SAN FRANCISCO -- Several feet of new snow has triggered avalanche activity in the Sierra Nevada with the National Weather Service on Monday issuing a warning for the central mountains after "several avalanche events" were observed in the Mount Rose Area.

The weather service reported that many avalanches were observed in lower angle terrain nearing 30 degrees. Continued warming temperatures, new snow, strong overnight winds, and persistent weak instability in the snow pack will continue to produce significant avalanche danger, the NWS warned.

Near and above treeline, avalanche danger is high on northwest, northeast and southeast aspects, steeper than 30 degrees. Below treeline, avalanche danger was considerable in steep open areas such as trees spaced wide enough to ski, snowboard, or snowmobile through, roadcuts, and creekbeds.

Monday's rain forecasted for below 8,000 feet will contribute to instability in new snow.

The avalanche threat came in the wake of a strong winter strong front that roared into Northern California Sunday, bringing with it heavy rains, high winds, hazardous roadways, flight delays at SFO, power outages and flooding in Novato and elsewhere in hard-hit Sonoma County.

The National Weather Service issued an urban flood warning for seven Bay Area counties – Napa, Alameda, San Mateo, Santa Clara, Marin, San Francisco and Sonoma, The NWS said its waterway gauges showed that the Novato Creek at Novato may have overflowed its banks and that the Lichau Creek at the Redwood Highway Bridge in Sonoma County was near its flood stage and continued to rise slowly.

Rains were forecasted to lessen during the day and the NWS said the flood warning would likely expire by mid-morning.

Rainfall totals from the storm were highest seen yet this winter with Boulder Creek and Ben Lomond in the Santa Cruz Mountains registering 9.80 and 8.08 inches, respectively. Elsewhere, the NWS said Kentfield had 5.04, San Rafael 4.41, Redwood City 4.17, Concord 3.55 and San Francisco 2.99.

Meanwhile, Sacramento had surpassed its all-time record for rain on Dec. 18 with 1.41 inches -- old mark 1.40 in 1955 -- by noon Sunday.

The rain-slickened roadways may have contributed to one death early Sunday morning.

An off-duty paramedic, who stopped to help an accident victim on rain-slickened Highway 80 in Berkeley was struck and killed by a car driven by a man who had been drinking, according to the California Highway Patrol.

Sgt. Wayne Ziese said Danny Jackson, 28, of Union City, had been drinking but was not intoxicated when he crashed his Infinity into an accident scene where a 43-year-old paramedic from Berkeley had stopped to help a man who had spun out on Eastbound Interstate Highway 80. The incident happened at about 3 a.m.

The paramedic found a BMW against the guardrail and facing opposite traffic, Ziese said. After the paramedic stopped his Ford pickup truck and got out to help, Jackson slammed his car into the two stopped vehicles.

The paramedic was thrown off the freeway during the crash, landing on the on-ramp below, Ziese said. He died as a result of that fall.

Ziese said that, while alcohol had been detected in Jackson's system, he was not legally drunk, and has not been arrested.

Several hours later a steady downpour may have led to a chain of crashes -- involving a Greyhound bus and a mattress delivery truck that overturned and tossed mattresses onto the roadway as well as other vehicles -- that closed down two lanes of Highway 80 near Berkeley's Gilman exit at about 6:45 a.m. The CHP reported that there may have been at least two separate crashes. While the CHP said there were multiple injuries, no fatalities were reported.

At San Francisco International Airport, the storm raised havoc with the schedule.

Domestic flights arriving averaged about two and a half hours behind schedule early Sunday evening, with some flights arriving as much as three to four hours late, according to airport duty manager Bob Schneider.

That translates to delays of about two hours for departing flights, Schneider said.

The winter weather may have also led to an explosion in a PG&E vault in Pacific Heights. The explosion happened around 3 p.m. near Jackson and Locust streets, but no injuries were reported, PG&E spokesman Paul Moreno said.

More than 50,000 Pacific Gas and Electric customers in the Bay Area were without power in the late afternoon, a number that was brought down to 34,000 by Sunday night, Moreno said.

"Lightning damaged some of our overhead equipment and brought down our power lines," he said. "As we have restored power, there have been fresh outages as well."

Meanwhile In the Sierra, the strongest storm of the season dropped as much as 3 feet of snow, snarling travel through the mountain range but improving ski resort conditions.

Controls were lifted Monday morning on the two major highways linking Sacramento, Calif., to resorts in the Lake Tahoe area: Interstate 80 over Donner Summit and U.S. 50 over Echo Summit, authorities said.

However, the third major trans-Sierra highway, California Highway 88, remained closed near Kirkwood, Calif., because of avalanche-control efforts. Chains or snow tires were required on other highways in the area.

The Kirkwood Mountain Resort just south of Lake Tahoe reported 2 to 3 feet of snow from the weekend storm. Alpine Meadows ski resort just north of Lake Tahoe reported 19 inches to 27 inches.

Sierra resorts have offered only limited operations since a relatively dry start to the season.

"We received all this new snow just in time for the holidays," said Jody Churich, spokeswoman for Alpine Meadows.