In mid-December, a large upper level low pressure system developed over the northeastern Pacific Ocean. This low pressure system tapped into a subtropical moisture source, bringing an extended period of significant rainfall to Central and Southern California from the 17th through the 22nd. Numerous daily rainfall records were set during the period. By the end of the event, rainfall totals ranged from 5 to 10 inches across coastal areas to between 10 and 18 inches in the foothills and mountains with even local amounts as high as 24 inches. With such significant rainfall, numerous reports of flooding were reported across the area, including several mountain roads being washed out. Due to the subtropical nature of the air mass, snow levels remained rather high through the event although between 12 and 24 inches of snow fell at resort level.

Flash Flood in LOS ANGELES, CALIFORNIA
From 22-DEC-10 15:36:00 to 22-DEC-10 18:00:00 (PST-8)
Location: VALYERMO
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Heavy rainfall produced flash flooding in the mountains of Los Angeles County. California Highway Patrol reported that Big Rock Creek Road at the intersection of the Angeles Crest Highway was washed out by flash flooding and mud flows.

Flash Flood in LOS ANGELES, CALIFORNIA
From 22-DEC-10 14:49:00 to 22-DEC-10 17:00:00 (PST-8)
Location: DOMINGUEZ
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Heavy showers produced significant flash flooding in south central Los Angeles County. California Highway Patrol reported significant flooding at the intersection of Vermont Avenue and West 190th Street with over one foot of water and numerous vehicles stalled in the high water.

Winter Storm in LOS ANGELES COUNTY MOUNTAINS EXCLUDING THE SANTA MONICA RANGE, CALIFORNIA
From 22-DEC-10 17:00:00 to 22-DEC-10 17:30:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Significant snowfall was received at the resort level (above 7000 feet) across the mountains of Los Angeles County. Mountain High resort reported between 12 and 24 inches of new snowfall. Along with the snowfall, southeasterly winds gusted between 45 and 55 MPH and produced dangerous winter storm conditions.

Winter Storm in VENTURA COUNTY MOUNTAINS, CALIFORNIA
From 22-DEC-10 17:00:00 to 22-DEC-10 17:30:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Significant snowfall was received at the resort level (above 7000 feet) across the mountains of Ventura County. Based on surrounding reports, high elevation areas received over 12 inches of snowfall. Along with the snowfall, southeasterly winds gusted between 45 and 55 MPH and produced dangerous winter storm conditions.

Flash Flood in LOS ANGELES, CALIFORNIA
From 20-DEC-10 20:00:00 to 20-DEC-10 20:30:00 (PST-8)
Location: LLANO CRYSTAL ARPT
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A weather spotter reported flash flooding and debris flow near the community of Wrightwood. Flume Canyon Road near the intersection with the Angeles Crest Highway was completely washed out by the heavy rainfall.