Winter has come and gone: What was left behind?
Dont look now, but another winter has come and gone. Although the 2008 water year does not officially end until Sept. 30 (the historic low-point for water flow and reservoir storage in the Sierra Nevada), for all intents and purposes the rainy season is over. It was an interesting winter in that two separate storm periods over the course of five weeks produced the bulk of the seasons precipitation. The heavy and persistent snowfall during January and into early February made national news and most likely helped boost our winter economy by ramping up the motivation of winter sports enthusiasts to visit local resorts. Besides ensuring great skiing and snowboarding conditions, the big snowstorms also allayed fears of another dry winter like we experienced in 2007, which turned out to be one of the driest on record according to the Central Sierra Snow Lab at Soda Springs near Donner Pass.
This past winter was slow out of the gate, with much-below average snowfall during November (Squaw Valley picked up only 8 inches all month). It was Californias 16th driest November in 113 years. December was no barnburner when it came to snowfall, but several decent storms before the all-important Christmas holidays saved the day economically. The storm door blew open dramatically in early January, when a powerful low-pressure system walloped California with high winds and heavy rain and snow. Low snow levels minimized flooding, but destructive wind gusts uprooted trees and knocked out electric power to millions of people. Another round of storms pounded the Pacific Coast in late January into early February, some of which reached bone-dry Southern California. The 2007 water year had been the driest on record for the southland, but in the last week of January 2008, Los Angeles picked up more rain than the city had received for all of 2007.
By early February, state hydrologists felt optimistic about the healthy Sierra snowpack and forecasted normal flows for the spring runoff. Unfortunately, the storm track shifted after the first week of February and the intense, snow producing weather pattern was gone for good. Ski resorts enjoyed one more significant storm, a healthy 5-foot barrage of snow during the third week of February, but for the rest of the season Squaw Valley received a meager 35 inches of additional snow on the upper mountain at 8,200 feet in elevation. The dry March was followed by an even more parched April, so it now appears that this spring (March, April, May) will go down as the driest on record for the Northern Sierra. The lack of late winter precipitation diminished the snowpack and reduced runoff forecasts. On April 1 the statewide snowpack looked good with just under 100 percent of normal, but by May 1 water content had plummeted to just 67 percent of average. The Central Sierra snowpack water content is even less, at 61 percent of normal for the date. To make matters worse as far as this years runoff is concerned, the desiccated soil conditions we inherited from 2007 has further reduced runoff as some of the water is being absorbed into the parched ground before it reaches our streams, rivers and lakes. Runoff into streams and reservoirs is 55 to 65 percent of normal. To give a comparative idea of how dry 2007 was, on May 1 of that year the snowpack was just 29 percent of average. Considering some of the major ski resorts pulled in about 36 feet of snow this winter, it may seem counter-intuitive that the Snow Lab has received less precipitation this year than during the seemingly snowless winter of 2007. During water year 2007, a total of 45 inches of precipitation was recorded at Soda Springs. This year, despite the impact of many impressive snowstorms, only 43 inches of water have been measured at the Snow Lab. The average annual precipitation on Donner Summit is about 54 inches. (Precipitation is snow melted for its liquid content combined with rainfall measured in the gage.) The difference between 2007 and 2008 is less about precipitation and more about snowfall. The 2007 season was noteworthy for the lack of natural snow cover in the Sierra; the Snow Lab measured only 275 inches (23 feet) total snowfall that year. This season was significantly better with 345 inches (almost 29 feet) at that location. Average annual snowfall in the Donner Summit region is about 34 feet.
Precipitation, however, is different from snowfall. For example, the snowiest winter near Donner Pass (measurements began in 1879) was 1937-38 when 819 inches were recorded. But that winter actually ranks 25th on the all-time precipitation list, because the abundant snow had relatively little moisture. In 1938, the most significant storm events were cold (Gulf of Alaska origin) and the snow was often dry and powdery, very much like this year. Dry snow has greater loft and will measure deeper than wet snow, but it wont contain more water. Take for example a Pacific storm that may generate one inch of water during its passage over the Sierra. If the storm is cold, more snowfall will be measured than if it is warm, even though the same one-inch of precipitation will have fallen. Sierra snowstorms average about a 1:12 ratio, meaning that for every inch of water available a foot of snow will fall. A lower ratio than that makes for relatively wet snow, often called Sierra cement. However, if the storm is cold enough, that ratio can climb to 1:18, and sometimes greater, meaning that 18 inches or more snowfall will be measured. The Wasatch Mountains in Utah commonly get snowfalls in the 1:20 ratio and greater, thus earning their mountain resorts the bragging rights of great skiing in champagne powder.For a while there this past winter, Tahoe resorts were chest-deep in Rocky Mountain powder due to the low temperatures associated with a strong Gulf of Alaska storm track. The Pacific moisture tap ended very early this year, but winter sports enthusiasts can appreciate the quality, and quantity, of snow that did fall in 2008. Mark McLaughlin’s column, “Weather Window,” appears every other week in the Sierra Sun. He is a nationally published writer and photographer whose award-winning books, “The Donner Party: Weathering the Storm,” “Sierra Stories: True Tales of Tahoe, Vol. 1 andamp; 2,” and “Western Train Adventures: The Good, the Bad andamp; the Ugly” are available at local stores. Mark, a Carnelian Bay resident, can be reached at email@example.com
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