Tuesday, November 16, 2010

Snow? and the Olympic Rain Shadow Reversal

Late last week long term weather models were indicating the formation of a deep low sliding down the coast with cold temperatures and high pressure in the interior of British Columbia. That model was for ten days into the future meaning this next weekend. That long term forecast has held up so far - pretty amazing how well weather models are working. The odds are still a bit long that it will snow this weekend, but it will be getting colder. Essentially a pressure gradient is setup that causes the cold interior air to be drawn out towards the coast. CBC radio (Canadian Broadcasting Company) often will say "outflow winds expected" during these events and everyone on the BC coast knows what that means - cold winds blowing down the the fjords and deep valleys along the coast.

For my town tucked up adjacent to the Canadian border it means outflow winds down the Fraser River Valley. The Fraser extends deep into the interior of British Columbia and hence makes a great conduit for air flow out of the interior and into the border area between Vancouver and Bellingham. This micro climate can really be a bit of a shock. Whatcom County low land areas go from typical mild 40s and 50s and wet marine weather to weather that is more like Minnesota with temperatures dropping to the 0 F degree range and winds of 50 mph. The worst I have experienced here was -5 F with steady 70 mph and gusts to 100 mph. Even a few miles can make a big difference. Locally people know that being north of Smith Road means way higher northeast winds when these conditions develop. The small border town of Sumas sitting right where the Fraser Valley begins to widen really gets blasted by these winds. Twice I have had drilling projects near Sumas during outflow events and the drillers arriving from south Puget Sound in both cases were not prepared for the weather conditions.

The outflow winds out of the Fraser create other micro climate issues beyond Whatcom County. The San Juan Islands get hit by these winds as well. And at the Olympic Mountains the rain shadow effect gets reversed. More typical rain storms coming off the Pacific collide with the south and west sides of the Olympics and areas to the north and northeast are in a rain shadow with yearly precipitation well under 20 inches. But during the northeast air flow events the air uplifts along the northern side of the Olympic Range and the north slopes get lots of snow. This phenomenon shows up in the local probability snow forecast for next Monday.

If this long term forecast holds up, Bellingham will get a light snow Monday and Port Angeles will be getting 5 inches with very heavy snow on the slopes of the Olympics as the air flows up the slopes. The Press Expedition that traveled into and through the Olympic Range experienced this phenomenon with days of snow even at low elevations during the early days of their trek.

After some delay it appears that the La Nina pattern of storm systems sending low pressure systems down along the coast with cold high pressure in land is staring to get set up so lots of snow chances are on the way.   

No comments: