How El Niño Impacts Seasonal Snowfall in the United States

(MORE: How El Niño Influences Winter Temperatures)

To examine how this periodic warming of the equatorial eastern Pacific Ocean water could have such an impact, we parsed through NOAA's seasonal snowfall data for 50 U.S. locations for which sufficient data exists and snowfall is typical at least 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 or 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 50 locations, we had 25 El Niño, 22 La Niña, and 21 neutral seasons of snowfall data. One admitted drawback to this study is the limited sample size of strong El Niño seasons (eight 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, it's 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 as just one factor loading the dice for or against a snowier or 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 might be small for several locations (scroll down for our city dataset used in this study), the general picture makes sense.

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

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 in either La Niña or neutral seasons.

El Niño's Impact

Again, unsurprisingly, we have a clear dichotomy. Namely, El Niño tends to produce less-snowy-than-average seasons in the northern tier of states, but a snowier-than-average southern tier.

As we saw in the first map, though, 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 mid-Atlantic.

This would appear to be due to cases where the stronger subtropical jet stream can occasionally couple with the polar jet stream, cranking up winter storms with a northern reach that extends into 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.

Now, let's lay out all the data for each of our 50 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, which made a significant impact on snowfall in some places.

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

In seven of the nine Northeast cities examined, strong El Niño seasons dominated by a negative NAO produced the snowiest seasons. Four of those cities saw over a foot or more of additional snow, compared to average, in these negative NAO seasons. However, as mentioned before, there were only two El Niño/negative NAO seasons in the database (1957-58 and 1965-66).

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 southward in the eastern U.S., bringing ample cold air. A powerful subtropical jet stream, typical in strong El Niño seasons, would interact with the cold air, perhaps coupling with the nosediving polar jet, making East Coast snowstorms more possible.

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

There are exceptions, however.

Just days before Valentine's Day 1983, the "Megalopolitan Snowstorm" dumped 10 to 30 inches of snow from West Virginia to southern New Hampshire, including the highly 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).

In late-January 2016, Winter Storm Jonas set snowstorm records from Baltimore to Allentown, Pennsylvania, to New York City. That winter had a record-tying strong El Niño, but not as persistent North Atlantic blocking as 1982-83.

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

The north-south split also shows up to some degree in the Midwest and Plains 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 (9 inches fewer) and Marquette, Michigan (15 inches fewer), leading the way.

Most of the region saw at least some statistically significant reduction in snowfall when considering strong El Niños alone, including Chicago (12 inches fewer), Cleveland (12 inches fewer), and Bismarck, North Dakota (11 inches fewer).

Indianapolis saw the strongest reduction with strong El Niños in the Midwest, regardless of NAO; the city's seasonal snowfall dropped more than 50 percent from the long-term average of 26 inches to around 11 inches in strong El Niños.

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

(Note: Government Camp, Oregon, is near Mount Hood in the Cascades. The Paradise Ranger Station is in Mount Rainier National Park, Washington.)

In addition to a snowier South and less snowy North, El Niño offers some surprises in the West.

For instance, taken together, La Niña edged out El Niño seasons at Tahoe City in California's Sierra. Two strong El Niños in the 1990s, 1991-92 and 1997-98, produced vastly different snow totals – 59 inches and 233.5 inches, respectively – at Tahoe City.

While El Niños, in general, take a sizable bite out of Alaska snow, the 1965-66 strong El Niño season, dominated by a negative NAO, brought heavy snowfall in both Fairbanks (123.9 inches) and Juneau, Alaska (149.7 inches). Seven years later, 131.4 inches fell in Juneau during another strong El Niño, proving strong El Niños can still generate heavy snow in parts of the 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 5 feet more snow, on average, in a positive NAO-dominated season than a negative one.

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 is about four times as likely there 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 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.

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 almost 19 inches more snow, on average, when a strong El Niño is accompanied by a negative NAO. More North Atlantic jet-stream blocking means a better chance of East Coast snowstorms. This southwestern Virginia city picked up more than double the average snow (43.1 inches) in the 2009-10 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 10 inches more snow in a positive NAO strong El Niño season than an average season. Almost three times the average snow blanketed the city in the 1982-83 strong El Niño season (47.9 inches).

Again, keep in mind these are 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.