Tropical Storm Helene: "Flash flood emergency" explained

Tropical Storm Helene brought more than a dozen concurrent "flash flood emergencies" across the Carolinas and Georgia on Friday — an unheard-of occurrence.

Why it matters: Flash flood emergencies are the most dire type of National Weather Service flood alert, issued only when flooding poses a threat that is catastrophic, life-threatening and imminent.

What to expect: NWS guidelines could call for a flash flood emergency designation when:

• Water has or is going to rapidly rise so much that it will threaten lives or homes;
• Water levels are reaching major flood stage or are unusually high;
• Or when the failure of a dam is imminent and poses extreme danger to communities downstream.

Zoom in: A never-before-seen extensive set of flash flood emergencies went into effect for Asheville, North Carolina and surrounding areas, where historic flooding is underway, bringing a high threat of landslides and flooding that could eclipse past events.

• The National Weather Service warned the storm would be "one of the most significant weather events to happen in the western portions of the area in the modern era."
• By Friday afternoon, rainfall totals in western North Carolina were as high as 20 inches.
• "A historic rainfall event is underway for the southern Appalachians and vicinity with widespread/locally catastrophic flooding expected," a NWS Greenville-Spartanburg forecast stated.

Atlanta saw its first-ever flash flood emergency after recording more than 11 inches of rain in two days — the highest rainfall on record spanning 48 hours there.

• Peachtree and Nancy creeks spilled over their banks, flooding homes and apartments, washing out roads and trapping people in homes.

By the numbers: There were 29 flash flood emergency declarations in all of 2023, according to The Weather Channel, including one in coastal South Carolina and another in south Georgia.

• None were recorded in North Carolina.