“Cry Me A River”
I got a Facebook message early this week from a friend in Sacramento CA that said after over 200 days with no rain, she got 4.83 inches in a 24-hour period from the latest extreme rainfall that occurred over northern California. Others have reported up to a foot of rain in three days. If you follow the news, you may have heard the term “atmospheric river” used to describe the torrential rains and flooding that have occurred this week in San Francisco and other parts of Northern California. In this post, I want to explain what atmospheric rivers are and how they affect rain climatology in the Western U.S. as well as other parts of the United States and the world.
Tahquamenon Falls–Autumn. Source: Wfgc, Commons Wikimedia.
What is an “atmospheric river”?
The term “atmospheric river” first appeared in the modern scientific literature in the early 1990s. Since it was first used, there has been a lot of discussion about what the term actually means. Commonly, it is seen as a band of very moist air flowing into a coastal area, bringing the potential for a lot of rain to the region that is at the downwind end of the flow. In some respects, it is like being on the receiving end of a firehose streaming high-intensity water right towards you! After a lot of discussion by meteorologists (described in this Bulletin of the American Meteorological Society article) the official definition in the Glossary of Meteorology became:
Atmospheric river – A long, narrow, and transient corridor of strong horizontal water vapor transport that is typically associated with a low-level jet stream ahead of the cold front of an extratropical cyclone. The water vapor in atmospheric rivers is supplied by tropical and/or extratropical moisture sources…
…click on the above link to read the rest of the article…
