HOLLYWOOD, Florida (CNN) -- Hurricane Katrina will make a "big shift" to the west on its way across the Gulf of Mexico and is expected to reach dangerous Category 4 intensity before making landfall Monday afternoon in Mississippi or Louisiana, the National Hurricane Center said Friday.

"I just don't see any reason why this will not become a very, very powerful hurricane before it's all over," hurricane center director Max Mayfield said at a news conference.

Category 4 storms on the Saffir-Simpson scale include dangerous sustained winds between 131 and 155 mph (210-249 kmh) and can create a storm surge of 13 to 18 feet (4 to 5 meters).

A storm of that intensity can cause extensive damage to buildings and destroy mobile homes, as well as flooding areas lower than 10 feet (3 meters) above sea level as far as 6 miles (9.6 kilometers) inland.

Forecasters said the storm is expected to keep getting stronger, reaching Category 3 intensity on Saturday, with winds of at least 111 mph (178 kmh).

Katrina hit Florida's east coast Thursday evening as a Category 1 storm. It pounded South Florida -- flooding streets, toppling trees and leaving at least six people dead.

At one point, more than a million residents and businesses were without power.

At 8 p.m. ET on Friday, Katrina was a Category 2 hurricane with sustained winds near 100 mph (161 kmh). It is expected to gain power from the warm waters of the Gulf of Mexico over the weekend.

The storm is moving west-southwest at about 8 mph (12 kmh) and its center is about 100 miles (161 kmh) west of Key West, Florida.

A tropical storm warning remained in effect for the Florida Keys and Florida Bay in an area stretching from Key Largo south and west to Key West and the Dry Tortugas.

Mayfield said the various computer models used to track the likely route of Katrina showed the storm moving west. That's potentially good news for residents of the hurricane-weary Florida Panhandle, who appeared to be in the storm's sights earlier Friday.

However, Mayfield cautioned against Floridians taking too much reassurance from those models, noting that "this is still three days away from landfall, and they could shift right back again."

"Everybody from southeast Louisiana through the Florida Panhandle really needs to pay attention," he said.

States of emergency

In anticipation of a possible landfall, Mississippi Gov. Haley Barbour and Louisiana Gov. Kathleen Blanco declared states of emergency Friday.

Blanco said "very well-coordinated evacuations" were planned that will be enacted "if there's a direct threat."

New Orleans is of particular concern because much of that city lies below sea level.

"It's always a huge concern, because there's a very large lake, Lake Pontchartrain, that sits next to New Orleans, and if the hurricane winds blow from a certain direction there are dire predictions of what may happen in that city," Blanco said.

Robert Latham, director of the Mississippi Emergency Management Agency, said evacuations of tourists along the coast could begin late Saturday afternoon, followed by mandatory evacuations of coastal residents on Sunday. The National Guard had been activated to help with storm preparations, he said.

The last time Mississippi or Louisiana saw landfall from a storm classified as Category 4 or stronger was in August 1969, when Hurricane Camille roared ashore with winds in excess of 155 mph (246 kmh), killing 143 people.

Red Cross emergency officials also were keeping an eye on Katrina, said the Red Cross' Renita Hosler.

"If it were a Category 4 storm, the scale and scope of what we would do would be much greater," Hosler said. "We would have more emergency response vehicles already pre-positioned the area. We would have more Red Cross kitchens identified and ready to open."

In the Gulf of Mexico, six oil companies operating offshore facilities evacuated at least 150 people as a precaution. However, most of those employees were described as "non-essential" to production, and rigs and platforms continued to operate.

At least 12 platforms and nine oil rigs in the Gulf have been evacuated -- a small portion of the 953 manned rigs and platforms operating there, according to the Interior Department's Mineral Management Service.

Friday afternoon, the Air Force began evacuating aircraft from at least two bases in the Florida Panhandle to minimize any possible damage.

Massive power outages

Florida power officials Friday afternoon said they had restored power to nearly 400,000 Florida Power and Light customers who lost electricity as the storm cranked its way across southeast Florida and the Florida Keys.

At the height of the storm, 1.4 million customers were without power. About 90 percent of customers in Miami-Dade and Broward counties should have their power back by Tuesday, although it may take as long as a week to get some customers back on line in the hardest hit areas, the company said.

Florida Gov. Jeb Bush said he has requested a federal disaster declaration that would bring aid to Broward, Palm Beach and Miami-Dade counties. Other counties could be added, he said. (See recent video of the aftermath and flooding.)

On Friday, Katrina continued to hamper the U.S. Coast Guard's search for a family of five missing at sea.

Four people were killed in accidents involving fallen trees in Broward County. The death toll climbed when a man's body was recovered from a boat docked on a small island near Dinner Key Marina, Miami police reported.

Another man was found dead in a capsized houseboat half a mile from the marina.

The Keys, which were not evacuated, were pounded all day Friday by rain and tropical-storm force winds, and another 5 to 8 inches of rain are expected. Forecasters said they would not be surprised if rainfall totals reach 15 to 20 inches in the Keys.

Katrina first rolled ashore between Florida's Hallandale Beach and North Miami Beach, bringing punishing winds and torrential rains.

While Katrina battered Broward County with wind gusts as high as 92 mph, it dumped more than a foot of rain on parts of Miami-Dade County to the south.

Insurers already are estimating that Hurricane Katrina will cause from $600 million to $2 billion in damage. (Full story)

Damage reports included an overpass under construction over Florida Highway 836 west of Miami, which collapsed. There were no reports of injuries, Miami-Dade Mayor Carlos Alvarez said.(View video of the downed bridge.)

CNN's John Zarrella, Jason Carroll, David Mattingly, Rob Marciano and Jacqui Jeras contributed to this report.

Copyright 2005 CNN. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed. Associated Press contributed to this report.

Commentary:
Some will argue they had no idea this storm was huge and getting bigger? They'll even argue there was nothing they could do. Poppycock. The levee's were built to withstand a Category 3 hurricane and this pup was projected to be a Category 4 on the Friday before it hit. States of Emergency were declared on Friday but it took Bush until the next Wednesday to return to work.