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Rain forecast from atmospheric rivers poses challenge, but advancements are being made

San Diego was never forecast to get the amount of rain that has fallen in Santa Barbara and some of the LA area over the last 24 hours. But local forecasts had San Diego getting up to an inch. Today, KPBS reporter John Carroll looked into why the rain forecast here fell short.

The forecast said high winds and up to an inch of rain in San Diego Tuesday. The winds showed, but the rain? That was a different story.

How much water you got from this latest atmospheric-river charged storm depended on where you live.

For an example, as of noon Tuesday, San Onofre had received a little over an inch of rain, but San Diego International Airport received just three-tenths of an inch.

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Still, National Weather Service meteorologist Alex Tardy told KPBS we will get more rain before the storm moves out.

“We’ll probably pick up a half inch this afternoon in San Diego, but places like Escondido? Maybe an inch — one inch of rain," Tardy said.

Tardy said it’s tough to make a pinpoint forecasts where an atmospheric river is going to dump heavy rain, until just a few hours before it comes on shore. A difference of just 50 or 60 miles can translate into a significant difference.

“If you take a look at like one location east of LA where they got five inches — six inches of rain, and then you hop down to, let’s say Irvine, where they got just an inch ... That’s just the nature of not just the forecasting, but the nature of how narrow these rain corridors can be," Tardy said.

Roland Lizarondo
The boardwalk at Mission Beach is shown with the Pacific shoreline in the background on January 10, 2023.
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One question asked is whether newer and faster computers could make better forecasts in the future. Tardy said it was possible, but the drive for accurate forecasts is geared toward obtaining better data.

California scientists are now taking a page out of their East Coast counterparts’ playbook, the ones that fly planes into hurricanes. They are flying planes into the atmospheric rivers to learn more about how they form and develop.

Roland Lizarondo
A Lifeguard jet ski sits on the sand at Mission Beach on January 10, 2023.

“And the end result will be what you’re talking about — better data and more precise forecasting with higher confidence and longer lead time," Tardy said.

That longer lead time will allow Californians to prepare for everything from torrential, mudslide-inducing rain to gentle scattered showers.