Southwest Florida: Worst and best scenarios from Hurricane Irma

Update, 1:40 p.m. Saturday:

MIAMI — Southwest Florida is on track to get a catastrophic strike from Hurricane Irma and potential storm shifts could create a worst or best scenario for the region.

The hurricane is expected to move along or near the southwest coast Sunday afternoon, according to a National Hurricane Center advisory released at 11 a.m. 

The best case scenario under that forecast: Irma moves inland or too far off shore. The latter is becoming very unlikely, said Chief Hurricane Specialist Michael Brennan.

The worst case scenario: If the center of the storm stays off shore and intensifies from Category 3 to Category 4 it will drive water into the coastal line ahead of storm. After Irma goes by and moves to the northwest its backside will continue to drive more surge.

[Evacuees around Florida rush to Orlando as Hurricane Irma approaches]

[Hurricane Irma: Melbourne Beach issues curfew]

The bottom line: If you're in an evacuation zone, leave, Brennan said. The area extending from Captiva to Cape Sable could get 10 to 15 feet of flooding, enough to fill the first floor of a home and cause people to drown. You should also take into consideration the waves that would top that water.

"We’re looking at a prolonged period of huge water and storm surge flooding into the Southwest coast," Brennan said.

He also advised people "not to get caught on the category because that’s just one aspect of the storm."

"That’s just the wind," Brennan said. "The storm surge is affected by the size of the storm. And of course Irma is such a large storm. You can see how big its circulation is. It’s gonna move a lot of water around."

Update, 11:30 a.m. Saturday:

MIAMI — Hurricane Irma is "a very dangerous situation that's developing for the Florida Keys and for the West Coast," a National Hurricane Center official warned Saturday morning.

Irma, which is located 175 miles southeast of the Keys, is expected to pick up strength over the warm waters of the Florida straights and off the West Coast, Chief Hurricane Specialist Michael Brennan said. The Category 3 storm is projected to turn northwest.

"The immediate concern is the Florida Keys and Southwest Florida," Brennan said of the 11 a.m. advisory.

Storm surge is the biggest concern. 

The area extending from Cape Sable to Captiva can expect 10 to 15 feet of surge, which is more than the 6 to 12 feet projected Friday.

"That's certainly enough to drown somebody, to put water on the first floor," Brennan said.

Brennan warned residents in evacuation zones to leave before it's too late. The National Hurricane Center projects Irma will come over the Keys overnight and early Sunday and over the West Coast on Sunday and into early Monday.

All of Florida can expect up to 20 inches of rain, Brennan said.

Update, 9 p.m. Friday:

MIAMI — As Hurricane Irma is projected to be on a collision course with Florida on Saturday, one question lingers: will the southwest or southeast part of the state receive the brunt of its Category 4 winds?

Irma's forecast track from the National Hurricane Center shifted west Friday toward Naples, which means the Florida Keys will be heavily impacted. While one side of the southern peninsular part of the state is expected to received Irma's strongest winds, the other side may be getting Category 3 or 2 winds,said the center's acting Director, Ed Rappaport.

"Where that Category 3 or 4 occurs is not clear," he said. "It could be on the east coast or west coast. It will be one or the other with the other one getting Category 2."

Southwest Florida and the Tampa area can expect life-threatening 6 to 12 feet of storm surge.

[Hurricane Irma track update, radar loops]

[Hurricane Irma: What to expect and when in Brevard]

Irma's western shift wasn't previously expected under the National Hurricane Center projections but a 25-mile shift is well within a 100-mile margin of error two to three days before a storm, Rappaport said.

Under Friday's forecast, Category 3 winds are expected up to Lake Okeechobee with the Treasure Coast receiving Category 2 or 3 winds, Rappaport said. The hurricane will gradually weaken as it moves up to a Category 2 in Central Florida and Category 1 in North Florida.

Stronger winds in Central Florida would still be possible if the storm shifts east, and the threat of storm surge still is important, Rappaport said. About half of all hurricane victims drown.

Update, 5:16 p.m. Friday:

Here are some of the questions our USA TODAY Network staff and readers asked during a Facebook Live interview with Michael Brennan, chief hurricane specialist at the National Hurricane Center.

The interview took place Friday afternoon, so this information can change in the next few hours.

When should you be 100 percent ready for Hurricane Irma (that includes putting up shutters, seeking shelter or evacuation if there's an order)?

If you live in Southwest Florida: By mid-afternoon Saturday. Tropical storm conditions arrive in the area Saturday evening.

If you live on the Treasure Coast: By Saturday evening. All areas south of Lake Okeechobee will see worse conditions Sunday morning to afternoon.

If you live north of Lake Okeechobee (Central Florida and Brevard County): You have all day Saturday, and tropical storm conditions could begin Sunday during the day.

Tallahassee area: The storm is three days out. If it stays to the east, you will be on the weaker side of the hurricane, but you could get a direct hit if it changes direction.

Which areas are at greater risk of storm surge?

Southwest Florida, from Venice down the coast, covering Naples and could extend inland into Fort Myers and Port Charlotte. Of particular concern are Captiva and Cape Sable, which could experience 6 to 12 feet of above-ground water. To put into perspective, that could be twice as tall as an adult. 

Why should Southwest Florida be particularly concerned?

The center of Irma is moving very close to the Southwest coast, which is at risk of seeing core winds. You should prepare to see major winds of the hurricane, which can cause catastrophic damage as a Category 4 storm. "We are talking about building failures, roof damage, trees knocked down and severe damage to infrastructure," Brennan said.

Will Irma weaken as it moves up Florida?

Irma is expected to still be a hurricane by Monday but weaker along both coasts and in the core.

If the core moves east and close to the Atlantic coast, winds could be stronger. If it moves west and stays off shore, it won't weaken as fast as it would if it stayed in the Florida peninsula.

If it moves over the peninsula, it would weaken below a major hurricane, which is a category 3. But, remember, that’s still a major event and storm surge still is a concern. About half of all hurricane fatalities happen because of storm surge and drowning.

I live in Brevard County. Should I evacuate to a hotel in Orlando?

You should only leave if you been ordered to do so by emergency managers. Other than that, stay put in a safe place, the National Hurricane Center said.