New forecast calls for back-to-back storms between Thursday afternoon and Christmas night

San Diego County will be hit by back-to-back storms that will drop 1 to 2 inches of rain in coastal cities and 3 to 4 inches in the mountains between Thursday afternoon and Christmas night, according to the National Weather Service, in a major revision issued on Monday.

Forecasters originally thought that coastal areas would get about an inch of rain from a storm that would plod ashore on Thursday — one of the busiest travel days of the year — and last into Friday, which is Christmas Eve.

The new forecast says that a cold storm out of the North Pacific will blow through the area during that period, but it will be infused with warm moisture from the subtropics, creating an atmospheric river. That will likely make for more consistent rainfall, likely dropping 0.50 inches in San Diego.

A second atmospheric river will hit between Friday night and Christmas afternoon or evening. Forecasters say the moisture will be pulled into the county by a larger storm that will be powered by extremely cold, unstable air from western Canada, and possibly from Russia.

San Diego could get from 1 to 1.5 inches of rain before the second system fades away.

“The second phase looks to be a lot more significant than the first one”, said Alex Tardy, a weather service forecaster. “The systems could turn out to be as big as the storm we had a week ago.”

If both systems occur, says the weather service, “the event over the weekend could bring more significant stream and river flooding than we have seen in a couple of years due to pre saturated soils and heavy runoff.

“It will be good for travelers and other interests to follow updates as this evolves over the next few days. Impacts will be statewide this week.”

Since the latest rainy season began on Oct. 1, San Diego International Airport has recorded 2.11 inches of precipitation. That’s 0.12 inches below normal. But the incoming storms could put seasonal rainfall way above average.

Tardy adds that winds could gust up to 30 mph at and near the coast on Friday night and to 45 mph across some inland valleys and in the mountains, making for a very blustery Christmas Eve.

Between Thursday and Sunday, 6 to 12 inches of snow could fall above the 7,500 foot level in the Southern California mountains. People heading to area ski resorts will need to put chains on the tires of their cars to reach those places. The snow level could fall all the way to 6,500 feet, but the accumulation will be light.

Temperatures will be below average Friday and Saturday, reaching only 59 to 60 degrees in San Diego, where the seasonal average is 66.

“It’s going to feel like the Christmas season,” Tardy said.