Weather Window: Octoberand#8217;s monster storms strike the Sierra
October 27, 2010
TAHOE/TRUCKEE, Calif. and#8212; For the second year in a row, a warm, wet Pacific storm pummeled the Northern Sierra before Halloween. Whatand#8217;s going on? Climatologically, October is not a very productive month when it comes to precipitation in the Sierra. At the Central Sierra Snow Laboratory near Donner Pass, the 10th month averages less than 3 inches of precipitation, about six percent of the water expected there annually. But this October has been so wet that even before the most recent storm pounded the region, Reno had already set a new record for the wettest October ever since 1871.
The subtropical-influenced moisture plume that surged into Northern California and deluged the region with near record amounts of rain last weekend was driven by a wicked fast jet stream approaching 200 mph. Entrained in the flow were the juicy remains of a Philippine typhoon. When the moisture slammed against the towering Sierra Range, the resulting downpour was impressive.
In the San Francisco Bay Area and the Sacramento Valley, the rain and wind played havoc with trees, traffic and power lines, but the relatively short-lived event did not cause serious damage. Downtown Sacramento picked up 1.60 inches of rain, but once the air mass began its lift over the Sierra, rainfall amounts increased exponentially. The greatest recorded in El Dorado County was more than 7 inches at Grizzly Flat, with 9.20 inches measured at Carson Pass (Alpine County). Preliminary reports indicate that Blue Canyon (Nevada County) picked up 9.81 inches, but even that hefty total was exceeded by the more than 11 inches measured near the Hell Hole Reservoir west of Lake Tahoe.
The higher amounts of precipitation along the Sierra west slope is caused by a natural phenomenon called orographic uplift. As wet air is forced over the range, it cools and releases additional moisture. This is illustrated by the difference in average annual precipitation amounts between Sacramento (20 inches) on the valley floor and Blue Canyon (68 inches), near 5,000 feet in elevation.
Contrary to the historic drying trend of autumn, since the end of September the water flow from Lake Tahoe into the Truckee River has gone up, not down, with a dramatic spike during the recent storm. Heavy rain falling on the Tahoe watershed jacked the lake level several inches, a huge amount of water and an additional cushion against the lake sinking to the natural rim where surface water fails to reach the Truckee River.
In addition to the heavy rain, extreme winds ripped across the region. On Squaw Peak, elevation 8,700 feet, the anemometer peaked out at 132 mph. At the 9,650 foot level of Slide Mountain, gusts reached 126 mph. The upper elevation winds spilled over to lake level where a 56 mph wind gust tore through the Tahoe Keys. Along the eastern Sierra in western Nevada, down slope gusts reached 76 mph. In Washoe Valley, screaming winds snapped trees in half, blew down freeway signs and toppled a tractor trailer.
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Last weekand#8217;s weather system shared the same synoptic signature as the wet and windy storm that drenched the region in October 2009. But neither event can match the record-setting Columbus Day storm of October 1962, when 7 inches soaked San Francisco and nearly 2 feet of rain fell on the mountains in three days. That epic storm disrupted baseballand#8217;s World Series between the San Francisco Giants and New York Yankees, a prolonged and soggy event eventually won by New York. Fortunately, this year the monster October rainstorm arrived before the World Series opened in San Francisco with the Giants back in it, against the Texas Rangers.
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 http://www.thestormking.com. You can reach him at firstname.lastname@example.org
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