A tornado warning was issued for the first time in San Francisco as thunderstorms swept across the region, toppling trees and causing severe power outages.

The warning was issued just before 6 a.m. local time (2 p.m. GMT) on Saturday for northern San Mateo County and parts of downtown San Francisco.

According to the PowerOutage website, the storm caused power outages for more than 221,000 people in northern and central California.

The city last experienced a tornado in 2005, but it struck without a clear radar signature, so no warning was issued in that case, National Weather Service (NWS) meteorologist Roger Glass told The Associated Press.

More than a million residents in San Francisco and its suburbs woke up Saturday morning to alerts on their cellphones urging them to “seek shelter now.”

Winds of up to 60 miles per hour (96 km/h) swept through the area, the NWS said.

But a few minutes later, NWS Bay Area said the warning had passed and the storm had moved out of downtown.

Authorities assessed the damage to determine whether a tornado had occurred.

The last time a tornado warning was issued for nearby San Mateo County was in 2011.

The NWS Bay Area warned residents to continue to stay away from the coast over the weekend as a coastal flood warning is in effect until Monday.

San Francisco’s tornado warning came just a week after the city received a tsunami warning for the first time

The brief warning was issued for northern California and southern Oregon after a magnitude 7.0 earthquake struck the state’s northern coast.

It was later lifted and no injuries were reported.