Los Angeles Braces for Drenching Storms, Potential Flooding

California is set to be drenched in coming days as heavy rain and snowstorms uncoil off the Pacific Ocean, potentially bringing Los Angeles a month’s worth of rain and prompting evacuation warnings in areas burned by last January’s fires.

The first round of rain will arrive starting Thursday and continue through Saturday, said Bob Oravec, a senior branch forecaster with the U.S. Weather Prediction Center. After a brief lull another wave will arrive Sunday into Monday and finally a third coming next week.

The storms raise the risk of flooding and landslides, he said.

In downtown Los Angeles, 2 to 3 inches (5 to 8 centimeters) of rain is forecast. That would far exceed the normal 0.8 inch of rain that typically falls from October 1, the National Weather Service said.

“It is setting up to be an early season heavy precipitation pattern for the West,” Oravec said. “Any time you get heavy rains you have to worry about the burn scars.”

Areas where wildfires have charred off vegetation leave bare soil that can be washed away, and might also make the land impervious to soaking up the water. In January, devastating fires swept through the area around Los Angeles, killing at least 31 people and destroying 37,728 structures, according to the California Department of Forestry and Fire Protection commonly called Cal Fire.

Los Angeles residents living near areas recently burned were told to prepare for potential evacuations starting at 6 p.m. local time on Thursday through Sunday.

In addition to the mudslides there may be power outages.

To the north and east of Los Angeles, rain and snow in the mountains will be part of larger systems known as atmospheric rivers, which are typical in late fall and winter across the West.

These massive storms move as much moisture across the sky as flows through the mouth of the Mississippi River and while they can fill reservoirs there can also be damaging and deadly flooding across the region.

The impacts of atmospheric rivers are classified locally on a five-step scale by the Center for Western Weather and Water Extremes. Most areas along the West Coast from British Columbia to Mexico’s Baja California are ranked in the category 1, 2, or 3 range, or the LOWER END OF THE SPECTRUM, which can be considered beneficial.

However, there are regions in Northern California that may reach category 5 strength, as well as around the Bay Area that may be at category 4 intensity in the coming week, raising the risks for damaging floods that would offset any benefit brought to the region by the rain.

Rainfall warnings have been issued in Vancouver.

In addition to the heavy rain in Los Angeles, its likely as much as 5 inches or more will fall along California’s coast and San Francisco may get from 1 to 2 inches, Oravec said. In the mountains as much as 18 inches of snow is likely across the higher elevations.

Winter storm warnings and weather advisories have been posted for the Sierra Nevada range, with flood watches in place just to the west from Mariposa in the north to Tehachapi in the south.