Helene Recovery, Day 26: new storm data helps put this flood into perspective

It's another in a series of crisp and clear autumn days, exactly the kind of weather we need for drying out. After a few incredibly bright nights around the full “supermoon,” the moon has waned from its peak and is rising later. It was finally dark enough on Friday night to see comet A3 in the western sky, and for the past three nights we’ve shared the experience with friends. The starry sky is not as intense as it was before power returned to the region, but it’s still dark enough to make out the milky way – and the comet, its own faint milky smudge. Through binoculars it is bright and clear, as it trails about two hours behind the sun – setting over Deep Gap around 9. 

We’ve swung from one of our wettest months in history (33.5” rain in September) to one of our driest (.09 so far in October, with little precipitation forecast in the coming week). It’s the climate crisis in a nutshell: hotter and drier spells, punctuated by record bouts of precipitation. The dry weather is a resumption of what had been a fairly dry summer. It’s as if the high water of the flood was some kind of fever dream. The debris in the treetops, far removed from current water levels in each river and creek, look like the result of a ghostly, invisible force that flung stuff into the trees. The river is back to an average flow (about 1’ in height, or about 100 cubic feet/second). Hannah Branch Road’s mud is a distant memory – now clouds of fine river dust are sent airborne every time a vehicle passes. If the dry spell continues as the remaining leaves come down, we’ll start worrying about wildfire.

Many people are asking me about the ecological impacts of Helene. In future posts, I will take a stab at the geological and ecological implications of Helene. First though, let's talk about the weather a bit more and we’ll dive deep into the weeds (i.e. data) to answer the question: why did Helene cause such destructive flooding? I’ll start with a basic overview of Appalachian weather, and will share a nugget of data that I finally had the time and bandwidth to uncover. It’s the puzzle piece that explains the magnitude of the flood that we experienced. 

I’ve dedicated years to developing an intimate relationship with this valley, its river, and especially its forests. I’ve also studied the weather that governs our ecosystems. Many people who are older and have a longer history in the area have memories of the flood of November, 1977. My own experience with flooding in this valley began with the twin floods of Sept 2004 caused by tropical storms Frances and Ivan. Ever since, I’ve thought a lot about what systems cause the biggest floods in this valley, and what might be a theoretical “max” flood.

Earlier this summer, I initiated an effort to mark historic flood levels on a tree next to the USGS river gauge. Like hunger stones in Central Europe, and ancient tsunami monuments in Japan, these marks were intended to remind people what the river is capable of during an extreme event. Placing the "9/27/2024" sign will be a solemn occasion, and it will mark a jump in magnitude – into a realm that was previously unimaginable. 

When I posted here about the new signs, I cited a TVA study from 1960 calculating potential for severe Appalachian flooding. A flash flood can occur on a small watershed like our own little stream, Hannah Branch (~100 acres), or one of the creeks descending from the Blacks, like Locust or Shuford or White Oak (~1,000 acres each), due to an intense, stalled thunderstorm. A flash flood like this happened on August 2, when a severe thunderstorm erupted over the headwaters of the South Toe. My NWS rain gauge in Celo, at the very northern edge of the storm, registered a mere ½” of precipitation. But over the high southern peaks of the Blacks, from Potato Knob to Mount Mitchell, a downburst dumped 3” of rain in a 45-minute period (recorded on the NADP rain gauge that I help monitor on Clingman’s Peak). Fourteen miles downstream, precisely two hours after the deluge, the South Toe River surged from a bucolic 91 cubic feet/second to 2400 cfs in just 15 minutes. An Inn guest lost her wallet and sunglasses to the rising river as she rushed from the river up to the road. Such storms are possible in any of our smaller watersheds. The smaller the watershed, the more it is affected by an intense local storm. The entire South Toe above the USGS gage is a medium-sized watershed (~30,000 acres), and it is unlikely that such a thunderstorm would be large enough to cause major flooding (over 12’ or 20,000 cfs) on the river. For major floods, we are dependent on a larger scale event, like a tropical storm.  

In Southern Appalachia, winter weather typically comes in the form of rain (rarely snow) from the Gulf of Mexico (southwest), and frequent bouts of cold Canadian winds and occasional snow from the northwest. Summer weather (roughly April-Oct) generally comes in the form of rain from the Gulf and sometimes from the Atlantic (southeast). The most impactful storms draw moisture from the warm waters of the Gulf, and the biggest of these are the tropical storms that form in or wander into the Gulf from the Carribean or Atlantic. Hurricanes moving inland from the Carolina (east) coast can wreak havoc on the mountains, but more typically, great Appalachian storms come ashore along the Gulf coast between Louisiana and the Florida panhandle. A storm that moves north out of the Gulf and into the Tennessee Valley drags its wet and powerful eastern flank across the Blue Ridge Mountains. 

Around here, the Blue Ridge escarpment serves as the continental divide in this region – rain that falls on the southeast side makes its way to the Atlantic, rain on the northwest drains into the Mississippi and the Gulf. The escarpment is at its steepest where the foothills of Buncombe and McDowell Counties meet the highlands of Yancey. In some places, the land rises nearly 5000’ from the foothills to the crest of the Blue Ridge. In a storm, megatons of Gulf moisture streams northward, ride up the mountainous land, condense into dense clouds, and fall as rain – a process called orographic lift. The ascent from warm Marion to chilly Mt. Mitchell wrings vast amounts of water out of a Gulf storm. During Helene, the rain that fell east of the divide is what dissolved the Buck Creek hillside that I checked out last weekend. But Helene was a fast moving (and huge) storm, so the moisture streamed for miles north and east of the mountain ridges, dumping 31” immediately on the other side, and spraying water so thoroughly into the mountain valleys beyond, that over 20” fell over nearly the entire swath of Yancey county, to the Tennessee border.  

In contrast to popular opinion, our little part of WNC is not a temperate rainforest, but we do average about 55” of precipitation per year. During Helene and her Predecessor Rain Event (PRE), we received half of our average annual rainfall in 2 days. However, this alone was not enough to produce the catastrophic flooding that we experienced. It was enough to cause big river flooding in the 1 million+ acre watersheds of the French Broad/Catawba/Nolichucky, and landslides/debris flows (especially given the tropical storm force winds that uprooted trees at the top of these slides), but 2 to 2.5 feet of rain spread evenly over two days would not have devastated creek and small river communities (South Toe, Micaville, Pensacola). What caused that devastation was the concentration of rain over a few crucial hours between 7 am and 10 am on that fateful Friday.

At the time of an October, 2015 “atmospheric river” event that caused historic flooding in South Carolina, and threatened the mountains, I calculated what rate of rainfall would flood the South Toe Valley. I’m no hydrologist, so it’s a simplified calculation that assumes 100% saturation – all the water falling from the sky ends up in the river. This is the scenario that played out on Friday morning, 9/27, so it was a good test of the formula, and the calculations held up well. The formula is a simple calculation of the volume of water falling over the watershed, concentrated into the flow of the river. According to this formula, a sustained rainfall rate of one inch/hour will result in a flood similar in magnitude to the 2004 flood (~28,000 cfs). This is where we were in the wee hours of Friday morning, and I’ve finally accessed the hourly data from the Busick RAWS rain gauge:

Time in Busick	Hourly rainfall
2-3 am	1”
3-4 am	.93”
4-5 am	1.03”
5-6 am	1.33”
6-7 am	1.53”
7-8 am	1.83”
8-9 am	2.88”
9-10 am	2.03”

From 2 pm Thursday to 2 am Friday, rain fell at a rate of .5-.75” per hour. From 2 am to 5 am, it increased to one inch/hour, and according to anecdotal reports (the streamgage ceased reporting at 5:30 am), the river reached that 2004 level a couple hours later, likely between 7 and 8 am. But then came the rain that pushed the flood from historic levels to unprecedented catastrophe. In the three hours between 7 and 10 am, it rained 6.74”, including nearly 3” in the 8-9 hour. I have no idea how or when USGS will calculate the volume of water at the crest of the flood, but it seems likely that it was in the order of 2-3x the 28,000 cfs 2004 flow.

As you can see in the following precipitation frequency chart for Celo, we endured a 1000 year event (7” in 3 hours), immediately on the heels of another 1000+ year event (24” over the previous 2 days). Of course the problem is that these models are pre-climate crisis models. Many are now asking, is a 1000-year flood now a 100-year flood? A 10-year flood? As this flood's mud dries to dust, we are all left in a cloud of conjecture.