California's Wet Pattern Returns; Mountain Snow to Add to Impressive Western Snowpack

Jon Erdman
Published: March 22, 2017

Two more rounds of rain and mountain snow are on the way to California through this weekend due to a pattern change that began at the start of this week. This has ended a more than two-week respite from one of the wettest winters on record in the Golden State.

(MORE: 5 Reasons March Weather Is Frustrating)

An upper-level ridge of high pressure that was responsible for the mainly dry and warm conditions in the West has now moved east.

In its place, a southward dip in the jet stream, or trough, is now in place over the eastern Pacific Ocean, steering the storm track back toward California through this weekend.

Current Satellite, Atmospheric Moisture

(MAPS: Weekly Planner)

Two More Rounds

The first Pacific cold front pivoted into the West Coast Monday-Tuesday and has spread its moisture through the Great Basin and northern Rockies.

Roadway flooding was reported in the Sacramento area on Tuesday, and funnel clouds were spotted near Turlock. A severe thunderstorm caused wind damage in Twain Harte, which is located southeast of Sacramento in the Sierra Nevada foothills.

Showers and few thunderstorms are expected to continue through Wednesday in California eastward to Utah, western Wyoming and northern Arizona. Snow levels will stay below pass levels in California's Sierra Nevada mountains.

Current Radar, Temperatures, Conditions

(MAPS: 7-Day Rain/Snow Forecast)

Another Pacific front should come ashore Friday and then slide into the Great Basin and Rockies this weekend. That storm will be accompanied by another burst of moderate to locally heavy rainfall, mountain snow and gusty winds.

Behind that system, another cold front will sweep into the Northwest Sunday, but its moisture will also extend southward into the northern half of California.

The Second System Late This Week

How Much Rain, Mountain Snow?

The good news is that none of these storms appear to be as strong or slow-moving as some of the atmospheric river events that occurred in January and February.

Rainfall Outlook

The additional storms will bring the heaviest rain to the coastal ranges of northern California, southern Oregon and the Sierra foothills below snow level, where over 2 inches total is likely.

Other low elevations of northern California will likely total at least an inch of rain through this weekend, including the Bay Area and Sacramento.

Rain will also reach Southern California, with some higher elevations picking up an inch of rain through Saturday.

(FORECAST: Los Angeles | San Diego)

Locally heavier amounts are possible where thunderstorms may occur.

Some mudslides, rockslides and debris flows are possible in spots with each round. Remember, in parts of Southern California, it takes much lighter rain rates over recently-burned slopes to trigger debris flows.

Rain and Snow Outlook Through Sunday

Snowfall Outlook

The combination of all these systems will bring multiple feet of snow to the Sierra through Sunday.

(MORE: Where March and April Are the Snowiest Months)

Over a foot of snow is also possible in the Cascades, Siskiyous and parts of the Rockies.

This is the first decent snow event in South Lake Tahoe, California, since March 5.

Adding to an Impressive Spring Snowpack

By late-March standards, the water content estimated in the Western snowpack is the highest in several decades.

According to the National Resources Conservation Service SNOTEL network, Leavitt Lake, at an elevation of 9,604 feet in the central Sierra south of Lake Tahoe, was estimated to have a snowpack of just under 20 feet: 234 inches.

The estimated water equivalent of that snowpack – just over 100 inches – was easily the highest snow-water content for any March 19 on record in their database dating to 1989.

Water content in the Wasatch and northern Rockies also was generally at least 33 percent above average for this time of year.

Estimated snow-water content percent of average for March 20, 2017, in western U.S. river basins.

The recent warmth has led to rapid snowmelt and flooding in parts of the Great Basin, from eastern Washington and the chimney of Idaho to northern Nevada.

Numerous U.S. Geological Service streamflow gauges are running in the highest 90 percentile for late March

For example, flow along the Snake River in Twin Falls, Idaho, on March 20, was over double the previous daily record set in 1928, according to the U.S. Geological Survey.

(MORE: Increased Spring Flood Risk in Parts of West)

Meanwhile, many California reservoirs continue to run above average for late March. 

San Luis Reservoir, just southeast of the Bay Area, was at 99 percent capacity as of March 20.

Gates at the damaged Oroville spillway in northern California were opened again last Friday to make room for this week's rain. 

Lake Oroville was 19 percent below capacity as of March 20, but still 10 percent above average for this time of year.

The last six months ending March 19 was the wettest such period on record in at least four cities in the West, according to the Southeast Regional Climate Center.

Sept. 20 through March 19 Precipitation Records Set
(Data: Southeast Regional Climate Center)
 2016-17 PrecipitationPrevious RecordAverage
Pocatello, Idaho14.23 inches11.90 inches (1983-1984)6.17 inches
Reno, Nevada13.12 inches11.00 inches (1913-1914)5.12 inches
Sacramento, California27.81 inches27.58 inches (1982-1983)15.45 inches
Spokane, Washington18.11 inches16.77 inches (1973-1974)9.97 inches

MORE: California Flooding - February 2017 (PHOTOS)

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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