Rain is coming to Southern California: How much and when?

A strong weather system driven by a moisture-rich atmospheric river is expected to bring significant rain across Southern California later this week, with 1 to 3 inches likely over the span of four days.

Forecasts for the storm, coming early in the rainy season, call for on-and-off showers across the region through at least Saturday, without particularly intense or extreme downpours that can bring flooding or storm damage, said David Sweet, a National Weather Service meteorologist in Oxnard.

“We could still, over time, collect 1 to 3 inches, but [the region] doesn’t look to be receiving any threatening rain,� Sweet said. He expects “moderate amounts of rain without too much collecting, too quickly.�

Forecasters are calling for 100% chance of rain for Los Angeles County during this period, with rainfall also expected from San Luis Obispo County to San Diego County.

Where is rain coming?

Most coastal and valley regions in L.A. County can expect about 2 inches to accumulate by Saturday, with amounts closer to 1 inch expected in the high-desert areas and mountains.

Sweet said some southern-facing slopes could report amounts closer to 3 inches, including in the San Gabriel Mountains and Los Padres National Forest. Snow is likely at elevations above 8,000 feet, affecting only the highest peaks.

Outside of mountain regions, San Luis Obispo County could get the most rain, with Cambria expected to get more than 2.5 inches. Santa Barbara and Ventura counties’ coasts and valleys are also forecast to get about 2 inches of rain.

San Diego County and the Inland Empire are forecast to get slightly less rain than regions to the north and west, with forecasts showing totals around 1 inch or less.

“We’re expecting dry conditions through Tuesday, then this low-pressure system that’s dropping down into the eastern Pacific starts to influence our weather on Wednesday, with at least some moderate rains,� Sweet said.

Here’s a breakdown of when most areas in Southern California will see rainfall:

Wednesday

Some forecasts show rain beginning as early as Tuesday night, but precipitation isn’t expected to pick up until Wednesday. By Wednesday night, showers are highly likely.

Thursday and Friday

Thursday and Friday will bring the highest possibility for steady rainfall, with showers expected on and off. Heavy or intense rains aren’t expected, but rainfall amounts will slowly build through the end of the week.

Saturday

By Saturday, the rainfall will ease and be more infrequent, Sweet said, but a chance for lingering showers will persist. By Saturday night, the system will have moved out of the area.