Weather Window: Is September Sierra snowfall a winter predictor?
Special to the Sun
TAHOE/TRUCKEE, Calif. and#8212; Even as we enjoy a late season, record-breaking September heat wave, thereand#8217;s no denying the changing colors in foliage and the inevitable approach of autumn. Shorter days and cooler temperatures usher in the fall season in the Sierra, as we slowly segue into another winter at Tahoe.
The unusually strong high pressure system that jacked up regional temperatures during the last week of September pushed Truckeeand#8217;s afternoon highs to a summer-like mid-80s, but overnight temperatures consistently fell to near freezing at the Truckee Tahoe Airport. These extreme daily temperature swings are classic for relatively arid regions where low humidity, cloud-free evenings and calm winds encourage any residual heat to dissipate into the atmosphere in a process known as nocturnal radiational cooling. With lower levels of humidity in the air at higher elevations, there is nothing to trap and hold daytime heat, so as soon as the sun sets temperatures plummet.
These nighttime surface inversions (atmosphere is inverted in the sense that the coolest air is near the ground, with warmer air at higher altitudes) are common in the Martis Valley, where cold, dense air flows down-slope under the influence of gravity, draining off surrounding terrain into the valley where the airport thermometer is located. The nationand#8217;s record for a diurnal (daily) temperature swing occurred at Deeth, Nev., on Sept. 21, 1954, when the temp soared from a frigid 12 degrees to a sizzling 87, an increase of 75 degrees. Itand#8217;s important to note that this radical diurnal swing was caused by daily local cooling and heating, not by any frontal passage.
September is an important transition month when it comes to mountain hydrology. In the Sierra the water year runs from Oct. 1 to Sept. 30 because the beginning of October represents the historic low point for water flow and reservoir storage in the high country. In the lower elevations of California, the water year follows the fiscal calendar from July 1 to June 30. The storms of 2010 delivered 61 inches of precipitation to the Central Sierra Snow Lab at Norden near Donner Summit, about 116 percent of average. Snowfall there totaled 509 inches, 25 percent above average. Regional ski resorts also reported above normal snowfall last season: Squaw Valley picked up nearly 47feet; Kirkwood almost 45 feet and Mammoth 46.5 feet.
The month of September normally contributes little to our annual precipitation or snowfall tallies. At the Central Sierra Snow Lab, September averages four days with measureable moisture totaling one inch of precipitation and only 1.6 inches of snow. From early October on, the chances for measurable precipitation increase daily, with the likelihood of rain and snow at its peak from late December through February.
On Sept. 8, 2010, an unusually strong and cold low pressure system smacked the Truckee-Tahoe region with rain and dropped overnight low temperatures into the mid 20s. The same system dusted Mount Rose near Reno with snow. September snowfall accumulations of more than several inches are infrequent but not rare, especially at the upper elevations. Interestingly, most of the biggest September snowstorms of record led to below normal winters. In 1884, September snowfall at Norden measured 11 inches, but the following winter had a season total of only 202 inches, far below the annual average of 409 inches. In both 1901 and 1923, September snowfall tallied at 14 inches, but the winter of 1902 delivered only 373 inches and 1924 a meager 200 inches, the driest ever. More recently, a 7.5 inch snowfall at the Truckee Ranger Station in September 1986 also led to a below normal winter. September snowfall may make skiers salivate, but statistically weand#8217;re better off without it.
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 email@example.com
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