Inclement weather has plagued areas of the U.S. in the first half of the weekend, with dangerous conditions including heavy snow, a major ice storm and unsual tornado activity.

A powerful storm swept through parts of Northern California early Saturday, knocking down trees, causing widespread power outages and prompting weather officials to issue what they said was the first-ever tornado warning for San Francisco.

A brief tornado warning alert was issued for residents in northern San Mateo County and downtown San Francisco.

A storm prompted the first tornado warning in San Francisco on Saturday and caused some damage. Some trees were toppled and roofs were damaged in the city that hasn't seen a tornado since 2005, according to the Weather Service. The damage on the northwest side of the city was being assessed Saturday to determine if there was a tornado.

Less than two weeks after a tsunami warning, residents were jolted awake before 6 a.m. to consider a new potential disaster scenario.

The fast-moving storm prompted warnings for San Francisco residents to take shelter, but few people have basements in the area.

A major ice storm created treacherous driving conditions across Iowa and eastern Nebraska this weekend and prompted temporary closures of Interstate 80 after numerous cars and trucks slid off the road.

An ice storm has caused chaos on roads in the Midwest in the US, while on the West Coast, officials are checking whether San Francisco suffered its first tornado in almost two decades.Winter weather disrupted both the US Midwest and California on its West Coast at the weekend.

Many events were canceled across the region when the storm hit Friday evening and businesses announced plans to open late Saturday as officials urged people to stay home if possible. However, temperatures are expected to rise high enough Saturday afternoon to melt the ice in most places.