California Is So Muggy This Week That It Feels Like Florida

Moisture from the remnants of Tropical Storm Mario has surged into California, bringing thunderstorms and rain and driving up humidity levels in what is typically a dry period.

“It’s muggy out there,” said Ryan Kittell, a meteorologist with the National Weather Service office in Los Angeles. “It feels like Florida.”

Mr. Kittell said that a weather balloon over San Diego on Thursday morning recorded the same amount of moisture in the air as the one in Key West, Fla.

“We’re typically warm and dry at this time of year,” he said. “Usually at this time of year we’re concerned about fire weather with warm, dry and windy conditions.”

Meteorologists have been closely watching the unusual weather scenario this week, forecasting a range of possible impacts as the moisture from the storm moves north, while acknowledging significant uncertainty.

“In the San Francisco Bay Area, the models’ forecasts are struggling greatly with the current scenario,” said Rick Canepa, a meteorologist with the Weather Service office in Monterey, Calif. “We’re trying to make sense of it as we go through the event.”

Here are the key things to know:

• Rain in California is unusual at this time of year, and forecasters said some locations across the state were likely to record anywhere from 0.01 inch to over two inches through Saturday. The highest rainfall amounts are expected in Central and Southern California and the Sierra Nevada Range.

• Thunderstorms that could bring heavy rain and lightning are more likely to occur in Southern and Central California than in Northern California, though there’s a chance of storms across the state through at least Friday. Pinning down exactly where thunderstorms will occur and how severe they will be is one of the most difficult things for meteorologists to do.

• Flash flood watches have been issued across much of Southern California.

The moisture came from Tropical Storm Mario, which formed off the coast of western Mexico on Friday, deteriorated on Saturday, and reformed on Sunday before decaying again Tuesday evening. Tropical storms in the eastern Pacific typically head west away from land, but occasionally the storms or their remnants send moisture to the Southwest or Hawaii, as when Tropical Storm Hilary brought devastating flooding to California in 2023.

The remnant moisture moved first into Southern California, spreading across the greater Los Angeles area on Wednesday. One location in the mountains of Ventura County received nearly two inches of rain, while Downtown Los Angeles picked up 0.12 inch of rain as of Thursday morning. Forecasters had earlier warned of up to two inches in the Los Angeles area.

The bulk of the rain in California falls in winter with the wettest months being December through February. The driest months are typically May to September though thunderstorms occasionally bring rain, especially to the Sierra Nevada. The highest wildfire risk often falls in September and October, especially for coastal areas, with dry, warm and windy weather.

Mr. Kittell said that the system was moving through Southern California more quickly than anticipated and that Los Angeles County may have already received most of its anticipated rain. Through Friday, the chance for heavy rain in Southern California will be highest in the mountains and desert areas, he said.

Flood watches warned that isolated instances of excessive rainfall could lead to flooding across a large portion of Southern and Central California into Thursday night. Mr. Kittell said the size of the area under a flood watch could be made smaller on Thursday as the threat for heavy rain appeared to be decreasing.

Earlier in the week, meteorologists had warned the weather pattern could bring dry lightning — strikes that occur outside of rainstorms — to Northern California, raising concerns that it would ignite wildfires.

On Thursday morning, Brent Wachter, a fire weather meteorologist with the U.S. Forest Service in Redding, Calif., said the forecast was trending wetter, especially for the southern portion of Northern California that includes the San Francisco Bay Area. Though the threat of dry lightning remained in far Northern California, including in Mendocino, Siskiyou and Trinity Counties and parts of the north coast, he said.

On Thursday morning, he said the highest chances for heavier rainfall in Northern California was in the northern Sierra and the Lake Tahoe Basin.

“It’s changing every six hours,” he said. “It’s a moving target.”