CURRENT ISSUE: December 2005
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Floods of Decembers Past
By Laura EdwardsWestern Regional Climate Center
Last month's weather brought an eerily familiar feeling to many who experienced the rains and floods of Decembers past, in 1955 and 1964, and New Years 1997. Fortunately, December of 2005 was not as deadly, and communities were better prepared.
There were many similarities in the synoptic, or large scale conditions, of December 2005 and 1955. In 1955, a ridge was positioned just east of California, instead of its climatological location in the eastern Pacific, and settled there for an extended period of time. This stationary pattern is also called a blocking pattern. This allowed multiple low pressure systems to progress inland and release precipitation over California. Similar conditions were set up in 2005, with an inland ridge.
Figure 1 (right). Marysville was protected by its levees in December 1955. Unfortunately many evacuees went to Yuba City, where a levee gave way. http://americahurrah.com/Flood55/Marysville.htm
Another similarity between 1955 and 2005 is the extreme wet conditions in the central and northern portions of the state and dry in the south. In December 1955, numerous stations south of a line from Bishop to Los Angeles reported below average precipitation, and as low as 0% in the far southeast. Last month brought dryness to the same area, with even more stations reporting 0% or near 0% of average precipitation in the Southeast Desert climate division.
The flooding in both years was caused by rain, but for somewhat different reasons. In 1955, the existing snowpack was melting due to the warm temperatures, in addition to the extreme rain events in the middle of that month. At times in the higher eleveations, snowpack was reported to melt 4 inches an hour during heavy rainfall. In 2005, snowpack was below to near average due to many of the previous storms bringing rain instead of snow, even at elevations up to 9000 feet. The system that came in on New Year's Eve 2005 dumped several inches of rain on already saturated soils. In 1964, a third case was presented when an already frozen ground could not absorb rainfall, so heavy precipitation simply ran off into the streams and rivers.
These warm temperatures and very wet storm systems were tapped from far in the Pacific, south of Hawaii. The moisture comes as rain in many of these similar type of synoptic cases. Luckily last month, California had the added advantage of flood protection in reservoirs. Since the floods of 1950 and 1955, new reservoirs, designed in part for flood protection but also water supply, have been constructed, such as Folsom, Don Pedro, New Exchequer, and New Melones. These dams may well have spared dozens of lives in December 2005 and early January 2006, as 67 people perished in the 1955 floods.
One climate factor that may also have played a role in these floods is the El Nino-Southern Oscillation. In 1955, a strong La Nina event was evident, and 1964's flood was in the midst of a weak-moderate La Nina. This season again, the Climate Prediction Center and others have told us of weak La Nina conditions devloping in the equatorial Pacific.
Figure 2 (left). April Adkins of Guerneville, December 30, 2005. http://www.cbsnews.com/stories/2005/12/29/national/main1168973.shtml
Streamflows were extraordinary in December 2005. Guerneville was in the news over the New Year's weekend, as the Russian River overran its banks. Early on January 1, 2006, the maximum streamflow was set at 85,800 cubic feet per second, with the gage height at 44.2 feet. On December 23, 1955, the Russian River at Guerneville peaked at 49.7 ft, 5.5 feet higher than this last flood. Most gages were all still below the peak of the 1955 floods. Twitchell Island's levees were overtopped due to high winds sustained at more than 60 mph.
Despite having the 4th wettest December on record in the northern Sierra Nevada, this author has not heard report of any deaths due to the events of last December. Fortunately, lessons have been learned from previous floods to prepare communities and protect lives. Months like these make it very interesting to be a climatologist. 50 years sometimes seems like a long time until the next "big one", but this climatologist think it's worth the wait.
References:
1. U.S. Department of Commerce, Climatological Data, December 1955, Vol. LIX, No. 12.
2. Cole, Robert O. and John P. Scanlon, 1955. Flood-producing rains in Northern and Central California, December 16-26, 1955. Monthly Weather Review, December 1955, pp. 336-347.
3. Andrews, James F., 1955. The weather and circulation of December 1955. Monthly Weather Review, December 1955, pp. 327-335.
4. Daily Weather Maps, November 28, 2005-January 1, 2006. http://www.hpc.ncep.noaa.gov/dailywxmap/index.html
5. USGS Real-Time Water Data, http://water.usgs.gov.
Last updated 1/19/06.