In the West, precipitation — rain combined with the water equivalent of melted snow — is measured in water years as opposed to calendar years. In California, the water year runs from Oct. 1 to Sept. 30. Historically, early October is the time of minimum stream and reservoir levels in the Golden State, as well as the traditional start of the rainy season. Since the water year ended at the end of September, it’s a good time to review the winter weather of 2018.
Despite some early high-elevation snow during the third week of September 2017, winter 2018 was definitely slow out of the gate weather-wise. And like in much of the Intermountain West last season, snowfall came in fits and starts with few big dumps. A decent storm just before the Thanksgiving holiday weekend blessed Tahoe resorts with between 2 and 4 feet of snow, but then December continued the dry trend with less than 10 inches falling over only three snow days. By New Year, just 69 inches of snow had fallen in the high country. Advanced snow-making systems proved their weight in gold last winter.
January and February were two more anemic months with only 48 inches and 39 inches of snow, respectively. But the Storm King finally attacked and March roared in like a lion. A major storm on the first four days of the month slammed the mountains with close to 7 feet of snow in the upper elevations. The weather remained cold and unsettled and a series of low-pressure systems swarmed the region mid-March, dumping nearly 12 feet of the white stuff on the major resorts. It was the single-biggest storm surge of the year and guaranteed a solid spring ski season. Some called it a “Miracle March.” Indeed, it was the fourth snowiest March in Squaw Valley’s history. Nearly 19 feet of snow fell that month, which finally eliminated the need for snowmaking on the slopes and opened up previously sketchy terrain.
The very wet March transformed a bone-dry season into a fairly respectable winter season, close to an average year for precipitation in the Tahoe Sierra. Winter 2017-18 was one of the most challenging for western ski resorts since the snow-starved winter of 2015, but the Tahoe Sierra did better than most. Squaw Valley tallied a total of 34 feet at its 8,000-foot measuring stake, slightly less than average.
At the Central Sierra Snow Lab near Donner Pass at elevation 6,900 feet, snowfall came in at 267 inches (22.3 feet), which is about 70 percent of normal for that location. An average winter dumps about 35 feet. But the reduced snow accumulation belies the fact that it was indeed a wet winter in our region — 61 inches of precipitation were measured at the lab, compared to a norm of 55 inches. That gives the Tahoe Sierra two wetter-than-average winters in a row.
The news wasn’t as positive over much of the rest of the state. During the 2018 winter, the Northern Sierra 8-Station Index totaled 41 inches of precipitation, a significant deficit of about 10 inches below normal. The index is comprised of eight measuring stations from State Route 50 to Mount Shasta. The region is a critical watershed for water supplies in much of California, especially in the populated south where rainfall totals last winter were half or less of normal. Statewide, the snowpack was just 58 percent of average on April 1, statistically the time of year when the Sierra snowpack has reached its maximum water content. A depleted snowpack reduces the amount of spring runoff leading to reservoir storage deficits. Fortunately, despite the dry winter overall, storage is still healthy thanks to the extreme, heavy-hitting winter of 2017, the wettest of record in the Tahoe Sierra.
California’s erratic weather
California has always experienced erratic weather, but climate-change computer models predict even greater variability in precipitation to be the new norm moving forward. In the past decade or so, the trend has been for extended dry periods during the winter interrupted by extreme precipitation events. That pattern has always been a signature of California’s climate, but the irregularity is becoming more acute and increasing the state’s vulnerability to drought and flood.
Atmospheric scientists understand the futility of accurate long-range weather forecasting, but that doesn’t mean we can’t look forward to what might be in store for this upcoming winter. In September, the Climate Prediction Center released an El Niño Advisory for winter 2019, with a 65 to 70 percent likelihood of at least a weak event. Due to its expected low intensity, extended forecasts give the Tahoe Sierra equal chances of an average winter for precipitation, with a greater likelihood for warmer than normal temperatures.
Scientists use the terms La Niña and El Niño to describe an atmospheric-oceanic oscillation known as ENSO, or El Niño Southern Oscillation. La Niña and El Niño reflect changing sea-surface temperatures in the equatorial Pacific Ocean and represent the two primary phases associated with ENSO. El Niño describes the condition when equatorial ocean temperatures are warmer than normal and La Niña is used when sea-surface temperatures are cooler than normal.
There always seems to be a lot of hype in the media when ENSO events are expected in the Pacific Ocean, but these episodes are poor predictive tools when it comes to forecasting weather. Take the winter of 2016 for example, when a “Godzilla El Niño” was supposed to hammer southern California with biblical rains and floods, and the winter turned out to be drier than normal, despite its metrics being the strongest El Niño measured since World War II. The upcoming event is expected to be weak, which statistically does us no favors in our neck of the woods.
In a review of how weak El Niño events affect the North Lahontan hydrologic basin, which includes Lake Tahoe and the Truckee River drainage, meteorologist Jan Null indicates that statistically we average 93 percent of normal for precipitation during events of that magnitude. During the most recent occurrence in 2015, the Tahoe Sierra came in at just 86 percent of normal.
The reality is that no one knows what this winter will be like weather-wise, but if we get our average snowfall in a timely manner, everything will be just fine.