Several relatively weak weather systems have brought light precipitation and upper elevation snow to the Tahoe Sierra this November, but the first major storm of the 2018 winter season barreled in last week. Moisture from a moderate to strong Atmospheric River drenched the lower elevations of the Lake Tahoe watershed with up to 10 inches of precipitation, enough to pump up Big Blue’s water level by 4 inches in less than 48 hours. Although it was mostly rain at lake level (6,225'), elevations above 8,000 feet were hammered with up to 40 inches of snow, which enabled several ski resorts to open with limited terrain in time for the Thanksgiving holiday.
October 25, 2017. Patches of snow from the previous winter remain on Mt. Tallac (9,735'). Last week's storm dumped more than 2 feet of snow on the upper portions of the mountain, guaranteeing that the old snow will roll into winter 2018. "Tallac" is a Washoe Indian word meaning Great Mountain.
Before the recent storm the Tahoe Sierra and nearly all of California were drier than normal. These data represent measurements from the SNOTEL system, which tallies the snow water equivalent in mountain snowpacks. The moisture pattern also reflects the current Climate Prediction Center forecast for the upcoming winter. The long-range prediction is partially based on La Niña conditions that are expected to develop. La Niña-influenced winters are generally wetter than normal in the Pacific Northwest and Rocky Mountains, with drier than average conditions for the Southwest and across the southern states. The Tahoe Sierra averages 94% to 108% of normal precipitation during weak La Niña episodes, just about average.
The Atmospheric River unloaded so much rain, Lake Tahoe's water level rose about 4 inches in less than 48 hours. That's the equivalent of 14 billion gallons. The lake's current elevation of 6,228.16' above mean sea level is within spitting distance of Big Blue's maximum legal limit of 6,229.1'. Another couple of strong ARs could raise the lake past that legal limit. To create more storage, water officials have jacked up the discharge rate through the Tahoe Dam from 70 cubic feet per second to 875 cfs.
Typical with strong Atmospheric Rivers, high winds preceded its arrival.