National Weather Service Meeting Braves a Forecast
WESTERN COLORADO – “I hate to say it, but it still looks like a No Niño winter,” said Joe Ramey, a meteorologist with the National Weather Service in Grand Junction, addressing a roomful of snow and avalanche professionals at the 2012 Avalanche and Winter Weather Coordination Meeting last week.
Ramey defined a No Niño as a “neutral ENSO” (El Niño Southern Oscillation), rather than a warm-ocean El Niño or a colder-ocean La Niña. No Niño winters tend to be “wild cards,” Ramey said, often producing dry winters for western Colorado, but also generating what seems to be, in weather records going back to 1950, an unusual number of “extreme weather events” – very dry or very wet seasons.
Bottom line, said fellow meteorologist Mike Meyers, “neutral ENSOs make predicting the winter harder.”
The assembled snow people hung on Ramey’s every word and PowerPoint slide. They had also come to hear Chris Landry, of the Center for Snow and Avalanche Studies in Silverton, talk about dust on snow. And Ethan Greene of the Colorado Avalanche Information Center, who talked about new features of the CAIC interactive website. And NWS forecaster Matt Aleksa, who examined the particular mysteries of the big Presidents Day storm last February.
But it was Ramey’s look to the future, and his backup reasoning, which held everyone rapt. (His opening slide showed a four-panel Calvin & Hobbes strip. Calvin’s dad is raking leaves. Calvin: “Can we burn these leaves?” Dad: “No, that pollutes.” Calvin, upset: “But how can we appease the mighty snow demons if we don’t sacrifice any leaves?! We’ll have a warm winter!”)
Actually, in addition to burning leaves, Ramey said, he looks to three big weather factors for clues to Colorado’s upcoming winter. The first is ENSO; then there’s the Pacific Decadal Oscillation, and finally the Arctic Oscillation.
“The ENSO is our strongest tool for the national winter outlook,” Ramey said, of the organization that measures ocean temperatures in the Eastern Pacific along the equator. Warmer than average, you get El Niño conditions; cooler, and it’s La Niña. A No Niño is right in the middle, on the fence.
ENSO influences the path of the jet stream, which brings stormy weather across the continent from west to east. El Niño nudges the jet stream north, favoring the northern Rockies and northern Colorado; La Niña tends to produce a more southerly track, sometimes well south of us. No Niños, and there have been 19 of them since 1950, Ramey said, “seem to allow the jet to set up in many different configurations. Sometimes Colorado is in the storm path, sometimes not.” The probabilities for No Niño winters are maddeningly vague: 33 percent chance of above-average precipitation, 33 percent normal, and 33 percent below average, Ramey said.
The second factor is the Pacific Decadal Oscillation, kind of an ENSO of the north Pacific. It’s not nearly as fickle as ENSO, though. ENSO phases last somewhere between six and 18 months before they switch from cold to warm, and vice versa. A PDO phase lasts a couple of decades, thus the name. The waters off North America are currently in a cold phase. Cold-phase PDOs tend to influence our weather in the same way that La Niñas do, though the correlation isn’t as strong.
Ramey’s third factor is the Arctic Oscillation, an even more mercurial phenomenon than ENSO. “We can only forecast the AO out about two weeks,” Ramey said.
The Arctic Oscillation, in its positive phase, “acts like a kind of noose,” keeping cold air – and storms – spinning around the top of the globe. In the negative phase, though, “the noose loosens and allows cold air outbreaks [down south].” Last year, the AO stayed positive for most of the winter, with one significant negative-phase outbreak coming in early February. That’s when people were skiing the amphitheater at Red Rocks outside Denver, and when the season’s single biggest storm brought 19 inches to Powderhorn and 27 inches to Steamboat overnight Feb. 19-20.
So, Ramey summarized, our current conditions are: neutral ENSO (No Niño), cold Pacific Oscillation, and Arctic Oscillation in positive (noose) phase.
What’s the outlook for this winter?
Using data from seven sites around Colorado – Steamboat, Winter Park, Breckenridge, Aspen, Crested Butte, Silverton and Telluride – Ramey looked at all of the No Niño winters, with a special emphasis on the four that have occurred in the last 15 years. Those were the winters of 1996-97, 2001-02, 2003-04 and 2008-09.
Looking just at precipitation, the No Niño years showed near-normal accumulations through November. Ramey found a big spike in snowfall for the month of December, then a big dip in precip through the end of March. And near-normal snowfall resuming in April.
For the seven chosen sites, 1996-97 was generally a wet winter, 2001-02 was extremely dry, 2003-04 was dry, and 2008-09 was dry south and on the Front Range. All four had snowy Decembers and Aprils.
Ramey also ran the numbers for No Niño winters following a La Niña, like the situation this year (and in 1996, 2001 and 2008), and for No Niños accompanied by very cold Pacific Decadal Oscillations (1990, 2008 and this year). The results were similar.
But, he asked rhetorically, “In a changing climate regime,” i.e., global warming, “do previous seasons well represent a future season?”
At this point in his presentation, Ramey needed to make clear the forecaster’s dilemma: Weather is so complicated; we still don’t know with any certainty what will happen. “Your ability to accurately predict the coming winter has not increased one iota” thanks to this presentation, he said with a smile.
Ramey had one more forecast tool up his sleeve: Extreme Event Analysis. “What were the ENSO conditions during the wettest and the driest seasons since 1950?” He found that No Niños produced a few of the wetter winters on record and many (five of the top 10, including the top two) of the driest seasons for the seven sites.
“Extreme Event Analysis indicates an extreme year is possible,” Ramey said – either extremely dry or extremely wet. “A second extremely dry year in a row would be unprecedented in the climate record since 1950,” he noted. “But it can’t be ruled out.”
(The final panel in the Calvin & Hobbes cartoon goes like this. Dad to Calvin: “I don’t know whether your grasp of theology or meteorology is the more appalling.” Calvin, stomping off: “I guess I’ll go light some candles around the toboggan and beg for mercy.”)