Today’s round of questions, my smart-aleck replies and the real answers:

Question: There have been various estimates as to the magnitude of the Helene flooding in the French Broad watershed. Some have said it was the 100-year flood. Others have said it was the 500-year flood. The determination is actually fairly straightforward. The calculation can be done for each specific river-gauging station. It is based on the historic record of each year’s highest stage (river level) as recorded at that gauge station. Its accuracy is obviously a function of the length of the record. The longer the record, the greater the statistical soundness of the estimate. The 100-year flood, as you probably know, has nothing to do with 100 years. It is just the flood level that has a 1 percent chance of occurring in any given year. The 500-year flood has a .2 percent chance. The longest gauge record for the Asheville region, based on the U.S. Geological Survey national water data dashboard, appears to be the one on the French Broad River at Asheville. Gauging data are also available on the USGS WaterWatch website. One site says there are discharge data starting in 1985. Another site says the record extends 128 years.  Neither site currently appears to be capable of downloading the data set. Can you ask whether anyone has actually determined the nature of the Helene flood at that station? It probably wasn’t the 100-year flood or the 500-year flood, but rather something in-between. A followup question would be how the 2024 flood will cause a reset in the determination of the 100-year flood level for that gauging station.

My answer: I’m going with “one-bazillion-year flood,” mainly because I never want to see another one of these. 

Real answer: This is a topic that keeps recurring here, and from what the experts tell me, it certainly seems like a 1,000-year rain event. Whether that translates into a 1,000-year flood event remains a little bit of a gray area.

Nathan Pennington, Buncombe County’s planning director, opened this can of worms at the Nov. 20 daily Helene briefing during a discussion about floodplain management and the National Flood Insurance Program. As Pennington mentioned, the county partners with the state on FEMA flood mapping, with the upshot being the state draws the flood maps for Buncombe.

He showed maps of the Swannanoa area that show the most at-risk section, marked in purple and called the floodway, which are areas that “almost always flood during a rain or storm event.” The next area, in blue, is the 100-year flood area.

“Development is allowed in this area, but standards must be met,” Pennington said, noting that, as the reader said, this means these areas have a 1 percent annual chance of flooding. 

Next, in green, was the 500-year flood area, which as Pennington and the reader noted, simply means it has a .2 percent annual chance of flooding.

“What we saw during this event was flood water that so far exceeded the 500-year (flood), it’s very likely we experienced a 1,000-year event, which is not even mapped,” Pennington said.

It’s important to note he said “very likely,” and that he was referring to one area of Swannanoa.

As the reader noted, the U.S. Geological Survey does track river gauge data, so I checked in with public affairs specialist Alexandra Hays to see if it had made a determination, based on the gauge the reader mentioned or any others.

“So, the National Weather Service actually sets the historical designations of ‘100-year-flood, 500-year-flood,’ etc., so to find out how they determine these, you’d have to reach out to them,” Hays said.

More on that in a second.

Like everyone who addresses this topic, Hays also stressed the notion of how “100-year flood” can be misleading. In fact, the USGS has prepared a document on this aptly titled, “The 100-year flood.” 

“Hydrologists don’t like to hear a term like ‘100-year flood’ because, scientifically, it is a misinterpretation of terminology that leads to a misconception of what a 100-year flood really is,” the article states, noting that instead hydrologists would say it’s a flood having a 100-year recurrence interval. That’s where the 1 percent chance comes in.

“That being said, we use the term ‘Annual Exceedance Probability’ to track the flooding at individual streamgages, and our teams are still in the process of collecting the needed data to compute the AEPs in areas affected by Helene,” Hays said. “One of our surface water specialists tells me we are going to publish a report of the AEPs for our streamgages in the area for Helene, but it will be a few months before that is available.”

Next, I called up the National Weather Service and spoke to meteorologist Jake Wimberley. Regarding the flood information being given out, Wimberley noted there were “a lot of differing numbers” coming out, because rainfall varies from point to point.

“There were unofficial statements that were really extreme in terms of how rare the event was, so I don’t want to lead you astray,” Wimberley said.

The best source is the National Oceanic and Atmospheric Administration’s data compendium called the Atlas 14 project. That includes analyses that show how much rain would fall at a given point, and how frequently that is expected to recur, which is where the idea of the 100-year event comes from.

Wimberley pulled up data for Asheville Regional Airport, the official recording station for the area. 

“So, the airport got 14.19 inches of rain during Helene — the precipitation event that led up to Helene, and the actual passage of the storm,” Wimberley said, noting it was technically listed as a four-day event, although rainfall was very light on that first day.

The 1,000-year measurement for a four-day event would be 11.6 inches.

“So we were over that,” Wimberley said. “And this is for one specific point. I could look at different points and come up with different numbers.”

Indeed, we know rainfall was heavier in certain areas. For instance, Busick in Yancey County recorded 30.78 inches, and Mount Mitchell State Park recorded 24.20 inches, according to a Nov. 7 article on NOAA’s Climate.gov site. 

“Estimated rainfall totals from Helene across the southern Appalachians had an Annual Recurrence Interval greater than 1,000 years over a wide area; meaning there is less than a .1 percent chance (annual exceedance probability) of that happening in any given year, according to NOAA’s National Water Center,” the article states.

So that’s indicative of a 1,000-year event.

The North Carolina State Climate office also had an article about Helene that cited the Atlas 14 information, noting that it is “imperfect due to its lack of recent updates.” The office used a three-day measurement of rainfall.

“In Asheville, the three-day total of almost 14 inches goes well beyond the 1-in-1,000 year total for a 72-hour period, which Atlas 14 cites as 11.4 inches,” the article states. “Likewise, the 24.41 inches over three days at Mount Mitchell is off the charts compared to the noted 1-in-1000 year amount of 16.5 inches.”

So, as Wimberley puts it, “The full event in total looks like there were multiple places that it was more than a 1,000-year” rainfall event.

“They haven’t come up with data for more than a one in 1,000-year event,” he added.

Clearly, that much rain translates into a lot of flooding, but it’s tougher to nail down that “1,000-year” designation.

“These major, major rainfall events usually translate to some flooding,” Wimberley said. “But I think it’s more accurate to say this was a 1,000-year rainfall event, as opposed to a 1,000-year flood event.”

Now, as far as Helene causing some kind of “reset” with the data, I contacted David Easterling, director of the National Climate Assessment Technical Support Unit at the National Centers for Environmental Information in Asheville.

“Bottom line is that the National Weather Service is producing a new version of NOAA Atlas 14, which will have both Volume 1 with new values based on the historical observed data, and Volume 2, which will provide estimated values for the future based on climate model projections,” Easterling said.

That means it will include projections related to climate change. Easterling did offer one caveat, though.

“I suspect the Weather Service is far enough along in their data analysis that they may not include Helene, since they expect to release Atlas 15 in 2026,” he said.

Question: We live in Weaverville, in the Reems Creek Golf Community, and we were all incredibly fortunate not to have suffered much in the way of damage to our homes. Our tree-covered streets and homesites, however, lost dozens if not hundreds of trees. While many of them, due to the extensive rainfall before Helene ever hit, were simply toppled from the roots, there were also many that were snapped clean in half at mid-trunk, often with a corkscrew pattern in the wood. This leads us to think that there might have been micro-bursts of mini-tornadoes spawned by the storm. Has there been any official determination to that effect? 

My answer: I have micro-bursts of energy these days. It’s the only way I get the Answer Man columns done.\

Real answer: Michael Rehnberg, a National Weather Service meteorologist, said we did not have documented tornadoes in the mountains with Helene. The strongest winds hit around sunrise Sept. 27.

“We had a swath of really strong winds that moved right up the Upstate and then over the state line from South Carolina into North Carolina, and affected a lot of areas in the mountains and the foothills in North Carolina as well,” Rehnberg said. “But those were non-tornadic winds.”

Rehnberg noted that the wind gusts data for Helene has not been certified by the National Centers for Environmental Information, so the numbers are still considered preliminary.

“But all across the Upstate, at least, of South Carolina, and into the southern mountains of North Carolina, there were varying reports of winds over 60 miles an hour,” he said.

That’s clearly strong enough to take down trees, especially with the amount of rain that had fallen over three days, leaving the ground saturated. That can turn root balls into mud, making even large trees vulnerable.

“So when you combine that with higher wind speeds, you can see just kind of the mass devastation — I mean, hundreds, thousands of trees down across these large areas,” Rehnberg said. 

The winds were also most devastating on south-facing slopes, as that’s how the storm traveled, from south to north.

A microburst is unlikely to have occurred with this event, Rehnberg said. In a microburst, contracting air aloft descends very suddenly. 

“The physics behind that are really closely associated with the way that thunderstorms develop, as opposed to tropical systems,” Rehnberg said. “But that’s not to say that you couldn’t have had very high winds, certainly winds capable of producing the really erratic gusts at the surface.”

Our terrain can play a role with high winds, though, and gusts reaching up into the 60s or maybe higher can cause major, and unusual, tree damage.

“In particular, when you’re dealing with the kind of terrain that you have up in the mountains, what can start out as a simple, straight line wind can simply become very erratic just because it’s responding to all of the rolls and dips in the actual terrain up there,” Rehnberg said. 

He would attribute those twisted-looking trees to that phenomenon, he said, not tornadoes.

I’ll note that several sources online, including one academic study, suggest trees will start to break under wind speeds of about 90 mph. But I suspect some other factors could come into play at times, including health of the tree.

Asheville Watchdog is a nonprofit news team producing stories that matter to Asheville and Buncombe County. Got a question? Send it to John Boyle at jboyle@avlwatchdog.org or 828-337-0941. His Answer Man columns appear each Tuesday and Friday. The Watchdog’s reporting is made possible by donations from the community. To show your support for this vital public service go to avlwatchdog.org/support-our-publication/.

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