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UC San Diego creates new system to rank atmospheric rivers for storms

System is similar to hurricane and tornado ratings
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Posted at 8:34 AM, Feb 14, 2019
and last updated 2019-02-14 15:19:07-05

LA JOLLA, Calif. (KGTV) - Scientists and meteorologists have a new tool to measure atmospheric rivers that roll into the West Coast.

Researchers at UC San Diego devised a ranking system for the atmospheric rivers, using data on how much water vapor they contain and how long they'll be in a particular area.

An atmospheric river occurs when a long, narrow band of heavy moisture is pushed along by storm systems or heavy winds. They typically provide 60 percent of the rain that falls in coastal California each year.

San Diego gets between three and five atmospheric rivers a year, on average.

"They’re trying to rate it in terms of how strong, how rare, how big and ultimately how much impact will it bring," says National Weather Service Meteorologist Alex Tardy, who reviewed the paper on the ranking system before it was published last month.

The system is similar to ones used for hurricanes and tornadoes, with a 1-5 scale, where 1 is a "Beneficial" rain fall and 5 would be "Hazardous."

Meteorologists still haven't put the scale to use this winter because there's debate over how to use it.

Some want it as a forecasting tool, using the data points in the scale to predict what's coming, like a hurricane. Others say you need to see the damage and flooding a storm causes before giving a ranking, like the Enhanced Fujita scale for a tornado.

"Forecasting has a lot of variables," says Tardy. "When we observe it, we can be more objective. We can see how much moisture’s coming, and we can see the wind speeds. What we can’t see is the actual impact."

Still, using the info on the scale as a predictor can help people prepare and let first responders know what range of damage to expect. It will also allow people to make an easier comparison to past storms.

"I think in general, the colors and the values might be a little easier to grasp than saying, 'Hey, in San Diego, we’re going to get 2.2 inches of rain,'" says Tardy.

The National Weather Service is still working out the particulars of the system. They hope to begin using the scale in forecasts starting next year.