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From major wildfires just over four months ago, and now major flooding, Northern California seems to leap from one perilous state to another. This time, rainfall from a “potent atmospheric river”, as described by the National Weather Service, caused flooding to over 3,000 properties in Sonoma County. This atmospheric river – a flowing column of condensed water vapor pumped up from the Tropics which can be up to 375 miles (603 kilometers) wide – started delivering rain and snow into the region late on Sunday, February 24.

The small town of Guerneville (pop. ~4,500) fared worst, reporting nearly 21 inches (529 millimeters) of rainfall in just 72 hours by 5 p.m. local time on Wednesday, February 27. The source of the town’s flooding was the Russian River, which flows from Mendocino County through to Sonoma County, reaching a maximum level of 45.5 feet (13.9 meters) at Johnson’s Beach, near Guerneville. This exceeded the defined 40 feet (12.1 meters) threshold for a major flood at this point, with local media reports stating that this is the worst flooding since New Year’s Day in 1997, when the river rose to 45 feet (13.7 meters). The nearby Napa River also crested at 26 feet (7.92 meters), one foot above the flood stage.

The town of Guerneville, which was originally built on a meander in the river, on February 27 was declared by the Sonoma County Sheriff’s Office “… [as] officially an island …” as all roads in an out of the town were flooded. 4,000 residents in both Guerneville and Monte Rio (pop. ~1,200) were under evacuation orders until Friday, March 1.

Guerneville: A History of Flooding
Flickr Jessica Merz
Russian River floods at Guerneville, Northern California – picture taken on December 24, 2005. Image credit: Flickr/Jessica Merz

This area is no stranger to flooding, with the Russian River towns nestled in a steep valley near the end of the river’s 110-mile (180 kilometer) length. On Valentine’s Day 1986, after six days of continuous rain, the Russian River crested at Guerneville with a record 48.9 feet (14.9 meters), and local reports state that 1,000 properties were damaged or destroyed. Events in 1995 and 1997 saw the river reach at or over 45 feet (13.7 meters), together with significant events in 2006 and 2016. Less severe floods are commonplace. Just two weeks prior to this event, the town had experienced a flood. One Guerneville resident even recorded a song mentioning how frequently the town floods. But it was the severity of this event that took the area by surprise.

California is no stranger to atmospheric rivers either. NOAA reports that between 30 to 50 percent of annual precipitation on the U.S. West Coast comes from atmospheric rivers (ARs), with just a handful of these “rivers” producing this sheer volume of water. As the plume of water vapor cools over California’s mountain ranges, the vapor turns into rain or snowfall.  In addition to prolific rainfall, snow has fallen in abundance, generating causes for optimism as snowpack is vital for the state’s water supply.

The Sierra Nevada are now at 153 percent average snowpack for the year, with these aggressive ARs in February almost doubling the snowfall from January. At the Department of Water Resources Phillips Station at Sierra-at-Tahoe, snow depth tops 115 inches (2.9 meters), with reports of snow depths at 20 feet in places.

But what was good for snowpack obviously was not so good for the towns along the Russian River, as it is these larger, more powerful ARs that cause problems with extreme rainfall, flooding and mudslides. Factors such as AR-induced precipitation and flooding, as well as snow accumulation and melt, all need to be considered in flood modeling and thus are accounted for in the RMS U.S. Inland Flood HD Model.

The drama of the recent floods temporarily shifted the focus away from the fact that California needs the water. This wet winter has been a state-wide phenomenon. There are tales of snow that has fallen at much lower levels to normal, and snow that has fallen on beaches in LA. The recent heavy rains, cold weather and consistent rainfall since late November last year has now taken the state from being entirely in drought to 67 percent drought-free according to U.S. Drought Monitor.

Dampening Down Wildfire

But will a wet California winter result in a less ferocious wildfire season in the state? Areas close to Guerneville have seen major wildfires over recent years, including the Mendocino Complex fire last summer, and the Wine Country fires in 2017. And Northern California saw the devastating Camp Fire last November. Just three weeks after the Camp Fire, Butte County witnessed flooding as the scared landscape could not absorb as much rain as normal, leading to increased runoff and the potential for flash floods and disastrous mudslides or landslides.

California winters need to be wet, to counterbalance the Mediterranean conditions resulting in very dry summers where little or no rain falls. However, it might also just be the case that a wet winter produces a different type of wildfire fuel. Looking back at the very wet season that ended in early 2017, California saw 159 percent of its average annual rainfall, and excessive rain damaged the Oroville Dam spillway. Even after this wet winter, 2017 then produced a devastating fire season, with 1.26 million acres burned. The California Department of Forestry and Fire Protection (CALFIRE) suggests that wet weather generates an abundance of grass fuels that could burn easy and fast. If combined with a hot, dry summer, this fuel overgrowth can exacerbate the size, spread, and intensity of a wildfire.

This wet winter will help to improve very dry areas, but in terms of wildfire, all eyes will be on whether the California summer will recreate the same conditions seen in the last two summers – hot, dry and windy conditions that interact with a huge range of natural environments across the state, combined with the threat of ignition sources. This wet spell could just mean different fuel types will burn.

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Holly Widen
Holly Widen
Product Manager, Model Product Management, RMS

Dr. Holly Widen is a Product Manager in the Model Product Management team, focusing on the U.S. Flood suite of products. She joined RMS in 2016 upon completion of her Doctorate in Geography from Florida State University, where she studied tornado risk and vulnerability using applied spatial statistics. She has co-authored over ten peer-reviewed journal articles and is a member of the American Meteorological Society and the American Association of Geographers.

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