high then slides offshore through 
Monday. A warm front approaches from the southwest Monday night, 
then lifts to the north Tuesday, followed by a slow moving cold 
front crossing the area during the middle week. High pressure may 
briefly build in to close the upcoming work week, before another 
frontal system approaches for the weekend.
Low pressure emerges off of the Mid-Atlantic coast and moves 
northeast, passing just south of the 40N/70W benchmark. The storm 
will pass by relatively quickly as there will be a lack of 
downstream blocking to slow it down.
PCPN types, particularly over coastal areas, remain a challenging 
forecast with boundary layer temperatures marginally supportive of 
snow. This along with a slight increase of QPF for some of the 
models complicates headline decisions. Models have generally nudged 
north with the track of the low with the 12z suite. Most noticeable 
however is that although the NAM has nudged north, it's thermal 
profile in the boundary layer has warmed up significantly over Long 
Island and to a lesser extent elsewhere. The rest of the models are 
fairly similar to their respective previous runs regarding the 
boundary layer temps and temps farther aloft. So with only NAM 
having a notably changed, changes to the snowfall forecast will be 
greater over coastal sections of the city and Long Island, including 
the North Fork. Snow amounts for these areas have been lowered.
For a few hours within both sides of midnight, the strongest 
frontogenetic upward lift shifts through the tri-state area with the 
heaviest PCPN. Light onshore winds will also be shifting towards 
north during this period. Therefore expecting boundary layer and 
surface temps to drop a degree or two at this point, shifting the 
boundary from all snow to a rain/snow mix farther south. Banding 
potential exists, however the window of opportunity for this to 
occur seems brief with the quick advancement of frontogenetic 
forcing and the orientation of the jet streak aloft. Best chance for 
banded PCPN appears to be over Long Island and SE CT, but thermal 
profiles at its onset might not be supporting all-snow before 
cooling down.
A blend of NAM 2-meter temps with SuperBlend looked good for surface 
temps through the event - generally on the colder side of guidance.
Thermal profiles support all snow for all areas north of the city 
and most of NE NJ. Mixing with rain could still occur for part of 
the CT coast as well as areas within about 10 miles of the city. A 
mix of rain and snow is otherwise forecast, and not even out the 
question for a complete changeover to rain over parts of Long Island 
and southern parts of the city. Should the warming trend of NAM 
hold, may very well need to make mention of a complete changeover in 
these areas and lower snow amounts.
A winter storm warning remains in effect where it had already been 
posted, with the exception of northeastern Suffolk County where a 
winter weather advisory is in effect. Did not have enough confidence 
to downgrade elsewhere. All areas that had been in a winter storm 
watch are now also under a winter weather advisory. The winter wx 
advisory over Orange and Putnam Counties remain, however a good 
portion of Putnam County might exceed 6 inches.
Ridging at the surface and aloft occurs on Sunday, with high 
pressure shifting offshore on Sunday night. Dry weather through the 
period with a mostly clear sky. Sided with the colder side of 
guidance for temperatures due to a fresh snow pack.
Anomalously strong Bermuda/East coast ridging develops early this 
week and continues into midweek, in response to deep western 
troughing. The western trough will gradually shear NE in piecemeal 
fashion for the late week/weekend, resulting in a transition to a 
bit more suppressed but active confluent sw upper flow.
At the surface, high pressure slides off the coast on Monday, and 
then becomes established under the upper ridge by midweek. Meanwhile 
a slow moving frontal system will cross through the Mississippi 
river valley the first half of the week. Its warm front likely 
approaches Monday Night with rain and then gets pushed north of the 
region on Tuesday. An unseasonably warm SW flow is likely Tue/Wed 
under deep layered ridging, with highs around 70 possible for NYC/NJ 
metro and areas N&W on Tue and lower to mid 70s on Wed. Meanwhile 
south coastal areas may struggle to get out out of the 50s due to 
the cold water temp. Coastal advection stratus/fog and nocturnal 
radiation fog is possible during this time as well.
Models in general agreement with the eastern ridging breaking down 
enough, to allow a stream of shortwave energy to approach and push 
an approaching cold front towards and through the region Wed night. 
This likely brings another round of rain to the region Wed night 
through Thursday.
Potential for brief ridging Friday, before next shortwave and 
associated frontal system affect the region on Saturday with another 
round of rain.
Temps during the period expected to average well above average, with 
high temps potentially 25-30 degrees above normal Tue and Wed for 
NYC/NJ metro and areas N&W..
Low pressure approaches this evening, and passes south of Long 
Island tonight. 
VFR through 23Z. IFR/LIFR develops between 23z and 04z as 
precipitation overspreads the area. Initially, a rain/snow mix is 
expected at NYC and Long Island terminals, but will transition over 
to all snow after midnight. Timing continues to be a bit uncertain 
and could be a few hours off from current forecast. P-type should be 
all snow at all other terminals. The snow gradually tapers off with 
improving conditions from west to east early Sunday morning. 
Snowfall rates could reach 1 to as much as 2 inches per hour during 
the heaviest snowfall. Total accumulations of 4 to 7 inches are 
forecast. Amounts could be lower at KJFK, KLGA and KISP, if more 
mixing were to occur.
Southeast winds around 10kt will continue through evening, then as 
low pressure passes south of Long Island tonight, will shift back to 
the NE. Expect Northeast winds to gust to around 20 kt during the 
day on Sunday.
.Sunday afternoon...VFR. Gusts to 20kt. 
.Monday...VFR becomes MVFR or IFR in rain and possible fog, 
especially late.
.Tuesday...MVFR or IFR possible in the morning and again at night, 
otherwise VFR with SW wind G20KT possible.
.Wednesday...Mainly VFR. Chance of showers/MVFR.
.Thursday...sub-VFR in rain.
SCA has been issued for the ocean waters for Sunday morning into 
early afternoon with increasing winds and seas. There could be some 
gusts to 25 kt Sunday morning over the Eastern Sound and eastern 
bays, but coverage didn't warrant an advisory. Sub-sca conds 
otherwise tonight and from late Sunday afternoon through Sunday 
Generally sub-advsy winds expected Mon through Wed, but a persistent 
15 to 20 kt SW flow may have sea building to SCA levels Tue/Wed. 
Seas should subside Wed night into Thu as a weak cold front passes 
through and winds become offshore.
Around 1/2 to 3/4 an inch of liquid equivalent is expected Saturday 
night with locally higher amounts. With most, if not all of this 
falling in the form of snow, no hydrologic impacts are expected.
No significant hydrologic impacts are expected from Sunday through 
the end of next week.
NYC NOAA Weather Radio Station KWO-35 (162.55 MHz) is off the 
air for an extended period of time.
CT...Winter Storm Warning until 7 AM EST Sunday for CTZ005>012.
NY...Winter Weather Advisory until 7 AM EST Sunday for NYZ067-068-
     Winter Storm Warning until 7 AM EST Sunday for NYZ069>073-078-
NJ...Winter Storm Warning until 7 AM EST Sunday for NJZ002-004-006-
MARINE...Small Craft Advisory from 6 AM to noon EST Sunday for ANZ350-