The ninth atmospheric river in a three-week series of major winter storms was churning through California on Monday, leaving mountain driving dangerous and the flooding risk high near swollen rivers even as the sun came out in some areas.

Heavy snow fell across the Sierra Nevada and the National Weather Service discouraged travel. Interstate 80, a key highway from the San Francisco Bay Area to Lake Tahoe ski resorts, reopened with chain requirements after periodic weekend closures because of whiteout conditions.

“If you must travel, be prepared for dangerous travel conditions, significant travel delays, and road closures,” the weather service office in Sacramento said on Twitter.

The University of California Berkeley Central Sierra Snow Lab tweeted Monday morning that it had recorded 49.6 inches (126 cm) of new snow since Friday.

A backcountry avalanche warning was issued for the central Sierra, including the greater Tahoe area.

A barrage of atmospheric river storms has dumped rain and snow on California since late December, cutting power to thousands, swamping roads, toppling trees, unleashing debris flows, and triggering landslides. Monday’s system was relatively weak compared with earlier storms, but flooding and mudslide risks remained because the state was so saturated, forecasters said.