Comparison of seasonal snowfall over previous El Niño winters


[NOAA by Rebecca Lindsey] Forecasters continue to predict an El Niño this winter in the tropical Pacific Ocean. An El Niño watch was released earlier this summer in June. One of the current “downstream” impacts of El Niño is above-average winter precipitation in the southern United States, the result of a stronger than usual Pacific jet stream.

When it comes to winter precipitation, of course, what most of us are really curious about is snow. Is El Niño likely to mean more or less where you live? These cards provide a clue. They show the difference between the average snowfall during the cold season months from October to April.

This first map above examines El Niño during the ten strongest winters from 1950-51 to 2008-09. The following maps below show all El Niño winters and the ten weakest El Niño winters. The places where the cold season snowfall was above average are colored in blue, while the places where the snowfall was below average are in brown.

Most of the areas that experienced snowier than average cold seasons during strong El Niño years are in the southern half of the country, with the largest deviations from the average west of the Mississippi: California, Nevada , Utah, Colorado, New Mexico and Arizona. This north-south divide is consistent with the influence of a strong Pacific jet stream causing winter storms in the southern United States. In the high countries of the region – the Sierra Nevada of California, the Colorado Plataea of ​​Arizona, the Sangre de Cristos and the Sacramento Range of New Mexico – precipitation from these storms will often fall as snow.

The difficulty in using El Niño to predict winter climate impacts is that El Niño cannot guarantee a given snow or temperature configuration; it just tips the odds that way. Another problem is that the strength of the El Niño event does not really predict the strength of the impacts. A strong El Niño makes it more likely that there will be a snowier-than-average winter in the northern Colorado Front Range and the Eastern Plains, for example, but that that means a little more snow or a lot is not predictable. depending on the strength of El Niño alone.

Another thing we need to remember is that statistically speaking, the 20 or so El Niño events we have to work with aren’t that many, especially when we categorize them more into “strong” and “weak” categories. With just ten years to analyze, even an unusual winter storm in a strong El Niño year could leave a confusing mark on the overall trend. Meanwhile, during weak El Niño years, other natural variabilities can smother the El Niño signal.

The most reliable impacts of El Niño on snowfall in the United States are therefore not necessarily the most important on the “strong events” map, but rather one of the above or below average patterns that appear. on the the two cards, regardless of their strength. This would include models such as below-average snowfall from the Pacific Northwest to the Great Lakes, for example, and snowier than average conditions in the high plains of eastern Colorado, Nebraska, Kansas and northern Texas, as well as the central Atlantic and Appalachians of southern West Virginia, Virginia and North Carolina.

The analysis was carried out by Stephen Baxter of the NOAA Climate Prediction Center, using satellite snow data from the Rutgers Snow Lab. (It’s a reminder of a similar analysis he did showing snow patterns during La Niña winters last year.) He and his colleagues are working to expand the datasets on seasonal snow anomalies. to include measurements up to last winter. The more events scientists have for their analysis, the more confident they can be that the patterns they find are really El Niño-related, and not just happenstance. Baxter believes the update may be particularly interesting for uncovering possible links between El Niño and snowfall in the northeast: it will incorporate two major El Niño events (2009-10 and 2015-16) during of which there were also major winter storms in the northeast. .

Edited for WeatherNation by meteorologist Mace Michaels

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