Wednesday, December 21, 2011

Where is La Nina?

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 .25 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.

12 comments:

Aaron Hill said...

Cliff, is there a relationship between the sea surface temperatures and how/where the ridge shifts?

Elf said...

You had the same title on a post last year too. You're becoming La Ninas defense attorney. Why?

Its a dreadful phenomenon. Half of me loves this beautiful weather. The other half screams at the overhype media outlets upon its arrival.

Cliff, I admire and love your mission to promote Pacific NW weather, but its kinda been a tough sell lately. All I can recall is December 2008 and heatwave of 2009 (Neutral year) Weather lovers shouldn't root for El Nino Or La Nina.

BTW, checkout the 90 day storm tracks. Hundreds of lines in the NH and not one in WA. Oh the irony. http://www.cpc.ncep.noaa.gov/products/precip/CWlink/stormtracks/strack_NH.shtml

Fixed Carbon said...

Cliff: I love that figure showing La Nina over the north Pacific. What does the El Nino figure look like? Thanks, Don

ILoveWinter! said...

One can only hope we not only return to a wetter pattern, but also cold, snowy, windy...in other words...WINTER! Sure hope we aren't dissapointed, yet again!

Leif said...

Could that "some reason" in fact be the late freeze of the open Arctic waters. The north Pacific is one of two major entry points for warm air to enter the rising air above the open Arctic, the other being the North Atlantic. As winter settles in the continental land mass cool cools much faster than the open water so that air must sink and go someplace. South would be the obvious call IMO. Thus bringing cold continental Arctic air further south and warn moist Pacific air further north. Anchorage AK has had rare "Winter Rains" in December and temperatures similar to ours here in NW WA at times this winter. Wondering where the Pacific NW rain went, look further afield. Wondering why? I would suspect Climatic Disruption might be a good place to start.

ip said...

The rain had to go somewhere...

Just talked to my dad in Costa Rica, this is typically their dry season, and he said that they've had day after day of rain there. Wonder if the overall cycle is related somehow?

WildernessFactory said...

I've been in the weather doldrums for a couple years now. I was living in Seattle the year after all the snowstorms, and it never snowed, barely rained, just cloudy and 40 everyday. I moved back to Minneapolis and left for San Antonio two days before the Metrodome collapse and the 100 inches of snow. Just moved back to Minneapolis and it's 40 degrees everyday, a dusting of snow. Boooooooooring.

smokejumper said...

Cliff, your synopsis on our current conditions is right on. But I believe its something beyond la nina.

The whole global patterns have been erratic. A lack of cold artic air is a start. Second, the lack thereof hasn't been drifting south.

The whole country has been barren. Unconfirm stat, but I read only 5 percent of the US below 3000ft will see a white Christmas. Bizarre.

Ferdi said...

I was supposed to leave for Hawaii the first of this month, but when I saw the long range weather models it looked so nice I decided to postpone my trip until the end of the month. I have not been disappointed. But if we don't get some rain in January and February we are going to have dry wells this summer.

2◻2BA☆ said...

Off-topic, but I was sad to see I can't buy a Kindle version of your book. Please sell the eBook!

kdscatt said...

According to the Weather Channel, there is a Positive Atlantic Oscillation (PAO) occurring now, whereas last winter had a Negative Atlantic Oscillation (NAO), that led to record snowfall and cool wet weather pattern here, but not that much lowland snow.

The PAO coupled with the LaNina apparently caused more of a neutral pattern (generally a bit below normal temps and abnormally dry here in Dec.). However, the persistent upper ridge of the last few weeks has broken down, allowing a zonal pattern to set up for the next week or so.

If the PAO persists over the next few months, how does that typically affect the LaNina pattern ? From what I have already seen this past month, it appears that drier would be the answer, but there are likely other complexities that have not been accounted for.

Unknown said...

Big Shift is right. Breezy wind forecast turned into 45-55mph Christmas. The trees haven't been exercised much lately. Lots of power outages east Renton Highlands area. Ours was about 4 hours outage so not bad. Just surprised to see some sort of meso low development that wasn't well advertized in the models. I have to admit I didn't look at the UW high res at the time so I don't know whether those runs caught it or not.