What to Know: Potential Megaflood in California

NBC Bay Area’s Rob Mayeda takes a look at the potential seasonal impacts from the Sierra to the rest of California.

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How would California be impacted by a catastrophic flooding event also known as an “ArkStorm” or megaflood?

New research by Science Advances suggests that there will be much higher rain rates and Sierra runoff potential due to a warming climate, as noted by Dr. Daniel Swain’s research. There will also be a risk for more intense storms and an increase of flooding risk due to a warming climate.

The “ArkStorm” scenario refers to a period from December 1861 to January 1862, when a series of power atmospheric river boosted storms impacted California, leading to rapid, overwhelming runoff that left portions of Sacramento and the San Joaquin Valley filled with water, like a giant inland lake.

What might happen if the same scenario were to occur in a warmer climate, capable of holding more water vapor, potentially boosting rain rates and raising snow levels with current and future storms?

For the Sierra, it could lead to a 200% to 400% boost in runoff potential versus previous storms. This scenario also can occur even as the drought becomes more persistent, known by atmospheric sciences like Dr. Swain as "precipitation whiplash."

While there could also be fewer storms and shortened rain seasons the storms may access.

The 2021 rain year supports the outlook, especially when you consider the weather seen in Northern California, more specifically near Sacramento. The city experienced its driest stretch on record followed up by the wettest 24-hour period last year.

All of this occurring within a period of persistent, more frequent drought in the last 10 years.

A new report details the potential impact of megaflood events in California.

For the Bay Area, the storm that brought record daily rainfall to Sacramento, sent rivers running far lower than average to flood stage within 24 to 36 hours. Marin County had 8 to 12 inches of rainfall, which aside from localized flooding was a welcome sight for Marin Water District reservoirs.

The October 2021 atmospheric river storm also set records in the air, with the highest precipitable water content for an October storm reaching the Bay Area.

In total, the storm brought more than 7% of the state’s annual rainfall in a single event as well, according to the Center for Western Weather and Water Extremes at Scripps Oceanography.

As the report suggests, in a warmer and warming climate, future storms may see higher rain rates and higher snow levels, amplifying the runoff and flood risk out of the Sierra.

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