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There are indications that the upcoming winter will be earmarked as an El Niño winter. However, there is more to a snowfall forecast for a winter season than just the presence of an El Niño or La Niña.
The strength of an El Niño or La Niña, along with the interplay of other oscillations must also be considered.
The El Niño Southern Oscillation, (ENSO) as it is known by atmospheric scientists, is the fluctuation in sea-surface temperatures over the tropical Pacific Ocean. When these waters are cooler than average, like this past winter, it is known as La Nina. When these waters are warmer than average, we are in the El Niño phase of the cycle. When the waters are near average, the condition is considered to be a neutral phase.
Essentially, surface water temperatures over such a large body of water have a profound effect on weather patterns around the globe, by causing a shift in the strength and position of the storm tracks around the Earth.
The National Oceanic and Atmospheric Administration (NOAA) is not making any bold claims as to whether or not an El Niño is coming this winter with a 50 percent chance of such an event forecast.
AccuWeather.com Long Range meteorologists, headed by Paul Pastelok, are projecting an El Niño this winter. However, the key is the strength of the feature.
"Based on what the patterns we are already seeing over the Pacific Ocean, we believe that an El Niño is beginning to set up and we may have a weak El Niño signature by late in the summer," Pastelok said.
How strong the El Niño becomes seems to have an interesting outcome for snowfall for the northern United States.
Pastelok's crew is projecting a weak to moderate El Niño for this coming winter.
According to a study done by Ralph Fato, during a "weak" El Niño, many winters have brought above-normal snowfall for cities such at New York, Boston, Philadelphia, Hartford and Chicago. However during strong El Niño winters, snowfall trends to well-below average.
Image by Ralph Fato. Click for larger image.
During a weak El Niño, there is generally a balance between southern warmth and northern chill, resulting in ample energy and moisture for storms.
During a strong El Niño, the storm track sets up in such a way to allow warm air to overwhelm the Midwest and Northeast; therefore, storms tend to favor rain rather than snow.
There are other oscillations that also have effects on the weather patterns. This past winter, in addition to the La Nina, there was an unusually strong and persistent positive phase of the Arctic Oscillation (AO).
The AO usually fluctuates back and forth between positive (strong jet stream near the Arctic Circle) and negative (weak jet stream near the Arctic Circle) over the course of a winter.
In short, when the AO is strongly positive, the jet stream is strong from west to east around the Arctic Circle and cold air cannot escape southward to the mid-latitudes.
"The persistent, strongly positive AO this past winter was highly unusual and had a profound effect on temperature and snowfall over much of the U.S. and southern Canada," Pastelok said.
Since the clash of cold air with warm air is a key ingredient for storms and cold air is needed for snow, a strongly positive AO greatly reduces the chance of such an occurrence.
Generally, the AO cannot be predicted more than a few weeks in advance.
"However, odds are greatly in favor of the AO either being less strongly positive or at least being negative at times this coming winter, compared to last winter," Pastelok said.
A negative AO is a weaker circulation around the Arctic Circle and allows cold air to drive southward.
The strongly positive AO throughout this past winter prevented a feature called the Greenland Block from setting up. The Greenland Block tends to lock in sufficient cold air in the northeastern U.S. for approaching storms.
The less strongly positive AO and occasional blocking should at least create more opportunities for snow this winter.
Much of the rest may be up to the strength of El Niño.
"At this point it is tough to say whether cities like Baltimore, Boston, Chicago, New York, Philadelphia and New York will have more or less snow than average, because of the question of the intensity of El Niño," Pastelok said.
If a weak El Niño develops, there is a chance many of these cities will have near- to above-average snowfall. Most cities would tend to pick up more snow this winter, when compared to last winter's "snow desert".
If a strong El Niño develops, there is a significant chance many of these cities will once again end up with little snow this winter.
This story was originally published Tuesday, June 12, 2012 at 12:00 noon EDT.
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