Heavy Rain Makes Dent in California Drought

Heavy Rain Makes Dent in California Drought

An ‘atmospheric river’ of Pacific moisture washed over the northern half of California this past Thursday through Sunday (February 6-9). It was the first (and strongest) such event since March of 2012. Here are some details of the storm precipitation and how it has impacted the deficits of the current rainfall season.

PRECIP UPDATES February 11th Some truly amazing amounts of rain fell over the coastal mountains north of San Francisco and also in the northern and central portions of the Sierra. In Sonoma County, just north of San Francisco, 13.98” of precipitation was observed at Guerneville over the course of February 6-9 and 20.17" at Monte Rio in Sonoma County. In Marin County, 20.86” fell at the Middle Peak RAWS site (elev. 2,300’) on Mt. Tamalpais and, at the low-elevation site of Kentfield, 11.71” fell. The top amount in the Santa Cruz Mountains south of San Francisco was 9.58” at Felton. In Napa County 13.56” fell on Mt. St. Helena (at about 4,000’) and in the town of Napa (near sea level) 8.11” accumulated. The Oakland Hills saw as much as 8.14” on Bald Peak. My home gauge here in the city of Oakland (at 314’) picked up 5.16”, the greatest storm total since 5.59” on March 13-16, 2012.

In the Sierra heavy precipitation was observed in the hills and mountains north of and including the Yosemite Park area. Some signature amounts of 16.60" were measured at Four Trees and 15.56" at Huysink and 6-8” totals were common for the entire Northern and Central Sierra. What has been unfortunate is that the snowline remained very high during the entire storm period, generally above 8,000’. Tahoe City (on the shores of Lake Tahoe, elevation 6,229’) received 6.20” of precipitation of which only 3” fell as snow. A site 3 miles SSW of South Lake Tahoe picked up 10.76" of rain. Above 8,000’ some prodigious snowfall was reported at some of the ski resort summit locations, like the 82” Kirkwood Resort claims to have fallen at their summit location at 9,300’. Squaw Valley says they picked up 38” in 24 hours and a 66” storm total at the 8,200’ level of their resort. Homewood Ski resort upper mountain picked up 63", and Alpine Meadows 61". Mt Rose ski resort in Nevada received 51" at their 8,260' level.

The latest graphic from the California Snow Survey Department shows a nice little tick up over the past few days in Sierra snow water content, enough to finally get slightly ahead of the record dry 1976-1977 season:



Sierra snow water content as of February 10th. The top graph is for the northern Sierra region, middle graph for the central Sierra, and bottom graph for the southern Sierra. California Department of Water Resources.

At lower elevations a big improvement in the seasonal precipitation deficits has taken place at a few locations in northern California. Santa Rosa has seen its % of normal precipitation to date go from 12% on February 5th to 40% today (February 10th).

Below is a table of how the situation has improved across the state over the past five days:



Change between February 5th and February 9th in % of normal precipitation to date for select California cities. Table produced by Jan Null of Golden Gate Weather Services.

What is clear from the table above is that the rainfall has not had much impact on most of the central and southern portions of the state where the worst drought conditions prevail.

Below is a map of California with the locations of the cities in the table above. Compare this to the latest drought monitor map and you can see that the region under ‘exceptional’ drought conditions (as of February 4th) have not been the prime beneficiary of the recent storm rainfall. It will be interesting to see what the new drought report due out on Thursday (Feb. 11) shows.





Since this was the first significant rainfall of the winter season, the ground has soaked the water up like a sponge and thus the rain will, at last, turn the brown hills green and put an end to the ridiculous mid-winter red flag warnings of December and January. How much of the precipitation has run off into the major reservoirs is not yet clear (as of Monday Feb. 10). We can, however, see a nice little uptick at one site, Folsom Lake:



Jeff Masters has another interesting graphic of the Folsom Lake water level in his latest post.

Christopher C. Burt
Weather Historian