The Impact of El Niño on Seasonal Snowfall

El Niño, the periodic warming of the equatorial eastern Pacific Ocean, can have a number of effects on weather around the world, from heavy rain to extreme drought, persistent warmth to stubborn cold, and inactive versus hyperactive tropical cyclone seasons.

(MORE: El Niño's Myths vs. Facts)

Does El Niño also influence how snowy your winter is?

To answer that, we examined NOAA seasonal snowfall data for 51 U.S. locations for which sufficient data exists and snowfall is at least typical once a year. 

We grouped these seasonal snowfall totals into El Niño, La Niña (its opposite, namely, a cooling of the equatorial eastern Pacific Ocean) and neutral (neither El Niño nor La Niña) seasons.

Since no two El Niños/La Niñas are alike and the intensity of each matters for impacts, we further examined moderate and strong El Niño seasons, based on the categorization by Jan Null, a consulting meteorologist with Golden Gate Weather Services. 

For most of the 51 locations, we had 23 El Niño, 20 La Niña, and 22 neutral seasons of snowfall data. One admitted drawback to this study is the rather limited sample size of strong El Niño seasons (five such cases), given NOAA's Oceanic Niño Index dates only to 1950. 

Finally, given El Niño/La Niña is not the sole driver of the atmosphere at any time, we thought it would be interesting to examine another atmospheric influencer during the winter months, the North Atlantic Oscillation (NAO).

Put simply, a positive NAO typically means cold air will drain from west to east across Canada, rather than plunging into the eastern U.S. Conversely, in a negative NAO, more blocking of the upper atmospheric pattern over the north Atlantic Ocean sends cold air deep into the eastern two-thirds of the nation.

We used December through March NAO values calculated by Dr. James Hurrell at the National Center for Atmospheric Research. 

Day-to-day and week-to-week variability in the weather can still deliver a potent snowstorm in a location where a season-long El Niño would work against that. Think of El Niño, then, as just one factor loading the dice for or against a snowy/less snowy season.

With that in mind, let's see if any patterns emerge.

Snowiest Seasons: El Niño, La Niña, or Neither?

While it should be noted the difference between El Niño, La Niña and neutral season snowfall may be small for several locations (scroll down for our city dataset used in this study), the general picture makes sense.

Namely, the more active southern-branch jet stream raises the odds of a snowy season in the Sierra, central and southern Rockies, including the Front Range, central and southern Plains, and from the Piedmont of the southern Appalachians to the Mid-Atlantic states.

On the other end of the spectrum, La Niña favors heavier snow in the Cascades, northern Rockies, and adjacent lower elevations.

The northern Plains, Great Lakes, Ohio Valley, interior Northeast and New England tend to be snowier either in La Niña or neutral seasons.

El Niño's Impact

Again, no huge surprise, here; a clear dichotomy. Namely, generally below-average snowfall in the northern tier of states and a snowier-than-average southern tier during El Niño. 

However, as we saw in the first map, El Niño is actually snowier in the High Plains of the Rockies as far north as Wyoming, in the central Plains as far north as Nebraska, and as far north as the Ohio Valley and Mid-Atlantic States. 

This would appear to be due to cases where the stronger southern-branch jet stream can occasionally couple with the northern branch jet stream, cranking up winter storms whose northern reach extends in those areas. 

The diminished snow in El Niños from the Pacific Northwest to the northern Plains, Great Lakes and New England is largely due to the polar jet stream's diversion north into western Canada, keeping the region warmer and/or drier than average.

Digging Into the Metrics

Now, let's lay out all the data for each of our 51 locations, organized by region. 

In each column you'll see seasonal snowfall averaged through all El Niño, strong El Niño, La Niña, and neutral seasons. The final column shows average seasonal snowfall over the entire period examined, for comparison purposes.

As mentioned above, we also parsed out the strong El Niño seasons into those dominated by either a negative or positive NAO. As you'll see, that made a significant impact on snowfall in some places.

Bolded data for each location highlights the snowiest seasons, on average.

Northeast

Arguably, the most interesting finding here is the sharp contrast in strong El Niños depending on whether they're dominated by a positive or negative NAO. 

Meteorologically, this makes sense. More North Atlantic Ocean blocking of the jet stream in a negative NAO pattern would allow the polar jet stream to plunge south in the eastern U.S., bringing ample cold air. A powerful southern branch jet stream, typical in strong El Niño seasons, would interact with the cold air, perhaps couple with the nosediving polar jet, making East Coast snowstorms more possible. 

The 2009-2010 season featured a moderate El Niño dominated by strongly negative NAO. The result: the snowiest season on record in Philadelphia (78.7 inches), Washington, D.C. (56.1 inches), and the second snowiest season in Pittsburgh (77.4 inches). 

Conversely, with the exception of northern New England, all other cities tended to see their least snow during strong El Niño/positive NAO seasons. These seasons often featured the polar jet stream pushing cold air east across Canada, rather than plunging deep into the East. With the dominant southern jet stream unable to meet cold air or interact with the polar jet, few Northeast snowstorms would result.

Except for one recent storm.

Just days before Valentine's Day 1983, the "Megalopolitan Snowstorm" dumped 10-30 inches of snow from West Virginia to southern New Hampshire, including the heavily-populated I-95 Northeast corridor. This happened despite one of the strongest El Niños on record and a mild season lacking in north Atlantic blocking (positive NAO).

The bottom line in most of the Northeast is to watch the degree of blocking (NAO) in a strong El Niño. Unfortunately, predicting the amount and location of North Atlantic blocking can be very difficult beyond a few weeks out.

Midwest

The north-south split also shows up in the Midwest during El Niños, with lower snow totals from the Dakotas to the Great Lakes and higher totals in the central Plains. With the subtropical jet dominating in stronger El Niño winters, the far north would trend either too warm, too dry, or both. 

Minnesota, Wisconsin and Michigan show rather significant reductions in snow during El Niño, regardless of strength, with the Twin Cities (8 inches less), Duluth, Minnesota (10-11 inches less), and Marquette, Michigan (27 inches less) leading the way.

Most of the region saw at least some statistically significant reduction in snowfall when considering strong El Niños alone, including Chicago (15 inches less), Cleveland (19 inches less), and Bismarck, North Dakota (almost 15 inches less). 

Interestingly, the Central Plains saw increased snowfall in strong El Niño seasons. However, in the cases of Des Moines, Omaha and Wichita, those were heavily weighted toward strong El Niño/positive NAO seasons, which were the snowiest of all seasons for each city.

Omaha, for example, picked up about 17 more inches of snow during a strong El Niño/positive NAO dominant season than their long-term average of 27.5 inches. 

West

(Note: Big Bear Lake is in the San Bernardino Mountains of Southern California. Government Camp, Oregon, is near Mt. Hood in the Cascades. The Paradise Ranger Station is in Mt. Rainier National Park, Washington.)

Other than the general snowier South, less snowy North theme, El Niño does offer some surprises in the West.

For instance, not all strong El Niños dump heavy snow in the mountains of California, as evidenced by Big Bear Lake's paltry 6 inches of seasonal snow in 1965-66.

That same 1965-66 strong El Niño season, dominated by a negative NAO, also featured heavy snowfall in both Fairbanks (123.9 inches) and Juneau, Alaska (149.2 inches). Seven years later, 131.4 inches fell in Juneau during another strong El Niño. So, strong El Niños can still generate heavy snow in parts of our 49th state.

Flagstaff, Arizona (elevation ~ 7,000 feet) exhibits an incredible contrast in snowfall for strong El Niños, depending on the NAO, picking up over 7 feet more snow, on average, in a positive NAO-dominated season than a negative NAO season. 

Denver and Casper, Wyoming, show similar, if not quite as large, strong El Niño/positive versus negative NAO contrasts as Flagstaff.

A UCAR/COMET study originally done in 2009 by Weather Underground meteorologist Bob Henson and updated in 2015 by Matt Kelsch for nearby Boulder, Colorado, found a 20-inch-plus snowstorm, there, is about four times as likely during an El Niño than a La Niña.

In a positive NAO-dominant season, a southward dip in the polar jet stream would more often occur in parts of the West, Rockies or Plains. Add an active subtropical or southern-branch jet in a strong El Niño and just enough cold air, and you could get more lumbering upper-level lows socking the Four Corners with snow, then hammering the High Plains.

South 

As mentioned earlier, most southern locations that pick up snow at least once each season see more of it in El Niño seasons, and many of these cities see even more snow when the El Niño is strong. 

Particularly impressive are the spreads between positive and negative NAO seasons during strong El Niños.

Roanoke, Virginia, picks up over 20 more inches of snow, on average, when their strong El Niño is accompanied by a negative NAO. More North Atlantic jet-stream blocking means a better chance of East Coast snowstorms. This southwest Virginia city picked up over double their average snow (43.1 inches) in the 2009-2010 moderate El Niño/strongly negative NAO season.

With just enough cold air plunging down the High Plains, a polar jet stream dip in the Rockies and a supercharged subtropical jet, Amarillo, Texas is swamped by 21 inches more snow in a positive NAO strong El Niño season. Almost three times their average snow blanketed the city in the 1982-83 strong El Niño season (47.9 inches).

Again, keep in mind these are just averages, which often include outliers. As investors often say, "Past performance doesn't guarantee future returns."

While the historical probability of a snowy or less snowy season may exist during a strong El Niño, it is just one factor at work in our complex ocean-atmosphere system.  

(MORE: Winter Storm Central | Wunderground El Niño Infographic)

Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7. Follow him on Twitter and Google Plus. 

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