From the Fort Collins Coloradoan (Jacy Marmaduke):
…there’s another factor at play this season to complicate an already inexact science regarding El Nino. And it’s leaving even seasoned forecasters unsure what this winter will look like.
It’s called the Pacific Decadal Oscillation, and it only happens about every two decades. The combination of the PDO with a historic El Nino means the normal El Nino pattern might be thrown off course.
“In my 27 years, I don’t know if I’ve ever seen anything like this,” said Mike Baker, meteorologist and climate service focal point with the National Weather Service in Boulder…
The last two big-time El Ninos took place in 1997-98 and 1982-83. Both periods started out warmer and drier than usual for Northern Colorado but were punctuated by a small number of heavy snowfalls in the area.
Fort Collins averages 15 inches of rain and 47 inches of snow a year. Looking at each of the four years individually, those years brought above to well-above rain and two well-above average, one average and one below average snowfall. The most rain was in 1997, when Fort Collins last experienced a major flood, with 25.24 inches. That year also saw 75.9 inches of snow, second in those years only to 1983’s total of 81.7 inches.
A Christmas Eve blizzard in 1982 dropped about 24 inches of snow on Denver, although Fort Collins only received about 4 inches. A late October snowstorm in 1997 slammed Denver with more than 24 inches of snow and Fort Collins with about 18 inches.
Northern Colorado is less affected by El Nino than southern and southwestern Colorado, and even Denver. That’s because it’s further from the storm track, or the Pacific jet stream —a river of wind in the upper atmosphere that picks up storms.
“These little weather disturbances are carried along like leaves in a river, and these little disturbances when they move across an area produce the weather,” Baker said. “That’s why we don’t see weather all the time. We have to wait for one of those little leaves of energy to come by in the jet stream.”
The Pacific jet stream sets itself up along the coast where the water is relatively warmer, which is why storms come from the south during El Nino years.
Here’s where things get weird, though. Remember the Pacific Decadal Oscillation, the thing that happens every 20 years? It causes warmer water in the northern part of the Pacific, all the way from the Pacific Northwest up to Alaska.
So right now, the entire Pacific coast is warm and there’s no temperature gradient, which means the jet stream is wandering around like an awkward party guest, unsure where to sit down.
Whether Fort Collins gets any big snows depends on where the jet stream eventually finds a seat.
“We always tell the media, if the jet stream sags 50 miles, we’re gonna see nothing here along the Front Range,” Baker said. “If it sags a little farther north, we’re gonna get clobbered.”
Forecasters say the precipitation outlook for the next three months is indeterminable. The jet stream’s location might be farther north than normal for an El Nino period, its movement toward the U.S. might be delayed, or it may not set up at all.
“The fact of the matter is it’s always unpredictable,” said Nolan Doesken, state climatologist at the Colorado Climate Center. “People get all excited about a strong El Nino as if this will absolutely predict the rest of the late fall and winter … But it’s just one modifier of the otherwise beautiful and complex atmospheric-oceanic circulation system.”