Tuesday, December 20, 2011

Snow on Christmas?

We are only 6 days away from Christmas! The big question each time December 25th rolls around is whether or not we will be seeing a white Christmas. Living in Northwest Oregon, we already have a handicap getting snow down to the valley floor. The Pacific Ocean is too close and acts as a moderating influence to our weather. It just keeps temperatures too warm for a persistent snow threat.

Portland's coldest month is January, where the mean average temperature is 39.9 degrees. That is "comfortably" above the freezing mark, making it tough for snow to fall. Let's look over the Cascades at Redmond's coldest month, that's December. The average mean temperature is 32.7 degrees. So why the difference between Portland and Redmond? Elevation plays a key role. Portland sits at roughly 200 feet above sea level. Redmond is at nearly 3,000 feet in elevation. The thermodynamic state of the atmosphere cools as you gain elevation. So you'd expect Redmond to be colder than Portland. Another issue is "continentality".  The land heats and cools much faster than the ocean, so the closer to a body of water you are, the more temperate the climate may be. That is exactly what we deal with here in Portland. We are just close enough to the Pacific Ocean to be impacted by its moderating factors.

So let's answer the question: what is the possibility of a white Christmas in Portland? You actually have a better chance of seeing snow on Christmas if you live in Downtown Portland rather than out near the Portland International Airport. On average, Downtown Portland has a 4% chance of seeing a white Christmas. And at PDX? If you had a worse chance, it'd be no chance... only a 1% chance of seeing the white stuff near the airport. Portland's most recent "white Christmas" was back in 2008, where close to 1" fell on the 25th to go along with the nearly 10" that was already on the ground (as a side note, December 2008 saw 19" of snow!). What about Redmond's chances? Since 1949, Redmond has had 12 white Christmases and on average has a 24% chance of seeing snow on the 25th. 

Just for fun, Portland's snowiest day is January 19th where since 1884 it has snowed 15 times and on average there is a 12% chance for snow that day. 

What does this Christmas have in store? This December has been very dry. The airport has reported only a tenth of an inch of rain through the 19th. That is nearly 3.39 inches below normal. The driest December on record was in 1976 where only 1.38 inches fell. So we are well on our way setting a new record. We remain locked in this very dry weather pattern. All month long, high pressure has been planted out over the Pacific Ocean, leaving us dry. The weather maps continue to hint at this high pressure staying more or less in place.
Christmas Eve Morning
Christmas Morning
Rain by Presents Time
The high pressure clearly shows up on Christmas Eve morning. The ridging sends any rain up over the top of us and keeps us dry and mainly sunny for the 24th. However, a weakness disrupts our nice weather by Christmas morning. That weakness will bring us some showers by the time you are opening presents. But temperatures just won't be cold enough to produce snow. Christmas average temperature in Portland is 45 degrees, I am aiming for 43. So no white Christmas this year!

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