Virginia Gov. Robert F. McDonnell (R) urged residents not to let their guard down just because Hurricane Irene has weakened, saying it is still a serious storm that will likely cause major damage in the state.

“When you see reports of winds going from 100 miles per hour to 80 miles per hour, people let their guard down,’’ McDonnell said at a news conference at the Emergency Operations Center in Richmond. “It’s absolutely the wrong thing to do. Nothing else has changed with the track, with the rainfall potential. The only thing that has changed is a slight decrease in wind speed. It is no time for people to relax.”

Eastern Virginia began experiencing tropical-force winds Friday morning. Those are expected to become hurricane force winds, which will last six to seven hours, McDonnell said. Some parts of the state will experience 30 hours of tropical-force winds, he said. State emergency officials also expect 6-12 inches of rain in parts of the state and flooding of up to nine feet because of the storm surge and high tide.

“This is a significant weather-making event that has potential for wind damage, flooding particularly in the low-lying areas in Hampton Roads,’’ McDonnell said.

Virginia Beach reported the death of one surfer late Friday afternoon, McDonnell said.

A tornado watch is in effect until 8 p.m. for much of southeastern Virginia. Tornado warnings are in effect in select areas, including Norfolk and Virginia Beach.

About 65,000 people in Virginia are without power, including 57,000 in Hampton Roads, 8,000 in Richmond, 1,000 in the Northern Neck and 1,000 in Southside.

“We expect that to go up dramatically as the eye gets closer and the hurricane force winds enter the commonwealth this afternoon,’’ McDonnell said.

Nearly 200,000 people in Virginia were subject to mandatory evacuations, including 40 percent of Hampton and a third of Norfolk. Shelters that have opened around the state can sleep 100,000 people.

In Hampton Roads, two key bridges were closed Saturday morning. The Midtown Tunnel is expected to re-open 9 p.m. Sunday. The Hampton Roads Bridge-Tunnel is expected to re-open at noon Sunday.

President Obama declare an emergency in Virginia, which allows the federal government to pay 75 percent of some costs and assist in certain missions. A team of 40 people from FEMA is in Virginia helping to respond to the storm.

“We are just beginning to see the effects,’’ McDonnell said. “Please stay off the roads. Stay home. Stay vigilant.”