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La Nina pattern emerging for Winter 2020-2021

JACKSON, Wyo. — Aside from a brief but abrupt cold snap earlier this month along with an unpleasant amount of smoke from wildfires west of here, a spectacular fall weather pattern is emerging in Jackson Hole just as the leaves are turning.

While warm days and cool nights with little rainfall will be the trend through the end of September and beginning of October, we’re also now getting to the time of year when people start to ask meteorologists about the upcoming winter.

Seasonal forecasts are always tricky because there are many variables that cannot be predicted far in advance, but at least one factor has emerged for the upcoming winter that can offer us some clues — a La Nina.

La Nina Phase

A La Nina pattern is the cold phase of the El Nino Southern Oscillation (the opposite of an El Nino pattern) and occurs when cooler than average ocean temperatures emerge across the Eastern Equatorial Pacific off the west coast of Peru and Ecuador.

During La Nina’s, average precipitation and atmospheric circulation patterns in the tropics become disrupted, and this in turn has a domino effect that influences the weather patterns in the mid-latitudes, including most of North America.

The current sea surface temperature anomaly map of the globe shows a classic La Nina signature developing with below-average ocean temperatures along the equator in the Eastern Pacific.

 

Sea surface temperatures as of September 20 show a classic La Nina signature developing in the Eastern Equatorial Pacific. Source: NOAA

Historically, La Nina winters tend to favor a more active storm track across the Pacific Northwest and Northern Rockies, including Jackson Hole — good news for skiers and snowboarders! Some of Jackson Hole’s deepest winters have occurred during La Nina winters.

Since Jackson Hole Mountain Resort snowfall records began in 1974-1975, there have been 16 La Nina winters over the past 46 winters. Of these 16 winters, 11 featured above-average snowfall in Jackson Hole, which is roughly two-thirds of the time.

 

Interestingly enough, recent winters seem to show a stronger correlation between La Nina’s and above-average snowfall. Over the past 15 years (since 2005), there have been seven La Nina Winters and six of these winters were snowier than average (only 2012 was below average).

One thing to keep in mind about La Nina winters: the weather we experience in the fall has no correlation with what we experience in the winter. Take 2016-2017 for instance, when November was so dry that JHMR had a delayed opening, before the pattern flipped abruptly and 150 inches of snow fell in December.

What about temperatures during La Nina winters?  The Moran weather station near Jackson Lake (which has the most reliable temperature records in the valley over the past 46 years) recorded below-average temperatures in 9 of the past 16 La Nina years, and the overall average over the 16 years was only 0.6 degrees below the average value of all winters — so not an especially strong signal.

The Pacific Northwest and British Columbia are more likely to experience below-average temperatures during La Nina’s, while in Jackson Hole temperatures tend to vary more from year to year.

Other Climate Trends that Factor into the Winter Outlook

There are many other climate patterns that emerge during a given winter, but they often change on an inter-seasonal basis and cannot be predicted well in advance. Unfortunately, this adds to the inherent uncertainty of winter forecasts.

However, one index aside from El Nino/La Nina that can be predicted is the Quasi-Biennial Oscillation, better known as the QBO.

The QBO is essentially a measure of trade wind patterns in the tropics that in turn have domino effects on the large scale patterns in the mid-latitudes as well. QBO fluctuates between varying strengths of westerly (positive) and easterly (negative) phases and identifying these trends in conjunction with El Nino and La Nina phases can offer some clues for upcoming winter patterns as well.

Currently, the QBO is in the process of transitioning from an easterly to a westerly phase (from negative to positive) and will continue to become “more positive” as we head into the winter.

Taking a look at past winters with similar QBO trends that occurred during La Nina’s, five “analog” years emerge: 1975-76, 1984-85, 1998-99, 2010-11 and 2016-17.

The two strongest matches both occurred in the past decade: 2010-11 and 2016-2017. This is a good sign if you like snow, as these were both huge winters in Jackson Hole with well over 500 inches falling at JHMR’s upper mountain.

Both years featured a La Nina pattern where the QBO was just starting to transition into a positive (westerly) phase during the fall.

Of the top five analog years, four of these years featured above-average snowfall at JHMR. Only 1984-1985 was below average.

 

Here is a map of precipitation anomalies from November through March during the top five analog years:

 

 

Jackson Hole, the Northern Rockies, Pacific Northwest and to a lesser extent, the Sierras and Wasatch, are more favored for an above-average winter based on the information about winter 2020-2021 climate trends currently available. Areas east of the Continental Divide in Wyoming, on the other hand, have no clear signal in either direction.

Temperature-wise, the top analog years show a slight bias toward below-average temperatures in Jackson Hole, though the records at the Moran weather station deviated very little from average during these years.

 

 

One thing to keep in mind is that temperatures are above average in the Northern Pacific currently, which could mean that we see a greater number of “warm” systems this year with higher moisture content snow.

However, La Nina’s also tend to be more favorable to arctic air intrusions from the north, so it could be that we see more variability between cold/dry pattersn, cold/low density snow patterns and warm/high density snow patterns, similar to what occurred in 2016-2017.

Patterns such as La Nina and other factors such as QBO and North Pacific Ocean temperatures offer some clues for the winter ahead, and can help with advance decision-making for water resource planning, agriculture, energy trading and of course, powder skiing.

However, winter forecasts must always be taken with a grain of salt as there are always factors that cannot be predicted in advance, and climate change could also be starting to impact La Nina and El Nino phases relative to historical conditions.

Alan Smith, Meteorologist, jacksonholeweather.com

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