Atmospheric river event brings flooding, landslides to Southern California (Photo Credit: Matt Roberts)

DISCUSSION: On January 8-9, California received a massive amount of rain across much of the state. However, the rain did not bring all good news as the National Weather Service had issued flood warnings as well as flash flood warnings across much of the Los Angeles and Santa Barbara areas. In addition, the National Weather Service had advised that there would be a slight chance of weak waterspouts in the Pacific Ocean near Los Angeles.  Northern California was not hit as bad as the storm brought weaker than expected winds to the San Francisco Bay Area. Flooding was minimal in the San Francisco Bay Area as it was mainly minor road flooding with a small closure on California Highway 121 due to a risk of landslides.

It was a different story in Southern California on the morning of January 9th, as a flash flood was observed on the Ventura River near where the Thomas fire was occurring. In addition, mudslides have been reported across Southern California resulting in multiple deaths as well as having portions of U.S. Highway 101 closed.  Much of the mudslides have been reported in the Santa Barbara area especially in the higher elevations due to a large amount of rain.
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The heavy rain across the state especially in Santa Barbara was due to an atmospheric river event. An atmospheric river event is when moisture was drawn from the warmer portions of the Pacific Ocean near Hawaii and is transported to the coast of California. This atmospheric river event had very high moisture content in the atmosphere up to about 13,000 feet. In addition, the heavy rain in the mountains are also due to the rain shadow effect as moisture condenses as it climbs over mountains and precipitates as the atmosphere cannot hold as much water as the temperature decreases. Forecast models had indicated a very strong upward velocity at both 700 and 850 millibar pressure levels (9500 ft and 5000 ft respectively) in the Santa Barbara mountains during the time of the heaviest rain.  

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©2018 Meteorologist JP Kalb