Helene sparking inland flooding disaster, widespread power outages in Southeast

Helene has left millions in the dark and prompted flash flood emergencies across the Southeast as the "once-in-a-generation storm" continues its rampage well inland following a Category 4 strike on Florida.

Key takeaways:

•Life-threatening, damaging and disruptive impacts from Helene are underway across the interior Southeast, following the storm's landfall southwest of Perry, Florida, on Thursday night.

•Damaging, hurricane-force wind gusts (74 mph or higher) have roared well inland across Georgia and into parts of South Carolina, leaving millions without power.

•Helene is unleashing a flooding disaster in some areas of the southeastern United States, especially in northern Georgia, upstate South Carolina and western North Carolina.

Helene's life-threatening, damaging and disruptive impacts are lasting long after the massive storm made landfall in the Florida Panhandle as a Category 4 hurricane on Thursday night. Heavy rain and strong wind gusts will spread through the southern Appalachians and into the Tennessee Valley into the weekend, AccuWeather meteorologists warn.

"Helene could be a 'once-in-a-generation storm' across western South Carolina and North Carolina, as well as northern and eastern Georgia," AccuWeather Senior Director of Forecasting Operations Dan DePodwin stated.

Because of the combination of impacts along the Gulf Coast as well as over vast areas of the interior Southeast, the AccuWeather RealImpact™ Scale for Hurricanes for Helene is a 4.

The RealImpact scale accounts for not only strong winds but also impacts from storm surge, flooding rain, the population affected and economic loss throughout the storm's duration while moving inland.

Typically, hurricanes lose wind intensity rapidly upon moving inland and transition quickly to tropical storms, tropical depressions and, finally, what AccuWeather refers to as tropical rainstorms. But sometimes, other factors can slow that weakening process down at the price of significant risk to lives and property.

As AccuWeather warned in advance, Helene maintained hurricane strength well inland, which resulted in widespread damaging wind gusts across Georgia and the Carolinas. Early Friday morning, Helene eventually became a tropical storm over central Georgia.

"Numerous trees can fall, blocking roads and resulting in widespread, long-lasting power outages," AccuWeather Chief Meteorologist Jonathan Porter warned ahead of the storm. The extent of fallen trees could hinder the arrival and progress of utility crews after the storm departs.

In some rural and heavily wooded areas of the Southeast states, power outages could last for weeks.

The likelihood of damaging winds is in addition to the potential for a number of tornadoes to occur as some hurricanes tend to produce tornadoes well after making landfall. The tornado risk typically exists to the east of the track of the center of the storm.

Flooding from heavy rain could be equally bad or worse than the winds in terms of life-threatening and damaging conditions.

"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," Porter added. Steep terrain in part of this zone can make matters worse.

The high risk of flooding will be due to the combination of rain that fell ahead of Helene during the first half of this week and another thrust of torrential rain into Friday as Helene moved inland. Flash flood emergencies were in effect for more than a dozen counties in western North Carolina, western South Carolina and northern Georgia, including the Atlanta metro area, on Friday morning.

Torrential downpours from Tuesday to Wednesday night, not directly related to Helene delivered 2-8 inches of rain from northern Georgia to upstate South Carolina, western North Carolina, eastern Tennessee, southwestern Virginia and southern West Virginia where flash flooding occurred.

"The risk is greatest in the steep terrain of the southern Appalachians where mudslides and rockslides can occur," Porter said, "Some communities may be cut off with no way to enter or exit for days if there is damage to infrastructure such as bridges and roadways."

In this area, 8-16 inches of rain can fall, with locally higher amounts and the bulk of the rain falling within only a few hours. The rain rate can exceed 2 inches per hour, which is enough to cause rapid, life-threatening flooding. The AccuWeather Local StormMax™ rainfall for Helene is 36 inches and may occur along the slopes of the southern Appalachians.

"Areas of significant urban flash flooding may also occur, such as in the Atlanta metropolitan area, should rain rates exceed a couple of inches per hour which can result in rapidly rising water and life-threatening flash flooding," Porter advised.

As runoff progresses downstream into larger waterways, a significant rise on some rivers is likely from the Carolinas and Georgia to Alabama and Tennessee.

Heavy rain is also forecast to reach farther to the north into parts of western Virginia, West Virginia and eastern Kentucky. This region is in serious drought, and any non-flooding rainfall might be welcomed.

Another pocket of heavy rain is forecast to develop mostly in response to a nearby non-tropical storm over portions of Arkansas and Missouri, where localized flash flooding can also occur this weekend.

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