Helene tracker: See projected path as storm continues to weaken over Appalachians

Hurricane Helene reached Florida's Gulf Coast late Thursday as an "extremely dangerous Category 4 hurricane,'' forcing mandatory evacuations for several counties as its deadly storm surge reached up to 20 feet , triggering millions of power outages in the Southeast.

Helene is the first known Category 4 storm to hit Florida's Big Bend region since records began in 1851, according to Colorado State University senior hurricane scientist Phil Klotzbach.

The storm weakened to a tropical storm early Friday morning, and weakened again into a tropical depression Friday afternoon as it barreled through Georgia, producing "historic and catastrophic flooding" over portions of the Southeast and southern Appalachians. As of the 2 p.m. ET Friday advisory from the NHC , the storm was located about 125 miles south-southeast of Louisville, Kentucky with maximum sustained winds near 35 mph.

The NHC said in the 2 p.m. advisory that a slowdown in forward speed is expected soon, with the depression forecast to stall over the Tennessee Valley Friday night and through the weekend. Continued weakening is expected and Helene is forecast to become extratropical later Friday, the NHC said.

Tropical-storm-force winds extended outward up to 345 miles east of the center of the storm, as of Friday morning, the NHC said.

Helene live updates: Storm wreaking havoc across Southeast; 8 dead; 4.4M in the dark

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When and where did Hurricane Helene make landfall?

Helene made landfall at about 11:10 p.m. ET Thursday near Perry, Florida, with 140 mph winds.

Tropical Depression Helene tracker

Tropical Depression Helene spaghetti models

Illustrations include an array of forecast tools and models, and not all are created equal. The hurricane center uses only the top four or five highest performing models to help make its forecasts.

How does a storm surge happen?

While Helene has weakened to a tropical storm, it is still unleashing life-threatening storm surge, winds and flooding rain to portions of the southeastern United States Friday morning.

In the open ocean, hurricane winds push water toward the center of the storm. Instead of piling up, the water spirals downward and flows outward.

As the water is pushed downward, some is pushed out to sea and some is pushed toward the coast.

As the storm approaches land and shallow water, the ocean floor blocks the outflowing water, causing the ocean to surge onto land.

The ocean floor and shape of the coast can influence the height of storm surge – the difference between water levels when a storm approaches. Along the Gulf of Mexico, wide, gently sloping continental shelves make the coastline more vulnerable to water piling up. On the Atlantic coast, narrower shelves with steep slopes produce a lower surge.

Contributing: John Bacon, Cheryl McCloud, Jeff Burlew, Christopher Cann, Jorge L. Ortiz , Javier Zarracina, Ramon Padilla and Stephen J. Beard, USA TODAY

Gabe Hauari is a national trending news reporter at USA TODAY. You can follow him on X @GabeHauari or email him at Gdhauari@gannett.com.

This article originally appeared on USA TODAY: Helene tracker: See projected path as storm continues to weaken over Appalachians