Weather Window | What makes a winter?
November 20, 2013
The Truckee-Tahoe region finally picked up some much needed precipitation this week when a low intensity "atmospheric river" surged into the region.
Unfortunately the relatively weak and warm storm systems lacked energy for forced lifting to cool the water vapor and condense it into a significant rain and snow event for the Northern Sierra. The moisture's sub-tropical tap northeast of Hawaii kept snow levels mostly above 7,000 feet for the bulk of the event.
However, as I write this, a sharp cold wave is projected to quickly dive into the region rapidly dropping temperatures and snow levels, so with any luck our local mountains might finally end up with at least the beginnings of a solid natural snow base to start the ski season.
CATCH THE PINEAPPLE EXPRESS
Last year the region also experienced an atmospheric river episode in November, but that weather system blew the doors off what we saw this week. Just as each winter storm has its own characteristics, "atmospheric river events" range from weak to epic. These "Pineapple Express" storm tracks generate virtually all the major winter floods in California and Western Nevada. In 2012 the surge of Pacific moisture was much more robust than this month, and the region was pounded with heavy rain and high elevation snow. Communities along the Truckee River narrowly averted severe flood conditions. Colder snowstorms followed in December and by New Year's Day, Squaw Valley's upper slopes had picked up nearly 21 feet of snow. The Northern Sierra snowpack stood at a healthy 140 percent.
When the Pacific moisture pipeline shut down from January through March, that abundant, early season rain and snow made all the difference in our lakes, reservoirs and on the slopes. Tahoe City received only 2.68 inches of water during the period — statistically the wettest time of year that averages more than 16 inches of precipitation there — which set a new record as the driest three-month stretch since measurements began in 1910.
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Despite the extended period without snow, Tahoe resorts did their best with their hefty early season snowpack and on-going snowmaking to provide a good mountain experience all winter.
WHAT'S IN A WATER YEAR?
If you consider the 2013 calendar year for San Francisco (which Bay Area journalists are currently doing with headlines claiming the driest year ever), the city is on pace to be the driest year since the beginning of records there in 1850. However, hydrologists, meteorologists, water districts, and even weather historians all subscribe to the Water Year when measuring and comparing annual precipitation. In California the lowlands calculate annual precipitation based on a July 1 to June 30 calendar cycle. In the Sierra, however, the water year runs from October 1 to September 30. San Francisco's 2013 water year actually ranks 43rd driest, not good but far from number one. The Central Sierra Snow Lab near Donner Pass reported a total of 45.7 inches of precipitation, a respectable 89 percent of normal in 2013. (Precipitation is rain and snowmelt combined.)
On the snow side of the equation, however, only 215 inches fell at the Snow Lab, a measly 53 percent of average. Last year's relatively mild temperatures during the bulk of the early season storm activity minimized snow packs near 7,000 feet, making it the fifth least snowiest there since 1878.
Beyond the coincidence between the start of 2013 and 2014 that both winter seasons opened with a decent October snowfall, followed by relatively warm moisture events in November, is there anything else that might give us a clue about weather patterns during winter 2014?
PICK A PATTERN
The lack of an El Niño Southern Oscillation event this winter (El Niño and La Niña oscillations) means that forecasters must look for other signals. For the past several years the Pacific Decadal Oscillation (PDO) has been trending negative, which implies cooler than average seas surface temperatures in the North Pacific Ocean.
The pattern tends to mimic a La Niña-like atmospheric circulation, which often leads to cooler and drier conditions in California. Researchers have analyzed data for winters that exhibited a negative PDO combined with ENSO-neutral conditions and the results aren't encouraging.
No one is predicting a winter like 2013, which produced one of the least snowy seasons on record, but between 1925 and 1998, on average, precipitation in the western United States was 8.9 percent below normal in these conditions.
At this time climatologists have yet to find a causal link between the PDO and seasonal climate or how it teleconnects with other regions, but the sobering fact is that ENSO-neutral phases during a negative PDO show the largest seasonal precipitation deficits in the western U.S. That's not a forecast.
Just an observation.
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 email@example.com. Check out Mark's blog: http://www.tahoenuggets.com.