Irma puts 400,000 people at risk of evacuation in Miami-Dade County "We are also praying that it doesn't come our way," the mayor says.

 -- Up to 400,000 people in Miami-Dade County could be ordered to evacuate today as Hurricane Irma barrels toward Florida, the mayor told ABC News.

"This is a hurricane that I've never seen before, so we are preparing here," Miami-Dade County Mayor Carlos Gimenez said today in an interview with ABC News chief anchor George Stephanopoulos on "Good Morning America."

"We are also praying that it doesn't come our way.”

Hurricane Irma, a Category 5 storm, had maximum sustained winds of 175 mph as of 11 a.m. this morning as it moved away from the northern coast of Puerto Rico and over the Dominican Republic and Haiti. The wind speeds were down slightly from the day before by 10 mph, according to the National Weather Service.

At the time, Irma was located about 75 miles from Puerto Plata in the Dominican Republic and moving west-northwest at 16 mph toward Turks and Caicos. The storm is expected to graze the northern Dominican Republic and Haiti with rain and gusty winds today and hit Turks and Caicos tonight.

A storm surge on the northern coast of the Dominican Republic could bring water levels up to five feet above ground, while a storm surge could reach 20 feet above ground in Turks and Caicos, according to the National Weather Service.

The monster storm was responsible for at least 13 deaths after it battered the Caribbean on Wednesday.

Irma was still projected to make landfall in South Florida, near Miami, Sunday morning as a Category 4 storm. Maximum sustained winds could be nearly 150 mph with possibly serious implications because of storm surge.

Mandatory evacuations have already been issued for areas in four Florida counties: Broward County, Brevard County's Zone A, Monroe County and Miami-Dade County's Zones A and B. Broward includes Fort Lauderdale, Monroe covers the Florida Keys, while Miami-Dade's Zones A and B include Miami Beach and Key Biscayne. Collier County has issued a voluntary evacuation of Marco Island.

In Miami-Dade County, the southeastern-most county on the U.S. mainland with more than 2.7 million residents, about 150,000 people so far have been ordered to evacuate. But the county mayor said that number could surge today depending on the information officials receive from the National Hurricane Center, which is tracking the Category 5 storm.

Miami-Dade County has also equipped several gas stations and supermarkets with generators so the retailers will be able to operate if the storm causes power outages. There's "plenty" of supplies and the county's gas stations will be replenished today, the mayor said.

Gimenez, a retired firefighter, added that Floridians have "learned a lot of lessons of the past" since Hurricane Andrew, a Category 5 storm that devastated the Bahamas and Florida in August 1992.

"We are well prepared here in Miami-Dade County," he told "GMA."