California Flood Threat From "Atmospheric River"
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 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 atmospheric river 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 atmospheric river stalls over a particular area, significant flooding can be the result. In fact, a study by Ralph et al. (2006) found atmospheric rivers 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 atmospheric river events.
With that in mind, one such atmospheric river 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 first storm arrived on Wednesday and brought rainfall from California to the Pacific Northwest. The combination of wind and rain led to hefty delays at San Francisco International Airport.
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.
Rain Forecast
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.
Heavy Snow Confined to High Terrain
This pattern will also produce heavy snow over the Sierra, not to mention parts of the Bitterroots, Tetons, and the Washington Cascades.
For Northern California our Winter Weather Expert Tom Niziol says, "This is a relatively mild pattern, therefore snow levels will be above 6,000 to 7,000 feet for most of the time with impacts mainly at pass levels. To put this in perspective, the highest elevation of I-80 through Donner Pass is about 7,230 feet."
Niziol adds, "Impacts ... spread north as the second system moves in Thursday through Friday bringing more precipitation, which will fall as heavy snow at elevations above 6,000 to 7,000 feet across the Sierra. The Cascades through the Bitterroots will also see heavy snowfall with accumulations exceeding a foot at elevations mainly above 6,000 feet."
No comments:
Post a Comment