Another round of atmospheric rivers is headed to California

USA

Two weeks after back-to-back atmospheric rivers brought powerful rainstorms to California, a new series is poised to bring even more excessive rain to the state in the coming days, potentially bringing flash flooding and deep snow that could make driving treacherous. .

The first storm, which is expected to start Saturday morning and last through Sunday morning, should be relatively weak compared to the storm that immediately follows. That storm, which began taking shape over the central Pacific Ocean Friday morning, is expected to begin hitting the state Sunday afternoon and last until the middle of next week.

In early February, an atmospheric river event caused severe wind and wave damage and mudslides, prompting multiple rescues as rivers quickly overflowed their banks.

As of Friday morning, the details of how these storms will unfold were still relatively unknown. They could change as the storm system approaches land and meteorologists better understand how strong it will be.

“Sunday's system is complicated, which makes it difficult to predict accurately,” forecasters at the National Weather Service's San Francisco office said early Friday.

Here's what you need to know heading into the weekend.

The forecasters expect Saturday's system will be lighter, but will act as a “primer” for the next storm. Each rain that falls will make it harder for the ground to absorb the next rain, increasing the risk of flooding.
The second system could bring widespread rain from Northern California to Southern California Sunday through Wednesday. It will most likely have enough force to drive the rain further inland into the valleys, but the heaviest rain will fall along coastal areas.
Winds are expected to be strong in the second storm, but as of Friday it was still not expected to be as destructive as the storm earlier this month.
High surf, including waves up to 20 feet, is expected in the second storm, potentially causing coastal erosion and damage to some structures.
By the end of the event, the Sierra Nevada above 6,000 feet is likely to receive another two to three feet of snow, with even higher amounts near mountain peaks. Forecasters warned anyone heading to the mountains over the long holiday weekend to prepare for travel to be difficult to impossible.

Why this prediction is complicated.

At the cruising altitude of most aircraft, a powerful wind region known as the jet stream ebbs and flows like a river. This weekend's jet stream could produce winds of 200 to 250 miles per hour, which will mainly impact the second storm system. The jet stream will send and drive the second system directly along the California coast Sunday through Monday. If the storm is right under some of those strong winds, it could quickly intensify. That couldn't happen either.

By the time the storm gets close to the coast, the jet stream should be weaker, leaving the storm to just meander toward land for several days. But where it stops and moves slowly will have a significant impact on which location gets the most rain.

The computer models that forecasters use to make predictions don't fully agree on how these details will unfold. As a result, they had only moderate confidence Friday morning in the impact this storm could have.

In the San Francisco Bay Area, forecasters said the most likely event would be two to six inches of rain in coastal areas. But there is a reasonable worst-case scenario in which this storm brings more than 6 to 8 inches.

Forecasters in Los Angeles shared a similar sentiment, saying they expected less rain in Los Angeles County than the early February storm — with one caveat. The forecast can be off by up to an inch in either direction. That may not sound like much, but it can be the difference between a small rain shower and a heavy rain shower in the city.