[californiadisasters] Super Soggy December (2010)

Super Soggy December

by Dave Bruno ~ NWS Los Angeles/Oxnard

Thanks in large part to a huge upper level low pressure system that brought rain to the region from the 17th through the 22nd, December 2010 will go down in the record books as one of the wettest, and in many cases the wettest, Decembers in history across southwestern California. Most of southwestern California received between 3 and 6 times the normal rainfall for the month, or 300 to 600 percent of normal.

The first few days in December 2010 were generally mild and dry across the region. A storm system moved into central California on the 5th. Its associated front brought rain to the region on the 5th into the early morning hours of the 6th. Rainfall totals generally averaged one third of an inch to three quarters of an inch across the region. Gusty south winds affected the mountains, with wind gusts of 40 to 50 mph common from the afternoon of the 5th through the early morning hours on the 6th.
Dry weather returned on the 6th and continued through the middle of the month.

A dramatic change in the weather pattern began after the 16th. A large upper level low pres-sure system moved into the northeastern Pacific, well off the coast of the Pacific Northwest on the 17th, then lingered there through the 21st. The circulation around the southern side of this upper low pushed very moist air with a subtropical con-nection into the region. This ribbon of moisture remained focused across southern California from the 17th through the 21st, bringing almost continuous rain to the region during that time. Several impulses rippling through the moist subtropical flow caused periods of heavy rain at times. On the 21st, the upper low began to shift eastward. Its associated strong frontal system moved across southwestern California late on the 21st through the 22nd, putting an exclamation point on what was a very wet week.

Many daily rainfall records were set across the region from the 17th through the 22nd. Some noteworthy records include the following:

On the 18th, daily rainfall records included 3.19 inches at Santa Maria airport and 2.80 at San-ta Barbara airport, both of which also set new rec-ords for any day during any month of December.
On the 19th, daily rainfall records included 2.80 inches at downtown Los Angeles, which was its 8th wettest December day ever. At Santa Barba-ra airport, the total of 2.79 inches set a new daily record. It was also its 2nd wettest December day ever, the wettest having been set the previous day. The daily total of 2.07 inches at Long Beach air-port set a new record for the day, and was its 3rd wettest December day ever. At Los Angeles air-port, the daily total of 2.23 inches set a new daily record, and was the 4th wettest December day ever.
On the 20th, a new daily rainfall record was set at Long Beach airport with 2.03 inches, its 4th wettest December day ever.

The week long siege of rain brought periods of flooding to the region, along with some mud and debris flows and rock slides, the most serious of which occurred on the 22nd. Significant flooding of the transition road between interstate 10 and high-way 57 was reported during the early morning of the 22nd, with a large amount of water in lanes and mud coming down from the hill sides. A small mountain side collapsed onto Little Tujunga canyon road at Sand Canyon Highway during the late morning hours, and rocks and debris covered all lanes at one point. During the afternoon hours of the 22nd, all lanes of Big Rock Creek Road in the Antelope Valley were covered by flooding and debris, and mud and heavy flooding was reported on an onramp to the 10 Freeway east of downtown Los Angeles. Three cars were trapped in flood waters on Avenue H east of Lancaster during the early afternoon hours, and five people had to be rescued. A funnel cloud was reported near the San Pedro area at 240 pm on the 22nd.

Rainfall totals for the week long storm were impressive, averaging 5 to 10 inches in coastal and valley areas...and 10 to 18 inches in the foothills and mountains, with local totals as high as 24 inch-es in the San Gabriel Mountains and the mountains of western Ventura County.

After a couple of dry days, another front swept through the region late on the 25th and into the 26th. The front brought rainfall totals of one half inch to one inch in most areas with locally higher totals, especially in the foothills.

The final weather system of the month swept through the region late on the 28th and 29th. Rainfall for this system ranged from 0.75 inches to 1.50 inches north of Point Conception, and between 0.50 and 1.00 inches south of Point Conception, with locally higher amounts in the foothills.

In downtown Los Angeles, the 10.23 inches of rainfall received in December was the second most in any December since records began in 1877. It also made December 2010 the 14th wettest of any month in history at the site. Rain-fall in December 2010 was equal to nearly 70% (67.5%) of the entire seasonal normal for downtown Los Angeles, which is 15.14 inches.

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View entire article here: http://www.wrh.noaa.gov/lox/spotter/documents/Newsletter_april2011.pdf

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