What La Nina? December sets records for cold and dry
I know, I know. There was much-promised fanfare when the news broke that we were about to go into our second La Nina winter in a row, as these "Double-Dip" La Ninas have some good history with a "wintry" winter.
But just like last year, La Nina has gone MIA through the autumn months (as in, missing in action, not to be confused with MIA, the airport code for Miami, which certainly would not be confused for a Seattle December.)
Oh sure, we had our little dance with snow in mid November and the week of Thanksgiving was pretty stormy, but we had a very slow start to November that is trying to be outdone by the ultra-slow start to December.
Just how slow? Through the first 12 days of the month, December has received 0.03" inches of rain total -- all pretty much courtesy of a thick fog that squeezed out some drizzle.
Normally, December is the third wettest month of the year, just barely behind January.
So as you might imagine, we're in some uncharted territory. Former KOMO weather intern Jason Phelps tells me that for Sea-Tac Airport, we've never gone the first 10 days of December with no day receiving more than 0.01" of rain. (The 11th eked out 0.02" of drizzly rain to thwart that statistic from going further.)
To put that in persepctive, our first 12 days of December have been drier than the first 12 days we had this July.
But it hasn't been just dry, it's been chilly as well, courtesy of a strong inversion. December has not reached 45 degrees yet this month -- another first for Sea-Tac to go the first 12 days without hitting 45. Pretty amazing when you consider we haven't had any kind of arctic outbreak or snow event, yet the month is running a full 5.5 degrees below normal for high temperatures.
The cold weather is due to persistent high pressure that has kept skies clear at night, allowing the day's heat to radiate back toward space. As the air cools at night, it gets relatively heavy (since cold air is denser than warm air) and that cold air will just sit along the surface until either some wind (which has been just about non-existent with high pressure around) or sun (which is at its weakest levels near the winter solstice) can break it up.
With neither working too well to clear us out, we've been left with cold but dry weather. And if the month were to end today, it'd be the second coldest by average temperature (35.9 so far, record is 35.2 set in 1990). And we've got just 19 days to add up enough rain to keep us from setting the driest December on record. (Record is 1.86" set in 1976, although it'll only take a couple of good Pacific storms will push us over that number.)
So, what happened to La Nina? Did it forget to show up?
No, officially, we are still in a La Nina pattern. In fact, latest data continues to suggest it's still strengthening, with a -1.0 degree cool anomaly recorded in the area of the tropical Pacific where La Nina counts. (Anything less than -0.5C is considered La Nina) That -1.0 is cooler than the three month running average of -0.7C so it's still there.
But we've been off just a tad on the weather set up. This big ridge of high pressure keeping us dry has set up shop a little further east than usual. Normally it's supposed to steer the jet stream up into Alaska then drive it southeast along the B.C. coast into the northwest, keeping a steady supply of cool, wet weather (see: March through May, 2011). But with the high further east, it's been keeping us "high" and dry while shifting the wet weather to the east into the Rockies.
Of course, winter fans know that all it takes is that ridge to move a bit more west and intensify a bit and that's a recipe for colder weather. It just appears that we'll have to be patient. Long range models do indicate a return to a more typical rainy pattern the week of Christmas but as for any La Nina fireworks? The Space Needle's annual New Year's display might come first.