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December 2017 Discussions & Observations Thread

Rtd208

By Rtd208,
in New York City Metro

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bluewave

bluewave

Posted
Posted
16 minutes ago, dmillz25 said:

That’s insane

The last 13 in December was way back in 2008. It last reached 11 in 2004....10....1993....and single digits 6 in 1989. So you can tell outside this year, cold has really been lacking in December. Especially when you consider the low for the month 2 years ago was 34 degrees.

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dmillz25

dmillz25

Posted
Posted
Just now, bluewave said:

The last 13 in December was way back in 2008. It last reached 11 in 2004....10....1993....and single digits 6 in 1989. So you can tell outside this year, cold has really been lacking in December. Especially when you consider the low for the month 2 years ago was 34 degrees.

Which is even more insane. Crazy to think that the following month we had a record breaking blizzard.

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bluewave

bluewave

Posted
Posted
17 minutes ago, dmillz25 said:

Which is even more insane. Crazy to think that the following month we had a record breaking blizzard.

All it took to get it this cold was one of the strongest December -EPO blocks on record. Just an amazing run of extreme NE PAC blocking since 2013.

 

IMG_0051.GIF.eeadece2db6823609c31afc3f9b52904.GIF

 

 

 

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donsutherland1

donsutherland1

Posted
Posted

This morning’s low temperature in New York City was 13°. That is the coldest minimum temperature since February 15, 2016 when the temperature also fell to 13°. It is the coldest December temperature since December 22, 2008 when the temperature dropped to 13°.
Monthly Mean Temperature for NYC Through:

12/27 37.8° (0.3° below normal)
12/31 35.0°-35.6° (12/27 estimate: 35.0°-35.8°)

Per aggressive sensitivity analysis, the estimated probability of a below normal monthly anomaly: 86% (12/27 estimate: 84%); more relaxed sensitivity analysis brings the probability to 99.8%.

The ensembles continue to suggest that an AO-/PNA+ pattern is possible to likely from the December 29-January 7 period. Such teleconnections have typically favored more frequent measurable snow events than climatology (about 80% more frequent). The implied figure is two measurable snow days during that 10-day period.

As teleconnections provide insight, but don’t always fully capture patterns, I ran a statistical analysis last night that includes but goes beyond the major teleconnections. Afterward, I grouped the values into a scale ranging from 0-5 (0 having the lowest frequency of measurable snow days and 5 having the highest). Based on the latest guidance, the figure for the 12/29-1/7 period would be a “4” (meaning much above normal probability of measurable snow events).

Afterward, I ran the numbers for the entire January 1950-2017 period for days with 2” or more snow, 4” or more snow, and all of January’s 10” or greater snowstorms. The percentage of events falling into the range of 3-5 was as follows:

Days with 2” or more snow: 89% (n=81)
Days with 4” or more snow: 95% (n=42)
Snowstorms with 10” or more snow: 100% (n=8)

If this is reasonably accurate, that means that if there is going to be a moderate or significant snow event, the 12/29-1/7 period offers a window of opportunity.

Great caution is required. It is possible that just has appears to have been the case between the Snow Advance Index (SAI) and predominant winter AO tendency, one could be dealing with something that looks good from a hindcasting standpoint, but not so great in a forecasting environment.

I continue to place much greater weight on the relationship between the teleconnections and frequency of snowfall events.

So, looking at the 12/29-1/7 period, the first potential measurable snow event will occur on 12/30-31. This should be a light event based on the latest guidance. The 0z GGEM and 6z extended RGEM show 1”-2” for NYC. The GGEM has been very consistent. The 0z ECMWF has about 0.5” and the EPS mean is a little less than 1” (5% threshold being 2.3”). One EPS member brought more than 4" snow to NYC. The NAM and to a lesser extent GFS have moved toward agreement about a light snowfall event. As things currently stand, my thinking is that NYC and the surrounding area will likely receive near an inch of snow +/- 0.5”. There could be some locally higher amounts of around 2” (especially to the north and east of the City).

The next measurable snow candidate for the period would be a potential coastal storm that would impact the region in the January 4-6 period. The 0z ECMWF brings a significant snowfall to the region. The 0z GGEM brought significant amounts just to the east of the City across coastal New Jersey, much of Long Island, and into southern New England. The 0z and 6z GFS developed the storm but kept it offshore. For now, there’s potential for the middle or latter part of the first week in January.

Finally, in what seems to be a rarity for the model, the latest CFSv2 run features colder than normal readings for January across the eastern two-thirds of the United States. Perhaps a screenshot might be in order to preserve the image for posterity.

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dWave

dWave

Posted
Posted
22 hours ago, SnoSki14 said:

That's a dangerously cold looking ball-drop forecast for the city. Temps could be in the low teens with a decent breeze and windchills at or below zero. 

Should be one of the coldest in a very long time.

Current forecast would have it as the 2nd coldest ball drop all time. The coldest is 1 in 1917.

Now its looking to be around 11 at midnight with a wind chill in the negative single digits.

 

I don't know if anyone would know but I'm curious what is the record for consecutive days with highs below 30. The record for consecutive highs below freezing in Central Park is 16.

https://twitter.com/NWSNewYorkNY/status/945820016682979328

 

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donsutherland1

donsutherland1

Posted
Posted

Up above, I discussed a statiscial "snowfall potential scale" for lack of a better name. I noted that values from 3-5 had above climatological frequency of measurable snow events and accounted for an overwhelming share of days (January 1950-2017) with 2" or more snow and 4" or more snow.

https://www.americanwx.com/bb/topic/50504-december-2017-discussions-observations-thread/?do=findComment&comment=4729262

I expect the figure to be at or above "3" from 12/29-1/7. For better insight, I ran a list of dates (January 1981-2010 base period) when the value was "3" or above for at least 10 consecutive days.
That data is posted at the end of this posted message.

Caveat: It remains to be seen whether the scale looks good from a hindcasting standpoint, but will break down in a forecasting environment. It supports the ideas from the demonstrated relationship between teleconnections and measurable snowfall frequency.

SnowScale12282017.jpg

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NEG NAO

NEG NAO

Posted
Posted
18 minutes ago, donsutherland1 said:

Up above, I discussed a statiscial "snowfall potential scale" for lack of a better name. I noted that values from 3-5 had above climatological frequency of measurable snow events and accounted for an overwhelming share of days (January 1950-2017) with 2" or more snow and 4" or more snow.

https://www.americanwx.com/bb/topic/50504-december-2017-discussions-observations-thread/?do=findComment&comment=4729262

I expect the figure to be at or above "3" from 12/29-1/7. For better insight, I ran a list of dates (January 1981-2010 base period) when the value was "3" or above for at least 10 consecutive days.
That data is posted at the end of this posted message.

Caveat: It remains to be seen whether the scale looks good from a hindcasting standpoint, but will break down in a forecasting environment. It supports the ideas from the demonstrated relationship between teleconnections and measurable snowfall frequency.

SnowScale12282017.jpg

It's difficult to come to any conclusions from the table above  regarding this pattern we are currently experiencing in  what to expect regarding snowfall since the snowfall numbers have a wide range in La Nina and also the other 2 Keys listed...........................so in other words expect anything from next to nothing to 2 or more feet of snow.

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donsutherland1

donsutherland1

Posted
Posted
5 minutes ago, NEG NAO said:

It's difficult to come to any conclusions from the table above  regarding this pattern we are currently experiencing in  what to expect regarding snowfall since the snowfall numbers have a wide range in La Nina and also the other 2 Keys listed...........................so in other words expect anything from next to nothing to 2 or more feet of snow.

The idea isn't to try to pin down amounts. Only the synoptic details can lead to determining specific amounts. The overriding idea is that the 12/29-1/7 period is likely to see a higher share of days with measurable snowfall than climatology. It could account for a disproportionate share of January's snowfall. It lends support to the AO-/PNA+ which argues for such an outcome. The implied number of days with measurable snowfall is 2 vs. the climatologically normal 1. Whether or not there is a moderate or big storm remains to be seen. However, 80% of the 6" or greater snowstorms and all of the 8" or greater snowstorms (January 1981-2010) occurred during the 10-day periods cited above. The lone exception was the January 21, 2001 snowstorm (6"), which occurred during a shorter 6-day stretch where the values were between 3-5.

The bottom line: Ignore Social Media speculation that the pattern is not a good one for snowfall. It is a good one. Nothing is guaranteed, but the odds are much higher than climatology.

 

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