Tuesday, January 10, 2017

Feet of Snow to Bury California's Sierra while Heavy Rain Floods Rivers For the Second Time This Week; Blizzard Warnings in Effect

Jon Erdman and Chris Dolce
Published: January 10,2017


Blizzard conditions will hammer California's Sierra with feet of snow, just days after an atmospheric river triggered the worst flooding in parts of northern California and western Nevada in 11 years. At lower elevations, yet another batch of heavy rain will escalate river levels through early Wednesday morning, which, for many central California rivers, will be the second flooding crest this week.
(INTERACTIVE: Latest Storm Reports)
The National Weather Service has issued the first blizzard warning for Lake Tahoe in nine years, in effect through early Wednesday afternoon.


California Winter Alerts
A powerhouse Pacific storm, followed by a colder system behind it, will pile on the rain and snow in California, but a break, at last, is ahead for parts of the storm-weary West.
(LATEST NEWS: California, Nevada Flooding)
Snow levels have plummeted to 4,000-5,000 feet, and heavy snow is now crushing the entire Sierra chain. Furthermore, strong winds, combined with snow, are raising the concern for blizzard conditions with this current Pacific storm.
(INTERACTIVE: Latest Radar, Alerts)

Current Conditions and Radar

Forecast Timeline: Pacific Storm Train Continues

Here's a general timeline of Pacific systems that will impact California and Nevada, as well as other parts of the West this week:
  • Through Wednesday: Mainly northern California; snow levels dropping to as low as 3,500 feet; still quite wet in elevations below snow level
  • Thursday: Most of California; snow levels continuing to drop to as low as 2,000 feet and colder air moves in; wetter in southern California
(MAPS: 7-Day Rain/Snow Forecast)
Beyond that, a much-welcomed dry out looks to be in shape from Friday into early next week.

Feet of Sierra Snow...Again!

Parts of the Sierra above 7,000 feet may see up to 10 feet of additional snow through Thursday morning. At lake level near Lake Tahoe, from 3 to 7 feet is possible, according to the National Weather Service (NWS).
(MORE: Where Snowfall Has Topped 200 Inches This Season)

Snowfall Outlook Through Thursday
Compounding that epic dump of snow, winds gusting over 100 mph are likely over the Sierra ridgetops. Gusts over 60 mph are likely over passes, and along the lee slopes of the Sierra in western Nevada, as well.
That combination of heavy snow and intense winds will create whiteout conditions, making travel impossible in parts of the Sierra through at least early Wednesday.
High wind warnings are also in effect for Reno/Sparks and Carson City, Nevada, where gusts over 60 mph are likely to down trees, trigger power outages, and could blow high-profile vehicles off the road. High wind warnings are also in effect across the Sacramento Valley, San Francisco Bay area and northern Coastal Range of northwest California.

Flooding Forecast

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

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.
The Napa River crested on Jan. 8 in the Napa Valley, and will likely see a second, lower crest associated with Tuesday night and Wednesday's Pacific storm. A flash flood warning has been issued for this river.
  • St. Helena, CaliforniaCrested at 18.82 feet on Jan. 8, but a second crest is expected Tuesday night. This crest could get close to major flood stage. Crop losses and impossible travel can be expected if the river returns to 19 feet. 
  • Napa, CaliforniaCrested at 26.81 feet on Jan. 8, roughly comparable to the Feb. 3, 1998, flood. A second, slightly lower crest is expected early Wednesday. A major flood is expected. 
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.
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.

Atmospheric River Storm Recap Jan. 6-9

In just 24 hours from Sunday morning through Monday morning, there were 105 reports of flooding, flash flooding or landslides in California and Nevada.
Here are some seven-day precipitation totals (rain, as of Tuesday morning, January 10, according to the NWS-California Nevada River Forecast Center:
  • 31.00 inches near Incline Village, Nevada (all snow)
  • 27.45 inches at Three Peaks (3350' elevation; southern Monterey County)
  • 24.99 inches at Nature Point (5150' elevation just south of Yosemite Nat'l Park)
  • 22.99 inches at Huysink, California (Sierra west of Truckee)
  • 21.88 inches at Downieville, California (northern Sierra)
  • 21.39 inches at Venado, California (Sonoma County)
  • 19.09 inches at Peppermint (7355' elevation; east of Porterville in southern Sierra)
  • 18.16 inches at Blue Canyon (5280' elevation in the northern Sierra)
  • 14.07 inches at Cisco (all rain)
  • 5.27 inches at Napa County Airport
  • 4.78 inches at Sacramento Int'l Airport
  • 3.86 inches at Downtown San Francisco
  • 3.63 inches at Carson City, Nevada
  • 3.07 inches at the Reno-Tahoe Int'l Airport
  • 2.45 inches at the Fresno Air Terminal
  • 1.39 inches at L.A. 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 from January 3-4, 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.
Parts of northern California's coastal range and Bay Area mountains had picked up over 6 inches of rain from this first event.
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 January 1-4, 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.

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