A major West Coast storm continues to dump heavy rains and feet of mountain snow in California, as the low pressure area taps into a corridor of ultra-moist air known as an atmospheric river.
The storm has led to mudslides in Monterey County, with continued concerns about the stability of lands in and around burn scars from recent wildfires. Meanwhile, in the Sierra Nevada Mountains, blizzard conditions continue, and several more feet of snow are possible before the storm finally pulls away from the region late Friday.
In Monterey County, at least two people were injured and 50 horses rescued after fast-moving mud and debris from the River Fire burn scar smashed into homes. Numerous flash flood watches remain in effect throughout the San Francisco Bay area, with the greatest risk of flooding on Thursday shifting slowly southward with time.
According to the National Weather Service forecast office in San Francisco, the heaviest rain has fallen in southern Monterey County – 9.45 inches as of early Thursday – and rain continues to fall.
A strong atmospheric river such as this event can transport an amount of water vapor that’s about equivalent to 7.5 to 15 times the average flow of water at the mouth of the Mississippi River, according to a National Oceanic and Atmospheric Administration fact sheet.
The heaviest rain is expected to focus south of San Francisco throughout the day, closer to Santa Cruz and Santa Clara, as the storm system responsible for the wintry mess digs further to the south.
Similar shifts are expected with the areas of heaviest snowfall, as snowfall rates of two to three inches per hour in the Tahoe Basin ease up, and blizzard conditions migrate south during the day. Blizzard warnings are in effect for the central Sierras through early Friday morning, and even if snowfall rates diminish in the Tahoe region, additional accumulations are still expected there.
“We also can’t rule out pockets of thundersnow with this storm,” the Weather Service said in an online forecast discussion.
Widespread snow totals of more than five feet were expected from the fire hose of moisture aimed at the tall peaks of the Sierra Nevada Mountains.
Mammoth Mountain had already picked up an estimated four to six feet as of early Thursday, according to the National Weather Service in Reno. An additional three to four feet was forecast by Friday afternoon, bringing isolated totals to 10 feet.
Images captured at the Mammoth Mountain Ski Area in Mammoth Lakes revealed visibilities down to barely 200 feet. In the highest elevations, the excessive snowfall rates of up to four inches per hour were combining with winds topping 75 mph to produce whiteout conditions.
“If you risk travel over the Sierra passes, you could be stuck in your car for many hours,” warned the Weather Service in Reno, stating that wind chills could dip to 20 below.
Meanwhile, the increasingly unstable snowpack is prompting avalanche concerns, and the Eastern Sierra Avalanche Center issued a backcountry avalanche warning for parts of the central Sierra Nevada along Highway 395. That highway was shut down overnight Wednesday from near Bishop, Calif., to the Nevada state line, and was expected to reopen on Thursday.
The extreme snowfall is reminiscent of a storm on Dec. 19-20, 2010, which dropped a whopping nine to 13 feet of snow atop Mammoth Mountain. Mammoth Mountain’s main lodge is at roughly 9,000 feet elevation, the summit towering to 11,053 feet. The altitude of the mountains in the Sierra Nevada is instrumental in their prolific snows, since they force the air, which is carrying extremely high levels of moisture, to rise, cool and condense, which results in a dramatic increase in precipitation rates. Such mountain-induced precipitation patterns are known as orographic enhancement.
On average, Mammoth Mountain sees about 17 feet of snow per year. Snow in the Sierra is expected to taper down by Friday afternoon.
California has been mired in a deepening drought, with wildfires igniting in Northern and Southern California earlier this month. The whiplash between extremely dry conditions and heavy rains is an example of a weather pattern shift that climate scientists expect to occur more frequently as the region continues to warm.