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Weather Wednesday: Where is California's rain coming from?

Find out about the weather pattern that's leaving the West Coast soaked.

KNOXVILLE, Tenn. — Since before the New Year began, the West Coast, especially California, has been barraged by back-to-back storms. This is due to weather phenomena called atmospheric rivers.

So what are atmospheric rivers? The National Weather Service defines them as "relatively long, narrow regions in the atmosphere, like rivers in the sky – that transport most of the water vapor outside of the tropics." If you've ever heard of the Pineapple Express, this is a type of strong atmospheric river that runs from Hawaii to the U.S. West Coast. 

They're somewhat narrow for a weather pattern, getting up to 500 miles wide, and quite shallow at only a few miles deep, but they can be thousands of miles long. The largest atmospheric rivers may carry almost 10 trillion gallons of water per day, the equivalent of two Amazon rivers.  

Most of the moisture remains as water vapor until it makes contact with the coastland. When the low-lying clouds are forced to rise over the Sierra Nevada mountains, rain is produced and falls over the Central Valley of California. The most destructive atmospheric rivers are those that stall over one area and pour on for days. These are the ones that can dump feet of rainfall and cause massive flooding and widespread landslides. 

While there has been dangerous flooding throughout the region, the precipitation from these storms is an essential source of rainfall and snowpack that is used year-round. 30 to 50% of West Coast rainfall comes from weaker atmospheric rivers.

A team of meteorologists, F. Martin Ralph et al., developed a scale based on water vapor content and duration, published in the Bulletin of the American Meteorological Society, Vol. 100. This can help meteorologists properly convey the benefit to the land vs the danger to the public. It reads as follows: 

  • AR 1 (Weak): Primarily beneficial.
  • AR 2 (Moderate): Mostly beneficial, but also somewhat hazardous. 
  • AR 3 (Strong): Balance of beneficial and hazardous.
  • AR 4 (Extreme): Mostly hazardous, but also beneficial.
  • AR 5 (Exceptional): Primarily hazardous.

Take a look at just how much rain the state has gotten in the past three weeks.

Credit: Tevian Whitehurst

More than 16 inches of rain in Oakland, which averages out to 5.5 inches per week – in an area that’s used to getting none. Records are breaking almost daily, and many cities are reaching their annual rainfall in just this month.

Credit: Tevian Whitehurst

The rain has also worked wonders on drought conditions – areas of extreme drought have diminished almost entirely.

Credit: Tevian Whitehurst

The snowpack in the mountains is recovering as well, as some spots have gotten nearly 100 inches of snow. Overall snow depth is now reaching back closer to 200+ inches.

These wet conditions have yet to let up, so we can only hope that everyone out there is staying safe and the state can make use of the excess water.

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