April 19, 2009 in Idaho Voices

Nature of storm makes a difference

Michelle Boss
 

Spokane’s weather made national news earlier this week. Now if you live in Coeur d’Alene, the Spokane Valley, or anywhere in between, you were probably confused upon first hearing the story.

Though many of us woke up this past Tuesday morning to find a snow covered ground, last season’s winter, plus the fact that it snowed last June would have meant that a little April white stuff was not particularly news worthy. The CNN video of snow plows pushing “mountains” of snow caught my eye, however. Across a very small area, from north Spokane, westward to the Five Mile Prairie, up to 12 inches of snow was received on April 14.

The difference from what these folks saw, and what was recorded at the Spokane and Coeur d’Alene airports was like night and day, and it was not your typical explanation of elevation differences which made for the vastly different snowfall amounts.

Climatologically speaking, higher elevations will see higher snow amounts during the winter season, and the proximity to the windward side of mountains will influence seasonal snow averages. Generally, for the latter reason, Coeur d’Alene will see more snow than Spokane, and the former reason results in much more snow at Spokane International Airport than what Spokane Valley sees.

Neither reason, however, would explain how Whitworth College picked up nearly 8 inches of snow, while most of Coeur d’Alene saw less than half an inch, and Spokane International officially recorded only a trace (less than .1 inches).

It was the nature of the storm that dropped the snow that made the difference. In the same way warm season thunderstorms can drop three-quarters of an inch of rain on one person’s lawn, while leaving the neighbor’s lawn across town parched, these “convective” type storms (ones with more intense vertical motions within them) can do the same thing with winter precipitation.

The snow comes down hard and fast over a small area, and even if temperatures hover a few degrees above freezing, any melting is overcome by the sheer intensity of the snowfall, and the snow can pile up quickly. From a forecasting standpoint, it is pretty much impossible to pinpoint exactly where such a “snow burst” will occur in advance.

Another feature called a “deformation zone” within an upper-level storm system can lead to a narrow band of extremely heavy snows, but again, it’s difficult to pinpoint exactly where this small scale feature will set up until the signature is seen on satellite or radar images. By then, folks under it are already feeling the effects.

So despite the fact that Spokane is only about 2 inches away from reaching the 100-inch seasonal snowfall mark, the last storm missed pushing the airport over the top.

Coeur d’Alene has received 145.2 inches this season, still about 30 inches less than last year, but well over the average of 60 inches.

Michelle Boss can be reached at weatherboss@comcast.net.


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