Everyone knows that it rains all the time in Seattle, especially during the early winter, but not this winter. During November western Washington basked in a 13-day dry spell the third longest rainy season dry spell on record ending up with about half the normal amount of rain. And, as of mid-month, December has brought a grand total of .30 inches. What's going on? Hi I'm dave Thurlow for the Weather Notebook.
The culprit is an atmospheric phenomenon known as the Rex Block, named for the meteorologist who first identified it. When a Rex Block sets up, a ridge of high pressure builds in the upper atmosphere. Rex blocks tend to have long stable reigns once they're in place, they stay in place, bringing essentially the same dry, cloudless weather for a week or more. And they don't go away quietly either: it takes a powerful storm to break a Rex Block down.
Rex Blocks have a ripple effect that can extend all across the country: when a block is in place, the atmospheric flow or the jet stream tends to split, with one branch dropping far to the south and the other curling up and around the block before diving down into the center of the country hence all the snow in the Midwest.
So what about the rest of the winter? The long-range outlook calls for normal temperatures and rainfall in the Pacific Northwest. But since normal means .20 to .25 inches of rain a day, Seattle obviously has a lot of catching up to do.
Our show was written today by David Laskin of Seattle, his books "Rains all the Time" and "Braving the Elements" can be found at our website, mountwashington.org. The Weather Notebook is a production of the Mount Washington Observatory in North Conway New Hampshire, an organization dedicated to weather research and education.