Life-threatening storm surges are expected to hit Florida's Gulf Coast this week with the arrival of Hurricane Milton, which was upgraded early Monday into a Category 4 hurricane over the southern Gulf of Mexico.

The storm first strengthened into a Category 3 hurricane by 7 a.m. and by 9 a.m. ET rapidly intensified into a Category 4, the National Hurricane Center said.

Data from Hurricane Hunter aircraft indicated Milton has estimated maximum sustained winds of 150 mph.

The storm is forecast to make landfall Wednesday evening in Florida, which along with the wider southeastern United States continues to recover from the impact of Hurricane Helene. Widespread evacuation orders are likely across Florida.

As many as 15 million people are under flood watches across the Florida Peninsula and 11 million are at risk for tropical tornadoes tomorrow and Wednesday.

Milton has already gone through multiple rounds of rapid intensification — including 65 mph in 24 hours. On Sunday at 8 a.m. ET, it was a 60-mph tropical storm.

On the current track, Milton is forecast to reach peak intensity Tuesday morning. However, it is forecast to weaken as it approaches landfall, which would be late Wednesday between 6 p.m. ET and midnight as a strong Category 3.

Milton’s greatest risk is storm surge. While it’s not clear yet exactly where the center of the hurricane will make landfall, the worst surge will be just south of the center and places could see 8- to 12-feet storm surges.

The hurricane center issued a storm surge warning early Monday for Florida's Gulf Coast from Flamingo on the southern tip to the mouth of the Suwannee River, including Tampa Bay. A hurricane watch is in place along the same coastline, from Chokoloskee near Everglades City, to the Suwannee River, while a tropical storm watch is in place further west to Indian Pass.

"A Storm Surge Watch means there is a possibility of life-threatening inundation, from rising water moving inland from the coastline," the agency said.

Milton will also dump more rain, including up to 15 inches across portions of the Florida Peninsula and Keys through Wednesday night, making flash flooding likely.

As of 9 a.m. ET, Milton was about 150 miles west of Progreso, Mexico, and about 735 miles west-southwest of Tampa.

The Mexican government has issued a hurricane warning for the Yucatan Peninsula coast from Celestun to Rio Lagartos.

"On the forecast track, Milton is forecast to move near or just north of the Yucatan Peninsula today and Tuesday, then cross the eastern Gulf of Mexico and approach the west coast of the Florida Peninsula by Wednesday," the hurricane center said in its 7 a.m. forecast.

Florida Gov. Ron DeSantis told a news conference Sunday that an around-the-clock operation to clear debris and fallen trees from Helene was underway ahead of Milton's arrival, to minimize the threat from flying objects. He said Milton is expected to make landfall in Hillsborough or Pinellas county Wednesday evening and pre-emptively issued an emergency declarations for 51 counties.

Hillsborough County issued a mandatory evacuation for Evacuation Zones A an B, for all mobile homes and manufactured housing starting at 2:30 p.m. Monday. Those residents should be in a safe location by 7 a.m. Wednesday. Nine shelters will be opened in mandatory evacuation zones.

Sarasota County also called for those in levels A and B (which includes barrier islands), and those in mobile or manufactured homes, to evacuate on Monday. Those living in level C “should be prepared to evacuate if the storm intensifies.”

In the coastal city of Anna Maria, south of Tampa, a mandatory order begins at midday. Pinellas County has begun mandatory evacuations for long-term care facilities.

However, state officials are stressing that people can leave without an order. “Have a plan, execute the plan,” DeSantis told the news conference. “You certainly can leave now. You don’t have to wait to get an evacuation order.”

Polk County, to the east of Tampa, has published a list of shelters, including three pet-friendly ones.

Many schools and colleges will be shut: All public schools in Collier County are shut from Monday through Thursday.

The National Weather Service office of Tampa Bay warned that coastal Gulf counties such as Sarasota, Pinellas and Lee counties, should plan for “catastrophic wind damage.”

“Act now to complete preparations before the wind becomes hazardous,” the office warned.

Milton is noteworthy for its very unusual path, approaching Florida from the west: Since 1850, only two storms have originated in the Gulf's Bay of Campeche and made landfall in Florida.

Further, for the first time on record, the Atlantic has three hurricanes simultaneously after September: Kirk, Leslie and Milton.

Hurricane Helene, which made landfall in Florida's Big Bend region Sept. 26, killed more than 230 people in six states.

Record-warm waters made more likely by climate change will fuel the rapid intensification of Milton as water in the Gulf of Mexico is 2-4 degrees Fahrenheit above average. Rapidly intensifying hurricanes are becoming more common in the warmer world.