Published: January 5,2017
Another multi-day siege of flooding rain and Sierra snow will take aim on California starting this weekend, which will eventually prove to be too much of a good thing for residents still thirsting for relief from a five-year drought.
Winter Storm Helena brought the first round of heavy mountain snow and rainfall to the state Tuesday into early Thursday. Flooding and some landslides were reported, while feet of snow piled up in the Sierra Nevada.
(NEWS: Flooding, Power Outages Reported)
Starting on Saturday and continuing through Monday, another very potent atmospheric river event will deliver even heavier rainfall to California, with rising snow levels resulting in a significant threat of flooding from snowmelt. The National Weather Service says that flooding with this event may be the greatest since December 2005.
Upper-level pattern over the Pacific Ocean this weekend expected to bring more heavy rain and mountain snow to California. Weekend Atmospheric River: Significant Flood Concerns
After a brief break Friday, another round of heavy rain and high mountain snow will take aim on the Golden State for several days beginning this weekend.(INTERACTIVE: Latest Radar, Alerts)
Flood and flash flood watches have been posted for parts of northern and central California, including the Sierra Nevada, where the National Weather Service cautions rapid rises on small streams, flooding of roads, debris flows and rock slides are possible. Rainfall totals of up to a foot are not out of the question along the western slope of the Sierra Nevada this weekend into Monday.
Flood Alerts
In the Yosemite Valley, the Merced River could top out near its record level set in a January 1997 flood that caused $198 million in damage. A closure of Yosemite National Park is possible in advance of the flood event, according to officials.
Rainfall Outlook Next 7 Days
(MET 101: Why Atmospheric Rivers are Hazardous and Essential)
A highly amplified upper-air pattern in the Pacific Ocean, featuring blocking high pressure near the Bering Sea and Alaska, and a downstream plunge of the jet stream off the Pacific Northwest coast, will likely park an AR over the state for what could be two to three days starting Saturday.
When these ARs stall over land and are lifted by mountain terrain, significant flooding often results. About 80 percent of California's major flood events can be traced to ARs, according to NASA.
The bottom line is the threat for more widespread river, urban flooding and landslides will rise substantially this weekend, particularly over northern and central California.
For now, the Southern California flood threat is lower, but at least a shorter period of heavy rain is still possible over the L.A. Basin before this atmospheric river event ends early next week.
This may be enough to trigger areas of flooding in typical flood-prone urban areas. Debris flows from recent wildfire burn areas are also possible.
Above snow level, ARs can dump some of the heaviest multi-day snowfalls you'll find anywhere in North America over the Sierra, and this should be no exception.
More additional feet of snow will impact the Sierra Nevada, but the heaviest snow will be confined to the highest peaks. That said, elevations as low as 5,000 feet will see additional snowfall Friday evening into Saturday evening before changing to rain.
Snowfall Outlook Next 7 Days
(MAPS: 7-Day Rain/Snow U.S. Forecast)
It has been five years since California's current long-term drought first began.
Owing to earlier-season high snow-level events, the first manual snow survey of the Sierra snowpack – an important parameter for monitoring replenishment of the state's reservoirs during the spring melt – found it to be only 53 percent of normal where it was taken at 6,000 feet elevation Tuesday.
The melting snow from this event will further reduce the snowpack, running off into rivers and streams.
Is This Really Anomalous?
This may sound like a lot of snow and rain, particularly if you don't live in California. But California winters are periodically wet at times."This is what we’re supposed to be getting," National Weather Service meteorologist Johnnie Powell told the L.A. Times. "This is normal rain and snow that we’re supposed to be getting in December and January."
Recent record dry winters plunged the state into its current drought status. In essence, that blocking high pressure we talked about earlier near the Bering Strait and Alaska had been over the West Coast in recent winters.
Current Drought Status
(MORE: 35 Strangest Weather Events I've Seen in My Lifetime)
This town nestled in the Sierra may easily exceed that average January total over the next several days.
These warm AR events, however, diminish the snowpack available to slowly melt and recharge reservoirs for the summer dry season, instead, triggering more serious flooding. Oddly, it's a net loss for California's water supply.
Let's hope for a Goldilocks scenario of building Sierra snowpack with minimal low-elevation flooding can finally push California out of its multi-year drought by the spring and summer.
Roound #1 Recap: Storm Totals From Winter Storm Helena
Rain
Several landslides were reported in parts of northern California Tuesday night and early Wednesday, particularly in Santa Cruz County. Up to two feet of water was reported near downtown Yountville, and two right lanes of the 101 freeway in downtown San Francisco were flooded by a couple feet of water.As of Wednesday morning, parts of northern California's coastal range and Bay Area mountains had picked up over 6 inches of rain.
Generally from one-quarter to one inch of rain had fallen in lower elevations of northern California, including San Francisco, San Jose and the city of Santa Cruz.
Snow
From Sunday into early Wednesday, parts of the Sierra Nevada picked up more than four feet of snow. The top totals were 56 inches at Boreal and Soda Springs.Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7.
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