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December 2020 General Discussions & Observations Thread


bluewave
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53 minutes ago, KEITH L.I said:

7 days out..I'd be surprised if that was the correct solution.Man you love to show warm maps lol  I don't think I ever seen you post a cold model run

I was posting cold model runs last week for this week when the models latched onto the big block over the North Pole. But the coastal sections had to deal with WAA around 850mb which I pointed out that lead to the sleet and the cement snowpack. You have to take the weather as it comes. What good would it be to only show the weather you like and ignore the kinds that you don’t. All different types of weather have an impact. There are many posters on this forum that are interested in a wide variety of weather. Imagine turning on you local weather forecast and they only showed a preferred type of weather and ignored all the rest.

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4 hours ago, Snowshack said:

That 1.61" seems really high - I didn't think we got anywhere near that much precip.   Certainly not questioning your report.

Understood.  Was more than I was expecting but not impossible.  I thought I picked a good spot but who knows   I don't know a better way to do it.  Even NWS climo stations report some unlikely stuff when it snows.

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28 minutes ago, jm1220 said:

Dec is typically when we’d get snowstorms in a Nina so this is evolving the way a Nina should. There’s been more blocking than some predicted which is great, but we have to keep that in place especially since part 2 of Nina winters are usually warm and snowless here. 

If the Nina dies out , that would be great.

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3 minutes ago, NEG NAO said:

This reminds me of last Monday when the NAM overdid snow accumulations in the area.........

Maybe but the models have so many short waves running around . Monday wave isn't far away from giving us some precip.

The wave last week produced some snow for inland areas.

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2 hours ago, bluewave said:

Flood potential on the 12z Euro around Christmas. Especially for the interior areas with a 20-40” snowpack. Euro has heavy elevated convection and temperatures into the 50s in those  areas. Maybe we can slowly melt our pack each day so there isn’t a sudden flash melt like they could get north and west of the area.

 

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Great point about the flood potential. 

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3 hours ago, jm1220 said:

Dec is typically when we’d get snowstorms in a Nina so this is evolving the way a Nina should. There’s been more blocking than some predicted which is great, but we have to keep that in place especially since part 2 of Nina winters are usually warm and snowless here. 

Yeah, this was our first La Niña with a NYC 6”+ snowstorm in the week before Christmas since 1995. While patterns have changed greatly since then, maybe we could get another shot a major snow event like we did in early January 1996. It may come down to how strong the wave break is with the Christmas storm. The one on December 1st resulted in the record block east of Newfoundland. That blocking eventually shifted poleward and we got our significant snow event this week. Models are going for another big block east of Newfoundland after the Christmas storm. Beyond that time, models may not yet have a handle on how the blocking east of the Maritimes progresses. We can remember how models greatly underestimated the blocking over the North Pole this past week from the day 10-15 period. It suddenly popped up in the 7-9 day forecasts.  So we need to watch for something like that near the end of December. That could potentially set the table for another significant snow event. The animation below is an excellent example of how blocking since early December shifted poleward over time.

https://www.cpc.ncep.noaa.gov/products/intraseasonal/z500_nh_anim.shtml

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Excellent posts this page - haven't had a chance to check anything else. I will begin a topic for 12/24-25 at about 715PM which will go without much ensemble support - always a concern, but the EC, GFS and WPC offers a good start 6-7 days in the future.  It will include damaging wind, potential for ''minor or at worst moderate stream flooding" and maybe an inch or 3 of snow on the back side, Christmas morning.  

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To date, the Arctic Oscillation (AO) has averaged -1.678 this month. It has been negative on 89% of days. It has been -1.000 or below 78% of the time and -2.000 or below on 39% of the days. A negative AO has been present in more than three-quarters of cases where Philadelphia and New York City saw 6" or greater snowstorms during the December 10-31 period, one of which occurred during December 16-17.

Since 2000, December snowfall exceeded 6" or more in 5/7 (71%) of cases in both New York City and Philadelphia when the AO averaged -1.000 or below in December. In Philadelphia, seasonal snowfall exceeded 20" or more in 80% of those December cases when 6" or more snow fell and 30" or more in 60% of those cases. In New York City, seasonal snowfall exceeded 30" or more in all those December cases with 6" or more snowfall and 40" or more in 80% of those cases. The recent snowstorm during what has been a blocky December suggests that seasonal snowfall prospects have increased especially from north of Philadelphia into southern New England. Nevertheless, there remains a chance that the current La Niña could adversely influence the pattern leading to a disappearance of the blocking. Were blocking to disappear, snowfall prospects would diminish. For now, blocking appears likely to continue into the start of January.

Temperatures will slowly moderate this weekend. Tomorrow will be fair and still cool. Sunday will be variably cloudy with some rain and snow showers possible. A strong storm could pass to the west of the region after midweek.

With the persistence of Arctic blocking, uncertainty about the closing days of December has increased. A possible piece of an Arctic air mass could even push into parts of the region on or just after December 25.

Frigid Arctic air is currently centered in northwestern Canada. Earlier today, near record and record cold was reported in parts of the Northwest Territories. At Norman Wells, the temperature fell to -53° (old record: -50°, 1961). At Yellowknife, the temperature fell to a near-record -48°.

Historic experience following exceptionally warm November cases suggests that a warmer than normal December remains the base case. Almost 90% of cases with a November mean temperature of 51.5° or above in Central Park (2020 saw the temperature average a record 52.9° in November) went on to record a warmer than normal December and just over three-quarters of such cases saw December register a monthly mean temperature of 40.0° or above. Sustained Arctic blocking will produce a much cooler outcome. The monthly temperature anomaly could wind up only somewhat above normal.  

The ENSO Region 1+2 anomaly was -0.1°C and the Region 3.4 anomaly was -1.2°C for the week centered around December 9. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.62°C and the ENSO Region 3.4 anomaly has averaged -1.43°C. La Niña conditions will likely prevail at least through the winter.

The SOI was +34.10.  

Today, the preliminary Arctic Oscillation (AO) figure was -2.035.

On December 17 the MJO was in Phase 5 at an amplitude of 0.790 (RMM). The December 16-adjusted amplitude was 0.845.

Based on the latest guidance, no significant stratospheric warming event is into the final week of December.

Since 1950, there have been five cases where a La Niña developed during June-July-August or afterward following an El Niño winter. 4/5 (80%) of those cases saw a predominant EPO+/AO+ winter pattern. The most recent such case was 2016-17. 10/11 (91%) of the La Niña winters that followed an El Niño winter featured a predominantly positive EPO. A predominant EPO+/AO+ pattern is very likely for winter 2020-21. It is likely that the Middle Atlantic and southern New England areas will see a warmer than normal winter with below normal snowfall. The large snowfall that just took place provides a strong contrary signal to the below normal snowfall idea. Should blocking continue, the probability of above normal snowfall would likely increase.

Based on sensitivity analysis applied to the latest guidance, there is an implied 54% probability that New York City will have a warmer than normal December. December will likely finish with a mean temperature near 38.0°.

 

 

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2 hours ago, MJO812 said:

Looks like the gefs has many different solutions in regards to the low developing along the front.

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It’s just FROPA. The GFS loves to show phantom anafronts and phantom lows developing off shore on cold fronts....none of which actually happen. It’s been doing this for years, seen this before, we all know how this movie ends....

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3 minutes ago, JoshSnow said:

Bro we’re going to flood than freeze. You know what I noticed I’m kinda new to this forum but 99% of what you say on here since I joined was wrong. I think you should post less and just post a lot less!

@bluewave was discussing the areas where 20”-40” snow just fell as having the highest risk of a flood threat, not here. 

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I am sorry but I do not see a real serious  flooding issue with rainfall in this upcoming event for area. Past history in our area with these type of  arctic frontal passages with a significant snow pack is  that a good portion of the snow pack melts because of the formation of  very dense fog (snow eater)  in the warm saturated air before frontal passage which helps limits convective rainfall events even with warmer temps.  I do however see some extensive snow pack  melting from the dense fog from the high dewpoints and somewhat warm winds right before frontal passage  but the frontal passage should be quick. I see the very brief heavy  rainfall as a squall line with  winds really gusty over 45 mph during and after the frontal passage . What concerns me is the front will move so quickly that you will have freezing rain/sleet and a significant flash freeze potential with a few inches of snow directly behind the front for most areas east of State College. The temps in eastern Pa/NJ  will go from the mid to upper 40's to below freezing in less than two to three hours. Just my thoughts from over 30 years of watching these events unfold. 

 

  I experienced the 1996 flooding and hit my house hard but that was a totally different animal than this arctic front.       https://www.dep.state.pa.us/dep/deputate/watermgt/GENERAL/FLOODS/fld96rpt.htm

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