Does Siberian Snow Cover in October Really Predict North American Winters?

Over the last several winters, an interesting phenomenon that has been discussed is that of Siberian snow cover trends over the month of October. The idea is that if snow cover is anomalously widespread over Siberia in October, the following winter in North America will lean towards being colder than normal. Similarly, if snow cover over Siberia is lackluster in October, temperatures across North America will tend to be warmer than normal during the following winter.

This theory has been advanced by Dr. Judah Cohen of MIT, and I have been particularly fascinated by the concept. In an effort to see how reliable this method could be, in this post I present an admittedly-crude analysis of Eurasian snow cover in the month of October from 1967-2018 and view the subsequent winter seasons.

I would like to make it explicitly clear that this is only meant to be a crude analysis of this phenomenon - I have a great deal of respect for Dr. Cohen, and there are doubtless other far more intelligent and far more able-minded scientists who can analyze this topic more closely and more accurately. This post is more of a preliminary and basic analysis on the topic. Feel free to read more about the concept as well: link

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Let's begin with an analysis of the data I will use in this analysis. I use data from Rutgers University's Global Snow Lab, which provides content on the extent of snow cover across different swaths of the Northern Hemisphere, as well as the Northern Hemisphere in the aggregate. This data is almost completely continuous, save for October data in the year 1969. As such, we will work with 51 samples in this analysis. For this post, I am using snow cover data for the Eurasia region specifically, over the month of October only. I am using data on an anomaly basis: per the Rutgers GSL, the average snow cover in the month of October in Eurasia is approximately 9,497,000 square kilometers.


I. Winters Following Top Ten October Eurasia Snow Cover Anomalies

First, I'll create a composite of winter-season temperature anomalies over North America for the winters following the ten years of highest positive snow cover anomalies in October over Eurasia. The composite is presented below. Click on any image to enlarge it.

Surface temperature anomalies for the ten winters following the ten Octobers featuring the highest Eurasia snow cover anomalies.
Source: ESRL

The ten years featuring the highest positive anomalies of snow cover in October over Eurasia, in order, are 1976, 2014, 2016, 2002, 1970, 2013, 2015, 1971, 1968 and 1972. I have graphed the composite surface temperature anomaly (in degrees Celsius) for the December-January-February period that followed each year in the above graphic. When putting these top ten years / top ten winters all together, the result is actually pretty in line with what this concept states, that stronger snow cover over Eurasia in the October preceding these winters should result in colder than normal winters for North America in the winters themselves.


II. Winters Following Bottom Ten October Eurasia Snow Cover Anomalies

Next, let's look at the temperature composite for the bottom ten years - that is, the winters that followed the ten Octobers where the snow cover anomaly over Eurasia was the lowest. The image is shown below.

Surface temperature anomalies for the bottom winters following the ten Octobers featuring the lowest Eurasia snow cover anomalies.
Source: ESRL

The picture becomes less clear-cut when looking at the ten winters which followed the ten Octobers with the lowest Eurasian snow cover anomalies. While the below-normal temperature anomalies in North America have become less centralized from what we saw in the first temperature composite, we do not see predominantly above-normal temperatures over these ten winters. Indeed, the majority of the country is actually at neutral or slightly below-normal temperatures, with much of Canada entrenched in firmly below-normal temperature anomalies.

However, I do see an area where this concept becomes useful.


III. 500-Millibar Geopotential Height Anomalies

I want to take a look at those same top-ten and bottom-ten winters, except this time instead of viewing surface temperature anomalies over North America we'll go over 500-millibar geopotential height anomalies over the Northern Hemisphere.

500-millibar geopotential height anomalies for the ten winters following the ten Octobers featuring the highest Eurasia snow cover anomalies.
Source: ESRL

500-millibar geopotential height anomalies for the ten winters following the ten Octobers featuring the bottom Eurasia snow cover anomalies.
Source: ESRL

Where the surface temperature anomalies were rather murky when trying to differentiate the winters following top-ten and bottom-ten Octobers in terms of snow cover anomalies over Eurasia, the picture becomes far more focused when expanding to 500-millibar geopotential height anomalies.

The first image of the above two shows 500-millibar height anomalies for those ten winters following the ten Octobers with the highest positive snow cover anomalies over Eurasia. That composite shows that the winters featured a firmly-disrupted tropospheric polar vortex, with strong ridging evident from the north Pacific through the Bering Sea, across the Arctic Circle and into western Europe. This led to the tropospheric polar vortex being forced to lower latitudes, as shown by the deep negative anomalies in North America, far western Europe, and northern Eurasia. In contrast, the second image - showing 500-millibar height anomalies for those ten winters following the ten Octobers with the deepest below-normal snow cover anomalies over Eurasia - shows essentially the opposite. A stronger than normal tropospheric polar vortex is observed across the upper latitudes, with generalized and widespread ridging prevailing around the mid-latitude regions.

This does seem to validate Dr. Cohen's theory, with more expansive snow cover over Siberia in October leading to a weaker stratospheric *and* tropospheric polar vortex in the Northern Hemisphere for the following winter.


To Summarize:
- A crude analysis of Eurasian snow cover data seems to confirm Dr. Judah Cohen's theory that anomalously high (low) snow cover over Eurasia in October corresponds to an anomalously weak (strong) stratospheric and tropospheric polar vortex in the following winter, with broadly-attendant chances for colder (warmer) than normal temperatures for the winter.
- While I was admittedly skeptical of this tool, the data in this preliminary analysis don't lie: this does seem to be a worthwhile tool for seasonal forecasting.

Andrew

2014-2015 Winter Forecast Update: Potentially Frigid Winter Ahead

This is a brief update to the temperature outlook for the 2014-2015 winter, where I'll close out discussions on the October-related patterns, such as the Snow Advance Index, Lezak Recurring Cycle, and October Pattern Index.

Rutgers

The image above shows snow cover anomalies over the Northern Hemisphere on October 19th. This is the latest we can get these charts, as data outages have lost snow cover records from October 20th to present day. Regardless, we can see the massive swath of above-normal snow cover extending across Eurasia, as the blues show. This snow cover has been expanding at a steady rate, and has maintained a well-above-normal stature.

We can apply this to seasonal weather trends in the coming winter. According to Judah Cohen, creator of the SAI concept, the rate of snow coverage across Siberia in the month of October can predict the following winter Arctic Oscillation (AO) phase, which can then play a big role in determining if we will see a cold winter. When snow coverage for October is well above normal, like this year, the AO will usually be negative in the following winter. Similarly, below normal snow coverage spells a positive AO for the following winter. Needless to say, the Snow Advance Index is favoring a potentially-strong negative AO this winter, which is likely to increase the risk of cold weather outbreaks here in North America.

OPI

Second, we have the October Pattern Index, or OPI. The above image shows anomalies of the October Pattern Index over the last several days, since the start of October. The OPI, the concept of which was brought about by a group of Italian scientists, says that monitoring of the atmosphere during the month of October can yield great hints at what the coming winter will bring. October is a month well-known for big winter-predictors showing their cards for the coming cold season (i.e. the LRC, and Judah Cohen's Snow Advance Index (SAI)), but it may interest many to know what the OPI may be one the best, if not the best predictor of the upcoming winter season out of the three mentioned above.

The explanation page of the OPI tells of the index's incredible accuracy, around 90%, of being able to predict the December-January-February Arctic Oscillation. In the winter, a negative phase of the Arctic Oscillation (AO) means the polar vortex is weak and is more prone to sending cold outbreaks to the mid-latitudes, while a positive AO indicates a strong polar vortex, hence a warmer winter increases in probability.

The OPI has been in sustained negative territory throughout the month, not once touching positive marks. This tells us with overwhelming certainty (or at least as much certainty as we can have at this point) that the coming winter's Arctic Oscillation will be negative, and this is backed up by the aforementioned SAI discussion.

Lastly, let's go over the Lezak Recurring Cycle.

HPC/WPC

Something I've discussed on here more than a few times is the concept of the Lezak Recurring Cycle, or LRC. The LRC was developed by meteorologist Gary Lezak, and discusses the idea that weather patterns which develop in October leave a 'footprint' of sorts that is repeated in a regular interval, between 40-60 days through the winter and following spring. In other words, the weather patterns that develop in October repeat themselves for the better chunk of the next year.

Since mid-September, we've seen predominantly below-normal temperatures for large swaths of the Central and East US. One of the more impressive atmospheric set-ups came with this very strong upper level low positioned just north of the US/Canada border. Notice how the influence of this low extends all the way to the Gulf Coast, per the contour lines. My worry is that this upper level low will end up being another piece of the polar vortex, like we saw last winter, that might push south and bring intense cold back once or twice through the coming winter. I'm not going to speculate any further, but some sharp Arctic blasts may be in the works for the next few months.
As we dip into a warm period right now, this will likely reflect in the LRC with some warm periods in the coming winter, but I'll go into that further down the road when we know how long this warmth might last.

To summarize:

- Indications are that a potentially frigid winter is once again in the works.

Andrew

Historically-Accurate Pattern Predictor Suggests Cold Winter Ahead

A winter predictor that has had incredible success in past years, commonly referred to as the October Pattern Index (OPI), is supporting another cold winter ahead.

The above image shows anomalies of the October Pattern Index over the last several days, since the start of October. The OPI, the concept of which was brought about by a group of Italian scientists, says that monitoring of the atmosphere during the month of October can yield great hints at what the coming winter will bring. October is a month well-known for big winter-predictors showing their cards for the coming cold season (i.e. the LRC, and Judah Cohen's Snow Advance Index (SAI)), but it may interest many to know what the OPI may be one the best, if not the best predictor of the upcoming winter season out of the three mentioned above.

The explanation page of the OPI tells of the index's incredible accuracy, around 90%, of being able to predict the December-January-February Arctic Oscillation. In the winter, a negative phase of the Arctic Oscillation (AO) means the polar vortex is weak and is more prone to sending cold outbreaks to the mid-latitudes, while a positive AO indicates a strong polar vortex, hence a warmer winter increases in probability.

Though we aren't to the middle of October yet, early indications are that the OPI is sustained in a negative phase, today plummeting below -3, off the chart as shown above. If the OPI stays in that negative territory for the rest of October, chances are the winter may have a negative Arctic Oscillation. The AO doesn't define the winter in terms of cold overall, but it can give a good hint at the temperature pattern for the coming winter.

Andrew

Long Range Climate Models Show Snowy Eurasia in October

Long range models are giving indications that the coming October may feature above normal precipitation in the Eurasia area, something that could then favor a colder than normal winter for the United States.

NOAA

The image above shows a global probability of above, below, and neutral precipitation anomalies. We can focus in on many areas of the world and try to decipher what they mean for the coming winter, but today, we’re going to keep our attention on the forecast over Russia and the general north Asia area.

This forecast, a probability forecast using a combination of roughly six or seven different climate models from various global meteorological agencies, shows a swath of above normal precipitation probabilities across a good portion of central Russia. In the forecast, we see the precipitation anomalies extending north of the 60N longitude line, an important line in our situation.

A few years ago, Dr. Judah Cohen created the Snow Advance Index, abbreviated as the SAI. Dr. Cohen claimed that by measuring snow cover anomalies in Eurasia, namely north of the 60N parallel, one could decipher temperature anomalies for the upcoming winter. For example, an October with sustained below-normal snow cover anomalies would favor a warmer than normal winter. Similarly, above-normal snow cover in Eurasia for the month of October would then tell us to anticipate a colder than normal winter. In past winters, I have seen the SAI perform quite well, the only exception being this past winter. Thus, I hold this index in fairly high regard in terms of accuracy.

As of now, guidance does favor a wetter than normal October over Eurasia, with some of that likely putting down snow cover in the process. While this is expected to change in the future, this could be an indication that we may expect a snowy October in Russia, which may lead to a colder winter here in the United States.

Andrew