TAHOE/TRUCKEE and#8212; Following the heels of the epic 2011 winter, the ninth snowiest since 1879, winter 2012 is off to a slow start. Both winters are influenced by La Nina conditions in the equatorial Pacific, but so far the similarity ends there. The highly amplified high pressure ridge in the eastern Pacific is currently shunting winter storms to the north and east of the Tahoe region. Last year a dome of higher pressure was centered further west, and its clockwise air flow steered cold wet storms down from the Gulf of Alaska and into the Sierra Nevada. Storms this year, however, have been tracking farther east, and overland into the Great Basin. The overland trajectory has starved these early season low pressure systems of moisture and energy.
The Tahoe region has been bone dry since the Thanksgiving holidays. Some pundits have claimed natural snowfall before the Christmas holidays is just a bonus, but the total lack of moisture since late November is actually a rare event. This is statistically a very active time of year weatherwise and it's highly unusual to go so long without a storm. Consider the time slot between Nov. 25, when a weak weather system brought a little bit of snow, and Dec. 15 when another inside slider-type storm sprinkled Donner Summit with less than a tenth of an inch of precipitation. During those 20 days, no precipitation fell at many locations throughout much of California. This year was only the third time since 1940 nothing was measured at Blue Canyon; only the second time since 1906 at Chico; and it had never happened before at Red Bluff since records began in 1892. It was only the fifth time in Sacramento since the Gold Rush era in 1850.
The primary culprit is the Arctic Oscillation (AO) which affects atmospheric pressure in the Pacific Ocean. In 2011 the AO was in a negative phase, which enhanced an extended period of lower pressure (storminess) in the mid-latitudes of the eastern Pacific (our region of the world). That negative phase contributed to last winter's persistent snowfall pattern. So far this year, the AO is in a positive phase which tends to support higher pressure (fair weather) in the mid-latitudes. The Climate Prediction Center is forecasting the AO to remain positive for the remainder of December.
It may seem counterintuitive, but precipitation (rain and the liquid content of snow) so far this water year is actually close to average. The Central Sierra Snow Lab at Soda Springs received 8.32 inches of precipitation in October and November, compared to 9.15 inches normal for that time period. What is missing is the nearly 10 feet of snowfall normally received at the lab by New Year's Day. At this time last year, the upper elevations at Squaw Valley had already been blasted with more than 20 feet of snow. Squaw Valley hasn't picked up any natural snowfall in more than a month. The upper elevations have received only 24 inches of snowfall since early October, and more than half that fell long before the resort opened. Fortunately, snowmaking technology has enabled many local areas to open quite a few trails for skiers and snowboarders to enjoy.
NOAA's prediction for the next three months calls for average precipitation in the Tahoe region. In general, the main impacts of La Nina-influenced winters are felt later in the season, from January to March. Forecasters at the NWS office in Reno, Nev. are looking at a possible pattern change by the middle of next week, but no and#8220;game-changingand#8221; storms are possible until then. Wait for the AO to switch negative and the eastern Pacific high pressure ridge to retrograde or flatten out to allow storms into the region. In the meantime, keep those snow guns fired up.
and#8212; Tahoe historian Mark McLaughlin is a nationally published author and professional speaker. His award-winning books are available at local stores or at www.thestormking.com. You can reach him at email@example.com.