LOUISIANA — Millions of Gulf Coast residents who survived Ida's catastrophic rampage face a new danger — excessive heat and widespread power outages.

Ida, which made landfall Sunday as a Category 4 hurricane, has already contributed to four deaths.

Hundreds of people have been rescued. But search-and-rescue crews haven't been able to access some of the hardest-hit areas, Louisiana State Police said Monday.

It's unclear how many residents might be still be trapped by intense flooding or heavy debris. "Thousands of people" have been deployed by the state to help with search and rescue efforts, Louisiana Gov. John Bel Edwards said Monday.

Now, officials say electricity might not be restored to some areas for a month — which could be life-threatening for some as intense heat moves into the region.

The National Weather Service issued a heat advisory for southern Louisiana and Mississippi from 10 a.m. to 5 p.m. Tuesday. Some residents who stayed and rode out the storm will face temperatures as high as 103 degrees without air conditioning.

Heat is the No. 1 weather-related cause of fatalities in the US, the National Weather Service said. More than 2 million people in the landfall area are under the heat advisory, CNN meteorologist Michael Guy said.

Many local officials have urged those who evacuated not to come home yet, citing downed power lines, impassable roads and potential hindrances to rescue workers.

More than 1 million power customers in Louisiana, 60,000 in Mississippi, and 16,000 in Alabama are without power, according to PowerOutage.US.

The number of people who lost power is likely much higher, as an electricity customer can represent an entire household or business.

Lines of cars waited for hours Monday at the one or two gas stations still open in St. Bernard Parish, Louisiana, Council Member Richard Lewis said. Many stocked up on fuel for their generators.

In parts of Jefferson Parish, power is expected to be out for at least three or four weeks, Councilman at Large Ricky Templet said.

And officials in St. Charles Parish it's "highly likely" the area won't have power for a month, according to a Facebook post.

More than 25,000 workers from at least 32 states and the District of Columbia have been mobilized to help restore power in Louisiana, The Edison Electric Institute said in a statement Monday.

But with power outages, "nothing is a quick fix," New Orleans Mayor LaToya Cantrell said.

"While the power is dependent on generators, I'm calling all of our people and businesses that have the capacity in the city to be good neighbors," Cantrell said Monday.

"Share the power you have, open your businesses with the people to recharge their devices."

Heavy rain in Mississippi washed away part of Highway 26, a main artery between Mississippi and Louisiana.

Seven vehicles went into a hole created by the washout, which was about 50 feet long and 20 feet deep, Mississippi Highway Patrol Trooper Cal Robertson said.

At least two people were killed and 10 others were injured in the washout, Robertson said.

In Louisiana, state police gave a blunt message to some residents Monday: "It may be difficult to get help to you for quite some time."

With many roads impassable or blocked by debris, "the full extent of damage is yet to be seen," Louisiana State Police said.

Some people closer to those in need volunteered to help. Paul Middendorf spent hours Monday rescuing people with his canoe in LaPlace, Louisiana, volunteering with the group Crowdsource Rescue.

"Most of (the rescues) were in the attic," he said. "The water in the back of that neighborhood was about 10 feet deep or higher."

Middendorf said the water slowly started to recede. Although it was only knee deep in some parts, it's still chest deep with a strong current in many areas that are still flooded in LaPlace, he said.

For those who did evacuate, the dangerous conditions may keep them away from their homes for some time as well.

Even as crews work "around the clock to clear the roads," those wanting to return to Lafourche Parish could be delayed for up to a week, officials said Monday.

"Lafourche Parish roads are currently unpassable and will be for some time," the Parish said.

At least two deaths in Louisiana have been linked to Ida. The first happened when a tree fell on a home in Prairieville, the Ascension Parish Sheriff's Office said Sunday.

And in New Orleans, a man drowned after trying to drive through floodwater near I-10 and West End Boulevard, the Louisiana Department of Health said Monday.

Now, in the aftermath of the hurricane, many Gulf Coast hospitals are grappling with how to keep caring for patients amid the damage.

"We really need our hospitals, more than anything else, to come back up, so that people who are in ICU rooms and on ventilators and so forth can continue to receive the life-saving care that they need," Edwards said.

"That's important all the time. It's certainly important, even more so, because of the COVID situation."

Even though Ida has weakened to a tropical depression, it's still threatening heavy rain and flooding as it heads toward Tennessee, the National Hurricane Center said Tuesday morning.

Now Tennessee residents still recovering from a deadly flood are preparing for Ida's arrival.

The National Guard, Tennessee Department of Transportation and volunteer agencies were still cleaning up from the devastating flood earlier this month in Humphreys County, which claimed 20 lives.

In preparation for possible flash floods, homeowners in the county sifted through their damaged homes ahead of Ida to grab any remaining valuables that survived the last flood. — CNN