Deluge Amidst Drought

December 2, 2014

The western U.S. has been suffering through drought for two years, but for the last year or more California has been the epicenter of the drought. Moisture conditions have been deteriorating in California for more than a year, and the most recent U.S. Drought Monitor paints all of California with a palette of red and orange. Almost 80 percent of the state is experiencing Extreme and Exceptional drought, with about 55 percent of the state in the Exceptional category. December 1 marks the start of the wet season in California, and typically about one-half of California’s annual precipitation can be expected to fall during December through February.

The northern portions of the state, at least, got a good start to the wet season last week. On December 2-3 a Pacific storm produced inches of rain over northern California. Three to almost six inches fell in the 24 hour period ending the morning of December 3 in central Marin County, north of San Francisco.

Marin County, CA precipitation for 24 hours ending December 3, 2014

  

December 2014 precipitation through December 9.

Percent of normal precipitation, December 2014 through December 9.

Three-day totals for December 2-4 from CoCoRaHS observers in northern California ranged from around 3 inches in San Francisco to 7.83 inches in Santa Clara County at CA-SC-3, Los Gatos 6.1 S. Rain fell across much of California with this system, but amounts dropped off to one to two inches and less in southern California.

The heavy rain in California caused, as might be expected, flash flooding in many areas. Even in southern California where rainfall amounts were modest by Midwest standards, the heavy rain and high rainfall rates caused flash flooding and mudslides. The rain occurred over saturated ground and in some cases barren ground from wildfires. Officials had to rescue dozens of people from vehicles caught in the water or mud, and homes needed to be evacuated in several areas due to the danger posed by mudslides. In northern California creeks and small rivers became raging torrents and sinkholes developed in some San Francisco streets.

During much of the fall and winter 2013-2014 a huge ridge of high pressure in the eastern Pacific deflected storms away from California. The pattern is quite a bit different this year, as the door is wide open to the Pacific. This steers storms and moisture into the western U.S.

500 millibar map for 4:00 a.m. PST December 9. Note the strong band of westerly winds in the eastern Pacific.

Another powerful storm is forecast to hit the coast the last half of this week. It will produce heavy rain, heavy (and much needed) snow in the Sierra Nevada, and high winds as it moves through. The main low pressure system will likely come ashore in northern California and southern Oregon early Thursday, but its effects will be felt along the entire west coast.

Forecast surface map for 4:00 a.m. PST Thursday, December 11, 2014.

Gale warnings are in effect are in effect for most of the California coast and storm warnings are in effect for the Oregon and Washington coastal waters. High wind warnings are in effect for inland areas of central and northern California. Blizzard warnings are in effect for the northern Sierra Nevada from Thursday night through Saturday morning.

Watches, warnings, and advisories in effect as of 7:20 p.m. PST December 9.

Rainfall (and water equivalent in the mountain snow) is expected to be heavy with this storm, perhaps in the range of 5 to 8 inches or more.

Quantitative precipitation forecast for the 72-hour period ending 4:00 p.m. PST Friday, December 12

The rain the last two weeks and the rain expected this week certainly helps, but don't look for any big changes to the Drought Monitor week to week. Snowpack in the Sierra Nevada, critical to western water supply, was only 25 percent of normal earlier this month. Reservoirs are far below capacity. Groundwater needs to be recharged as the rain percolates down through the soil. It will take a number of storms over the next several months to make an appreciable dent in the California drought. Whether or not that happens will depend on whether the "door" to the Pacific remains open, and for how long.