Hurricane watches could be posted for South Florida as early as Thursday, as the latest forecast showed a growing risk from one of the strongest Atlantic hurricanes in history.
“The threat of direct hurricane impacts in Florida over the weekend and early next week has increased,” the National Hurricane Center said. “Hurricane watches could be issued for portions of the Florida Keys and the Florida peninsula on Thursday.”
A hurricane watch, which is typically issued 48 hours before the storm hits, means hurricane conditions are considered possible within that time frame. If they are considered likely, the condition is upgraded to a hurricane warning.
“The odds are definitely looking worse for the east coast of Florida, given the latest set of model runs,” said Jeff Masters, meteorology director for the website Weather Underground, who used to fly on NOAA Hurricane Hunters. “It’s unusual to have the top four models all give the same track in advance, and they’re all doing that now. And they’re all saying it’s going to hit you guys squarely on Sunday.”
Miami-Dade County ordered evacuations to start Thursday morning in Miami Beach and other barrier islands, as well as evacuation zone A, which encompasses low-lying coastal neighborhoods on the mainland. Broward County ordered coastal and low-lying neighborhoods evacuated Thursday, with 14 shelters opening at noon.
At 2 a.m. Thursday Irma remained a Category 5 storm with winds of 180 mph ---down 5 mph from 11 p.m. Wednesday--- as it headed to the northeast of the Dominican Republic.
The question for South Florida is where that turn will take place. If the hurricane veers right soon enough, it could rake the Bahamas but avoid a direct strike on Florida. It could turn later and go up the Gulf coast or it could buzz right up the South Florida coast, putting millions of people in the path of a once-in-a-generation hurricane.
At roughly 400 miles wide, the hurricane is virtually certain to affect South Florida, whether through a few days of rainy, windy weather, a direct hit or something in between. Hurricane-force winds extend 50 miles from the center and tropical-force winds extend 185 miles from the center.
The storm, which currently exceeds Hurricane Katrina or Hurricane Andrew in strength, is forecast to weaken over the next two days. But it would be a still-formidable Category 4 storm, with winds of 145 mph or more, as it approaches South Florida.
The timing of the hurricane’s possible arrival is also uncertain. Tropical storm conditions, which means winds of 39 to 73 mph, are possible in South Florida starting late Friday or Saturday, according to the National Weather Service in Miami. If the hurricane comes close enough to deliver-hurricane-force winds, they could arrive Saturday or Sunday.
Gov. Rick Scott called up 900 more National Guard members Wednesday morning, after the forecast cone shifted overnight to place more of the state in danger.
South Florida began buttoning up for the storm, with a round of school closings, evacuation orders and shelter openings from the Keys through Palm Beach County.
The Broward evacuation covers neighborhoods east of Federal Highway, barrier islands, mobile homes and low lying area, County Mayor Barbara Sharief said a news conference Wednesday.
Although the evacuation is mandatory, there will be no enforcement.
“We’re not going to knock on doors,” Broward Sheriff Scott Israel said. “We’re not arresting people and we’re not pulling people out of their homes. We’re asking you to leave so you don’t become a victim.”
Palm Beach County announced plans to open 15 shelters Friday morning, as well as pet-friendly and special needs shelters, but has not announced any evacuation orders.
Although South Florida lies within the center of the forecast cone, the hurricane center stressed that the forecast of the storm’s likely path has become less clear at the three-day mark, when it could be near South Florida, due to a lack of certainty about the an upper-level trough over the southeastern United States. The storm is currently being held on a western course by a zone of high pressure to the north. But this high-pressure zone is expected to be eroded in next few days, as the low-pressure trough moves in, allowing the hurricane to hook to the north.
The governor’s activation of additional members of the Florida Army and Air National Guard brings the total to 1,000. The state’s remaining 6,000 National Guard members were told to report for duty by Friday.
At a news conference Wednesday in Marathon in the Florida Keys, the governor urged residents to obey evacuation orders.
“Storm surge and extreme winds are the biggest concern right now,” he said. “The storm is biggest, faster and stronger than Hurricane Andrew.”
Storm surge, the temporary rise in sea-level caused by a hurricane’s winds, has historically been the biggest killer, as ocean water floods neighborhoods and waves tower higher on top of the elevated sea level.
Irma has been forecast to bring storm surges of seven to 11 feet to the islands of the northeastern Caribbean, although the National Hurricane Center has issued no storm surge forecast yet for South Florida.
As the hurricane’s predicted course came closer to South Florida, various graphics have appeared on TV and the Internet showing a confusing and often frightening mix of possibilities for the storm’s path.
The tangle of paths known as the spaghetti models represent computer simulations constructed by various government agencies, using various assumptions about weather conditions over the next few days. A bunch of strands of this meteorological spaghetti typically represent a single model’s simulations, using different assumptions.
The National Hurricane Center’s cone of uncertainty, the widening graphic that shows the range of possibilities up to five days out, represents the agency’s best judgment, using the spaghetti models and the agency’s own analysis. Forecasters universally urge residents to focus on the cone, not any single strand that shows a particular path, since the margin of error widens substantially with time.
The average forecast error at four days out is 175 miles, and at five days out it’s 225 miles.
The storm, which carried enough power to register on devices that record earthquakes, made landfall on the islands of the northeastern Caribbean early Wednesday morning, pounding Barbuda, Antigua and other islands with heavy rain and some of the strongest winds ever recorded in an Atlantic hurricane.
Although the islands of the Caribbean, particularly the large ones, have acted as a natural shelter for South Florida, weakening many storms before they reach the state, this may not take place as much with Irma, the hurricane center said. Irma may experience only limited interaction with the islands, retaining Category 4 or 5 status as it enters extremely favorable conditions for a hurricane, with low wind-shear – the high-level cross winds that can disrupt a hurricane’s circulation – moist air and warm water, providing lots of energy for the storm.
Broward and Palm Beach county schools will be closed Thursday and Friday. The Florida Keys were ordered evacuated, with tourists told to start leaving by 7 a.m. Wednesday and residents told to start by 7 p.m.
“The hurricane force winds in Irma are wider than Florida,” said Bryan Norcross, hurricane specialist for The Weather Channel, who became famous as a Miami weather forecaster in Hurricane Andrew, in a tweet. “You won’t need a direct hit to get Wilma-type winds & storm surge on both coasts.”
The storm could produce eight to 10 inches of rain this weekend, generating local flooding but nothing on the scale of the catastrophic floods experienced by Houston, according to the South Florida Water Management District, which handles flood control from Orlando to the Keys.
The district is lowering the water levels in drainage canals to accommodate the rain, said John Mitnik, chief engineer for the district.
President Donald Trump declared an emergency for Florida, freeing up federal funding to help the state defend itself against the huge storm. Among possible emergency measures would be shoring up beach dunes, building emergency berms and planning for potential evacuations, according to a news release from the governor’s office.
“Our state emergency management officials are working with our federal and local partners to prepare for any potential impacts from this dangerous storm,” Scott said. “And it is crucial that we have access to every available resource to protect our families and communities.”
The storm’s strength places it in the top tier of Atlantic hurricanes, with wind speeds that exceed Hurricane Katrina and Hurricane Andrew at their maximum power, according to Phil Klotzbach, research scientist for Colorado State University's Tropical Meteorology Project. The strongest Atlantic hurricane ever recorded, Hurricane Allen in 1980, had winds of 190 mph.
Any approaching hurricane raises concerns about the Herbert Hoover Dike, which holds back the waters of Lake Okeechobee and could be vulnerable to a breach if a strong hurricane hits.
The Army Corps of Engineers announced plans late Tuesday afternoon to release water from the lake into the Gulf and the Atlantic.
The water level in Lake Okeechobee is at 13.67 feet, within the level considered optimal by the Army Corps of Engineers, which manages the dike.
In an interview last week Army Corps spokesman John Campbell estimated that a foot of rain falling into the basin north of the lake would cause the level to rise three to four feet as the water drains into it.
"We know that the dike has had performance issues when the lake is above 18 feet," he said. "When it's above 17 feet we are on a daily basis doing inspections."
The Associated Press contributed to this report.