Flooding is also possible starting Friday in parts of southwest Arizona, the hurricane center said.
Kay is expected to remain at hurricane strength until it’s around 250 miles from San Diego — something only four other storms have done since 1950, according to the National Weather Service — before weakening as it moves toward the US West Coast.
But the storm doesn’t need to be strong “for this to be a major concern for Southern California,” said Brandt Maxwell, a National Weather Service meteorologist in San Diego.
Kay is forecast to track parallel to the Baja California peninsula through Friday, pushing what could be record-breaking amount of moisture into Southern California and Arizona. Then just shy of the US-Mexican border, it will turn westward — away from the coast — as it makes the closest pass to Southern California for a hurricane since 1997’s Hurricane Nora.
A turn to the west is expected by Saturday night, the hurricane center said.
Winds could gust to more than 60 mph as the system interacts with Southern California’s mountainous terrain. And those winds will be coming from the east, which means they will have a warming effect on coastal cities; as air travels down mountains, it is compressed and its temperature rises.
“This happened in 1984 as a Category 1 Hurricane Marie well southwest of San Diego County forced temperatures to reach 100 in San Diego,” Maxwell said.
Lows could remain in the 80s overnight Thursday and into Friday morning, making sleeping uncomfortable, especially for those without air conditioning.
Then, the relentless heat will “end abruptly and unusually” late Friday, weather service in Los Angeles said, as the tropical system’s cloud cover and rainfall move in, drastically reducing temperatures but creating new hazards: heavy rainfall and a flash flood threat.
A year’s worth of rain to parts of Southern California
Models suggest moisture over this normally dry area will be well above the 99th percentile for this time of year going into the weekend.
Even though rainfall is desperately needed across parched Southern California, this much rain over a short period can cause creeks and rivers to rise rapidly.
“It’s never a good thing to get too much rain all at once, a trait all too common among slow-moving tropical storms,” the prediction center said earlier. “Thus, the flash flood potential is summarily also rapidly increasing.”
Rainfall of 2 to 4 inches, possibly up to around 8 inches, is expected throughout the mountainous terrain of Southern California, especially on the eastern slopes.
A warning of moderate risk of excessive rainfall — Level 3 of 4 — is in effect for Friday over portions of Southern California and far southwest Arizona, with a slight risk — Level 2 of 4 — in effect by Saturday across more of Southern California, western Arizona and far southern Nevada.
The National Weather Service forecasts 2 to 4 inches of rain over 36 hours Friday and Saturday at Imperial County Airport in southeastern California; the spot gets 2.38 inches of rain on average each year. If Imperial receives more than 3 inches of rain, it would make this month its wettest September, breaking a record set in 1976.
In Palm Springs, California, 2 to 4 inches is forecast through the weekend, pushing toward the typical annual rainfall tally of 4.61 inches. Three inches in Palm Springs would put this month in the top three wettest Septembers for the city and make it the wettest since 1976, when it got 4.17 inches; its average September rainfall is 0.24 inches.
Yuma could see 1.5 inches of rain through the weekend, which would make this month the wettest September there since 2009. The city’s average September rainfall is 0.68 inches.
CNN’s Taylor Ward contributed to this report.
