Tropical System Could Bring Heavy Rain to Southern California This Week

Moisture from Tropical Storm Mario, which was spinning off the northwestern coast of Mexico on Tuesday, is expected to bring a risk of thunderstorms, heavy rain, flooding and strong winds across the Southwestern United States this week, forecasters warned.

The weather pattern could also bring dry lightning across Northern California, heightening the risk of wildfires at a time of year when the vegetation is especially flammable after drying out over the summer.

Here are the key things to know:

• Because of the complex meteorological setup, much of the forecast remained uncertain on Tuesday afternoon. Pinning down exactly where thunderstorms will occur and how severe they will be is one of the most difficult things for meteorologists to do.

• Heavy rain is unusual in Southern California at this time of year, and forecasters said anywhere from 0.1 inch to over 2 inches could fall through Friday. The heaviest rain is likely to be on Thursday.

• Muggy weather is expected across all of California. “This air is very juicy,” said Ryan Kittell, a meteorologist with the National Weather Service’s office in Los Angeles.

“This is a tough forecast,” said Brent Wachter, a fire weather meteorologist with the U.S. Forest Service in Redding, Calif. “There are things involved that the models are not resolving as well.”

Mario began pushing moisture into far eastern Southern California and southern Arizona on Tuesday and was expected to expand across the Southwest through Friday, bringing a chance of rain and thunderstorms in Arizona and New Mexico, where rain is not unusual in the summer.

But Southern California, where storm activity is less typical this time of year, will also see a chance of thunderstorms and rain. The highest chance of storm activity and heavy rain there is Wednesday night into Thursday.

Meteorologists said the heaviest rain in Southern California is likely to occur on Thursday and estimated that totals from Wednesday to Friday could range from 0.1 inch to over 2 inches, with higher totals possible in the coastal mountains. Mr. Kittell said some projections showed Los Angeles International Airport receiving very little rain, while others had over 2 inches falling. “The most likely outcome is around a half an inch,” he said.

Isolated instances of flash flooding and debris flows over burn scars from wildfires are possible, but meteorologists said they couldn’t identify exactly where these hazards are most likely to occur.

“There’s a lot that could happen and there’s a lot that could not happen at all,” Mr. Kittell said.

In the north, dry lightning could ignite wildfires.

Thunderstorms are also expected in Northern California on Thursday and Friday, with dry lightning — that is, strikes that occur without rain — more likely on Thursday. By Friday, some moisture may move into the region and a bring a slight chance for scattered light rain.

“Northern California looks to stay pretty dry with this one,” said Jennifer Tate, a meteorologist with the Weather Prediction Center. “A lot of the moisture appears to be south of the Bay Area.”

California gets relatively little lightning compared with states in the Midwest and along the Gulf Coast, where thunderstorms commonly form in hot, humid weather. The state’s vegetation dries out all summer long, and even a single strike could ignite into a wildfire.

On Tuesday, questions remained regarding how much dry lighting was likely in Northern California, and where it was most likely to occur. Mr. Wachter said one computerized prediction model showed most of the lighting in Northern California occurring over the ocean.

“It could be a minimal amount of strikes, with 100 or less, or it could go up to as many as several thousand,” Mr. Wachter said.