From Cliff Mass Weather Blog:
A number of you have commented or emailed the same questions and comments that run like this:
" I thought it was a La Nina year--aren't they supposed to be wetter than normal"
"The National Weather Service said this fall would be wetter than normal--boy did they mess up!)
December is turning out to be one of the driest on record--in fact, there is a chance it could be the driest December since record-keeping began at a number of western Washington sites.
Take a look at the precipitation at Sea-Tac for the past 4 weeks (blue is normal, red this year):
Amazing...precipitation has almost been flat-lined since December 1--we are about 4 inches below normal, with only about .5 inches this month.
In general, La Nina years are wetter during the entire fall-winter season....but that is only on average. You can think of weighting a coin---instead of heads and tails being equally probable, heads is more likely. Throw the coin ten times and it could be heads eight times--BUT you STILL will get two tails.
This year it is more LIKELY to be wetter than normal, but some La Nina years HAVE been drier than normal.
Here is a plot that summarizes the situation for Washington State. The dots representation individual years, which can be El Nino (red), neutral (green) or La Nina (blue) years. The y axis gives the precipitation amounts for October through March. Yes, the blue (La Nina) years typically have more precipitation than the other years. But there is a considerable range for the La Nina, El Nino, and neutral years, with a number of La Nina years being relatively dry. This year may well be one of them--although it is too early to know what the final winter's total will be.
You can think of NWS forecasters as casino operators---over a period of time the house will win, but occasionally someone can walk away with a jackpot.
But there is something else... a characteristic of La Nina years is the persistence of a major ridge in the eastern Pacific (see NWS graphic below). Normally, that ridge is far enough west that we are in the
downstream trough...giving us cool, wet weather. This year, the trough has been farther east than normal...giving us drier than normal conditions. And we have seen the ridging day after day. For some reason the ridge is shifted--and so we have been generally dry and cool.
The current model runs show a return to more normal, wetter conditions soon.