California is under an atmospheric river, or ARK storm with the worst to come
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California is under an atmospheric river or ARK storm with the worst to come. This is a string of storms that battered California on Wednesday and Friday, and will continue through Sunday. Forecasters had warned of a string of intense tropical storms that would pass through northern California over a five-day period, but this is something that has not been seen in a long time.
The first storm arrived on Wednesday, bringing wind gusts of up to 40 MPH and substantial rain.
The second storm arrived on Friday, dumping substantial amounts of rain on top of saturated ground and bringing up fears of localized flooding in certain flood prone areas.
The third storm is expected to deliver the worst soaking and will last through Sunday. All three storms are expected to have dumped 4 to 8 inches in the valley and 12 to 18 inches of rain in the Sierras.
Three unusual events are going on. First, this weather pattern is normally not seen before January. Atmospheric rivers are massive columns of warm, tropical water that are picked up from the Pacific Ocean, carried for thousands of miles, and then dumped on California. In other words, this is an atmospheric river.
Second, atmospheric rivers can rapidly melt an existing Sierra snow pack. If there is no snow pack, the water will drain off right away. The current string of storms can inundate the valley, while the Sierras will be unable to freeze the rain and serve as cold storage for a slower spring snow melt and controlled runoff. Whatever these storms dump, it all will start to drain right away.
Third, the natural watershed that drains the massive Sierra mountain range will bring most of Northern California’s water to the Feather and American rivers. Those two rivers meet up at Sacramento, where the water either heads out through the delta to San Francisco Bay or it is sent southward via the central canal. Add in thousands of creeks, streams, sloughs and tributaries, and a lot of trouble can happen in a relatively short period.
Minor atmospheric rivers are traditionally an annual event in California. Those are called the “Pineapple Express”. However a major atmospheric river can be a huge problem. Scientists refer to a devastating atmospheric river as an “ARK storm” that could flood part of California’s central valley to a 20 foot depth.
The last ARK storm occurred in the 1800s and was estimated to have dumped several times the volume of the Mississippi River during 40 straight days of rain.
The United States Geological Service (USGS) has officially designated the current series of storms as an atmospheric river.
The USGS also discusses the ARk Storm Scenario, a major experiment that simulated the worst that could happen.
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