Huge Rainstorm Poised to Ease California's Thirst

The wettest storm to affect the U.S. West Coast since 2009 is gathering strength over the Pacific Ocean, and promises to bring much-needed drought relief to thirsty California Wednesday through Friday. Rainfall amounts of 3 - 8 inches are expected over most of Northern California, with snowfall amounts of 1 - 3 feet predicted in the Sierra Mountains. As noted by Wunderground weather historian Christopher C. Burt in his Monday post, California Drought Situation Improves, this week’s storm may be the strongest and wettest storm to hit the region since October 2009, when the last major ‘pineapple express’ soaked the state. California is already benefiting from widespread heavy rains that fell November 29th through December 6th, and most of California is now running a seasonal precipitation surplus—the first time they’ve seen such since December 2012.


Figure 1. Predicted precipitation for the 7-day period ending December 16, 2014. Image credit: NOAA.


Figure 2. December 9, 2014 values of Total Precipitable Water—the amount of rain that would fall if one condensed all the water vapor in a column of air over one’s head. A very moist flow of air originating in the subtropics and flowing past Hawaii—the “Pineapple Express”—was headed towards the U.S. West Coast. Image credit: University of Wisconsin.


Figure 3. Observed snow pack in California’s Sierra Mountains averaged just 25% - 45% of normal for the date on December 8. Image credit: California Department of Water Resources.

Insufficient snow still a concern
This week’s rains will further improve the dire drought situation in California, though not as much as one might expect, since near-record warm temperatures have been reducing the amount of moisture falling as snow. A heavy snow pack in the Sierras is critical to reducing drought in California, since melting snow provides a much-needed source of water in the subsequent summer dry season. The first week of December was one of the warmest on record for much of California, with the average temperature running about 10°F above normal. This was due, in part, because of near-record sea-surface temperatures along the Californian Pacific Coastline, ranging from 59°-63°F—at least 5°F above average, and close to record warmth for this time of year. Snow levels during last week’s storm were generally above the 7,000-8,000’-foot level, and the Sierra snow pack on December 8 was just 25% - 45% of normal for the date. Snow levels from this week’s storm will start off above 7,000’, but will then lower to 5,000’, as much colder air than was seen during last week’s storm moves into the state. Hopefully, this will put the Sierra snowpack close to normal depth by mid-December.

Jeff Masters