California's Drought to be Pummeled by an Atmospheric River With Feet of Sierra Snow, Flooding Rain Into Next Week

Jon Erdman
Published: January 3,2017

California residents still thirsting for relief from a five-year drought may get too much of a good thing into next week.
A persistent Pacific pattern, featuring an atmospheric river, will deliver multiple rounds of heavy mountain snow and flooding, lower-elevation rain in the Golden State.
It has been five years since California's current long-term drought first began. Owing to earlier-season high snow-level events, water content in the Sierra snowpack – an important parameter for monitoring replenishment of the state's reservoirs during the spring melt – was actually running lower than average on Jan. 3.

Basin-wide water content of the snowpack, relative to average, on January 3, 2017. At that time, the water content of the Sierra snowpack, circled in red, ranged from 69 to 89 percent of average for January 3.

Round 1: Winter Storm Helena

Through Thursday, Winter Storm Helena will turn its firehose of moisture on northern California, as well as other parts of the mountain West.

Current Radar, Temperatures, Conditions

Several feet of snow is likely in the Sierra, with soaking rain at lower elevations of northern and central California.
Flood watches have been posted for the Sierra foothills, generally below 5,000 feet through Thursday, where the National Weather Service cautions rapid rises on small streams, flooding of roads, debris flows and rock slides are possible.
(INTERACTIVE: Latest Radar, Alerts)
In general, most lower elevations outside the foothills should pick up around 1 to 2 inches of total rain through Thursday. Some coastal ranges, including higher elevations near the Bay Area, are likely to pick up several inches more.

Round 2: Weekend Atmospheric River

After a brief break, another round of heavy rain and mountain snow will take aim on the Golden State for several days beginning this weekend.
The culprit will be an atmospheric river (AR): a thin, long plume of moisture emanating from the tropics or subtropics.
(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.

Upper-level pattern over the Pacific Ocean this weekend expected to bring more heavy rain and mountain snow to California.

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.
Furthermore, snow levels are expected to rise in the Sierra, raising the possibility of rain falling on previous snowpack and, thus, the likelihood of flooding.
The bottom line is the threat for more widespread river and urban flooding will rise substantially this weekend, particularly over northern and central California.

Rainfall Outlook Next 7 Days

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. Six feet or more of total snow may accumulate in parts of the Sierra from the combination of Winter Storm Helena and the weekend AR.

Snowfall Outlook Next 7 Days

If that wasn't enough, the Pacific storm parade should continue well into next week, with more Sierra snow and some low-elevation rain.
(MAPS: 7-Day Rain/Snow U.S. Forecast)

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

Tahoe City, California, for instance, went snowless through the entire month of January 2015, a month that averages about 40 inches of snow. The previous January, only 4 inches fell.
(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.
With any luck, 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.
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.