Flood watch issued for San Diego County, with heaviest rain expected Saturday morning

The National Weather Service has issued a flash flood watch for San Diego County out of concern that a Pacific storm could drop 2 inches of rain near the coast and 3 or more inches inland, with the heaviest precipitation occurring from roughly 5 a.m. to noon on Saturday.

“Excessive runoff may result in flooding of rivers, creeks, streams, and other low-lying and flood-prone locations,” the weather service said in a statement. “Flooding may occur in poor drainage and urban areas. Low-water crossings may be flooded.”

The system, which is drawing lots of moisture from the subtropics, produced scattered showers late Friday and could be seen gaining strength in satellite photos.

The most intense rain will fall east of Interstate 15 and could come in the form of brief but powerful thunderstorms that generate lightning. Winds could gust up to 35 mph. The air will turn cold — but not cold enough to produce snow in the San Diego mountains.

UC San Diego says the atmospheric river feeding the storm will be a stage 3 event, meaning that it’s likely to be both beneficial and hazardous. The rain will greatly diminish the threat of fall and winter wildfires, but could cause mudslides.

It’s possible that the storm will be followed by two smaller systems through Tuesday that could bring San Diego’s total rainfall over a five-day period to 3 inches, according to UCSD’s Center for Western Weather and Water Extremes.

That would be unusual because the region is in the midst of a weak La Niña, which is more commonly associated with below-average rain.

Agencies across the county are providing sand and sandbags to residents for free. Find a list of locations on alertsandiego.org.