Published: March 21,2017
California will be soaked with more rain and heavy mountain snow after a more than two-week respite from one of the wettest winters on record in the Golden State.
A pattern change in the upper atmosphere this week will bring a return to conditions that have been more familiar this winter, compared to the weather so far in March.
(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 this week.
Current Satellite, Atmospheric Moisture
Two Rounds
The increasingly wet pattern this week will manifest itself mainly in two rounds.The first Pacific cold front pivoted into the West Coast Monday and spread its moisture through the Great Basin and northern Rockies into Tuesday. Showers are expected to continue through Wednesday in these areas.
Current Radar, Temperatures, Conditions
Given the cold, unstable air aloft, thunderstorms are a good bet into Wednesday in California and the Great Basin, some with small hail, potentially even a tornado. There were a couple reports of funnel clouds in California's Central Valley Tuesday afternoon, but no touchdowns were reported.
(MAPS: 7-Day Rain/Snow Forecast)
Strong wind gusts, particularly over the crest of the Sierra, are possible ahead of the cold front's arrival.
Right behind that system, another Pacific front should come ashore Friday, and then slide into the Great Basin and Rockies this weekend.
The Second System Late This Week
How Much Rain, Mountain Snow?
The good news is neither 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
Heavy rainfall triggered roadway flooding in the Sacramento area Tuesday afternoon, where more than a half inch of rain fell within 15 minutes.The combination of both 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 in each round, with some lower 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 Saturday
Snowfall Outlook
The combination of both systems will bring multiple feet of snow to the Sierra through Saturday.(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 will be 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.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.
| 2016-17 Precipitation | Previous Record | Average | |
| Pocatello, Idaho | 14.23 inches | 11.90 inches (1983-1984) | 6.17 inches |
| Reno, Nevada | 13.12 inches | 11.00 inches (1913-1914) | 5.12 inches |
| Sacramento, California | 27.81 inches | 27.58 inches (1982-1983) | 15.45 inches |
| Spokane, Washington | 18.11 inches | 16.77 inches (1973-1974) | 9.97 inches |
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