Another Megastorm is Due for California

Joseph Phillips | | WeatherWeather
Atmospheric river, megastorm to western us
World map of precipitable water. Arrow showing the direction of 1862 atmospheric river. Photo: NOAA

If you’ve never heard of California’s 100-200 year storm, you’re not alone. The last megastorm dumped 10+ feet of rain and snow on California during a 43-day ordeal. The atmospheric river, shown above, began in December of 1862 and lasted until mid-January. Skip forward 156 years and we haven’t seen anything near this level of intensity. The megastorm decimated much of the western United States all the way from Washington and Idaho down to Arizona.

Megastorm that hit california
Atmospheric river in late February 2019. Photo: SFGate

An atmospheric river is a unique climate event where moisture concentrates in the tropics and is rapidly transported by abnormal atmospheric conditions. The California winter of 2018/19 saw a couple of these events, as shown above, but nothing as extreme as the rains of 1862. Although the megastorm, nicknamed ARkSTORM, likely dumped incredible powder in the Sierra Nevadas, the storm flooded most of California’s Central Valley in feet of water.

Central Valley is a giant bowl surrounded by California’s mountain ranges. Credit:

The Central Valley of California is essentially a huge bowl trapped between the Sierra Nevadas and the Coastal Range. The only outlet is through the San Francisco Bay. This valley is widely known as the most fertile and productive agricultural region in the country, and that is because yearly floods spread fertile sediment across the basin. However, as humans settled in the valley, we built dams, levees, more dams, and other water diversions to prevent flooding in a normal year of rain. The megastorm toppled most of the water infrastructure and created a massive lake throughout most of the Central Valley.

megastorm floods sacramento
Downtown Sacramento during the flood of 1862. Photo: Weather Channel

The 300 mile-long freshwater lake slowly drained through the San Francisco bay, but only after causing massive destruction. Most cities in the valley were underwater, and the governor reportedly commuted in a rowboat from his governor mansion to the capital. Our water infrastructure today is better than it was in the 1800s, but would definitely fail if another ARkSTORM hit.

It is a question of when, not if. Just how the San Andreas fault in Central and Southern California is “overdue for a big quake”, California is due for another Megastorm.

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