Snow telling what this winter holds
October 30, 2017
As the last of the leaves fall and the days and nights turn colder and occasionally wetter, thoughts for many Roaring Fork Valley residents and visitors turn to ski slopes packed with powder and perfect corduroy groomers. Others would just as soon see milder winter temperatures and not so much snow, especially in the lower elevations.
But making winter snowfall predictions can be a bit of a wildcard for the Central Rocky Mountains, a region that’s often stuck in the middle with whichever dominant weather pattern takes shape heading into the winter months.
The latest U.S. Winter Outlook from NOAA’s Climate Prediction Center shows a short-lived and rather weak La Niña pattern potentially emerging for a second straight year.
La Niña is when the water surface temperatures in the eastern equatorial Pacific Ocean are colder than normal, which tends to favor colder temperatures and more precipitation across the northern tier of the United States and drier conditions across the south.
The opposite, called El Niño, is when those ocean surface temperatures are warmer than normal. That tends to produce more precipitation across the southern tier of the U.S. and temperatures more supportive of heavy snows in the Rocky Mountains.
“If La Niña conditions develop, we predict it will be weak and potentially short-lived, but it could still shape the character of the upcoming winter,” Mike Halpert, deputy director of NOAA’s Climate Prediction Center, said in the Oct. 19 Winter Outlook report.
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For central Colorado, whether it’s a La Niña year or an El Niño year, the dividing line between those two tiers often gets drawn in a swath from the Interstate 70 corridor on the north to the Elk Mountains on the south, placing Glenwood Springs, Aspen/Snowmass, Vail and other central mountain ski resorts smack dab in the middle.
The big wildcard for Colorado comes down to other influences related to the jet stream that are more difficult to predict more than one or two weeks before they occur, but have everything to do with the way winter storms track.
“Wildcard is a good way of putting it,” said Jim Pringle, a meteorologist with the National Weather Service in Grand Junction, echoing the term used by colleague Megan Stackhouse just last month in describing the El Niño conditions that appeared to be shaping up at that point in time.
Just how that storm-tracking flow sets up is anyone’s guess.
An Arctic Oscillation influences the number of arctic air masses that penetrate into the south, while the Madden-Julian Oscillation affects the number of heavy rain events along the West Coast that can translate to big snowstorms when they hit the Rockies.
“Each year, even with La Niña, it can be significantly different,” Pringle said. “It just depends on where that jet stream sets up when the storms come through.”
A similar winter forecast emerged last year, leading to predictions of greater-than-normal snowpack in the northern reaches of Colorado, and normal to below-normal snowfall in the southern mountains.
“What we had were precipitation totals that were a little reversed from what you would expect in a moderate to long-term La Niña pattern,” Pringle said.
Last winter, most of the mountains north of I-70 had anywhere from 75 percent to 100 percent of normal snowpack, while the mountains south of I-70 received had snowpack levels well above average.
So, it ended up being a decent season as far as getting enough snow with a good moisture level, Pringle said. That also translated to full reservoirs in the spring and less-severe drought conditions during the summer.
Historically, weak La Niña years have produced mixed snowfall results. In 2010-11, for instance, the central and northern mountains ended up with a whopping 110 to 200 percent of normal snowpack, while the southern mountains saw only 60-90 percent of normal snowpack, Pringle noted.
The winter of 2008-09 was similar to last winter, he said, but the La Niña winter of 2000-01 was the complete opposite, where the northern and central mountains only saw 50 to 85 percent of normal snowpack, and the southern mountains ranged from 90 to 120 percent.
“The last five episodes of a weak La Niña did tend to favor northern Colorado in terms of snowfall, but a slight shift in the jet stream flow can make all the difference in the world for places like Glenwood Springs and Aspen,” Pringle said. “If it’s anything like we saw last year, most of the mountain areas will have at least normal or slightly above normal snowpack.”
With the La Niña pattern, the entire middle part of the United States falls into an “equal chance” category, “meaning an equal chance for above-, near- or below-normal temperatures and/or precipitation because there is not a strong enough climate signal in these areas to shift the odds,” according to the NOAA Winter Outlook.
“Despite the outlook favoring above-average precipitation this winter, drought is likely to persist in parts of the northern plains, although improvement is anticipated farther west,” it predicts. “Elsewhere, drought could develop across scattered areas of the South, mainly in regions that missed the rainfall associated with the active 2017 hurricane season.”
NOAA’s seasonal outlooks give the likelihood of temperatures and precipitation compared to averages, but do not project seasonal snowfall accumulations.
“While the last two winters featured above-average temperatures over much of the nation, significant snowstorms still impacted different parts of the country,” Halpert said in the report. “Snow forecasts are generally not predictable more than a week in advance because they depend upon the strength and track of winter storms.”
The U.S. Winter Outlook will be updated on Nov. 16.