Winter 2015 at Lake Tahoe-Truckee: a roller-coaster ride so far
February 23, 2015
TAHOE-TRUCKEE, Calif. — It's been an unusual winter so far at Truckee and Tahoe, with unprecedented dry conditions twice broken by "Atmospheric Rivers" that delivered prodigious moisture in short periods of time.
A quick look at the quirky weather pattern this season provides a snapshot into these Dr. Jekyll-Mr. Hyde swings between sun and rain.
Let's start with the good news. The heavy precipitation over the past weekend (Feb. 6-9) was desperately needed in the Lake Tahoe-Truckee watersheds.
The first significant snow in more than six weeks was wet and heavy at local ski resorts, but snow depths at the highest elevations were boosted by up to 2 feet.
The big winner this time was Mt. Rose-Ski Tahoe, which pulled in a storm total of nearly 4 feet of fresh snow.
At the resorts, there was no bottomless Rocky Mountain powder for the fat board crowd, but after the driest January on record, the new snow greatly improved ski and snowboard conditions for both locals and visitors.
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And the rain that fell made a decent contribution to our depleted lakes and reservoirs.
Atmospheric River storms, known colloquially as a "Pineapple Express," are a big deal when it comes to our winter weather.
On average, about 30-50 percent of annual precipitation on the West Coast occurs in just a few AR events, and they are a vital component in our winter climate.
These relatively narrow and slow moving bands, about 250 to 300 miles wide, draw deep subtropical moisture across the Pacific Ocean past Hawaii.
Driven by high-altitude winds, an AR can move huge amounts of water in the atmosphere. A strong event can transport an amount of water vapor equivalent to the volume of water flowing at the mouth of the Mississippi River.
California typically experiences five or six ARs each winter, and they can collectively provide nearly half of the state's freshwater supply.
The strongest ARs transport the most water and bring the most extreme winds. Sometimes these powerful storms can stall over the Sierra watershed and create catastrophic floods and damage.
Recent examples include the 1986 and 1997 floods that were both generated when ARs dumped more than 30 inches of rain in a relatively short period of time.
The high-elevation rain also melts the existing snowpack, which increases water runoff. This combination of heavy rain and melting snow causes most of the floods in California and western Nevada, and are known as "wet mantle" events.
Extremely high winds are often associated with stronger ARs, and the most recent storm was no exception. Sierra ridge gusts approached 150 miles per hour, and along the eastern Sierra Front near Reno and Carson City, wind gusts at 60 to 70 mph were widespread.
At times, extreme gusts up to 90 mph roared through many western Nevada valleys. The fierce wind toppled large trees, blew down power lines, overturned tractor-trailers, and caused significant property damage.
While the recent storms were a big boost for conditions at local ski resorts, it did not materially help the Sierra snowpack.
Due to the storm's subtropical origin, snow levels remained mostly above 7,500 feet, meaning that the majority of the precipitation fell as rain.
The base of Homewood Mountain Resort picked up more than 3 inches of rain in just one 24-hour period.
A recent snow survey in the Lake Tahoe basin indicated that the meager snowpack contains less than 20 percent of the average water content for the date.
One reason that number is so low is that two of the six snow survey locations have no snow at all. Lake Tahoe's snowpack is the lowest for Feb. 1 since snowfall telemetry sites were installed in the 1980s.
With such dismal numbers, hydrologists are already warning public agencies and farming interests to prepare for a fourth year of below-normal surface water supplies.
The month of January really hurt us water-wise. Normally January is the wettest month of the year, but in 2015, virtually no precipitation fell, and South Lake Tahoe's temperature averaged nearly 9 degrees above normal.
At the Central Sierra Snow Lab at 6,900 feet near Donner Pass, station manager Randall Osterhuber measured only 1.4 inches of snow last month, compared to the January average of 81 inches.
In San Francisco, the month of January in 2015, 2014, and 2013 all rank in the top five driest since records began in 1850.
— Tahoe historian Mark McLaughlin is a nationally published author and professional speaker. His award-winning books are available at stores or at thestormking.com. Mark can be reached at firstname.lastname@example.org. Check out Marks blog: tahoenuggets.com.