Most people of the ski and snowboard persuasion know a La Nina winter is on the way. The reality set in for me when a powder alert from Larry Schick at Ski Washington showed up in my e-mail on Oct. 23.
Schick, in his inimitable “Grand Pubah of Powder” style, described a developing storm in language only a powder hound could appreciate. He called the low-pressure system offshore an “explosive, extratropical, midlatitude cyclogenesis.” It was enough to make me wax my skis.
“Explosive cyclogenesis” is meteorologist-speak for a major winter storm. This one dumped about a foot and a half on the local hills. It was an exciting preview to what promises to be a productive season. After last winter’s miserly El Nino, the pendulum is swinging the other way.
Whether or not we’re skiing on Thanksgiving weekend depends on what is happening thousands of miles away. In the Pacific Ocean along the equator, buoys measure the water temperature down to 600 meters below the surface. Those buoys detect a massive pool of water, extending from the international date line to the coast of South America, at temperatures down to 10 degrees Fahrenheit less than normal.
Cooler ocean temperatures along the equator influence convection. Convection is rising water vapor that condenses into clouds, a fundamental influence on our climate, according to John Livingston, meteorologist in charge at the National Weather Service in Spokane.
“With cooler sea surface temperatures you get less convection,” he said. “Less convection off the coast of South America is another signal for La Nina. The convec- tion is displaced to the west toward Indonesia.”
Livingston said there’s a link between increased convection around Indonesia and storm tracks in the northern hemisphere.
“When the convection sets up far to the west in the Pacific, it tends to push a strong high pressure system into the gulf of Alaska, which pushes the storm track down across us, like what happened in the winter of ’08-09.”
People in Spokane were shoveling snow off their roofs in the winter of ’08-09. But for epic proportions it was pretty average, according to Livingston.
“The mountain snowfall in the winter of ’08-09 was good, but it wasn’t huge,” he said. “The precipitation amounts were near normal. In fact, La Nina winters in ’07-08 and ’08-09 brought lots of snow to Spokane, but we didn’t get a mountain snowpack much above what would be considered 100 percent of normal.”
If our back-to-back La Nina winters were normal statistically, could we be in for a powder bonanza in 2010-11? The Climate Prediction Center at the NWS forecasts an increased chance of above-normal precipitation and below-normal temperatures through the winter, all the way to spring – with a caveat.
“What the forecasts do is shade the probabilities in three categories,” Livingston said. “With precipitation there’s a 40 percent chance of above normal, 33 percent chance of near normal and 28 percent chance of below normal.”
Will we be skiing on Thanksgiving weekend? The outlook for November has equal chances among the three probability categories for precipitation and temperature.
“Forecasters have nothing to hang their hat on as far as whether they can skew it above or below,” Livingston said. “We translate that to mean temperatures should be near normal. We’re normally cooling off, so chances are storms will have snow for our local mountains and be on the edge for snow in the lower elevations.”
So normal can be a good thing.
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