Winter storms keep lashing California with various degrees of impact, and on the doorstep is another atmospheric river primed to drop torrents of rain on waterlogged ground.
For all the record-breaking precipitation, the flood damage to property and harm to people has been minimal (with localized exceptions such as Guerneville).
“The big billion-dollar disaster has been avoided,” said Marty Ralph, director of the Center for Western Weather and Water Extremes (CW3E) at the University of California at San Diego’s Scripps Institution of Oceanography.
The credit for flood prevention goes to the state’s systems of flood control and water retention, some of which were constructed more than a century ago.
Also there’s been luck seen in the randomness of meteorology, as no single metropolitan region has found itself consistently under a deluge. In February, measurable rain was recorded on 17 of 28 days in Sacramento and San Francisco, on 13 days in San Diego and 12 in Los Angeles. (Which also goes to show how much California there is to California: The coastline measures 840 miles — longer than the straight-line distance between Washington, D.C., and St. Petersburg, Fla.)
This latest storm — making landfall early Tuesday between Monterey and Los Angeles — is the work of the 20th atmospheric river to strike Southern California during the 2018-2019 water year, which began in October.
According to CW3E research, that number isn’t out of line with a wet year. In 2016-2017, there were 27 atmospheric rivers during a winter so laden with snow and rain it ended a drought that had lasted as long as a decade, according to some experts. During the winter of a dry year, Ralph said, the number averages about 12 to 15.
There have been so many atmospheric-river-fed storms in Southern California that smaller events are escaping notice — such as the weekend’s spritzing of the region with an inch-plus of rainfall.
But this winter is notable, said Ralph, namesake of the new rating system for atmospheric rivers, for the number of such events categorized as 3′s (“strong”) or greater. So far in 2018-2019, there have been two such events. Since 2013-2014, there have been three years with no strong atmospheric rivers.
What’s due to strike isn’t expected to pack an undue amount of moisture. But the duration and location of the storm could make it a soaker.
“It still looks like the area of deepest moisture will stall for a 6-12-hour period somewhere . . . Tuesday night, likely the western portion but not certain yet,” the National Weather Service office serving the Los Angeles area said Monday.
Even a seemingly mild rain, given enough time parked over one spot, can wreak havoc. The area the Weather Service considers likely to be under a fire hose for up to half a day is the scorched mountainsides of the Thomas Fire, in western Ventura and eastern Santa Barbara counties.
Plus, the storm is expected to pack strong southerly winds that will gust headlong into California’s Transverse Ranges — which run east-west, or perpendicular to the axis of the atmospheric rivers’ flow — meaning orographic effects are in play.
“Assuming all these factors come together,” the Weather Service said, “there is a reasonable chance that rain rates will reach or exceed burn area debris flow thresholds.” This means enough rain may fall hard enough for a torrent of muddy debris in areas scarred by previous wildfires.
The risk of debris flow has prompted evacuation orders for the areas burned by the Thomas, Whittier and Sherpa fires.