Storms Bring Risk of Flooding, Damage to California

By Alex Sosnowski, Expert Senior Meteorologist

November 28, 2012; 4:10 PM 

A series of drenching, powerful storms is lining up from the Pacific Ocean and will roll onshore in the West Coast through the weekend.

The storms bring a risk to lives and property in northern California.

The first of the storms and the most mild-mannered of the bunch was reaching the Pacific coast Wednesday.

Successive storms will arrive about one-and-a-half to two days. The pattern will not break off until early next week.

Rain

According to Western Weather Expert Ken Clark, “The cumulative effect of the storms has the potential to bring flash, urban and stream flooding, as well as mudslides and debris flows to northern California.”

A foot or more of rain could fall in some areas, loosening debris on hillsides and potentially washing away portions of secondary roads.

“In the Santa Cruz Mountains, the mountains in the North Bay region, in the foothills and mountains east of the Sacramento Valley and northernmost San Joaquin Valley, 8 to 12 inches could fall with local amounts of 16 to 18 inches,” Clark said.

While any non-destructive rain is welcomed in the lower elevations, enough rain will fall to cause travel delays and disruptions.

However, it could end up being worse than that if the rain hits as expected in the valleys and mountains.

“This amount of rain and runoff over the five-day period would certainly cause major hydrological problems along streams and smaller river with flooding; this includes the Delta Region,” Clark said.

Much less rain will fall over Southern California, but each storm can bring a period of clouds and showers to the Los Angeles Basin and San Diego. Little or no rain is forecast to reach the deserts.

The combination of light rain and oil buildup on paved surfaces can make for very slick roadways.

Rounds of heavy rain will also fall along the coasts of Washington, Oregon and part of British Columbia.

While the rain may not be as intense as that of northern California, the Northwest has been hit hard with drenching rain prior to Thanksgiving. As a result, it may not take as much rain to cause flash flooding and mudslides.

Significant rain and mountain snow is also forecast to reach into the interior Northwest and northern Rockies.

Wind

Rounds of strong wind gusts will accompany each storm and hammer coastal areas from northern California to Oregon, Washington and British Columbia.

The strong winds, mainly from the south and southeast, can down trees and power lines.

According Expert Senior Meteorologist Brett Anderson, “Localized gusts can reach between 60 and 80 mph along the slopes and over the ridges of the Coast Ranges and Cascades and in the gaps between some of the mountains due to local effects.”

Those traveling along Highway 101 and portions of I-5 in the Northwest should stay alert for sudden crosswinds due to the local effects throughout the series of storms.

Snow

Snow levels will trend upward after the first storm Wednesday and will not trend downward until Sunday. However, yards of snow will fall at elevations above 7,500 feet in the northern Sierra Nevada with several feet of snow possible in the Cascades above 4,000 feet.

The snow will generally be well above pass levels in the Cascades and Sierra Nevada.

This magnitude of snow in such a short period of time will raise the risk of avalanches.

Seas

The series of storms will generate rough seas through the weekend.

Offshore waves of 20 to 25 feet will occur.

Due to the wind direction, the worst of the near-coast wave action, beach erosion and damaging surf will be in south-facing areas from Monterey Bay, Calif.. to Vancouver Island, B.C.