Late Thursday, Hurricane Helene struck Florida’s Gulf Coast as a “very dangerous Category 4 hurricane,” leading to mandatory evacuations across multiple counties due to a deadly storm surge that peaked at 20 feet, resulting in widespread power outages affecting millions in the Southeast.

According to Phil Klotzbach, a senior hurricane scientist at Colorado State University, this marks the first recorded Category 4 hurricane to hit Florida’s Big Bend area since 1851.

The storm had weakened into a tropical storm early Friday morning, then downgraded further into a tropical depression by Friday afternoon while moving through Georgia, causing “historic and catastrophic flooding” in parts of the Southeast and southern Appalachians. By the NHC’s 2 p.m. ET Friday update, Helene was positioned about 125 miles south-southeast of Louisville, Kentucky, with maximum sustained winds near 35 mph.

The NHC indicated in its 2 p.m. advisory that a reduction in forward speed is anticipated soon, predicting the depression will stall over the Tennessee Valley on Friday night, continuing into the weekend. Ongoing weakening is expected, and Helene is predicted to transition to an extratropical storm later on Friday, as noted by the NHC.

As reported by the NHC Friday morning, tropical-storm-force winds were extending as far as 345 miles east of the storm’s center.

Where and when did Hurricane Helene make landfall?

Helene came ashore at approximately 11:10 p.m. ET Thursday, close to Perry, Florida, with winds measuring 140 mph.

What causes storm surges?

Despite being downgraded to a tropical storm, Helene continues to pose a significant threat with dangerous storm surge, winds, and heavy rainfall in parts of southeastern United States on Friday morning.

In the ocean, winds from a hurricane push water toward the storm’s center. Instead of accumulating in one spot, the water spirals downwards and flows outward.

As water is forced down, some of it is pushed out towards the sea while the rest is directed toward the coast.

As the storm approaches shallower waters near land, the ocean floor obstructs the outflowing water, prompting the water to surge onto the beach.

The terrain of the ocean floor and the coast’s shape can affect the height of the storm surge, which is the difference in water levels as a storm approaches. On the Gulf of Mexico’s coast, extensive, gently sloping continental shelves increase the risk of water accumulating. Conversely, the Atlantic’s steeper and narrower shelves result in a lesser surge.