Hurricane warnings are in effect from just north of Tampa to southeast of Panama City, Florida. Storm-surge warnings cover the Florida Gulf Coast from the southern tip of the Everglades to near Mexico Beach in the panhandle.

Florida Governor Ron DeSantis (R) declared a state of emergency in 61 of the state’s 67 counties ahead of the storm; only six counties in southeastern Florida were not included. Mandatory evacuation orders have been issued for part or all of at least 26 counties between the panhandle and the peninsula’s west coast. Florida State University in Tallahassee said it will close its campus through to Sunday, among other school closures in the state.

“Preparations to protect life and property should be completed by early Thursday before tropical storm conditions arrive,” the Hurricane Center wrote.

Helene will be an exceptionally large storm with a broad wind field.

In fact, its size could be in the top 10% of hurricanes observed in the region, according to the Hurricane Center. That means its surge and wind impacts will be both greater and wider-reaching than most hurricanes. Tropical storm-force winds could extend more than 200 miles (322km) from the centre, and tropical storm warnings even extend into Miami, far from where Helene will come ashore.

These warnings also stretch into northern Georgia, including Atlanta, South Carolina and western North Carolina.

In all, hurricane and tropical storm warnings affect 36 million people from Florida to North Carolina.

Helene is forecast to barrel inland at high speeds, allowing severe winds to penetrate much farther into Georgia than typical. It could be Georgia’s most serious weather event in quite a while, with gusts of 80 to 90mph (128-144km/h) possible over southern portions of the state.

Downed trees and power outages are expected to be widespread in both northern Florida and Georgia.

Flooding rains should also affect portions of the Southeast and the Tennessee Valley. The southern Appalachians, where the National Weather Service has declared a rare “high risk” of excessive rainfall, could see up to 18 inches (45cm). “Catastrophic and life-threatening flash and urban flooding, including landslides, is expected across portions of the southern Appalachians through Friday,” the Hurricane Center wrote.

In addition to probably becoming the strongest hurricane to make landfall in the United States this year, Helene is poised to become the fifth hurricane to hit Florida since 2022 and potentially the ninth major hurricane, rated Category 3 or higher, to make landfall along the Gulf Coast since 2016.

Where is the storm now?

The centre of Helene was 430 miles (692km) southwest of Tampa and heading north at 12mph. Its peak winds were 85mph – a 40mph increase – making it a Category 1 hurricane. The Hurricane Center wrote that the storm showed the beginnings of a “ragged eye” in satellite imagery, a sign of intensification.

The storm’s centre was entering the warm waters of the Gulf of Mexico where “the environment appears primed for significant strengthening,” the Hurricane Center wrote.

Heavy rain streaming out ahead of the storm was developing from the eastern Gulf Coast into the Appalachians, where the National Weather Service declared a high risk of flooding. Multiple flash flood warnings were in effect in eastern Tennessee. This heavy rain falling ahead of the storm should increase the flood danger from the storm itself Thursday night and Friday by saturating soils and raising river levels.

Into Thursday, the storm’s gusty winds and downpours are expected to surge north over the Florida Peninsula and toward the panhandle. The wind will become more biting throughout the day, especially in northern Florida. The surge should increase by late morning, with conditions deteriorating more rapidly in the early afternoon. When the eyewall, or ring of violent thunderstorms surrounding the storm centre, hits the Big Bend, probably during the afternoon or evening, destructive winds will plough ashore.

Where and when will the storm make landfall and how strong will it be?

The storm is most likely to make landfall in the Florida Big Bend sometime on Thursday evening. While there remains an outside chance of a small path deviation west or east, which could pose more direct threats to the eastern panhandle or areas closer to Tampa, confidence is reasonably high in the projected landfall location.

Helene has another roughly 24 to 30 hours over the exceptionally warm waters of the eastern Gulf of Mexico, which contain near-record oceanic heat content – or hurricane fuel. Rapid intensification could ensue right until the point of landfall. That’s why it has become more probable the storm will make landfall as a Category 4 with sustained winds of at least 130mph.

What impacts should Florida expect?

Winds: Within the eyewall, gusts up to 130mph or so are possible within a few miles of the coastline. The National Weather Service will probably issue extreme wind warnings or special alerts for the immediate onset of tornado-like wind speeds found in the eyewall. Outside the eyewall, gusts of 75 to 110mph are still probable across extreme northern Florida and central and eastern parts of the Panhandle. Gusts over 70mph are likely in Tampa and could easily exceed 90mph in Tallahassee.

Surge: The greatest surge will be found where onshore winds are the strongest – near the storm’s core and to the immediate north and east. That’s where 15 to 20 feet of water will be pushed ashore, inundating ordinarily dry ground. A 5-to-8-foot surge is probable in Tampa Bay, potentially bringing the worst inundation to the area since the 1980s. The storm surge will arrive abruptly and build quickly because of Helene’s exceptional forward speed, which could be close to record-setting.

Flooding rains: A general 6 to 12 inches of rain, with some locally higher totals, is expected. Rainfall amounts will be tempered by Helene’s swift forward speed. Still, serious inland flooding is possible in spots.

Tornadoes: An isolated tornado is possible Thursday across the Florida Peninsula.

What impacts should the Southeast, Southern Appalachians and Tennessee Valley expect?

Winds: A serious windstorm is possible in Georgia, especially in the south. This may be among the greatest wind threats in years in southern Georgia. Widespread winds gusting to perhaps 90mph are possible. There’s even a chance the winds hold together into southwest South Carolina. Otherwise, the remainder of Georgia and western South Carolina could see gusts over 60mph. Tropical storm-force winds are possible elsewhere in the southern Appalachians and Tennessee Valley but gusts should diminish as the storm progresses to the north and west.

Flooding: There is a high risk for flash flooding and excessive rainfall in the southern Appalachians, particularly in the mountains of northeast Georgia, northwest South Carolina and southwest North Carolina. That’s where the terrain may focus rainfall and lead to totals up to 18 inches. Otherwise, widespread amounts of 6 to 12 inches are probable throughout much of Georgia, South Carolina, eastern Alabama, the Cumberland Plateau and Middle Tennessee. Atlanta could see half a foot or more of rain.

Tornadoes: Helene’s spin will probably result in a few tornadoes in the system’s spiral rain bands Thursday, particularly in the northern half of the Florida Peninsula, eastern Georgia and eastern South Carolina. All those regions are included in a Level 2 out of 5 risk of severe weather drawn by the Storm Prediction Center, but a Level 3 risk is up for Savannah, Georgia, and surrounding areas.