Monday, November 15, 2010
Is it Going to Snow Over the Lowlands?
Several of you have asked the potential for cold and snow this weekend, and this morning the National Weather Service put out a special statement regarding the possibility of a change to more winter-like conditions. And today, I got several calls from the media about it. And yes, the local TV stations are already talking about it online and on air. It is never good to get excited about snow too early--there are a few ways to get snow in the lowlands and so many ways for it to go wrong.
Anyway, we are now close enough to the weekend, and the models are in good enough agreement, and I am comfortable talking about what I see happening.
Bottom line: forget lowland snow this weekend but lots of snow in the mountains this week. Strong NE winds will blow through the Fraser Valley into Bellingham and NW Washington and colder air will spread over the region on Sunday and Monday. Clear evidence of the classic La Nina pattern. Maybe snow on late Monday or Tuesday, but too early to be sure. At this point, the Mayor can keep his snowplows in the garage and Jim Forman of KING TV can get his famous parka dry cleaned this weekend. And there will be loads of active and interesting weather during the next week!
But lets start with tonight. A low pressure center is now passing north and northeast of us, bring a strong N-S pressure difference and gusty winds (see image). During the past hours winds have gusted to 30-40 mph over the area, with some minor damage. Nothing special in November. Perhaps more special are the stronger (30-65 mph gusts) winds predicted for tonight and tomorrow morning over the eastern slopes of the Cascades. My lights are flickering right now and my Uninteruptable Power Supply (UPS) is going on and off!
But the weather fun is just beginning!!
During the next two days the upper level pattern is going to change radically--to one reminiscent of many La Nina winters--one with a very high amplitude ridge over the eastern Pacific and troughing and cold air in our area. Here is the situation on Wednesday morning aloft. Huge N-S ridge extending to the Aleutians and a high-amplitude trough to its east.
At the surface, there is a strong low center NW of the tip of Vancouver Island and REALLY cold air move southward into northern BC.
On Wednesday and Thursday trough and low slowly slip southward. And a strong,cool, moist flow hits the Cascades and Olympics...the result? Several FEET of new snow at pass level and above (see one 24-h period below ending 5 AM Thursday). Good for Thanksgiving skiing!
But then the weekend. The low slowly weakens and moves south. No cold air over us on Saturday. On Sunday at 4 AM the surface pressure pattern and low-level temperatures (shaded) and winds (the barbs) are shown below. The cold air is still mainly stuck in British Columbia--with one exception---air starts flowing through the Fraser River Valley into Bellingham and NW Washington, accompanied by strong NE winds (see figure)
The problem for snow lovers is that the precipitation over the weekend falls as rain over the lowlands since the air is too warm. And then when cold air moves in later on Sunday , things have dried out. Here is the predicted snow for the 24-h ending 4 AM on Monday. The only snow is over far NW Washington and southern BC. Remember this is sustained winds..really roughly 20 minute sustained winds..you must multiply by at least 1.5 to get gusts.
The situation changes on Monday and Tuesday, but that is too far out to be sure about anything. The cold air will be in place and showers could bring snow in a convergence zone or other feature. And the upper level pattern looks much more like the canonical snow pattern. So keep tuned. And consider that there is still some uncertainty for the forecasts over the weekend...we are still days out and a modest shift in positions of some features could have a significant impact.
By the way, there was a lot I didn't mention...like strong SE winds over NW Washington on Wednesday and Friday..and the strong coastal winds with the low on Friday. Or the strong westerly wind surge into the Strait of Juan de Fuca tonight. My colleagues at the National Weather Service will be worn down when this week of active weather is done!
Posted by Cliff Mass at 7:23 PM