Epic Storm Pounds Southern California With Heavy Rain, High Wind and Huge Waves

The epic assault of the winter of 2016 - 2017 on California continued on Friday morning as torrential rains, damaging winds and huge waves from a massive and powerful Pacific storm system pounded the Golden State. Carrying with it an “atmospheric river” of moisture originating in the subtropics near Hawaii, Friday’s storm was most dangerous because of its heavy rains. According to the National Weather Service in Oxnard, California, Friday and Saturday could be the heaviest two-day rainfall event for the entire L.A. Basin since Dec. 19 - 20, 2010. Rainfall amounts of 2 - 6 inches are expected along the coast west of Los Angeles, from Santa Barbara to Ventura, with 5 - 10 inches likely in favored foothill and mountain locations. As of 7:10 am PST Friday, Santa Barbara had received 0.97” of rain, causing a sink hole to open up on Olive Street. The coast about 50 miles northwest of San Luis Obispo had received over 3” of rain as of 9 am PST, according to radar estimates. Soils are already wet in Southern California from previous storms, and the heavy rain is likely to runoff quickly and create dangerous mudslides, rockslides and flash floods. In wildfire burn-scarred areas, debris flows are likely. In anticipation of debris flows, an Evacuation Warning has been issued for the greater Sherpa Fire Burn area inland from El Capitan Beach State Park. In the high mountains, the precipitation will fall as snow, with 1 - 2 feet of snow expected above 8,000’ elevation.

Strong winds were sweeping the area Friday morning, with numerous wind gusts in excess of 50 mph in the foothills of Los Angeles County and Santa Barbara County. The top wind gust as of 7 am PST Friday was 60 mph, recorded at both Grass Mountain and Mill Creek in Los Angeles County. Wind gusts as high as 75 mph are predicted at higher elevations near San Diego. The strong winds were being driven by the extremely low central pressure of the storm. According to a tweet by NWS San Diego, the storm’s expected central pressure of 986 mb on Friday evening, when it will be over San Francisco, will rank as the lowest pressure of any Central California coastal storm during the past 30 years (for the period February 7 - February 28.)


Figure 1. An atmospheric river of moisture extended from Hawaii to Southern California at 6 am PST Friday February 17, 2017, as seen in this satellite-derived measurement of Total Precipitable Water (TPW)—the total amount of water that would fall on the ground if one were to condense out all of the water vapor in the atmosphere. Image credit: University of Wisconsin SSEC.


Figure 2. Predicted 66-hour rainfall amounts in southern California ending at 10 pm Saturday, February 18. Image credit: NWS Los Angeles.

California’s beaches receiving a pounding
Today’s storm is bringing south to southeasterly gale-force winds to the coastal waters of Southern California, which is churning up huge battering waves. At Santa Rosa Island in Channel Islands National Park, about 100 miles west of Los Angeles, sustained winds as high as 41 mph, gusting to 60 mph, were recorded between 5 - 8 am PST Friday. Significant wave heights of up to 11.2 feet were recorded Friday morning at buoy 46054, located 44 miles west of Santa Barbara. Breaking waves of 6 - 9 feet are expected along the coast of Santa Barbara on Friday, increasing to 10 - 14 feet on Saturday. These battering waves, which are riding up on top of a storm surge of more than one foot in some locations, will cause beach erosion and dangerous swimming conditions. Fortunately, due to the phase of the moon (third quarter), water levels along the Southern California coast are about two feet lower than would have been the case if this storm had hit during a full moon. Thus, coastal damage is not likely to be severe.


Figure 3. Big waves affecting beach access and ocean bluffs at the Isla Vista, California study site in March 2016. Image credit: David Hubbard, UCSB.

California beaches received a record pounding during the winter of 2015 - 2016
This week’s storm comes on the heels of unprecedented coastal damage wrought along portions of the California coast by the El Niño-fueled storms of the winter of 2015 - 2016. According to Patrick Barnard, a geologist with the U.S. Geological Survey and lead author of a February 2017 paper, Extreme oceanographic forcing and coastal response due to the 2015–2016 El Niño, published in the journal Nature Communications, winter beach erosion in California was 76 percent above normal, and most beaches were eroded beyond historical extremes. Added David Hubbard, a UCSB marine ecologist and paper co-author, “The winter wave energy equaled or exceeded measured historical maximums along the West Coast, corresponding to extreme beach erosion across the region. The waves that attacked our coast, generated from storms across the North Pacific, were exceptional and among the largest ever recorded. But the lack of rainfall means that coastal rivers produced very little sand to fill in what was lost from the beaches, so recovery has been slow." If strong El Niño events become more common in the future, as some studies suggest, the California coast will become increasingly vulnerable to storm damage. Sea level rise will make these damages even greater. The authors concluded that “Water level anomalies of 7–17 cm above normal were measured across the US West Coast during the El Niño winter of 2015–2016, similar to anticipated global mean sea-level increases expected within the next few decades. Therefore, the 2015–2016 El Niño also provides an indication of future background coastal water-level conditions and the associated beach hazards that will become more common during typical winters.”

Jeff Masters