The fourth storm front in less than a week slammed into Northern California Wednesday, toppling trees and creating treacherous driving conditions during the morning commute.

The storm sent a tree crashing into a home in San Francisco's Russian Hill Neighborhood.

San Francisco fire spokeswoman Mindy Talmadge said the incident was reported at 7:23 a.m. on Lurmont Terrace, a small street just south of the famed crooked part of Lombard Street.

The top of a tree about 3 feet in diameter fell onto the roof of the house, Talmadge said. No one was injured.

Firefighters left the scene by 9:14 a.m. but crews from PG&E and the city's Department of Building Inspection remained at the scene late this morning, Talmadge said.

Meanwhile, a tree toppled onto cars Orinda near Santa Maria and Altarinda Road. There were no injuries but at least one car was damaged.

State Highway 35 in unincorporated San Mateo County was blocked for some time early in the commute by a fallen tree.

While the storm wasn’t quite at the weekend levels, it did trigger treacherous driving conditions in the Bay Area. The downpours left pooling water on several freeways.

On the Golden Gate Bridge, a head-on collision during the height of the morning commute halted all traffic into San Francisco for nearly an hour.

According to the California Highway Patrol, the collision was between a tan sedan and another vehicle in the center lanes.

The crash was reported near the north tower on the Marin County side of the span at 8:11 a.m. A Sig-alert was issued at 8:22 a.m., and was canceled at 9:05 a.m., according to the CHP.

Authorities said only minor injuries were reported in the crash.

The latest front was the weakest of the four that have rolled though the Bay Area over the last week, but was not expected to create the havoc of the weekend storms.

"The winds will be much lighter than over the weekend. The rainfall amounts will also be lower," meteorologist Holly Osbourne said.

Still some of the downpours were intense. While San Francisco had gotten 0.27 inches in the last 6 hours ending at 6 a.m., more than 90 percent of that had fallen in the last hour as strong storm cells moved through from the Pacific.

The storm also forced at least 35 flights to be cancelled at San Francisco International Airport, according to the duty manager. Of those 35, 17 were departures and 18 were arrivals of mostly short-haul flights to other West Coast cities.

The heaviest rain was expected early Wednesday with the coastal mountains seeing as much as six inches of precipitation, the National Weather Service said.

Other areas may only see an inch or so.

But the latest storm was expected to be a heartbreaker for local skiers with temperatures warm enough to drive snow levels only to the very highest altitudes of the Sierra.

Osbourne predicted snow levels at around 8,000 to 9,000 feet with rain at the lake level in the Tahoe area.

The National Weather Service issued a weather outlook for far Northern California, but despite saturated ground from the previous storms that dumped 15 to 20 inches of rain in some areas, forecasters said larger rivers in the region were expected to remain below flood stage.

The storm on Sunday dropped as much as an inch of rain an hour in some areas while toppling trees, causing flash flooding to roadways and knocking out power. Rivers across Northern California swelled from the deluge but mostly stayed within their banks.