MONTHLY WEATHER SUMMARY
By Bill Mork
California State Climatologist, California Department of Water Resources
December 2004

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December 2004 was warmer than normal in much of the State except close to normal in Southern California.  The greatest departures from normal were in the Sacramento Drainage and Northeast Interior where average temperatures were mostly 4 to 5 degrees above normal and as much as 7 degrees above normal at Markleeville, Shasta Dam, and Susanville.  This warmth was caused by a strong upper level ridge of high pressure which hung out near the West Coast December 10 - 26 with long periods of fog in the Central Valley and occasional spurts of Santa Ana winds in the Southland.

However, the month will be longer remembered by the strong Pacific storms in Northern and Central California on December 6 - 9 and even stronger storms up and down the Golden State on December 27 - 29 with record rainfall pelting the Southland on December 28.  Little did we know that those heavy rains in late December would set the stage for devastating landslides in January as even heavier rains pounded Southern California the first 10 days of the New Year.   Monthly  precipitation totals ranged from below normal in the Northeast Interior to well above normal in Southern California and the Southeast Desert.  The greatest percentage of normal precipitation was in downtown Los Angeles where the monthly total of 8.77 inches was 459 percent of the normal of 1.91 inches.  The Northern Sierra 8-Station Precipitation Index had 10.9 inches liquid for the month, 130 percent of average.

A strong Pacific weather system followed by a warm advection pattern with subtropical moisture brought heavy rain and snow to Northern and Central California December 6 - 9.  Largest remote sensor precipitation totals include 12.00 inches at Elk Valley in the Smith River basin, 11.48 inches at Honeydew in the Mattole River basin, 9.86 inches at Brandy Creek west of Redding, 9.32 inches at Brush Creek in the Feather River basin, 8.76 inches at Leggett in the Eel basin, 8.44 inches at Venado in the Russian River basin, and 5.31 inches at Blue Canyon in the American River basin.  Cooperative observer and city totals include 9.64 inches at Shasta Dam, 7.96 inches at Paradise, 7.11 inches at Crescent City,  6.18 inches at Kentfield, 5.22 inches at Mount Shasta, 5.10 inches at Grass Valley, 4.95 inches at Santa Cruz, 2.88 inches at Fairfield, and 1.12 inches at Sacramento.  Strong south to southeast winds on December 6 - 7 produced wind gusts to 87 mph at Los Gatos, 82 mph at Kregor Peak, 58 mph at Mt Tamalpias, 56 mph at Angel Island and Redding, 48 mph at Golden Gate Bridge, 47 mph at San Francisco International Airport, and 41 mph at Sacramento International Airport.

A strong Pacific weather system brought heavy rain, mudslides, and high winds to Southern California December 27 - 29.  Largest storm totals include 16.11 inches at Upper Matilija Canyon in Santa Barbara County, 14.79 inches at Opids Camp in Los Angeles County, and 14.49 inches at Old Man Mountain in Ventura County.  Downtown Los Angeles had 6.85 inches of rain during this period.  The 5.55 inches which fell on December 28 made that the wettest calendar day in December since weather records began in July 1877 in downtown Los Angeles. This was also the third wettest calendar day in downtown Los Angeles weather history and the wettest day since 5.71 inches fell on January 26, 1956.  The wettest calendar day in downtown Los Angeles was March 2, 1938 with 5.88 inches.

The late December storm produced 3 to 5 feet of snow in the Sierra Nevada with some locations buried with 7 to 8 feet of snow.  The heavy snow helped the water supply in western Nevada but also hindered transportion in and out of Californa as up to 27 inches of snow fell in Reno.  By December 28, a deep low pressure area squatted off the coast of Oregon and produced strong west to southwest winds aloft which gave very heavy, orographic snowfall to the Sierra Nevada.  By December 31, the weather observer at the Central Sierra Snow Laboratory at Donner Summit had measured 93 inches of snow. The 6.3 inches liquid made the water content about 15 percent.  Seasonal snowfall by the end of the month and year had reached over 200 inches compared to an average of about 120 inches.

Page last updated 1/18/05.