The narrow tendril from Hawaii to the U.S. West Coast shown here in GOES water vapor imagery at 9 am PDT Saturday, 7 April 2018, is actually a Pineapple Express atmospheric river that carried a record-setting amount of moisture into the western U.S. Photo: NWS / Spokane, WA

By Bob Henson 
9 April 2018

(Weather Underground) – If you love atmospheric extremes, but you hate to see people in harm’s way, you couldn’t ask for a more pleasing event than the phenomenal infusion of moisture into the western U.S. from Friday into Saturday. No deaths or serious injuries were reported from the weekend rains, although Yosemite National Park was closed over the weekend as valley roads were inundated by up to 4 feet of water. The storm brought rain to unusually high altitudes for early spring, which led to a slight net loss in snowpack across the Sierra, but the rains also helped push the major reservoirs of central and northern California even closer toward full capacity.

The most striking aspect of this storm is something that’s much less obvious: the amount of water vapor that streamed from the central tropical Pacific well into the western United States. Simply put, this storm brought more moisture into parts of the West—by far—than any other winter-type storm on record, going back to when radiosonde launches began measuring conditions through the depth of the atmosphere in the late 1940s.

This event was a weird mash-up of winter and summer elements. One way to see this is through precipitable water (PW), or the depth of water that could be squeezed out of an imaginary column of air directly above a point. The table below shows six stations that have just recorded their highest values of PW ever observed in the six-month period from November to April. Some of these records were broken by huge margins on multiple dates. [more]

What Gave the West Its Soggiest Winter-Type Atmosphere on Record?

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