California and Nevada Pummeled By Flooding Rain, Mudslides and Rockslides; Rain Heads South Until Tuesday

Jon Erdman and Chris Dolce
Published: January 8,2017

Another multi-day siege of heavy rain and Sierra snow is pounding California and adjacent parts of Nevada which has resulted in flooding, mudslides and rockslides. The heaviest rain will slide south into Southern California early Monday, while another blast of heavy rain will arrive in northern California Tuesday.
This very potent atmospheric river event will continue to aim a fire hose of moisture at the West Coast into Monday. The rain combined with snowmelt due to high snow levels is leading to rises on rivers and other waterways. The National Weather Service says that flooding with this event may be the greatest since December 2005 for some locations.
As of early Sunday evening, there have been numerous reports of flooding, mudslides and rockslides in northern and central California. Significant flooding was reported in the Sonoma County Sunday afternoon. Some trees have been downed by gusty winds as well. A woman was killed by a falling tree in San Ramon on Saturday. For more the latest impacts, see the link below.
(LATEST NEWS: California, Nevada Flooding)
Rainfall totals of 3 to 7 inches have been recorded along the western slope of the Sierra Nevada and in some locations north and south of the Bay Area in the 24 hours ending Sunday morning 8 a.m. PST. Through Sunday evening, some locations had totals approaching 10 inches in central California. Thankfully those locations will get a break on Sunday night into Monday as the heaviest rain moves south, but it will return.

Flooding and Landslide Reports Last 24 Hours

(INTERACTIVE: Latest Radar, Alerts)
This is the second round of heavy precipitation to impact California in the past seven days. Winter Storm Helena brought the first round of heavy mountain snow and rainfall to the state Tuesday into early Thursday.

Forecast: Major Flood Concerns Into Monday; Another Round Arrives Tuesday

Heavy rain and high mountain snow will continue to pummel the Golden State and adjacent parts of Nevada into Monday.
The wintry side of this onslaught is associated with Winter Storm Iras and extends northward into the Pacific Northwest, and east into the Great Basin.
(FORECAST: Winter Storm Iras)

Current Conditions and Radar

Flood and flash flood watches have been posted for parts of northern/central California, western Nevada and southwest Oregon, 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 into Monday.

Flood Alerts

Given the warmer air that has moved into the region, snow levels will be as high as 8,000-10,000 feet in parts of the Sierra Sunday into early Monday, leading to melting snow. The Carson, Consumnes, Merced, Susan and Truckee Rivers could all experience some flooding from the heavy rain and snowmelt runoff. Snow levels are not expected to drop much overnight, so flooding will continue as heavy rain falls below 8,000 feet.
Here are a few examples of the flooding we could see.
Truckee River in Reno, Nevada

• Forecast to be just under major flood stage late Sunday into early Monday which is 13 feet. When the river reaches 12.5 feet, some flooding occurs in the Reno area and the airport begins seeing impacts. More serious impacts occur when the river reaches 13 feet.

Truckee River in Vista, Nevada (just east of Reno)

• Forecast to be near major flood stage which is 21 feet by early Monday. Major flooding and significant damage are possible from Highway 395 to Nixon, including Sparks, Lockwood, Tracy and Wadsworth. The Sparks industrial area is flooded up to seven feet deep. Many bridges are flooded over the Truckee River.

Truckee River near Truckee, California

• Forecast to be in major flood, climbing to over 7.0 feet by later Sunday into early Monday. This current forecast crest is similar to the flood of December 2005. At this level, major damage would occur to low-lying structures in the Alpine Meadows area.

Carson River near Carson City, Nevada

• Forecast to be in moderate flood over 10.5 feet by Monday. The National Weather Service (NWS) river impacts at 10 feet includes " Damage to roads, bridges, crops, irrigation systems and buildings in lower areas. Several homes and businesses flood in lower parts of Genoa, Carson Valley, Stewart and Dayton. Transportation begins to be affected, with water flowing over US Hwy 395 in places."

Napa River near St. Helena, California

• Crested just over moderate and record flood stage at 18.82 feet on Sunday afternoon. Travel will remain nearly impossible in the area into early Monday.

Merced River near Yosemite, California 

• Forecast to crest at over 10 feet on early Monday in minor flood stage. At 10 feet, a level that is expected to be reached overnight into Monday, water rises to the base of the swinging bridge downstream from Chapel Meadow.

A shorter period of heavy rain is still possible over the L.A. Basin early Monday. Up to 2 inches of rain is possible in coastal portions of Southern California through late Monday, and totals could approach 4 inches in the Santa Lucia Range.

Rainfall Outlook Through Wednesday

Gusty winds will also accompany the stormy weather, which could down trees given the saturated ground conditions. Power outages are also possible.
After Monday, another round of wet weather will sweep into northern and central California in association with another couple areas of low pressure Tuesday into Wednesday. Locally heavy rain could contribute to some additional flooding.
Snow levels will be lower, however, preventing any additional snowmelt. Several feet of snowfall will pile up in the Sierra Nevada later Monday through Wednesday. Travel will be greatly impacted through major passes.

Snowfall Outlook Through Wednesday

Flood Threat Caused by Atmospheric River

The culprit for this flood threat is the aforementioned 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, has parked an AR over the state this weekend.
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.
These warm AR events, however, diminish the snowpack available to slowly melt and recharge reservoirs for the summer dry season, instead. Oddly, it's a net loss for California's water supply.
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.

Round #2 Rainfall Totals in California and Nevada: 

• 9.60 inches near Vernado, California
• 8.06 inches in the Sierra Nevada at Peppermint, California (Elevation 7,385 ft)
• 6.45 inches near Carmel Valley Village
• 5.97 inches near Inverness, California
• 5.94 inches in Mt Saint Helena, California 
• 4.10 inches near Lemmon Valley, Nevada

Round #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.