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Major Nor'easter snow storm (possible top 20) Noon Wednesday-Noon Thursday Dec 16-17, 2020


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Edit topic:  525A Wednesday 12/16: No change in thinking including uncertainties along I95.  I just cannot shake the basics: S+ axis is usually 90 miles left of the 850 low and the northern half of the 700MB low and the deformation zone near 500 MB with contribution from the nose of the 850MB  50-65 kt easterly jet (moisture transport). This favors I84 and banding casts northwest to a central PA - east central NY area (roughly IPT-ALB) where 20-25" seem probable within 60 miles of that axis - at least in the high terrain, but must consider mountain shadow lesser amounts in the leeward valleys.  There a multiple models showing that axis.  In our forum, I think most everyone will see a period of sleet near or after midnight and that the bulk of the snow occurs in a 6-10 hour period prior ~ midnight, but that there will be a tail of moderate to heavy snow toward dawn with still 1-4" after sunrise Thursday on top of whatever occurs before sunrise---the heavier 4" potential ne PA, extreme nw NJ, se NYS and CT {where 5" may occur in 2-3 hours after sunrise Thursday}.  Drifts to 4 feet possible ne PA, se NYS and northern CT where 18" accumulates since winds will gust 30-40 Mph at times overnight. NYC CP still 6-10" in my mind but shakey and may be too high because of only 10-1 snow ratios at best, sleet and possibly rain compacting as well as temps near 33F at times during the night.  The power outage concerns continue near I95 with 45-50 MPH gusts (9P-6A). Coastal flooding - please see NWS offices for minor or moderate near the time of high tide Thursday morning. Finally, even if it rains or is above freezing w LI and Monmouth/northern Ocean County NJ tonight--- there could be sudden icing near sunrise Thursday when the wind switches from northeast to north and drops temps 5-10F in an hour,  and iceing the residue  and make travel very hazardous again in leftover new snowfall. Otherwise temps in the I84 corridor ~20-25F throughout the event.  Will add some graphics by 6AM on wherever the last page of the thread.  NOT a top 20 for NYC...best chance is MPO-POU axis in our forum.  This is my last topic update for this event. 


Edit Topic: 445 AM Tuesday 12/15: NO change in thinking.  Except for the outlier south GFS/GEFS, all systems appear go for a major snowstorm as outlined below in the 6AM Monday topic update. Will detail in the latest page by 545AM. 00z/15 EC/NAM/RGEM/UKMET/GGEM/UKMET operationals on board! There will be multiple (~12 graphics of supporting model documentation-most probabilistic and 2 day top 20 snowfalls in the available period of record).


Edit Topic headline at 6AM Monday 12/14/20.  Please monitor NWS Watch-Warnings for this storm and associated statements and still uncertainty on where highest impact.  There is/was some overall slight southward shift in the guidance in the past 24 hours and so my uncertainty for the forum is northern CT-Catskills,  and LI/I95 southeastward. WPC still has 1.5"+ qpf just s of I80-LI and 3/4" into northern CT.

If you have travel plans Wednesday into Thursday I95 northwestward from VA through WV, PA, NJ, NYS and southern New England it's probably a good idea to consider changes, based on your NWS or other favorite media outlet forecasts. 

I think this will be a top 20 snow storm for a few parts of our NYC subforum (basically ~ 1 foot will prompt that review) but unsure where. For now between I95 to I84. Anticipating (not guaranteed) widespread closures or delays for all transportation systems I95 corridor northwestward Wednesday evening into Thursday morning,. For those who have access and wish to do so, you can calculate an experimental storm index to get an idea of what is probably coming. I don't want to presume more than Major yet (crippling is the next step up). Here is some information. https://www.ncdc.noaa.gov/snow-and-ice/rsi/?nesis

Still variability in northward extent of heavy 1 foot snow ranging from the NAM Adirondacks to the GFS-RGEM I80-LI. UKMET is still quite far north.  Ensembled r/s line looks to remain mostly south of I80-LI but awfully close, close enough to think sleet, rain or freezing rain mixes in for a time after the initial Wednesday evening thump for LI and s of I80.  EPS continues almost unyielding on the widespread 8-18" snowstorm (isolated 2 feet possible in the banding somewhere north of I80-LI).  Major power outages possible wherever the 6" of 32-33F wet snow accumulates along with wind gusts of 40-50 MPH. If this were to occur, it would most likely develop between 9 PM and midnight Wednesday evening per consistent modeling of snow onslaught and wind gusts increasing past 30 MPH.  Am targeting LI south coast and NJ coast but this will require 6" wet snow (32-33f).  Isolated wind gusts above 50 MPH possible on those coasts.  Snowfall rates of 1-3"/hr possible in northward transiting bands Wednesday-evening/ night.  Where these tend to park on the northern extent of the storm, will probably be the 2 feet of snow, especially with ratios drier than 10 to 1 (maybe 13 to 1). NWS has the multi model ratio blend tool and BUFKIT can assist for those who have that tool. There could be closing bands of moderate snowfall Thursday morning after sunrise into early afternoon as the comma head tends to fold southeastward behind the eastnortheastward departing  storm. 

Clearing the first few inches of snow around 8-10PM Wednesday will make it easier to remove the additional snow Thursday morning. Where it's wet-caution for those with heart conditions., 

This strong easterly surface gradient (northeast wind) between the cold 1035MB high over Quebec and and the probable 995-1000MB low near Cape May early Thursday will produce minor to moderate coastal flooding, for the Thursday morning high tide cycle. Follow NWS statements. 

No graphics update with this topic edit. 


Edit Topic time at 506AM Sunday... otherwise an 18-24 hour nor'easter coming with the uncertainties continuing as previously listed  below. HIGH impact late Wednesday afternoon into Thursday morning with many cancellations probable of whatever activities in this Pandemic. 

It's 5-6 days away but taking my cue from our participants, have separated Wednesday-Thursday (16-17) from the prior 5 day (14-18) topic.

Modeling is showing the potential for a 6"+ snowfall for a large part of the NYC forum area, somewhere between the I95 corridor and I90. There is model uncertainty regarding the intensity of the upper air and surface systems but enough information in the models to monitor this storm for possible high impact in parts of the NYC subforum.

Some potential exists for nearly a foot of snow in a small portion of the forum area, IF everything comes together correctly... but it's too early to be sure and it's best not to focus on the higher amounts yet.   Banding in the DGZ could make for as much snow along and north of I84 as may occur closer to the storm center in NNJ where the snow could be a lot wetter.  

Usually it takes -3 to -5C at 850MB this early in the season with wind off the 40's SST, for snow not to change to rain. So LI and the NJ coast are presently in the modeled greatest uncertainty region whether it's mostly rain or snow.

Astronomically high tides Wednesday and Thursday mornings seem destined to allow some coastal flooding within 2 hours of high tide, but this is going to be driven by storm intensity and wind direction.  Just too early have a good handle on flood potential but I would think minor at a minimum. 

Not posting a topic for late Friday-Saturday Dec 18-19 as that storm could easily pass to our south.  Just good enough to know that appears to be a highly unusual week, potentially, for the NYC forum to have up 3 different snow events of varying magnitude in a 6 day period.  

Added this afternoons NWS ensemble driven D6 chance of more than 3" of snow, which has increased considerably over the past 24 hours.  I also added the GEFS probability of half a foot of snow or more... use the legend color (greens-yellows edging past 65%) for your area of interest.  The Dec snowfall's in NYC since 2010, 2nd column from the right.  and The daily almanac for NYC CP-Wednesday Dec 16...the 5.3 " is the daily snowfall record. Small chance of daily record snowfall?? 


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Hazardous Weather Outlook

Hazardous Weather Outlook
National Weather Service New York NY
442 PM EST Fri Dec 11 2020

Northern Fairfield-Northern New Haven-Northern Middlesex-
Northern New London-Southern Fairfield-Southern New Haven-
Southern Middlesex-Southern New London-Western Passaic-
Eastern Passaic-Hudson-Western Bergen-Eastern Bergen-Western Essex-
Eastern Essex-Western Union-Eastern Union-Orange-Putnam-Rockland-
Northern Westchester-Southern Westchester-New York (Manhattan)-Bronx-
Richmond (Staten Island)-Kings (Brooklyn)-Northwestern Suffolk-
Northeastern Suffolk-Southwestern Suffolk-Southeastern Suffolk-
Northern Queens-Northern Nassau-Southern Queens-Southern Nassau-
442 PM EST Fri Dec 11 2020

This Hazardous Weather Outlook is for southern Connecticut,
northeast New Jersey and southeast New York.

.DAY ONE...Tonight.

Hazardous weather is not expected at this time.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday.

There is potential for a significant coastal storm Wednesday into
Wednesday Night bringing heavy snow and rain, strong winds, and
coastal flood impacts.


Spotter activation may be needed. Please safely report significant
weather conditions while following local, state, and CDC guidelines.


This Hazardous Weather Outlook provides a summary of potential
widespread hazardous weather events that may reach NWS warning
criteria. Most long fused NWS watches, warnings, and advisories in
effect are highlighted.

Please refer to the latest NWS forecasts for weather not meeting NWS
warning criteria.

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GEFS is split into 2 camps: the cold, weaker, progressive camp of the OP and the warmer (for coastal areas) amped bomb camp of the CMC/Euro, with the latter group being larger. Could be the difference between an SECS for I-95 and the coast and a glancing blow for the interior on one hand and an MECS for the interior and a mixed bag on the coast with I-95 on a knife's edge. Hopefully the confluence continues to build which could allow colder solutions for marginal areas even in the case of an amped system.

Edit: Here are the GEFS snowmaps. Some great hits in here:




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I always worry about these D5-6 forecasts.  And now i see some servers can't handle loading new model data in a timely fashion.  One downside i neglected to mention, in my haste to get this going...  overall weakening trend of the 500 MB trof, flattening and speeding up as it exits east of LI.  Progressive... Our numbers will in part be dependent on how much 850 low forms Ohio Valley.  I think we're going to need that. Check of the 12z/11 NAEFS looks good.  Also duration may only be 12-18 hours..???  and with cold air... comes dry air and I can see dry air eroding the northern part of the ensemble qpf (I90?).  I always like deep closed lows to our south. In this case 850 to our s looks good. That's about all for me tonight.   Oh,  don't think highest amount... be satisfied if it's 3" NYC-LI-coastal NJ and 6-9" elsewhere. Always look for the downside on these storms. That can reduce surprise disappointment. 


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3 minutes ago, JoshSnow said:

How much?

Taken literally about 2.2 at LGA but the BL is marginal this is an event that you’d have to be in the perfect spot for accumulating snow.  Monday will have a very small axis where the snow accumulates.  Especially if if only ends up far enough NW for the immediate coast   

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Just now, JoshSnow said:

John Manetta yes it was, we had a monster high to our north and a developing low off the stalled front south of us. Definitely very close that’s my opinion just because you think it’s not true doesn’t make you right by any means!

Jan 2016 was a Miller A El Niño fueled southern stream system. This (Weds) is a system redeveloping due to (hopefully) blocking and confluence north of us, so Miller B. Also should be moving quicker. 

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5 minutes ago, MJO812 said:

Few inches for NYC on the Nam for Monday

You want the Monday event to be more amped.  Even if it’s rain in most of the area.  PSU in the MA forum went through ensembles on the GFS and Euro and found almost unanimously the members that are amped Monday have the best solutions for Wednesday.  The only place which probably doesn’t want Monday to be amped are places like interior SNE such as Worcester 

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1 minute ago, CIK62 said:

3",  but only if timing is perfect.     925mb, 850mb T's drop at last possible moment to perhaps coincide with the precipitation.

The air mass just isn’t that good.  It’ll be a very narrow corridor who gets hit with decent amounts Monday.  If the air mass was even 3-4 degrees cooler in the 850-1000mb layer this would pretty much be a snow event across the board   

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