Parts of Northern California that saw up to half a foot of rain last weekend are bracing for more this week as a series of powerful storms up and down the West Coast threatens to bring scattered flooding, mudslides and debris flows in the lower elevations and heavy snow in the mountains.
And parts of Southern California, where an exceptionally dry winter helped fuel multiple wildfires last month, are expected to see rain from Tuesday through Friday that could raise the threat of mudslides in some areas.
The storms, known as atmospheric rivers, are ribbons of moisture carried by powerful winds, and the first of three in a row occurred this weekend; it stretched roughly 2,500 miles from Hawaii to California like a conveyor belt bringing tropical water vapor to the north. Nearly six inches of rain fell on Saturday and Sunday in places like Redding, Calif., which caused some roads to flood.
The second of these storms is expected from Monday into Wednesday. Flood watches were issued for roughly 13 million people across Northern California from Monday afternoon through Wednesday morning, including heavily populated areas like the San Francisco Bay Area and Sacramento Valley. (A flood watch means residents should be prepared for possible flooding; it does not mean that flooding will occur.)
That thin strip of moisture, only about 100 to 200 miles wide, might not be as long as the first, but for much of this week, it will meander along the West Coast like an uncontrolled fire hose. It is expected to drench Northern California before drifting south, bringing some rainfall to Southern California.
Forecasters warned that last weekend’s rain had left the ground saturated, essentially priming some northern regions for flooding to occur as new rain pours down this week. By the end of it, the Sacramento Valley and areas south of San Francisco may have seen another one to three inches of rain. North of the city, forecasters predicted an additional two to four inches.
A third atmospheric river is expected at the end of the week. While it is still uncertain exactly how that system will evolve, forecasters expect it to be weaker, with reduced totals of only one to two inches of rain. This third system is also likely to be aimed farther south.
Southern California will likely see some rain.
The recent and continuing rains have been focused on the northern part of the West Coast, but Southern California is not out of danger. The region would welcome the steady rainfall after dry conditions that helped fuel disastrous wildfires in and around Los Angeles last month, but too much rain, too fast, could lead to mudslides in areas that burned.
But forecasters believe the rainfall this time isn’t expected to be intense. One of the storms is likely to peak in Southern California from Tuesday night into Wednesday, with maybe an inch of rainfall. Los Angeles County may even see far less. But up to two inches of rain may fall on south-facing slopes, including in the Santa Barbara South Coast, Malibu and the foothills of the San Gabriel Mountains.
After a brief lull on Thursday morning, another system will arrive later, delivering widespread rainfall across the region. Rainfall from that storm is expected to taper off on Friday afternoon, and the rain threat should end by the evening.
Mountain snow may be welcome, but it could bring treacherous travel conditions.
The moisture-laden systems also cover the northern Rockies, where cold air allows for it to crystallize and fall as heavy snow. The Cascade Mountains and other interior mountain ranges, such as the Sawtooth, Bitterroots, Tetons and Absaroka, are expected to see significant accumulations.
Winter snow piling up across the West is a natural reservoir that will fill rivers through the drier months of the year. It is also a boost to mountain recreation as fresh snow days draw skiers, snowboarders, snowshoers and snowmobilers to the backcountry and ski resorts.
The Cascades could receive an additional six to 12 inches of snow through early Wednesday, while areas such as the Shasta and Siskiyou Ranges and the northern Rockies may see one to three feet.
From Thursday into Friday, snow is likely to extend into the higher elevations of the Sierra Nevada, creating hazardous travel conditions.
