More Heavy Snow, Rain Headed to Flood-Weary California and Nevada Through Thursday

Jon Erdman and Chris Dolce
Published: January 9,2017

An atmospheric river dumped torrential rain, even at high elevations, triggering the worst flooding in parts of northern California and western Nevada in 11 years, this past weekend.
(INTERACTIVE: Latest Storm Reports)
While the heaviest rain gave these hardest-hit areas a break Monday, more rain and heavy mountain snow is expected Tuesday-Thursday.
In just 24 hours from Sunday morning through Monday morning, there were 105 reports of flooding, flash flooding or landslides in California and Nevada.
(LATEST NEWS: California, Nevada Flooding)
Parts of the Sierra and coastal ranges of northern California have picked up over a foot of rain since Saturday morning, according to the NWS-California Nevada River Forecast Center.
(INTERACTIVE: Latest Radar, Alerts)

Forecast Timeline: Pacific Storm Train Continues

Current Conditions and Radar

Here's a general timeline of Pacific systems that will impact California and Nevada, as well as other parts of the West this week:

• Into Early Tuesday: Northern California; modestly wet
• Tuesday into Wednesday: All of California; wet in northern California, but not as bad as this weekend
• Thursday: Most of California; colder (lower snow levels) and not as wet

(MAPS: 7-Day Rain/Snow Forecast)
Beyond that, a much-welcomed dry out looks to be in shape from Friday into early next week.

Flooding Forecast

Flood watches and warnings continue in parts of southwest Oregon, much of northern California below snow level and western Nevada.

Flood Alerts

Most rivers have crested and are now falling, at least temporarily.
Here are some highlights:
Levels on the Truckee River were the highest since New Year's Eve 2005.

• Reno, Nevada: Crested early Jan. 9 at 12.3 feet, highest level since New Year's Eve 2005. Now falling.
• Vista, Nevada (east Reno metro): Crested at 19.5 feet Monday, the sixth highest level on record; Hundreds were evacuated and roads and bridges were closed in the Reno/Sparks metro area due to the swollen river.
• Truckee, California: Crested Jan. 8 at 6.53 feet. 

The Napa River crested on Jan. 8 in the Napa Valley, and will likely see a second, lower crest associated with Tuesday and Wednesday's Pacific storm.

• St. Helena, California: Crested at 18.82 feet on Jan. 8, and is now well below flood stage. A second crest is expected later Tuesday.
• Napa, California: Crested at 26.81 feet on Jan. 8, roughly comparable to the Feb. 3, 1998, flood. A second, lower crest is expected early Wednesday.

The Merced River near Yosemite National Park, California, crested Monday above the level at which the two main roads in the park become flooded (Northside and Southside Drives). This last happened on May 16, 2005.
Total additional rainfall through Thursday will average at least 3 inches, below snow level, in the Sierra foothills and coastal ranges of northern California. Another inch or two of rain is possible in lower elevations, including the Bay Area and Sacramento. Up to another inch of rain is possible in the Reno-Sparks metro area.
Generally lighter totals under one inch are expected in southern California.

Rainfall Outlook Through Thursday

Gusty winds will also accompany the stormy weather, which could down trees given the saturated ground conditions. Power outages are also possible.

Feet of Sierra Snow...Again!

A rather abrupt change will occur in the Sierra, following this weekend's inundating rain.
Snow levels will plunge below major pass levels, with multiple feet of snow falling again over areas deluged with rain this past weekend.
Parts of the Sierra above 7,000 feet may see up to 8 feet of additional snow through Thursday. At lake level near Lake Tahoe, from 2 to 5 feet is possible, according to the National Weather Service (NWS).

Snowfall Outlook Through Thursday

The NWS also warns of blizzard conditions over the Sierra passes Tuesday afternoon and evening.

Atmospheric River Storm Recap Jan. 6-9

As mentioned earlier, since the morning of Jan. 7, parts of northern California's Sierra and coastal ranges have picked up over 10 inches of rain. Here are some 72-hour rainfall totals, as of Monday morning:

• 13.51 inches at Huysink, California (Sierra west of Truckee)
• 13.37 inches at La Porte, California (northern Sierra)
• 13.11 inches at Nature Point (5150' east of Oakhurst in southern Sierra)
• 12.96 inches at Venado (Sonoma County)
• 12.59 inches at Three Peaks (3350' elevation; southern Monterey County)
• 11.87 inches at Peppermint (7355' elevation; east of Porterville in southern Sierra)
• 4.05 inches at Kings Canyon (5180' elevation; just west of Carson City, Nevada)
• 2.84 inches at the Napa County Airport
• 2.73 inches at the Monterey Airport
• 2.32 inches at Downtown San Francisco
• 1.88 inches at the Oakland Museum
• 1.71 inches at the Reno-Tahoe Int'l Airport

South Lake Tahoe, California, at an elevation of about 6,300 feet, picked up almost 2 feet of snow, then over 4 inches of rain, and will then pick up feet of snow this week.
The seven-day precipitation total ending early on the morning of Jan. 9 was the second wettest seven-day period on record in South Lake Tahoe dating to 1968, according to the NOAA Weather Prediction Center's forecast operations chief meteorologist, Greg Carbin.
A mudslide shut down a stretch of heavily-traveled Interstate 80 near Truckee, California, on Jan. 8.
Understandably, due to the weight of all that rain on existing snowpack, the avalanche danger was extreme in the Sierra. The Sierra Avalanche Center reported widespread avalanches on Carson Pass on Jan. 8.
All this rain prompted the California Department of Water Resources to open all gates of the Sacramento Weir on Jan. 9 for the first time in 11 years. This 101-year-old structure protects the city of Sacramento from excessive water flow, diverting it west of the city in the Yolo Bypass.
As if that wasn't enough, high winds also downed trees in several areas of northern California. A gust in the Sierra reached 173 mph on Jan. 8.
Sadly, these winds downed the iconic Sequoia "tunnel tree" in California's Calaveras Big Trees State Park, estimated to be more than 1,000 years old.

Round #1 Recap: Storm Totals From Winter Storm Helena Jan. 1-4

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.

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.
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.