Meteorologists use the term "atmospheric river" to describe a long, narrow plume piping deep moisture from the tropics into the mid-latitudes. One type of atmospheric river (hereafter, AR) you may have heard of is the "Pineapple Express", a pronounced plume tapping moisture from the Hawaiian Islands to the U.S. West Coast. Amazingly, according to NOAA's Earth System Research Laboratory (ESRL), a strong AR can transport as water vapor up to 15 times the average flow of liquid water at the mouth of the Mississippi River!
Suffice to say, if an AR stalls over a particular area, significant flooding can be the result. In fact, a study by Ralph et al. (2006) found ARs responsible for every flood of northern Calfornia's Russian River in a 7-year period.
That said, they're also important for western water supply considerations.
According to NOAA/ESRL, 30-50% of the average annual precipitation in the West Coast states typically occurs in just a few AR events.
With that in mind, one such AR is poised to soak parts of the West Coast this week. Let's get to the forecast details
Flood Threat Looms
The graphic at the top of this article depicts the upper-air pattern that will setup by mid- late week. Namely, a deep dip, or trough, in the jet stream will carve out over the eastern Pacific Ocean.
This will send a parade of frontal systems and upper-level disturbances into the West Coast through this weekend.
The soaking starts Wednesday as the initial frontal system sweeps into the West Coast.
This first system should be a quick mover. While rain and high-mountain snow is forecast primarily for California, but also into western Oregon and parts of western Washington, precipitation amounts Wednesday should be relatively "routine", for a Pacific frontal system in the wet season.
Then, the "atmospheric river" makes landfall.
Beginning Thursday, the upper-level pattern will begin to tap into an atmospheric river of moisture extending from just north and west of Hawaii to the West Coast.
Most importantly, that plume of moisture won't move appreciably for a couple of days, perhaps through Sunday, aiming its firehose of moisture at northern California and, perhaps, southwest Oregon.
Therefore, some locations, particularly in the coastal ranges of northwest California and the Sierra foothills, will likely pick up over 10 inches of total rainfall through this weekend, leading to flash flooding, river flooding and, in recent burn areas, debris flows.
Depending on exactly where the moisture plume sets up, this heavy rain could produce significant impacts (flash flooding, rock/mudslides, etc.) in at least parts of the Bay Area, as well.
Rainfall amounts in Southern California are expected to be much lighter, with totals over the 5-day period starting Wednesday around an inch or so expected.
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