Three atmospheric rivers aim U.S. West, heavy rain and higher elevation snow expected

A potent weather system will bring rain, locally, heavy with higher elevation snow to portions of California, southwestern Oregon, and the Northern Rockies. As much as 60 cm (2 feet) of snow may blanket the California mountains above 1.8 km (6 000 feet). Rain and snow will also impact much of the Inter-mountain West. 

This will be the first of three expected waves of rain to hit the region over the next 7 days.

A developing upper-level low will move onshore over Northern California by Thursday morning, May 16, continuing to move inland to the Great Basin by Friday, May 17, NWS forecaster Ziegenfelder said.

The system will produce rain over parts of Northern California that will expand into parts of the Pacific Northwest/Northern Intermountain Region and Central California/Great Basin by Wednesday evening.

Overnight Wednesday, May 15, the rain will move into Southern California and heavy snow will occur in the higher elevations of the Sierra Nevada. Rain will expand into parts of the Northern Rockies and Eastern Great Basin with showers and thunderstorms developing over parts of the Northern High Plains by Friday.

Snow will continue to fall over the Sierra Nevada and in some of the higher elevations of the Great Basin through Friday. 

"We’ve got an active storm track moving in," said Marty Ralph, director of the Center for Western Weather and Water Extremes at UC San Diego. "By May standards, the rain will be substantial."

Atmospheric rivers normally occur in the winter months, but every other year or so, they extend into May, Ralph said, adding that he isn’t aware of any unusual conditions in the Pacific that are generating the latest wave.

Since October, Washington, Oregon and California have received 47 atmospheric river storms, with 12 of them being strong or extreme, he said.

Featured image credit: GFS/TropicalTidbit