From the San Francisco Chronicle – Most of the models — including the European, American and Canadian models — are signaling this being a La Niña winter. This means that waters off the coast of the Americas will be running cooler than average, and high-pressure systems can form more easily over the West Coast. On the other side of the ocean, warmer and more humid air stays off the coast of East Asia. But there’s another weather phenomenon that might be a boon for drought — the Pacific North American Oscillation. The PNA signals a change in pressure patterns between weather systems in the middle of the Pacific Ocean and the West Coast of the U.S. and Canada. It captures how much a high-pressure system in the northeast Pacific wobbles between the West Coast and the middle of the ocean. A positive PNA phase indicates that the high pressure that normally sits in the middle of the Pacific Ocean is shifting toward the West Coast. This creates a barrier around the Western U.S. that shields it from storms coming out of the Pacific. Once the high pressure backs out of the Western U.S. and shifts back toward the middle of the ocean, a low-pressure system will tend to form over the West Coast. This is a negative PNA phase, and it creates an environment where low-pressure systems help pull in storms from the Pacific toward California. This negative phase would be ideal for an otherwise dry La Niña year. (more)