It happens every year like clockwork. October brings shorter days and colder nights. It’s also that time of year when weather prophets come forth with their winter forecasts — not a job for the meek. From ski resort managers and hydrologists to the average guy buying firewood, it seems that everyone wants to know: “What will this winter be like?” Besides putting faith in the folksy Old Farmer’s Almanac or observing how busy the squirrels are, can anyone really predict the weather months in advance?
The first winter-like storm for the 2018 season barreled into the Tahoe region in late September bringing cold, wind, rain and up to a foot of snow to the Tahoe Sierra. Technically, the precipitation from that storm was added to the winter of 2017. Some of that fresh September snow fell on patches left over from last year suggesting that there will be carryover snow from 2017 into this winter.
For the Tahoe Sierra, La Niña-influenced winters are a toss-up when it comes to precipitation.
The 2017 water year ended on Sept. 30 with impressive rain and snowfall totals tallied at the Central Sierra Snow Lab near Donner Summit. Last year’s 573 inches of snow ranks the 16th greatest since 1879. Due to frequent and powerful atmospheric rivers, precipitation totals — rain and the water equivalent of snow — reached 121 inches, making 2017 the all-time wettest winter since precipitation measurements began on Donner Pass in 1871. That 10 feet of water is more than double the average of 54 inches and nearly a foot more water than 1982, the previous No. 1 wettest year.
Last winter’s moderate La Niña event delivered major snowfall in the higher elevations and produced the best skiing and snowboarding conditions since 2011. The active weather pattern increased skier visits and boosted the bottom line for many resorts and local businesses. Finally, a damp, cool spring with several cold storms enabled a few resorts to open for skiing on the Fourth of July. The unprecedented storm pattern raised Lake Tahoe about 6.5 feet, the greatest seasonal rise for Big Blue since the Tahoe dam was built.
La Niña is the term used when equatorial sea-surface temperatures in the Pacific Ocean are cooler than normal. El Niño is used when temperatures are warmer than average. These temperature anomalies, coupled with atmospheric pressure changes, influence the storm-driving jet stream. Historically it leads to wetter conditions in the Pacific Northwest with drier weather in California, especially in southern California. Northern California can swing either way with Interstate 80 at about the pivot point.
The term El Niño means “Christ child” or “the boy,” and was first used by Peruvian fishermen in the late 1800s to describe the warm current appearing off the western coast of Peru around Christmas. La Niña is a play on words for “the girl” to describe the opposite weather phenomenon. Meteorologists call them ENSO cycles, for El Niño southern oscillation, a reference to the large-scale, atmospheric-pressure swings that are part of the pattern.
During this past summer, the cooler sea-surface temperatures in the Pacific Ocean warmed to average and above. But in recent weeks, cooler water returned to the eastern Pacific, which led the Climate Prediction Center to issue a La Niña Advisory for winter 2018. Confidence in the forecast is currently about 55 to 60 percent, with neutral conditions expected to return in the February-to-April time period.
For the Tahoe Sierra, La Niña-influenced winters are a toss-up when it comes to precipitation. Last winter was exceptional, but in 1924 cooler-than-normal surface temperatures cut off eastbound Pacific storms, which produced the driest winter on record. But similar conditions during the winter of 1907 altered the storm track and blasted the Sierra with a record-setting 884 inches or more than 73 feet at Tamarack, an 8,000-foot-level station south of Lake Tahoe.
As in 2017, La Niña conditions sometimes unleash torrential rain in the mountains and spawn lowland flooding. Western Nevada’s worst flood in history happened during a weak La Niña episode in early January 1997 and caused damage exceeding $619 million.
As always, the Dr. Jekyll and Mr. Hyde characteristics of La Niña will keep long-range forecasters guessing as we move into the winter months. Dr. Kelly Redmond, deputy director and regional climatologist of the Western Regional Climate Center in Reno, Nev. stated that La Niña conditions, “seem to raise the odds in the Sierra Nevada of wintertime floods. It can bring some dramatic weather to areas like Reno.”
Redmond studied the La Niña connection on the American River between 1933 and 2000 and found that “the girl” caused water flows on the American River to average 60 percent greater than in El Niño-influenced winters, indicating an increased chance for flood.
The reasoning behind the flood possibility is that La Niña episodes appear more likely to tap atmospheric rivers that funnel heavy precipitation into the Sierra. Ninety percent of winter floods in the mountains are these wet mantle events when high-elevation rain melts the existing snowpack. Indicative of this trend, some of the most notable flood events on the Truckee River have occurred during La Niña-influenced winters, including the washouts of November 1950, December 1955 and 1964. California’s top 10 floods have all come during La Niña years.
As of press time, the Climate Prediction Center’s forecast for December, January and February is partly based on the La Niña episode expected to build in the Pacific. The CPC is predicting above-average precipitation in the Pacific Northwest and northern Rockies. Tahoe Sierra and all of California, except the far northeast corner, fall into the equal chances category. Warmer-than-average temperatures are forecast for all the lower 48 states and Alaska.
The extent of the expected negative water-temperature anomaly and the timing of the ENSO pulse are critical to the storm track. Strong La Niña events produce more powerful polar jet streams with winters that tend to be very cold, but rather dry. The last strong, cold ENSO occurred during the parched winter of 1988-89.
One signature to look for as we move into winter is that statistically, La Niña years that start out dry, generally stay dry, averaging 50 to 90 percent of normal precipitation for the year, according to Elissa Lynn, a senior meteorologist with the California Department of Water Resources. Amazingly, the La Niña-influenced winter of 2017 started off with the wettest October in history and ended up with record-setting precipitation totals.