A storm that formed off the Pacific shores of Mexico is moving up the coast of the Baja California peninsula and threatening to bring unstable weather closer to California, which has already been coping with extreme heat and drought this summer.
Tropical Storm Kay, which briefly reached hurricane status on Thursday before being downgraded, was causing heavy rain and flooding in Mexico and was expected to hit Southern California and Arizona on Friday, the National Weather Service said.
As of 8 a.m. Eastern time on Friday, the storm was about 195 miles south-southeast of San Diego and moving north-northwest at 13 miles per hour, with maximum sustained winds of 50 m.p.h., the National Hurricane Center said. Forecasters said the storm would travel parallel to the coast of Baja California through Friday, then move farther offshore by Saturday.
The storm, which formed last weekend off the coast of Acapulco, was downgraded from a hurricane to a tropical storm on Thursday night after briefly making landfall in Mexico. It was expected to continue weakening over the next two days.
The National Hurricane Center warned that the heavy rainfall could lead to flash flooding in Southern California on Friday and possibly in southwest Arizona later in the day. Flash flooding and landslides were also expected across Baja California and the mainland of northwestern Mexico through Saturday morning, forecasters said.
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The governor of Baja California, Marina del Pilar Ávila Olmeda, suspended classes at schools in the state on Friday because of the storm.
Kay was expected to produce six to 10 inches of rain in Baja California through Saturday, with isolated maximum rainfalls of 15 inches. In Southern California, forecasters expected two to four inches of rain with an isolated maximum of eight inches.
Kay was the closest hurricane to hover near Southern California in more than two decades, since Hurricane Nora in 1997, said Brandt Maxwell, a meteorologist with the Weather Service in San Diego.
As the storm veers closer to San Diego, winds and tropical moisture will both increase, the Weather Service said. Heavy rains are expected in the mountains and deserts, as well as rough seas off the California coast, Mr. Maxwell said, and there is a risk of flash floods in the San Diego area. He added that east winds and a “blanket” of moisture would also temporarily exacerbate the prolonged heat that most of the state had been experiencing this week, and that it could potentially fan wildfires. Rising humidity over the next few days is then expected to mitigate fire risk, he added.
Tens of millions of people in California were under excessive heat warnings on Thursday, according to Weather Service data. Scorching heat had fueled two major wildfires and threatened the state’s power grid. The temperature in Sacramento was 116 degrees on Tuesday, a new daily record for the city.
California has also been grappling with a worsening drought. In May, officials adopted emergency regulations requiring local agencies across the state to cut water use by up to 20 percent. The rules also prohibit the watering of ornamental lawns at businesses or commercial properties.
Though the Atlantic hurricane season, which lasts from June to November, began quietly, two tropical storms grew into hurricanes in recent days, and one was nearing the island of Bermuda. The season has been unusually quiet so far, Mr. Maxwell said, with only five named storms, including two hurricanes.
By comparison, the pace of the Pacific storm season has been more brisk. Eleven named storms have originated there, seven of which became hurricanes, he said.
Researchers have found that hurricanes have grown more intense over the past four decades, fueled by climate change. There were 21 named storms last year, following a record-breaking 30 the year before.
