(This story, first posted Thursday, will be updated through Friday morning. It was last updated to reflect the 8 a.m. Friday National Hurricane Center advisory and computer model forecasts from overnight.)
Hurricane Irma, the massive, record-setting hurricane, continues on a collision course with South Florida where severe, if not catastrophic, damage is becoming inevitable. By early next week, Georgia and the Carolinas could also be in the storm's crosshairs.
Thursday night, as the storm charged ever closer to the U.S. mainland, the National Hurricane Center issued hurricane warnings for the Florida peninsula from Jupiter Inlet southward and around the peninsula to Bonita Beach, including the Florida Keys.
"Severe hurricane conditions are expected over portions of the Florida peninsula and the Florida Keys beginning late Saturday," the National Hurricane Center said early Friday.
A storm-surge warning was issued for the same area of Florida under the hurricane warning because of the potential for water to rise 5 to 10 feet above normally dry land at the coast. The Hurricane Center said this would bring the risk of "dangerous" and "life-threatening" inundation.
Hurricane and storm surge watches extended north of the warning area on Florida's east and west coast up to Sebastian Inlet and Anna Maria Island.
Closer to the storm's immediate location, hurricane warnings remained in effect for the Bahamas, and northern Cuba.
Having just raked across the Turks and Caicos islands, the storm was chugging along at 16 mph toward the west-northwest Friday morning, about 450 miles southeast of Miami. Irma was forecast to pass between the Central Bahamas and north coast of Cuba Friday, according to the Hurricane Center.
Packing peak winds of 150 mph, Irma was an extremely dangerous Category 4 storm Friday morning. Through Thursday night it had produced Category-5 winds (157 mph+) for two and a half days.
It is possible the storm could regain Category 5 intensity if it remains clear of large land masses, which would disrupt the storm. There is a scenario in which the center could scrape Cuba's north coast, which would weaken it. However, Irma is still to pass over some of the warmest water in the world (nearly 90 degrees), which could allow for restrengthening.
The Hurricane Center said to expect fluctuations in the storm's intensity but that, in most scenarios, "Irma is expected to remain at least a Category 4 hurricane until landfall in Florida."
It urged residents of Florida to rush preparations to completion.
"This hurricane is as serious as any I have seen," tweeted Eric Blake, a forecaster at the Hurricane Center. "No hype, just the hard facts. Take every life saving precaution you can."
Meanwhile, two other hurricanes were intensifying in the eastern Atlantic and southwest Gulf of Mexico — Jose and Katia. On Saturday, Jose could hit some of the same small islands in the northern Lesser Antilles decimated by Irma, including Antigua and Barbuda.
Potential effects on Florida and the Southeast U.S.
In South Florida, this storm is being taken as deadly serious. Coastal areas are being evacuated, shelters are being established, and food and gas supplies have dwindled. Although there is uncertainty in the track and the exact path of the violent eye wall, where winds are the strongest, it will be difficult for the state to avoid a disaster: It's just a matter of how severe.
Tropical-storm-force winds are expected to reach South Florida by Saturday as Irma approaches from the southeast. Then, the all-important northward turn is still expected to take place early Sunday, when the storm would make landfall and unleash its worst effects. Exactly where the north turn occurs is the critical question for Florida.
As of Friday morning, the most likely scenario based on computer-model guidance was that the storm will track between right up the spine of Florida to near its East Coast.
Models, however, can shift. The difference between a track just off the east coast and just off the west coast is only 150 miles, and the average error in hurricane forecasts this far in advance is nearly that big. It is still not out of the question that Irma could track north up the west coast of the Florida peninsula.
If the storm tracks up Florida's east coast, then Miami, Fort Lauderdale, West Palm Beach, Melbourne, Daytona Beach and Jacksonville will take devastating hits. If it runs up the spine of the peninsula, the storm will be quicker to decay but hurricane-force winds would reach both coasts. If it buzz-saws up the west coast, then Key West, Naples, Fort Myers, Tampa and Tallahassee would face severe effects.
When Irma makes its closest approach to Florida — most likely early Sunday — the Hurricane Center predicts that it will produce Category 4 winds. Here is its description of the kind of damage Category 4 winds would inflict:
Catastrophic damage will occur: Well-built framed homes can sustain severe damage with loss of most of the roof structure and/or some exterior walls. Most trees will be snapped or uprooted and power poles downed. Fallen trees and power poles will isolate residential areas. Power outages will last weeks to possibly months. Most of the area will be uninhabitable for weeks or months.
Note that such extreme winds are typically confined to the eye wall, which is only about 10 to 15 miles wide. That is why the exact track is important in terms of where the most severe wind damage concentrates.
Irrespective of exactly where Irma tracks, it appears inevitable that many coastal population centers in Florida will experience a devastating storm surge of 5 to 10 feet above normally dry land, inundating roads, homes and businesses. The biggest storm surge will occur immediately north of the storm center.
Over the Florida peninsula, 8 to 20 inches of rain is forecast, with the heaviest amounts most likely in the southeast.
Beyond Florida, there is a risk for destructive winds and a serious storm surge up to Georgia, South Carolina and North Carolina, but the details greatly depend on the track over Florida.
The worst case for these states would be if Irma narrowly misses the east coast of Florida, stays over warm water and then hits them while maintaining its strength. A potential landfall along the Southeast coast would be Monday. However, a number of models early Friday suggested the storm would pass over Florida's spine and into interior Georgia, such that there would be no second landfall from Irma along the Southeast coast.
"There is a chance of direct impacts in portions of Georgia, South Carolina, and North Carolina, but it is too early to specify the magnitude and location of these impacts," the Hurricane Center said early Friday.
As the storm tracks up the Southeast coast and then inland, heavy rain will expand over a large area, along with the potential for flash flooding, although it's too soon to pinpoint what areas will receive the most rain.
Effects on the Turks and Caicos and Bahamas
Thursday evening, the center of the storm passed very close to the Turks and Caicos, producing potentially catastrophic Category 5 winds and 8 to 12 inches (locally up to 20 inches) of rain. The storm surge was of particular concern, as the water had the potential to rise 16 to 20 feet above normally dry land in coastal sections north of the storm center, causing extreme inundation.
A devastating storm surge of 16 to 20 feet was also possible in the southeastern and central Bahamas into Friday, along with hurricane-force winds. The Hurricane Center noted that the southern Bahamas and northern Cuba could receive 10 to 15 inches of rain with isolated 20-inch totals.
Irma's path so far
Through early Thursday, the storm had battered islands from Puerto Rico to the northern Lesser Antilles.
While the center of Irma passed just north of Puerto Rico late Wednesday, a wind gust of 63 mph was clocked in San Juan early Wednesday evening, and more than 900,000 were reported to be without power. In Culebra, Puerto Rico, a small island 17 miles east of the mainland, a wind gust registered 111 mph in the afternoon.
Wednesday afternoon, the storm's eye had moved over Virgin Gorda in the British Virgin Islands, and its southern eye wall (the region of most powerful winds) raked St. Thomas in the U.S. Virgin Islands.
Early Wednesday afternoon, a wind gust to 131 mph was clocked on Buck Island and 87 mph on St. Thomas in the U.S. Virgin Islands.
Tuesday night and Wednesday morning, the hurricane passed directly over Barbuda and St. Martin in the northern Leeward Islands, the strongest hurricane ever recorded in that region and tied with the 1935 Florida Keys hurricane as the strongest Atlantic storm to strike land.
As Barbuda took a direct hit, the weather station there clocked a wind gust to 155 mph before it went offline.
The storm also passed directly over Anguilla and St. Martin early Wednesday, causing severe damage.
Irma's place in history
Irma's peak intensity (185 mph) ranks among the strongest in recorded history, exceeding the likes of Katrina, Andrew and Camille — whose winds peaked at 175 mph.
Among the most intense storms on record, it trails only Hurricane Allen in 1980, which had winds of 190 mph. It is tied for second-most intense with Hurricane Wilma in 2005, Hurricane Gilbert in 1988 and the 1935 Florida Keys hurricane.
The storm maintained maximum wind speeds of at least 180 mph for 37 hours, longer than storm on Earth on record, passing Super Typhoon Haiyan, the previous recordholder (24 hours).
Late Tuesday, its pressure dropped to 914 millibars (the lower the pressure, the stronger the storm), ranking as the lowest of any storm on record outside the Caribbean and Gulf of Mexico in the Atlantic basin.
The storm has generated the most "accumulated cyclone energy," a measure of a storm's duration and intensity, of any hurricane on record.
Without a doubt, the World Meteorological Organization will retire the names Harvey and Irma after this season. While there have been several instances of consecutive storm names getting retired (Rita and Stan 2005, Ivan and Jeanne 2004, Isabel and Juan 2003, Luis and Marilyn 1995), the United States has been hit by more than one Category 4+ hurricane in a season only one time: 1915. Two Category 4 hurricanes hit in Texas and Louisiana six weeks apart that year.
Credit to tropical-weather expert and occasional Capital Weather Gang contributor Phil Klotzbach for some of the statistics in this section.
Hurricane season update
We are nearing the halfway point of the hurricane season in terms of overall activity, and this season has ranked almost double what's normal (183 percent of average). This is largely due to the massive contribution from Irma, but also from Harvey as well as recent additions Jose and Katia. The June outlooks for an above-average season will almost certainly verify at this rate.