Hurricane Michael has strengthened into a Category 4 storm before it's expected to plow into Florida's Gulf shore with towering waves and roof-shredding winds as 500,000 people were under evacuation orders and advisories.

Hurricane Michael was packing winds of up to 130 miles per hour (210 km per hour), hours before it was set to make landfall on Florida's Panhandle or Florida's Big Bend where it potentially could unleash devastating waves as high as 13 feet (4 meters), the National Hurricane Center (NHC) warned.

'Weakening is expected after landfall as Michael moves across the southeastern United States,' the forecaster said.

Florida Governor Rick Scott called Michael a 'monstrous hurricane' with a devastating potential from high winds, storm surge and heavy rains - prompting President Trump to declare Florida a state of emergency.

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'Hurricane Michael will be here TONIGHT. This is your LAST CHANCE to get prepared for this monstrous and deadly storm. The state is not taking this storm lightly and neither should any family,' Scott warned.

Heavy surf from the approaching Hurricane Michael pounds the fishing pier on Okaloosa Island in Fort Walton Beach, Florida.
Heavy surf from the approaching Hurricane Michael pounds the fishing pier on Okaloosa Island in Fort Walton Beach, Florida.

Forecasters said Michael could potentially be the most powerful storm to strike the Panhandle in at least a decade. Parts of Florida's Big Bend area could see up to 12 feet of storm surge, while Michael also could dump up to a foot of rain over some Panhandle communities as it moves inland.

Scott warned that people in potentially affected areas should not take any chances against such a large storm, adding that 'no one's going to survive' such a wall of water.

'I can not emphasize enough. Hurricane Michael is forecast to be the most destructive storm to hit the FL panhandle in decades. It will be life-threatening and extremely dangerous. You cannot hide from this storm. You can rebuild your home, you cannot rebuild your life,' he said.

'I understand that evacuations are inconvenient, but this storm will bring torrential rain and heavy wind, along with dangerous storm surge throughout the panhandle. If you have been told to leave, you need to go. It could be the difference between life and death.'

The hurricane has started affecting flights in the Florida with multiple airports announcing they will be closing ahead of the storm making landfall. Airports in Pensacola and Destin-Fort Walton Beach will shut down from midnight and flights out of Panama City Beach and Tallahassee are already being canceled.

The National Hurricane Center is predicting Michael to make landfall somewhere over the Florida Panhandle or Florida Big Bend area on Wednesday.

The center warned residents along more than 300 miles of coastline, from the Alabama/Florida border to the Suwannee River in Florida, to brace for hurricane conditions.

As much as 1 foot of rain was also forecast across Florida, Georgia and South Carolina.

The destructive force on the storm's outer bands were felt out at sea on Monday night as passengers on board the Royal Caribbean's Empress of the Seas filmed the ship being tossed around by strong winds.

Footage posted on Twitter by Snapper Tams showed the cruise ship caught up in the storm southwest of Isla de la Juventud in Cuba. He tweeted that the storm heavily delayed their arrival into Havana because the ship was forced to sit for several hours waiting for it to move through the Yucatán Channel.

National Hurricane Center director Ken Graham, left, updates viewers on the status of Hurricane Michael.
National Hurricane Center director Ken Graham, left, updates viewers on the status of Hurricane Michael.

The Florida governor declared a state of emergency in 35 counties along the Panhandle and Florida's Big Bend regions. About 2,500 National Guard soldiers were assisting and Florida Highway Patrol made nearly 350 troopers available for deployment.

State offices, schools and universities were closed through the end of the week in Panhandle counties. Scott also warned caregivers at north Florida hospitals and nursing homes to do all possible to assure the safety of the elderly and infirm.

Lines at gasoline stations grew as people left and tolls were waived in a bid encourage evacuations. Residents who opted to stay emptied grocery store shelves of water and other supplies.