Welcome to West Coast winter 2018-2019, responsible for a snowbound Seattle (the most snow in February since the Truman administration), a soaked Los Angeles (its wet season rainfall exceeding 13 inches, more than 50 percent above average) and totally stoked skiers in between.
Through the weekend, the forecast models examining the atmospheric river event, due to arrive in force early Wednesday, were having a hard time pinning down intensity, location and duration. But the looming threat of warm rain sending torrents of snowmelt thundering into rivers, canyons, channels and arroyos has everyone a little on edge.
It was in early January 1997 that storms fed by the warm “Pineapple Express” melted snow high atop California’s mountain ranges, and that water coursed over saturated ground toward reservoirs already full because of an earlier wet month.
The atmospheric river now approaching California’s doorstep has been seen in computer models for more than a week, first showing up on scopes during the state’s soggy Super Bowl weekend. Since then, the forecasts occasionally have offered eye-popping numbers, like a 24-hour span in Los Angeles with more than six inches of precipitation, while another prediction, made Sunday, called for the AR to flow over Southern California for more than four days. Another prediction called for heavy rainfall in Southern California for 36 straight hours.
Current forecasts call for one to three inches of rain around Los Angeles.
Heavy rain and high winds are also predicted in the San Francisco Bay area.
The damage this atmospheric river can cause is all about timing.
While streaming toward the mainland from Hawaii is a significant amount of moisture — remains of the Kona low that sent hurricane-force winds over the Big Island’s tallest peaks during the weekend — a pair of low-pressure systems are sliding down the back (eastern) side of a high-pressure system over Alaska. A delay here, combined with an accelerated arrival there, means the difference between even more gnarly powder at the resorts or evacuation orders ahead of rising rivers.
No matter the when and where, this event will be the first real-time test of a new categorization system for atmospheric rivers. The scale, developed by a team led by the Center for Western Weather and Water Extremes, takes into account a river’s strength (as measured by the water vapor it carries) and how long the fire hose of moisture stays pointed at one area. It’s a nod to the reality that even a weak system can wreak havoc if it hangs around the house long enough.