24-hour rainfall totals over Western North Carolina. (National Weather Service)

The slow-moving remnants of Tropical Storm Alberto are raining down across the Southeast this week, triggering flooding and mudslides. Near Asheville, N.C., as much as 7 inches of rain fell in less than 24 hours, flooding rivers and streams and compromising the dam on Lake Tahoma.

Around 3 a.m., the National Weather Service issued a flash flood emergency for McDowell County downstream of Lake Tahoma. A mudslide compromised the dam and prompted mandatory evacuations downstream.

Downstream of Lake Tahoma, the Catawba River near Pleasant Gardens, N.C., approached record levels early Wednesday morning, 4.72 feet above flood stage. It was the highest level at which the river has crested since Hurricane Frances in September 2004. The Swannanoa River at Biltmore, N.C., continued to climb as of 10 a.m., Wednesday, nearly 5 feet above flood stage.

Wednesday’s rain is on top of what has been an extremely wet month in Western North Carolina. With more than 13 inches of rain so far in the month of May, Asheville is approaching its all-time wettest month. September 2004 is the wettest month on record in Asheville as the result of Hurricane Frances.

Engineers inspected the dam in daylight on Wednesday morning and found it was safe for people to return to their homes. Before that it was at a “level 1″ emergency, which means the dam has either failed or is about to fail.

In the meantime, more rain is in the forecast for the region with warm, humid air flowing from the Gulf of Mexico across the Southeast.

There are two factors contributing to this week’s torrential rain on the Eastern Seaboard — the remnants of Alberto and a ridge of high pressure over the Atlantic Ocean, called the “Bermuda high.”


High pressure over the Atlantic is forcing air from the Gulf of Mexico to travel over the Southeast and Mid-Atlantic.

Alberto brought deep tropical moisture up from the Gulf of Mexico earlier this week, after it made landfall just west of Panama City, Fla., on Monday. The storm killed two journalists in North Carolina who were covering the storm when a tree fell on their car. Since then, the storm has been slow to move out of the region.

High pressure over the Atlantic is driving Alberto’s track north, and it’s forcing winds to flow from the Gulf of Mexico — where the air is very warm and moist — to the Southeast and Mid-Atlantic. This pattern of moisture transport was also behind the catastrophic flooding in Ellicott City, Md., on Sunday.

“We have been stuck in a late-July weather pattern, one in which the jet stream has built a ridge of high pressure over the Mid-Atlantic,” the Capital Weather Gang’s Jeff Halverson wrote in an analysis of the Ellicott City flood. “This has allowed high levels of afternoon heat and humidity to build through our region. … The high humidity comes from the notorious Bermuda High, which pumps vast amounts of Atlantic moisture our way.”

So, even though Alberto is fading, the high humidity — and potential for heavy rain — will continue as long as the Atlantic high pressure remains.

The National Weather Service said the flash-flood watch will continue for the North Carolina mountains and foothills through at least Thursday morning.

“Flooding and mountain landslides will remain a primary concern,” the Weather Service wrote. “Given very saturated soils, it won’t take much for a few trees to topple over as well, even with minimal wind gusts.”