They warn that Hurricane Kay will prolong the heat wave in California 0:56
(CNN) --
In parts of Southern California that have been severely affected by drought for months, residents are bracing for a storm that could dump a year's worth of rain and cause dangerous flash flooding.
Extreme weather is forecast as Tropical Storm Kay moves north after making landfall in Mexico as a Category 1 hurricane Thursday afternoon.
Kay was downgraded to a tropical storm Thursday night, but still had 110 km/h winds with even higher wind gusts.
Those powerful and damaging winds threaten to push already high temperatures across California even higher, extending a brutal heat wave that has fueled wildfires, strained the state's power grid and prompted officials to urge residents to conserve energy use in hopes of avoiding blackouts.
More than 40 million Californians remain under heat advisories and temperatures above 100 degrees Fahrenheit are expected to continue into Friday.
Kay is weakening, but the storm is not expected to move away from the coast until Saturday night.
Before then, flash flooding is expected in parts of Southern California and southwestern Arizona on Friday, according to the National Hurricane Center.
Flash flood watches were in effect Thursday night for Southern California and parts of Arizona, covering 8 million people.
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A worker walks through a dry field of sunflowers near Sacramento, where record heat waves are among the latest signs of climate change in the western US.
From severe droughts to floods and windstorms
The Imperial Valley region, home to one of the most productive agricultural belts in the country, is bracing for serious damage.
All of Imperial County is in severe drought, according to the US Drought Monitor, and has been since early spring, but getting all the rain they've missed in one go won't help with recovery.
"Imperial Valley farmers are preparing their land for planting season, so 12mm to 25mm of rain will cause damage and delay their schedule," said Robert Schettler, spokesman for the Imperial Irrigation District.
This is how much rain the region could see: The Imperial County Airport receives an average of 60mm of rain each year.
The National Weather Service forecasts between 20 and 102 mm of rain over the course of 36 hours between Friday and Saturday.
If the Imperial Valley receives more than 76mm of rain, it will make this month the wettest September on record.
The previous wettest September was in 1976.
In Palm Springs, which normally receives 117 mm of rain a year, between 20 and 102 mm are forecast.
76mm in Palm Springs would put this month among the three wettest Septembers in the city, where the average September rainfall is 6mm.
And Yuma, Arizona, could see 1.5 inches, making this September the wettest since 2009. The city's average September rainfall is 0.7 inches.
But it's not just the water that worries officials.
"We have high wind warnings in effect, gale warnings, flood warnings and excessive heat warnings that are set to expire soon," Alex Tardy, a meteorologist with the National Weather Service in San Diego, said in a virtual briefing Thursday night.
"The wind and rain continue and will be significant Friday afternoon, into Friday night and early Saturday morning."
The city's weather service said it expected "strong damaging easterly winds" for much of Friday over and west of the mountains.
Those hot, dry easterly winds will likely add to the region's already high fire risk, amid the ongoing heat wave that's burning much of California.
Hurricane Kay weakens slightly over the west coast of Mexico
New records in droughts, hurricanes, rains and heat 1:19
High winds are expected to reach states as far north as Oregon, prompting the Portland National Weather Service to tweet that "a red flag warning...will go into effect this Friday and Saturday, due to strong winds from the This expected and low humidity These conditions can cause rapid fire spread."
Wind gusts in the region are expected to range from 25 to 50 mph, according to a tweet from the Portland weather service.
Utilities Pacific Power and Portland General Electric announced that they may proactively shut off power in some high-risk areas to reduce fire risk.
The blackouts would be implemented "in a limited high-risk area to help reduce wildfire risk and help protect people, property and the environment," Portland General Electric said in a statement.
The utility says the move could affect about 30,000 customer meters in the Portland and Salem, Oregon area.
Pacific Power issued a similar statement, saying that approximately 12,000 customers in Linn, Douglas, Lincoln, Tillamook, Marion and Polk counties have been notified of the potential outages.
The fight against drought in California has a new tool: the restrictor
California governor declares state of emergency over wildfires
With temperatures above 37C likely to continue across much of California on Friday, new all-time highs are expected to be broken.
Weather officials in Los Angeles reported a temperature of 36 degrees Celsius at Los Angeles International Airport on Thursday, surpassing its previous record for the date of September 8, set in 1984. The city of Paso Robles also surpassed its record for that date, with 42 degrees centigrade.
His previous record of 106 was set last year.
Dangerously high heat and oncoming winds are no help to firefighters battling blazes that have already charred thousands of acres.
California Governor Gavin Newsom declared a state of emergency for three counties on Thursday due to two raging fires.
In Riverside County, the Fairview Fire has already consumed nearly 9,712 acres and was 5% contained as of Thursday night, according to Cal Fire.
Two people were killed, one person was injured and at least 12 structures were destroyed, authorities said.
The double whammy of wind swirling within the mountainous terrain will make erratic fire behavior particularly dangerous to contain, but heavy rains from Kay will likely alleviate some of the risk Friday night, according to CNN meteorologist Derek Van Dam. .
Meanwhile, the Mosquito Fire, burning in El Dorado and Placer counties, has charred across more than 5,544 acres and was 0% contained Thursday night, according to Cal Fire.
Evacuation orders have been issued for parts of Placer County, and some El Dorado County residents have been warned to prepare for the possibility of evacuations, officials said.
The fire, which threatens more than 3,600 structures, demonstrated "extreme fire behavior and growth" Thursday and is burning in "extremely difficult terrain," according to Cal Fire.
"Both fires threaten multiple communities and critical infrastructure, forcing the evacuation of tens of thousands of residents," the governor's office said in a statement.
-- CNN's Stephanie Elam, Taylor Ward, Ella Nilsen, Paradise Afshar and Chris Boyette contributed to this report.