October 28-November 17, 2020 HISTORY
Winter weather:
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t happens every year like clockwork. October’s shorter days and colder nights trigger weather prophets — professional, as well as amateur — to come forth with their long-range winter forecasts. It’s also the end of the previous water year and traditionally a time to look forward to the upcoming storm season. In the West, precipitation (rain combined with the liquid equivalent of snow) is measured in water years as opposed to calendar years. In California, the water year runs from Oct. 1 to Sept. 30 because early October is historically the time of minimum stream and reservoir levels, as well as the traditional start of the rainy season. The 2019-20 winter was influenced by anemic El Niño conditions in the equatorial Pacific Ocean that helped establish extended atmospheric blocking patterns. Sea surface temperatures were slightly above normal and significant storms were few and far between. The Storm King tossed out a teaser snowstorm in late September, but then the next two months were warm and bone dry. In early November, Squaw Valley Alpine Meadows optimistically fired up snow-making operations, but warm temperatures limited production. Beginning Thanksgiving weekend, a parade of cold winter storms boosted the snowpack to 248 percent of normal at Tahoe Sierra resorts and business surged through the all-important Christmas and New Year holidays. During January 2020, however, a stubborn high-pressure cell shunted many storms away from the area and an extended period of low-level stratus clouds formed due to temperature inversions. In a mixing atmosphere, warmer air resides at the surface while the temperature cools with altitude. An inversion reverses that profile and colder air and moisture near the surface cause the formation of ground fog or low-level clouds. During such conditions, upper ski slopes are often sunny and warm while a damp and chilly low-level cloud deck plagues Lake Tahoe and Truckee. February 2020 set a new and unwanted weather record. Virtually no rain or snow were measured — only 2 inches of snow fell on the mountains at Squaw Valley — the driest February ever. My first mentor, Dr. David Ludlum, a Princeton University professor and America’s foremost weather historian, hit the nail on the head when he said, “The variability of California weather seems to be its greatest consistency.” It’s true. The Golden State’s climate is wildly unpredictable from year to year and the extremes of wet and dry reflect poorly on what can be expected from what’s considered normal or average. Finally, after months of sketchy snowfall, on March 14 the National Weather Service warned that the biggest storm of the winter was barreling toward the Tahoe Sierra. In just four days, ski areas picked up between 6 and 9 feet of snow. However, just as the storm arrived all resorts were forced to close for the season due to the Coronavirus pandemic. It was
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a heartbreaking end to a somewhat disappointing ski season. For 2020, Squaw Valleys’ aggregate upper mountain snowfall eclipsed 32 feet, but nearly 13 feet of it came after the resort shutdown. Precipitation for the region came in at a paltry 73 percent of average with much less in most of the state. Only Southern California did well.
The Dr. Jekyll and Mr. Hyde characteristics of La Niña will keep forecasters on their toes as we move through this season. For the first time in the 30 years that I’ve been chronicling Tahoe Sierra weather and climate, I cannot share vital snowfall and precipitation data from the Central Sierra Snow Lab located near Donner Pass. Randall Osterhuber, the longtime manager of the lab, retired last year and a replacement station supervisor was slow in coming. There were no observations made between Dec. 1, 2019, and April 1. Osterhuber will be missed. He was a consummate professional at his job for decades.
LA NIÑA PREDICTED So, what’s in store for this season? As of October 2020, National Weather Service’s Climate Prediction Center has issued a La Niña Advisory for the upcoming winter. Confidence that a strengthening La Niña event will persist through winter 2021 is relatively high — at about 75 percent. 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
Temperature inversion over western Nevada from Diamond Peak Resort. | Mark McLaughlin
with atmospheric pressure changes, influence the storm-steering 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 about the pivot point. Other factors at play during the course of a winter season, such as the Madden-Julian Oscillation and oceanic Kelvin wave activity, can also significantly impact surface and subsurface conditions across the Pacific Ocean. The Climate Prediction Center’s current seasonal outlooks for temperature and precipitation predict elevated chances for above normal temperatures from the central Sierra and points south. Precipitation is expected to be increasingly below average south of our region. California is divided into 10 hydrological regions. Truckee and Lake Tahoe are located in the North Lahontan region, which includes all lakes and reservoirs within the Truckee, Carson and Walker river watersheds. Generally, in our neck of the woods, La Niña events produce close to normal precipitation — within about 10 percent of the annual average. Although not guaranteed this year, strong La Niña events energize the polar jet stream with winters that tend to be very cold, but rather dry with a few extreme precipitation events. One signature to look for as we move into winter is that, statistically, La Niña years that start out dry generally continue that way, averaging 60 percent to 90 percent of normal precipitation for the year. The La Niña-influenced winter of 2017 started off with the wettest October in history and ended up with record-setting precipitation totals. The Dr. Jekyll and Mr. Hyde characteristics of La Niña will keep forecasters on their toes as we move through
this season. Dr. Kelly Redmond, former deputy director and regional climatologist at Western Regional Climate Center once stated that La Niña conditions “seem to raise the odds in the Sierra Nevada of wintertime floods.” Redmond studied the La Niña connection on the American River between 1933 and 2000 and found that they enhanced stream flow an average 60 percent greater than in El Niño-influenced winters. La Niña episodes appear more likely to tap atmospheric rivers that funnel heavy precipitation into the Tahoe Sierra.
Read more local history at TheTahoeWeekly.com 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, many of the most notable flood events on the Truckee River have occurred during La Niña-influenced winters. In fact, California and Nevada’s biggest floods have all occurred during cool, oceanic events. Tahoe historian Mark McLaughlin is a nationally published author and professional speaker. His award-winning books are available at local stores or at thestormking.com. You may reach him at mark@thestormking.com.
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