A check of the most recent hemispheric analysis shows a rather deep 500 mb trough and surface low off of the northwest coast.
|Fig 1 -- 12Z analysis of 500mb geopotential height (shaded) and mean sea-level pressure (contoured) on Dec. 18, 2010. From the HOOT website.|
The 500 mb trough extends all the way down through northern California and well out into the Pacific. The height gradient there (as well as the temperature gradient since temperature corresponds to thickness which roughly corresponds to height) hints at a jet stream oriented so as to bring lots of moisture up from the central Pacific and into the California coast. We can see this on the water vapor imagery.
|Fig 2 -- 2330Z GOES-W water vapor image on Dec. 18, 2010. From the HOOT website.|
Of course, this image is from nearly 12 hours after the above analysis, but it still shows a strong current of moisture being pulled out of the tropics and into the California coast. Note the enhancement of the water vapor right after the stream hits the coast--this shows where air is being lifted over the Coast and Sierra Nevada mountains. Since the water vapor channel on the GOES satellites tends to only pick up water vapor in the upper levels of the atmosphere, low-level moisture is often not as visible on these images. However, when moist air is forced to rise over the mountains, all that very moist air in the lower levels (since, after all, most of that moisture is coming from the ocean surface which is at the lowest possible level...) is lifted up to heights where it becomes more detectable by the GOES imager. Thus we see an enhancement of water vapor right as the air ascends over the mountains.
A quick look at the radar mosaic confirms large areas of moderately heavy precipitation falling across much of central and southern California.
|Fig 3 -- Base reflectivity radar mosaic at 2358Z on Dec. 18, 2010. From weather.gov.|
No wonder the Los Angeles forecast office has flood watches and advisories out for most of their forecast area--southern California doesn't usually get this much rain. But the ski buffs in the Sierra Nevadas must be loving all this new snow...
Of course, many people (at least, if they were the Seattle media) would blame this large amount of rain on the fact that we're currently in a La Nina pattern with cooler-than-normal sea surface temperatures across much of the equatorial central Pacific. It's true that the west coast is expecting a wetter-than-average season right now. The latest images from the Climate Prediction Center show how the sea-surface temperature anomalies have been and are remaining cold over the past few months--a pattern that defines a La Nina event.
|Fig 4 -- SST anomalies for several weeks ending the week of Dec. 8, 2010. Click on the image to see the animation. From the Climate Prediction Center.|
So this current deluge just reinforces the connection between a wetter-than-normal west coast and La Nina conditions over the Pacific. Is it proof of such a connection in and of itself? No. But we'll just say that because we're seeing La Nina conditions, this heavy rain isn't as unexpected as it would have been otherwise.