The epic winter of 2011 is finally winding down after pounding the Sierra with enough snow to set new records for seasonal accumulation at major Tahoe resorts. The official 2011 water year won’t end until Sept. 30, but because April, May, and June combined typically provide only 12 percent of annual snowfall and 16 percent of our annual precipitation (summer precipitation is statistically negligible), the worst is likely over.

There hasn’t been enough snow at the Central Sierra Snow Laboratory (CSSL) near Donner Pass this April to bump 2011 past 16th position on the list of snowiest winters since 1878, but the game isn’t over yet and the Storm King might have a few more surprises for us. In April 1880, Donner Pass was bombarded with a record 25 feet of snow, an impressive onslaught that boosted 1880 into third place for the all-time snowiest winter.

At the end of March 2011, the snowpack at Norden and Serene Lakes near Donner Pass approached 20 feet deep on the level, with the 17.2 feet recorded at the CSSL the fifth deepest since its establishment in 1946. The heavy snow loads pulled down power and phone lines, while propane leaks in the Serene Lakes community caused an explosion that destroyed a three-story house and forced a voluntary evacuation order.

The seemingly endless storms in March dumped 248 percent of the average precipitation for the month at Lake Tahoe, and left a snowpack nearly double normal. The deluge of precipitation (combination of rain and snow melted for its water content) did wonders for the water supply outlook for Lake Tahoe and the Truckee River basin, and decisively broke a four-year dry spell. In the Truckee River Basin, reservoir storage at major reservoirs were all at or above average, compared to last year when storage was only 67 percent of normal. On April 1, water values in the Lake Tahoe Basin snowpack were measured at 173 percent of normal for the date. Last year it was only 87 percent of average. Better yet, Lake Tahoe’s water level has already risen more than 2 feet since last October, and is forecast to rise another 2.30 feet to its high elevation of about 6,228 feet, just 13 inches below the maximum allowed by federal law. This dramatic rise of 4 to 5 feet in Lake Tahoe will rank as one of the top 10 greatest increases since the completion of the Tahoe Dam in 1907.

The enormous Sierra snowpack has the potential to generate flooding on both sides of the range. The San Joaquin River system and its tributaries appear most at risk due to above average runoff and unusually high water levels in its reservoir system. Historically, peak reservoir inflows from the melting snowpack in the central and southern Sierra don’t occur until mid to late May, so high water issues and flood damage will be a concern for another 8 to 10 weeks. Flood potential in the Truckee-Tahoe region is classified as above average on Lake Tahoe tributaries as well as the uncontrolled feeder streams on the Truckee River above Reno. Fortunately, despite the rapid rise this year of Lake Tahoe’s water level, there is still plenty of storage available in Big Blue, as well as in flood control reservoirs like Prosser, Stampede and Boca, to prevent major damage on the main stem of the Truckee River.

This spring’s massive snowmelt runoff appears controllable at the moment, but all bets are off if an extended heat wave engulfs the region or if a significant rain event should occur.

– Tahoe weather historian Mark McLaughlin is a nationally published author and professional speaker. His award-winning books are available at local stores or at http://www.thestormking.com. You can reach him at mark@thestormking.com.