Nonstop Rain Inundates Southern California
by Alex Tardy and Miguel Miller
by Alex Tardy and Miguel Miller
A plume of moisture across the east Pacific began to develop during the middle of December. Light precipitation began across the San Bernardino Mountains as early as the 15th, but then in-creased over the mountains and in the lowlands and coast on Thursday and Friday December 16th and 17th. By Friday night, 1 to 3 inches of precipitation had occurred across the San Bernardino Mountains and snow levels were near 8000 feet.
Steadier widespread rainfall began on the 17th and 18th across Orange County and the Inland Empire. Rainfall rates up to 0.30 inches per hour oc-curred across the south facing slopes of the San Bernardino Mountains. By the evening of Sunday the 19th, total pre-cipitation reached near 10 inches in Lytle Creek and the first significant flooding commenced. Runoff from the mountains also sent high waters into the Whitewater drainage bringing down boulders and large amounts of mud. Satellite imagery depicted anonymously high moisture levels across Southern California that extended southwest
into the tropics.
On Monday December 20th, the low pressure system remained anchored over the eastern Pacific and had moved little, but began to move towards the West Coast tapping into
the high moisture plume that existed across Southern California. Heavy rain moved into Orange County and spread southward over San Diego County. Rainfall rates increased
significantly to 0.50 to 0.75 inches per hour across the high terrain and up to 0.25 inches in the valleys. This caused more flooding on Lytle Creek where it left its banks and caused water to cross roads. Landslides resulted Monday and Monday night and several rescues were required due to high water flows across the entire area. Snow levels on Monday remained above 8000 feet but lowered to 6500 feet on Tuesday. Total precipitation amounts reached just over 20 inches in parts of the San Bernardino Mountains and 2 to 5 inches in San Diego County. The last significant and heaviest surge of precipitation moved into extreme southwest California during Tuesday afternoon and continued into Wednesday morning December 22. This rainfall brought 2 day totals just over 4 inches in downtown San Diego and 5 to 10 inches to most areas. The steady rainfall tapered
off Wednesday morning with additional showers and isolated thunderstorms that continued into Wednesday evening as the final cold front swept through the region. Significant flooding occurred in San Diego County Tuesday night and Wednesday when the San Diego River rose to just over 14 feet, the highest rise since 1980.
Major landslides and flash flooding impacted the communities of Laguna Beach, Apple Valley, along the Whitewater Channel in the Coachella Valley near Palm Springs, Highland, Corona, Loma Linda, La Jolla, and the city of San Diego from Tuesday into early Wednesday December 22th. Rainfall totals for the entire week of 5 to 10 inches were widespread along the coast and inland valleys. The mountains received totals of 10 to 28 inches, highest in the Lytle Creek drainage and Crestline area. Heavy wet snow accumulated above 6500 feet with amounts over 6 inches and as much as 2 feet above 7500 feet.
Some Background
This plume of moisture has been called the "Pineapple Express" or "Pineapple Connection" in the past, and has been called colloquially the "hose". But recent research names this phenomenon an "atmospheric river". California meteorologists have long known that these "rivers" can set up across the ocean for days. They are not unusual, and a few times a decade on aver-age they will sock Southern California. One such river produced more than 40 inches of rainfall in the mountains of southern California in only four days in early January 2005. That, in turn, caused a massive mud-slide in La Conchita, Calif., that took 10 lives and pro-duced widespread flooding. Many atmospheric rivers form in the mid-latitudes when cold fronts concentrate mois-ture into narrow ribbons as it is transported toward the poles. Occasionally, atmospheric rivers tap moisture directly from the tropics. In these situations, the likelihood of devastating rainfall and flooding—such as what occurred in La Conchita—increase significantly. The above graphic illustrates the isolated nature of this particular weather problem. California is the national capital of multi-day precipitation totals that exceed 16 inches.
Source: http://www.wrh.noaa.gov/sgx/newsletter/current-newsletter.pdf
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