(This post was updated throughout Sunday and last updated at 8:00 a.m. Monday to reflect the latest National Hurricane Center advisory and current conditions in Florida.)
Extremely dangerous Hurricane Irma first crashed into the Florida Keys on Sunday morning and then made a second landfall on Marco Island on Florida's west coast Sunday afternoon, unleashing violent wind gusts up to 142 mph and storm-surge flooding. As it plowed through northern Florida Monday morning, however, it weakened to a tropical storm.
At 8 a.m., the storm was centered 105 miles north-northwest of Tampa. The storm center was barreling north-northwest at 18 mph toward southern Georgia. Almost the entire state of Georgia was covered in heavy rain.
"Significant river flooding is likely over the next five days in the Florida peninsula and southern Georgia, where average rainfall totals of 8 to 15 inches are expected," the National Hurricane Center said.
Dangerous flash flooding due to heavy rain and storm surge was occurring around downtown Jacksonsville. And thanks to Irma's large wind field, a broad area was experiencing tropical-storm force winds and gusts up to hurricane strength.
Irma's peak winds of 70 mph make it a strong tropical storm, but much less intense than the Category 4 hurricane it was just one day earlier with 130 mph winds. The National Hurricane Center said Irma is expected to weaken to a tropical depression by Tuesday afternoon.
Coastal waters could still rise well above normally dry land along the coast of Florida, Georgia, and southern South Carolina, resulting in areas of flooding.
A flash flood emergency was issued for downtown Jacksonville early Monday morning, in effect through 12:30 p.m., due to heavy rain and storm surge. The Weather Service urged affected residents to move to higher ground. The Weather Service reported the storm surge flooding in Jacksonville set an all-time record.
Wind gusts in northeast Florida had reached up to 60 to 80 mph.
Irma's eyewall passed on the east side of Sarasota around 10 p.m. and was about 60 miles north of Tampa around 5 a.m., producing wind gusts around 75 mph.
Even on Central Florida's east coast, tropical-storm force winds and hurricane-force gusts were fairly widespread Sunday evening. At St. Lucie, a gust reached 99 mph and Cape Canaveral gusted to 79 mph.
The worst winds had passed this region just prior to 9:30 p.m. Sunday but gusty showers continued on the storm's backside.
Irma's eyewall passed through Fort Myers and Cape Coral just before 7 p.m., producing wind gusts of 88 and 101 mph and then passed on the west side of Port Charlotte between 8 and 9 p.m.
Josh Morgerman, a hurricane chaser positioned in Naples, described the scene: "Went thru violent, destructive winds. Screaming, whiteout, wreckage blowing by in fog." Then the calm eye moved overhead.
Before the arrival of the storm center, water was actually retreating from Naples to Tampa due to offshore winds from the east pulling the sea back. But forecasters warned residents that shortly after the storm's center passed to the north and winds blew back onshore, waters would rush back in rapidly causing severe inundation.
In Naples, as of 7 p.m., water levels were about four feet above normally dry land but the level was starting to stabilize around 8 p.m. Amazingly, it set its second lowest water level and highest water level all in the course of 8 hours.
In Ft. Myers, waters levels were rising through 10 p.m., but not as dramatically as they had in Naples.
In Southeast Florida, spiral bands continued to unleash tropical-storm-force winds. Even into the evening, winds were gusting up to 60 to 75 mph around Miami and West Palm Beach (7 p.m. gust of 75 mph), but they weren't as strong as earlier.
In the afternoon, sustained winds in Miami and Fort Lauderdale reached 50-60 mph through the early afternoon, gusting as high as 80 to 100 mph. Miami International Airport clocked a gust to 94 mph and an isolated gust hit 100 mph at the University of Miami.
Also during the afternoon the seas had risen several feet above normally dry land. Social media photos and videos showed water pouring through Miami's streets, in between high-rises, amid sideways sheets of rains.
Late Sunday afternoon, waters were finally starting to slowly recede around Miami.
While the core of the storm and worst winds passed the Keys early Sunday morning, the Weather Service warned storm surge flooding was ongoing as winds on the storm's backside shoved water over the islands. Gusts still reached 50 to 60 mph as of 7:45 p.m.
Early Sunday afternoon, the maximum surge at Cudjoe Key was estimated at 10 feet.
About 3.6 million customers were without power.
Particularly in South and Central Florida, torrential rain had fallen, with widespread totals of 6 to 10 inches and pockets up to 10 to 14 inches. Numerous flash flood warnings had been issued.
As the storm's spiral bands walloped Central and Northern Florida, the potential for tornadoes arose in the swirling air, and the Weather Service issued watches and scores of warnings.
Storm warnings in effect and predicted surge height and winds
Tropical storm warnings cover much of central and northern Florida (except for the far west panhandle), the entire Georgia coast, and the southern coast of South Carolina.
A storm surge warning remained in effect for much of Florida's west coast and northeast coast and extended north along the Georgia coast and southern South Carolina coast.
Effects on Florida
Conditions will continue to deteriorate Monday over Florida in the north part of the state as Irma plows northward. Conditions will slowly improve to the south.
Here's a guide to what is most likely and where …
Time frame for worst conditions: Sunday evening through Monday afternoon.
Hazard threats: Rain, tornadoes, wind, surge
The northeast portion of Florida will be spared the worst of Irma but won't escape unscathed. Sustained tropical-force winds of 40 to 55 mph will overspread the area from Daytona Beach to Jacksonville by Sunday evening, with the worst winds (gusts up to 70 mph) occurring overnight. Heavy rain will be a story line here as six to 10-plus inches of rain is expected to fall in a relatively short period.
As with other parts of the state, the tornado threat will peak overnight on Sunday as Irma's storm center tracks northward.
Storm-surge values will be elevated (two to four feet) but should result in only minor to moderate coastal flooding.
Potential effects on Georgia and the southeastern United States
Time frame for worst conditions: Monday morning through Tuesday morning.
Hazard threats: Wind, rain and, at the coast, storm surge
Hurricane warnings extend well into Georgia, covering over half of the state. Parts of southern South Carolina also are under a hurricane warning, with Irma poised to maintain its hurricane-force strength for several hours after landfall.
Sustained tropical force winds of 25 to 45 mph will spread over Georgia from south to north starting late Sunday night. The strongest sustained winds (40 to 50 mph) with gusts of 60-plus mph will move in on early Monday morning, lasting through Monday evening. This includes Atlanta, which is under a tropical-storm warning, where sustained winds of 25 to 40 mph with gusts up to 60 mph will occur from about 10 p.m. Sunday night to about 5 p.m. Monday afternoon. This could lead to downed trees and outages.
Heavy rain is also expected, with storm totals of six to 10 inches forecast, the bulk of which should fall Monday.
Storm surge along the Georgia/South Carolina coast will be a hazard, as well, with the Hurricane Center predicting a surge of four to six feet. Of particular concern is the duration of the storm surge. Persistent onshore winds will extend the surge component here, with elevated water levels potentially lasting up to 36 hours.
Irma's path so far
At 3:35 p.m. Sunday, Irma had made its second U.S. landfall of the day over Marco Island, where a wind gust of 130 mph was reported.
Earlier, the storm officially made its initial U.S. landfall at Cudjoe Key at 9:10 a.m. as a Category 4 hurricane. Winds over the Keys raged, gusting to at least 94 mph in Key West (before the wind instrument failed) and up to 120 mph in Big Pine Key. Witness video showed the rising storm surge flooding Key West streets.
Before its encounter with the Keys, Irma made landfall on the north coast of Cuba as a Category 5 hurricane just after 9 p.m. Friday, with maximum sustained winds of 160 mph. It became that country's first Category 5 hurricane since 1924. Fueled by the extremely warm ocean temperatures, Irma reintensified to the maximum hurricane classification level after weakening slightly on Friday afternoon.
As it scraped Cuba's north coast early Saturday, it produced a sustained wind gust of 118 mph, and a gust to 159 mph was reported at Falla, Cuba, in the eyewall of the hurricane.
On Friday, before making landfall along Cuba's north-central coast, Irma passed north of Haiti and then between Cuba's northeast coast and the Central Bahamas.
Thursday evening, the center of the storm passed very close to the Turks and Caicos, producing potentially catastrophic Category 5 winds. 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 and destructive winds had also probably battered the southeastern Bahamas, near Great Inagua Island.
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 people were reported to be without power. In Culebra, Puerto Rico, a small island 17 miles east of the main island, a wind gust registered 111 mph in the afternoon.
On Wednesday afternoon, the storm's eye had moved over Virgin Gorda in the British Virgin Islands, and its southern eyewall 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.
On Tuesday night and Wednesday morning, the hurricane passed directly over Barbuda and St. Martin in the northern Leeward Islands, the strongest hurricane 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 any storm on Earth on record, passing Super Typhoon Haiyan, the previous record-holder (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.
Irma's landfall pressure of 929 millibars in the Florida Keys was the lowest for any U.S. landfalling hurricane since Katrina (920 millibars) and for a Florida landfall since Andrew (922 millibars). It ranks as the seventh-lowest pressure of any U.S. landfalling storm.
When Irma crashed into the Keys early Sunday as a Category 4, following Hurricane Harvey's assault in Texas, it marked the first time on record that two Category 4 storms had made landfall in the United States in the same year.
Capital Weather Gang hurricane expert Brian McNoldy contributed to this report. Credit to tropical-weather expert and occasional Capital Weather Gang contributor Phil Klotzbach for some of the statistics in this section.