December set several records
Though winter didn’t officially start until Dec. 21, Coeur d’Alene definitely received more than its fair share of wintry weather last month.
When all the weather stats were tallied up recently, December went down in the record books in several categories. With 7.86 inches of precipitation, it was the second-wettest December since 1933, when 9.91 inches of precipitation fell. It was the fourth snowiest December, with 47 inches of snow falling. Finally, December came in third for having the most snow on the ground – 17 inches – with 21 days in the month with measurable snow. The two biggest snow days, as measured at the weather station of local climatologist Cliff Harris, were Dec. 1, with 6.4 inches of snow (a record for that day) and Dec. 26 with 6.5 inches (also a record). As a reminder, December normally receives 19.3 inches of snow.
What is just as interesting to me as the weather records for Coeur d’Alene, are the stark differences between the weather here and what happens in neighboring Spokane and even closer in the Spokane Valley. Where I grew up in the plains of Oklahoma, you could drive for hundreds of miles and the scenery would remain the same. Likewise, in forecasting winter storms, elevation and orography, or the lay of the land with respect to mountains, were not factors.
Shortly after moving here, I came to realize that I couldn’t always cut and paste Spokane’s official forecast to Coeur d’Alene, despite their close proximity. For comparison, Coeur d’Alene ended 2007 with 26.56 inches of total precipitation, coming in just above the average of 26.11 inches (the extremely dry summer really set us back). In Spokane, total precipitation for the year was only 13.97 inches, below the average of 16.67 inches. At the Spokane airport, where official observations are taken, only 20.1 inches of snow was measured for December, with a monthly total precipitation of 3.72 inches. The largest one-day snow total was a measly 1.9 inches. In the Spokane Valley, where Felts Field airport sits approximately 400 feet lower in elevation than Spokane International, total snowfall was even lower (though no official records are kept for that location).
What can account for such large differences? Elevation is only part of the puzzle. Spokane International is at 2,372 feet, Felts Field is at 1,952 feet, and the Coeur d’Alene airport is at 2,320 feet. It makes sense that Spokane Valley would be on the lowest end of the snow totals, as there are plenty of times when it is snowing in Spokane and raining in Spokane Valley, but why does Coeur d’Alene get so much more? In combination with elevation, orography is the key. To get moist air to precipitate, it has to be lifted. There are many ways this can happen in the atmosphere, but in the presence of mountainous terrain, the mountains play a key role. Air moving toward a mountain, or mountain range, cannot pass through the mountain, so it is forced to rise. This sharp rising motion enhances the precipitation process. In prevailing westerly flow, central Washington is located in the “rain shadow” of the Cascades where air is traveling down the mountain, warming and drying as it does so. Continuing eastward, the terrain begins a gradual upward slope, with increasing precipitation totals toward the Idaho border. Lying just west of the Rockies, North Idaho as a whole is in a favored upslope region. For this reason, Coeur d’Alene will always end up on the higher end of snow and precipitation totals when compared to Spokane.