Many of you have asked for the details of the long-term forecast: what is expected for this fall and winter. OK, you asked for it--and some of you may not like what you hear.
The most powerful tool we have for predicting months ahead of time is the correlation between El Nino/ La Nina and Northwest weather. You have heard me talk about this frequently on this blog--the tropical Pacific sea surface temperatures oscillate between cooler than normal conditions (La Nina) and warmer than normal conditions (El Nino) over a period of roughly 3-7 years. And it turns out there is a strong correlation between the temperature of the tropical Pacific and the general character of our weather.
During the past half year the tropical Pacific has decidedly turned toward La Nina, with cooler than normal temperatures now evident near the equator. Here is the latest graphic: blue indicates more than .5C cooler than normal.
The figure below shows you the time evolution of this cold water...take a look at the Nino 3.4 area, a zone in the equatorial Pacific that is most often considered. Last year we had an El Nino (warm anomalies--an anomaly is the difference from normal) and during the early summer we rapidly slid into colder temps. We now have a moderate La Nina and it may well become a strong one. Computer models of La Nina evolution run by the National Weather Service and others are predicting a continuation of the La Nina through the winter.
Now the interesting thing is that the temperature of the tropical Pacific influences weather throughout the Northern Hemisphere, including us. Perhaps some day I will tell you why that happens, but in this blog let me give you the bottom line.
La Nina years are generally associated with wetter than normal conditions during the fall and winter, something I bet you believe after the last few days of liquid sunshine.
Here are official NWS average precipitation anomalies (again differences from normal) for fall and winter. The Northwest is wetter than normal in both, and you will note that California is relatively dry.
Now what about temperature? Turns out there is little temperature signal for La Nina years during fall, but during winter (January-March) we are colder than normal, as shown in this graphic:So colder and wetter than normal in winter....what do you think that implies? Yes, more SNOW, particularly in the mountains (see the graphic, where blue indicates more snow than normal). A good year to risk a season pass at a local ski area.
It also turns out that there is a greater probability of lowland snow west of the Cascades during La Nina years. Now, if Seattle's Mayor McGinn knows whats good for him he would be sharpening those snow plow blades, securing lots of sand and SALT, establishing rational plans for plowing the city, and telling all snow plow operators to avoid his neighborhood. We lost one Mayor to snowappocalypse, two would be an embarrassment. I offered to build a SNOWWATCH web page for the city...no bites yet.