December 19, 2010 in Idaho Voices

It’s early winter, and we’ve seen it all

Michelle Boss
 

Who would have thought after a record snowy November in both Spokane and Coeur d’Alene, that the first half of December would bring everything from a big thaw via the Pineapple Express, strong winds, thunderstorms with hail, nearly two inches of liquid precipitation, but less than four inches of snow!

La Niña hasn’t suddenly disappeared, but the average snowfall for December in Coeur d’Alene is 19.3 inches, while on average Spokane sees 13.7. To have just an average December, and keep those ski resorts happy, mother nature will have to ramp it up for the remainder of the month in the snowfall department. According to the long range models as of Dec. 16, the pattern looks conducive to some snowier weather and for folks seeing a white Christmas across the Inland Northwest. According to statistics, the chances of either Spokane or Coeur d’Alene having at least one inch of snow on the ground on Christmas Day is right around 60 percent.

As if December thunderstorms were not strange enough, what about the tornado that was reported in Oregon this past week? It is rare to see tornadoes in Oregon to begin with, as the whole state averages only 1 or 2 per year. The ones that do occur are usually weak. The tornado that hit Aumsville, just south of Salem, on Dec. 14, however, was rated an EF2 on the Enhanced Fujita Scale (range EF0 to EF5).

The structural damage surveyed would be consistent with wind speeds of 110-120 mph. That same day, a waterspout was also reported off the Oregon Coast near Lincoln City, and winds there gusted to nearly 70 mph. While tornadoes are often explained as being caused by a “clash of air masses,” that explanation is not only overly simplistic, it is not always true either. Though the thunderstorms that rumbled through the Inland Northwest with heavy rains, strong winds, and hail occurred ahead of a strong cold front (which is the leading edge of colder air), the Aumsville tornado formed behind that front, under a deep upper level low pressure area (or cold pool of air at about18,000 feet).

Cold air over much warmer air results in instability and rising air motion – think about how a hot air balloon works. The intensity of this rising motion, coupled with adequate moisture, (and in this case, also an upper level disturbance moving through such an environment) can lead to severe thunderstorms which can sometimes produce tornadoes. In Washington, the only confirmed tornado this year was the one that occurred in Moses Lake back in May. There has never been a confirmed tornado in Eastern Washington or North Idaho in the month of December.

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