Fickle La Nina still has most skiers guessing about ski season

It’s official. After months of hearsay based on conjecture, La Nina is here. The Climate Prediction Center of the National Weather Service issued a La Nina advisory on Nov. 10. The agency announced that “weak” La Nina conditions are present and slightly favored to persist (about a 55 percent chance) through winter 2016/17.

The presence of La Nina is widely accepted as great news among skiers and riders. This optimism is based on our memories of how the climate has behaved in the past. In general, we have expected cooler than normal temperatures and greater than normal precipitation when La Nina conditions are present: a recipe for plenty of mountain snow.

The most well known among La Nina conditions is below average sea surface temperatures across the equatorial eastern central Pacific Ocean – right now about .9 degrees Fahrenheit below average. However, whether this weak La Nina delivers more mountain snow depends on a complex interaction of climate forces.

The easterly trade winds near the equator get even stronger than usual during La Nina. Stronger winds push warm surface water into the western Pacific. Meanwhile, cool water from deeper in the ocean rises up in the east central Pacific.

This ocean cooling off the coasts of Ecuador and Peru leads to low pressure and enhanced convection over the “Maritime Continent.” The Maritime Continent is what meteorologists call the region between the Indian and Pacific Oceans, including the archipelagos and seas of Indonesia, The Philippines, Borneo, New Guinea and the Malay Peninsula.

This region is the most powerful energy source in global atmospheric circulation. Within the Maritime Continent, the land mass has high mountains, and the seas are among the warmest on Earth. This produces a strong convection pattern and widespread areas of consistent thunderstorms, which pump huge quantities of moisture and heat high up into the atmosphere. The moisture and heat circulate north and collide with the polar jet stream at about 60 degrees latitude.

The low pressure enhancing convection over the Maritime Continent is opposed by stronger than normal high pressure in the east central Pacific, which deflects the polar jet stream far to the north. Ideally, the La Nina polar jet travels southeast from the Gulf of Alaska, passing right over the Pacific Northwest to dump heavy snow in the mountains.

The forecast this winter offers a slight chance for increased precipitation. But the colder temperatures we need for snow are anybody’s guess. How much can we expect our winter to be affected by a weak La Nina? Meteorologist Rex Block researched that question for his Inland Northwest Weather Blog. Block looked back on what has happened in the Spokane area during seven weak La Nina episodes from 1950 to 2009. Cooler than normal temperatures have been pretty reliable during this period.

Precipitation hasn’t been nearly as reliable. Some weak La Nina winters have been wet, some have been dry and others have been average. Block found that actual snowfall has been just about as variable, ranging from the record-breaking 97.7 inches in 2008-09 to a paltry 27.3 inches in 2005-06.

Will this winter be more like 2005-06, or 2008-09? Block’s data shows that the 2005-06 winter started cold. Snow fell in late November and early December. But after Dec 20, cold and dry conditions persisted through early February. In 2008-09, cold temperatures waited until mid-December to arrive, just in time for a rapid-fire succession of storms that dumped 74.5 inches of snow from Dec 17 to Jan 6.

Thanks to La Nina, I spent an unforgettable afternoon skiing four feet of powder off the South Hill bluff on Christmas Day 2008. At the time, I figured it could be a once-in-a-lifetime experience. So far, cold temperatures are taking their time to show up in the Inland Northwest, just like 2008-09. Maybe I’ll get another chance.