CNN
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California officials are urging residents to prepare for a powerful storm that will pummel the region with torrential rain later this week as the state continues to recover from massive amounts of snow that trapped mountain communities.
About 16 million people in central and northern California, including the San Francisco Bay Area and Sacramento, were under surveillance early Wednesday ahead of a storm that would drench the region with dangerous amounts of rain on Thursday — heavy snow in places on existing layers.
“The combination of heavy rain and melting snow can lead to flooding,” says the Weather Prediction Center said. “Creeks and streams in the western foothills of the Sierra Nevada will be most vulnerable to flooding from rain and snowmelt.”
The ominous forecast comes as much of California has been hit by several back-to-back rounds of heavy snow that trapped mountain residents in their homes, made roads impassable for days and knocked out thousands of residents as temperatures plummeted.
In addition to the heavy snowfall that overwhelmed the state last week, more than a foot of additional snow has already fallen this week in some mountainous parts of Northern California. And Wednesday is expected to bring more to that region, where lower elevations can see between 1 and 6 inches of snow with isolated totals of more than a foot of snow across the highest elevations of the Sierra Nevada mountain range.
Closer to the coast, officials in Marin and Monterey counties have begun preparations days ahead of the impending storm, which is expected to hit the area as a strong atmospheric river event.
For the Big Sur community in Monterey County, the emergency services agency went so far as to advise residents and businesses to stock up on essential supplies that would supply them for at least two weeks. The province has also made sandbags available for residents they need to protect their property.
The Marin County Fire Department will have personnel on hand for rescue efforts in anticipation of potential flooding, Fire Chief Jason Weber said.
“Our reservoirs are all full from storms earlier this year. Now that the reservoirs are full, we expect our creeks to rise faster and most of the rain to run off,” Weber told CNN.
Marin County, where a flood watch is in effect as of Thursday, is home to one of California’s urban search and rescue teams, and it will make its resources available to other counties if needed, Weber said.
This week’s expected atmospheric river event won’t be the first this year to ravage California. Late last year and into the new year, multiple heavy downpours from atmospheric rivers devastated much of the state – drenching entire neighborhoods and unleashing mudslides while killing at least 18 people.
Atmospheric rivers are long, narrow bands of moisture in the atmosphere that carry warm air and water vapor from the tropics. They can stretch for thousands of miles and dump rain and snow when they come ashore.
What Are Atmospheric Rivers?
Much of California will be affected by this week’s expected atmospheric river.
The Weather Prediction Center says parts of the state have a Level 3 or 4 risk — the second highest on the downtown scale — of excessive rain Thursday through Friday.
The storm is expected to drop some significant rainfall on top of some heavy snow packs. The National Weather Service expects widespread precipitation totals of 2 to 4 inches, with isolated amounts of up to 8 inches.
“The uncertainty lies in how much rainfall will be absorbed by the snowpack before a significant amount of water is released into the rivers,” the Weather Prediction Center said. “It is likely that some of the (precipitation) will simply be absorbed in the many feet of snow at the highest elevations, but lower elevations, generally below 5,000 ft, most likely do not appear to have the snow pack necessary to sustain the expected to absorb many inches of precipitation.”
In addition, the threat of heavy rain seeping into deep snow could cause the snow’s weight to increase, which could cause roofs to collapse, the forecast center noted. “Affected communities are being urged to remove existing snow from their roofs to mitigate,” the weather bureau added.