6:15 p.m. update

The powerful and unpredictable Irma is expected to pound the Big Bend for 18 hours or longer starting late Sunday, potentially as a major hurricane with devastating, triple-digit wind speeds.

Forecasters with the National Weather Service in Tallahassee said the most likely scenario has Irma making landfall around Tampa Bay on Sunday evening as a major hurricane. From there, it would move northwest, hugging the Gulf coastline and weakening over land before moving into the eastern Big Bend and into Georgia as a Category 1 hurricane.

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But they couldn’t rule out the possibility that Irma would miss Tampa Bay, maintain its intensity in warm Gulf waters and slam into the Big Bend as a major hurricane, with wind speeds of 111-mph plus.

"If the storm takes a westward track, there’s a possibility it could make landfall as a major Category 3 storm in the Taylor/Dixie County area," said Mark Wool, warning coordination meteorologist with the Weather Service. "And there’s no instance of that in the historic record. If the track shifts west, it could be a higher category hurricane here in town. That's the scenario we are hoping doesn't happen."

Even if Irma does make its way to the Big Bend in a weakened state, it's expected to unleash sustained tropical-storm-force winds from 40 to 70 mph, Wool said. Depending on Irma's exact track, eastern parts of Leon County could see higher winds than western parts.

"There's a very high likelihood that there will be widespread power outages just like last year, many trees coming down on power lines and people's homes and cars and blocking roads," Wool said.

As hurricane warnings went up Saturday for nearly the entire state, Leon County issued a voluntary evacuation for all residents. Emergency officials urged people to leave by 8 p.m. or shelter in place until Irma passes.

"This voluntary evacuation is not mandatory, but we have a responsibility to advise you at this time that it may be in the interest of your family's safety and comfort to do so," Leon County Commission Chairman John Dailey said during a Saturday news conference at the Public Safety Complex.

Mayor Andrew Gillum said city electric customers can expect major outages.

"We do expect widespread outages throughout the Tallahassee/Leon County area," he said. "If this will provide for you either discomfort or an unsafe situation, obviously you've got some options and some choices to make. For everyone else, we want you to continue to be patient, to continue to make preparations."

Sheriff Walt McNeil said deputies and Tallahassee police officers will be stationed throughout the city, in part to protect the property of people fleeing Irma. He urged people to stay off the roads beginning late Sunday.

"Get off the streets of our community before that storm hits," he said. "We don't want to have anybody on the roads after 10 o'clock on Sunday night."

Irma, a Category 3 storm with 125 mph winds, was moving west-northwest at 9 mph Saturday night through the Florida Straits, lashing Cuba, the Keys and South Florida.

The hurricane is forecast to dump 5-6 inches of rain on the Tallahassee area and bring storm surge of 4-6 feet at the coast. Forecasters said that could change, however, if Irma makes landfall in the Big Bend as a higher intensity hurricane.

5:45 p.m. update: Leon County is now under a Hurricane Warning until further notice. The National Weather Service says gusts could reach 80 mph.

4 p.m. update: City and county officials are calling for a voluntary evacuation of all of Leon County in preparation for what they call an "18-hour storm condition event."

In a short press conference, County Commissioner John Dailey and Tallahassee Mayor Andrew Gillum stressed that the recovery could be long and power could could be out for days, meaning it could be "in the interest of your family's safety and comfort" to evacuate.

"Because of the forseeable threat to life and property of Leon County citizens in what could be a long post storm recovery time, this voluntary evacuation is being issued to give citizens adequate time to leave the area if you choose to do so," Dailey said.

They said each family must determine whether they should leave or shelter in place.

They said by tomorrow evening they would ask that all motorists stay off city streets.

Gridlock Guide:Planning on evacuating? Create an exit strategy with this guide to traffic

2 p.m. update:

This just in from the City of Tallahassee: Brace for badness from Hurricane Irma. "Latest forecast track for Hurricane Irma predicts serious impacts to Tallahassee. Early impacts Sunday evening. Tropical Storm force winds expected just after midnight increasing to Hurricane force winds by Monday afternoon. Please use today to ensure that you, your family & your neighbors are prepared. Stay safe."

11 a.m. update: According to the National Weather Service out of Tallahassee, the latest for Hurricane Irma means an 80 to 90 percent chance for tropical storm force winds in Tallahassee and a 30 to 40 percent chance for hurricane force winds.

Evacuation orders have begun in coastal communities as the Big Bend prepares for the possibility of a direct hit from the giant storm.

Here's the latest on the storm's position and preliminary damage in Florida.

If you are planning to evacuate, here's your guide to the traffic gridlock with highway camera views.

Earlier update:

The forecast track for Hurricane Irma continued to shift to the west overnight, with the powerful storm expected to skirt up the west coast of Florida and potentially make a final landfall in the Big Bend.

If the current forecast track holds, Irma could make landfall around daybreak Sunday in the Keys before moving into the Gulf of Mexico and making a second landfall Sunday evening around Fort Myers, said Tim Barry, meteorologist with the National Weather Service in Tallahassee.

From there, Irma could move just east of Tampa before possibly heading back into the Gulf for a brief time. Irma would weaken over land as it heads toward Dixie County in the southeast Big Bend.

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"By the time it gets up to North Florida in our area, it should be either a strong tropical storm or a very weak Category 1 hurricane," Barry said. "We're looking at sustained tropical storm force winds, probably somewhere within the 35-40 mph range. Of course, we'll get higher gusts. It's not out of the question that we'd get hurricane-strength gusts."

The National Hurricane Center's five-day forecast track shows Irma maintaining its strength as a major hurricane, with winds of 111 mph or higher, as it closes in on the Tampa Bay area. The forecast shows Irma weakening over land as it moves north but maintaining tropical storm strength well into central Georgia or Alabama.

Tallahassee remains in the Hurricane Center's five-day cone of uncertainty, and a direct hit is not out of the question. The entire Big Bend, Tallahassee included, is under a hurricane watch.

“It is a little troubling that this track is being extended a little further west on each advisory, which is a big concern for us because if it were to continue to extend west, then there’s a possibility the center could stay out over the open warm waters, which would maintain most of (Irma's) strength of course,” Barry said.

Florida Gridlock Guide:Here's the latest traffic reports and live views from the state's highways.

During a news conference Friday, Leon County Schools Superintendent Rocky Hanna expressed doubts about models and forecasts, which just a few days ago showed Irma off the coast of Jacksonville.

“We’re preparing like the eye of the storm or what’s left is coming right over Tallahassee," he said.

Irma isn't expected to cause major flooding in the Tallahassee area. But predicted rainfall totals increased with the shift to the west, from 2-4 inches to 3-5 inches.

As of 5 a.m. Saturday, Irma was located near the north-central Cuban coast and heading west-northwest at 12 mph. The Category 4 hurricane had top sustained winds of 155 mph.

This morning, heavy rain and damaging winds were reported in South Florida as Irma’s outer bands began to come onshore. The National Weather Service in Miami urged residents in Key Biscayne, Coral Gables and south Miami to take cover.

In Leon County, officials issued a voluntary evacuation for people living in mobile and modular homes. The evacuation was announced Friday and went into effect this morning.

Check back with Tallahassee.com for more on Hurricane Irma.

Contact Jeff Burlew at jburlew@tallahassee.com or follow @JeffBurlew on Twitter.