When we think of major surges of moisture into the western United States, sometimes called atmospheric rivers, we often think about a connection to the subtropical
eastern Pacific. However, that's not always the case.
In the latest satellite derived water vapor loop (below), you can see that the moisture for the storm that is about to push into the western United States originates over the
western subtropical Pacific and is moving over the low-amplitude ridge just east of the dateline.
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| Source: SSEC |
Understanding these surges of moisture requires a very broad perspective, one that looks at the global scale over many days. Even the analysis above does not tell the whole story, but it will have to suffice as I'm blogging on the road right now.
It seems like many of the largest and wettest systems in the Pacific Northwest region occur during the early season, particularly November and December. I definitely think that a significant amount of this early-season moisture comes from the tropical western Pacific, some of it the distant remnants of tropical systems.
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