MONTHLY WEATHER SUMMARY
By Dr. John Lewis with Laura Edwards
Desert Research Institute and National Severe Storms Laboratory
December 2005

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The first two weeks of December were characterized by the same type of ridging pattern over the western U.S. that was predominant in November. A number of record low temperatures were reported in Central and Southern California on the 6th, including Palmdale (21), Stockton (26), and Santa Barbara (31). From the 7th until the 16th, December was fairly uneventful with few extremes in temperature and little precipitation. More record lows were reported beginning on the 16th, with Campo at 20 and Redding at 24.

By mid-December, however, the mid-tropospheric ridge moved inland and a series of synoptic disturbances moved from mid-Pacific to the Pacific Northwest on paths that skirted the California coast. Although the low pressure centers did not move over the state, the strong surges of moist air on the west side of the disturbances brought abundant precipitation into central and northern California. This deep layer moisture source can be traced to narrow bands (several hundred kilometers wide) that emanated from the equatorial Pacific, well south of Hawaii. This moist air that tracked into California was anomalously warm and thus most of the precipitation in the High Sierra was in the form of rain rather than snow. Many records were shattered on the 18th, particularly in the San Francisco Bay Area, including Kentfield with 5.04" (old record 1.75"), Concord with 3.50" (0.23), and Big Sur with 7.46" (2.60).

On the 19th-27st, warm air preceded the next system, with new high temperature records reported from Eureka to San Diego. Some areas of the Southland reached the upper 70s and 80s. The most records were made on the 21st, with at least 20 reported in the Los Angeles and San Diego areas. It was an exceptionally warm and dry Christmas for many in Southern California.

The persistent rain from four disturbances the second half of the month led to widespread flooding along the north coastal region, in the Sacramento and northern San Joaquin Valleys. This series of disturbances, with the abundant liquid precipitation, bore strong resemblance to the events of late December 1955 when severe flooding occurred in essentially the same areas of California. Daily precipitation records were broken again on New Year's Eve in the Central Coast, Sacramento and Stockton areas.

Page last updated 1/19/05.