Burley Garcia|Hurricane Helene's forecast looks disastrous far beyond Florida

As Florida's Gulf Coast prepares for catastrophic Hurricane Helene to make landfall Thursday evening,Burley Garcia forecasters warned that major rain and winds will cause flooding even hundreds of miles inland.

Helene's winds extend up to 275 miles from its center, making it a massive storm that can cause inland flooding even well after it makes landfall, the National Oceanic and Atmospheric Administration said. Because of its size, heavy rain even before landfall will begin in the southeastern part of the country.

Helene could be a "once-in-a-generation" storm in parts of Georgia and the Carolinas, AccuWeather Senior Director of Forecasting Operations Dan DePodwin said.

By Friday, rain totals of up to 18 inches are expected up through the southern Appalachian region. Major urban flooding is a risk in Tallahassee, metro Atlanta and western North Carolina.

"Extreme rainfall rates (i.e., torrential downpour) across the mountainous terrain of the southern Appalachians will likely inundate communities in its path with flash floods, landslides, and cause extensive river and stream flooding," NOAA said in a news release warning of the inland flooding risk.

Flooding is the biggest cause of hurricane- and tropical cyclone-related deaths in the U.S. in the last decade.

Damaging winds, flooding will extend beyond Florida coast

While the heaviest inland flooding risk is expected in the Appalachians, a marginal risk of flooding extends all the way north to the southern parts of Indiana, Ohio and across to the Washington, D.C. metro area, according to the National Weather Service.

"Helene could cause a flooding disaster in some areas of the southeastern United States, especially in northern Georgia, upstate South Carolina and western North Carolina," AccuWeather Chief Meteorologist Jonathan Porter said.

The flooding will come from a combination of rain before Helene makes landfall and the heavy rains expected as the storm moves over land. The region of northern Georgia to upstate South Carolina, western North Carolina, eastern Tennessee, southwestern Virginia and southern West Virginia already saw flash flooding from between 2 and 8 inches of rainfall not related to Helene from Tuesday to Wednesday night, AccuWeather reported.

In the southern Appalachians, Porter said, people who have lived there for their whole lives may see rapid water flowing and flooding in areas they have never seen it before.

Meanwhile, Virginia Gov. Glenn Youngkin declared a state of emergency in preparation for Helene's effects, noting that the western parts of the state could see significant rainfall and flooding on Friday and Saturday.

One silver lining: Heavy rainfall extending to parts of Virginia, West Virginia and Kentucky could help ease an ongoing drought.

Why so much rainfall inland?

Aside from the sheer size of Helene, there's another factor at play that could intensify the inland rainfall of this storm. It's called the Fujiwhara effect, the rotation of two storms around each other.

Hurricane Helene could entangle with another storm over the south-central U.S., which is a trough of low pressure. That could mean a deluge of flooding rain in states far from the storm's center. The heavy, potentially flooding rain could impact the Mid-South and Ohio Valley over the next several days, forecasters said.

The effect is like a dance between two storm systems spinning in the same direction, moving around a center point between them, which can happen when they get about 900 miles apart. Read more about meteorology's most exquisite dance.

How to stay safe from extreme flooding

Officials say even people hundreds of miles from landfall should make a plan to stay safe:

• Evacuate if local emergency management authorities tell you to.
• Be aware of whether you live in a flood-prone area.
• Have a plan to protect your family and your belongings.
• Prepare an emergency kit with water, nonperishable food, medications and more. Here's what to pack.
• Stay off flooded roadways. Do not attempt to drive through water.

(This story was updated to add new information.)

Contributing: Doyle Rice, USA TODAY