Winter officially begins today; however, I believe we’ve been having winter weather for some time. The cold east winds and the sleet and snow recently may be just the beginning of what we’ll see come January and February.
Of course, some of you had just a chilly rain as the frozen variety was somewhat elusive around the county. We remain on the cold side of the jet stream that has been aimed at California for some time now. Since last weekend, rainfall amounts are more than a foot in some places, with ten feet of new snow in the Sierras.
Back home, besides the winter solstice today, we had a lunar eclipse last night to round things off. It was likely tough to see due to cloud cover. Another low swings inland by Wednesday and will once again pull cool easterly winds down the Columbia River Gorge, bringing a threat of low-elevation snowfall.
My thinking now is that Christmas will be just green and wet, with colder conditions moving in by New Year’s.
It’s still plenty cold up in Alaska and in central and upper B.C., with temperatures far below zero — like 50 to 60 degrees below zero. Rainfall for the month locally is running above average and continuing our string of wet months this year.
I’ve had several inquiries about how it can snow when it’s so warm. The dry and cool eastern winds contribute to evaporative cooling. As a result, when snowflakes fall through the drier air aloft, they tend to release heat as they fall, chilling the atmosphere. Hence, snowflakes at around 40 degrees. If the dew points are low enough in the 20s and the precipitation heavy enough, the rain can easily change back to snow.
A formula to remember is if the dew point temperature and the ambient temperature average 32 degrees or less, the precipitation can be snow. For example, on Sunday, if it was 38 degrees and the dew point was 26 degrees thanks in part to the dry east winds, the average would be 32 degrees. So snowflakes can easily be in the air at 38 degrees.
Enjoy your week and keep warm and dry.
Pat Timm is a local weather specialist. His column appears Tuesdays, Thursdays and Sundays. Reach him at http://weathersystems.com.
