October blocking could hint at winter pattern

Autumn began just a few days ago, but the weather pattern has already adjusted to the fall-like pattern with temperatures falling into the 30’s and 40’s at night. The first cold nights of the year typically come in mid to late September, as the warm and humid days of summer fade. The cooler, more crisp air gives us the first small taste of the cold winter which is only a handful of weeks away. The pattern has begun its seasonal change, and we are beginning to feel the effects of it. Yet, the best hints of what the winter pattern may have in store for us may be hundreds and thousands of miles to our north — over the high latitude/arctic region.

Seasonal forecasting can be a tedious and intricate task for meteorologists. Many times, meteorologists and forecasters alike find themselves drawing upon past events, analogs, and other research for help with predicting the months ahead. This is especially true in the Autumn season, as winter forecasting begins. Meteorologists are about to enter a flurry of forecasting over the next few months (yes, we are somehow enjoying this benign and calm weather right now). So as we look forward to the winter, we can often use the preceding months as indicators of the pattern to come. The pattern itself can often tip its hand, giving us a clue as to what we may be dealing with a few months down the road.

The words “high latitude blocking” likely will instantly bring back memories of some of our stronger storms and more volatile weather patterns in recent memory. The Blizzard of 2010, Hurricane Sandy, the Nor’Easter just a few weeks afterward. They all occurred during episodes of high latitude blocking, with a negative NAO cor North Atlantic Oscillation (for a background and more information on the NAO, click here). Still, high latitude blocking episodes which occur months before these events can often serve as accurate predictors, foreshadowing in a sense, of the upcoming pattern a few months after. A few months ago, we published some research on the blocking pattern during the month of May helping to predict the pattern during the summer months.

For a bit of reference, high latitude blocking refers to blocking ridges which occur in the higher latitudes over Canada and the North Atlantic (background on atmospheric blocking can be found here). These ridges slow down the jet stream/weather pattern. The high latitude blocking over Greenland, the North Atlantic, or Canada during the winter months often leads to the displacement of very cold arctic air over the United States, very often the Northeast US. The more amplified/slower pattern also leads to the potential for large coastal storm systems and has historically featured snowier and colder periods in the Northeast US.

Using the same general research ideas as we did back in April, we can attempt to use high latitude blocking or the lack thereof during October to give us a bit of a sneak peek ahead to winter. As far as winter forecasting goes, the month of October (and November, in some cases as well) is immensely important. But the presence or lack of high latitude blocking has proven, over time to be a great indicator of the forthcoming pattern.

Using 2012-2013 as a raw example, high latitude blocking was observed during October 2012. The maps below use an average of the 500mb height anomaly over the 31 days of October. Compared to the 1981-2010 climatology, there was blocking ridges (above normal anomalies) observed in the high latitudes during October from the Pacific and Western Canada to Southeastern Greenland. Accordingly, the blocking pattern during the three winter months (December through March 2013) featured continued blocking over Greenland and the high latitudes.

Averaged 500mb geopotential height anomalies during October 2012 (left) and Dec-Jan-Feb-Mar 2013 (right).

Averaged 500mb geopotential height anomalies during October 2012 (left) and Dec-Jan-Feb-Mar 2013 (right).

Interested yet? The research goes deeper, and becomes more convincing. Using this correlation over one winter season is good, but not quite compelling enough. However, when drawing upon all years that featured blocking in October since 2002, it becomes clear that the correlation during 2012-2013 did not occur by chance. The blocking years included in the October research included 2002, 2003, 2004, 2005, 2006, 2009, 2010, and 2012. All featuring above normal geopotential height anomalies through the high latitudes during the month of October, pictured on the left side of the below image. Below normal height anomalies occurred to the south of this blocking over the Northern 1/3 of the US as a result.

During the winter months, pictured right, the blocking pattern persisted with the above normal height anomalies remaining fairly stout across Greenland and the NAO region.

Averaged 500mb geopotential height anomalies during October years with blocking (left) and the following Dec-Jan-Feb  (right).

Averaged 500mb geopotential height anomalies during October years with blocking (left) and the following Dec-Jan-Feb (right).

There is a flip side to every story, no? The blocking research works the other way around as well. When taking the years since 2002 that featured minimal blocking over the high latitudes during October, which were only the years of 2007 and 2011 for the purpose of our research, the non-blocking pattern continued through the winter months. In fact the below normal height anomalies which occurred over the high latitudes during those two Octobers became even stronger during the winter months.

Accordingly, above normal height anomalies were observed to the south over the Continental US during those years.

Averaged 500mb geopotential height anomalies during October years with no blocking (left) and the following Dec-Jan-Feb  (right).

Averaged 500mb geopotential height anomalies during October years with no blocking (left) and the following Dec-Jan-Feb (right).

To wrap up the research above, since 2002 the correlation between high latitude blocking in October and the winter months that follow is very strong. Whereas, high latitude blocking or above normal height anomalies in Canada or Greenland during the month of October can be stated as likely to persist on average through the winter. Additionally, the same can be applied for a lack of blocking — or below normal height anomalies in the high latitudes, which during the years of 2007 and 2011 occurred during October and also persisted through the winter months.

What exactly could this mean for the NYC area? Some additional research draws upon our snowiest winters on record and the NAO which preceded that winter during the month of October. Of the 23 years listed, 18 of them featured a -NAO, -AO (Arctic Oscillation) or both during the month of October. Only a handful featured positive values of either index.

NYC's snowiest winters, with the AO and NAO values during the preceding October.

NYC’s snowiest winters, with the AO and NAO values during the preceding October.

This brings us to the last week of September 2013, with some compelling research at hand. Ahead of us lies the month of October, which now seems to be an accurate predictor of the high latitude pattern to come during the winter. Despite all of this, the research isn’t certain. There are outlier years, years with major pattern changes, and of course years which feature no established blocking pattern in either direction. Still, our eyes will be turned to the high latitude pattern and resulting temperature pattern throughout the Continental US during the month of October. Although the leaves have barely begun falling and autumn is just a few days old, as we move towards winter October suddenly seems to be quite an important month.

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