The Climate Prediction Center has issued a La Nina Watch as sea surface temperatures in the equatorial Pacific have cooled and may continue to cool into the Winter months.
Cooler water temperatures in the Eastern Pacific causes the Polar Jet stream to move farther north. That carries the storm track into the Pacific Northwest and keeps more of the much needed Winter rain away from California.
La Nina is a contributing factor to our current mega-drought across the western states. It gives the Bay Area a dismal outlook for the chance that we may soon see an end to the drought.