Monday, October 27, 2014

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.

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.

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.

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.