Disastrous Effects of Hurricane Helene in the Southern Appalachian Mountains Including a Review of Mechanisms Producing Extreme Rainfall

Abstract

Hurricane Helene made landfall near Perry (Latitude 30.1 N) in the Big Bend area of Florida with a central pressure of 939 hPa. It moved northwards creating devastating damage and loss of life; however, the greatest damage and number of fatalities occurred well to the north around the City of Ashville (Latitude 35.6 N) where extreme rainfall fell and some of the strongest wind gusts were reported. This paper describes the change in the hurricane’s structure as it tracked northwards, how it gathered tropical moisture from the Atlantic and a turning wind profile between the 850 hPa and 500 hPa elevations, which led to such extreme rainfall. This turning wind profile is shown to be associated with extreme rainfall and loss of life from drowning and landslides around the globe. The area around Ashville suffered 157 fatalities, which is a considerable proportion of the 250 fatalities so far recorded in the whole United Stares from Helene. This is of extreme concern and should be investigated in detail as the public expect the greatest impact from hurricanes to be confined to coastal areas near the landfall site. It is another example of increased death tolls from tropical cyclones moving inland and generating heavy rainfall. As the global population increases and inland centres become more urbanised, run off from such rainfall events increases, which causes greater devastation.

1. Introduction

Most severe damage and loss of life in the United States from Hurricane Helene in September 2024 occurred in and near the state of Northwest Carolina 790 km from the point of landfall in Florida. Most damage and loss of life from landfalling hurricanes usually occurs near the point of landfall, so the aim of the study is to document how such an anomaly occurred [1,2,3]. The Black Mountains are a range in Western North Carolina in the southeastern United States. They are part of the Blue Ridge Province of the southern Appalachian Mountains and are the highest mountains in the Eastern United States. The southern and western extremes of the Black Mountains run along the Buncombe County line, which suffered most damage and loss of life from Helene.

The highest Measured Wind Gusts in the Appalachian Mountains from Ref. [4] were in the Black Mountains in Yancey County at Mt. Mitchell (Elevation: 2037 m Prominence: 1856 m) at 1227 UTC 27 September 2024, where gusts of 47.3 m/s (92 knots) and sustained one-minute wind of 36 m/s (70 knots) were measured. At Watauga County Banner Elk at 1200 UTC, 27 September 2024, the readings were 45.3 m/s (88 knots) gust and 28.3 m/s (55 knots) sustained wind, and in Haywood County Frying Pan Mountain at 1152 UTC, 27 September 2024, 39.1 m/s (76 knots) gust. Gusts were a little weaker In South Carolina where Laurens County Laurens recorded 34.5 m/s (67 knots), Beaufort County Beaufort 33.4 m/s (65 knots), Anderson County Anderson 32.4 m/s (63 knots), Aiken County Aiken 32.4 m/s and Pickens County Sassafras Mountain 32.4 m/s.

Mt Mitchell is the highest peak in mainland North America east of the Mississippi River, and the 92 knots (47.3 m/s) gust recorded there was the second-highest wind gust from Hurricane Helene. The highest measured wind gust was slightly more at 93 knots (47.8 m/s) in the United States recorded during Helene, which was measured by a buoy offshore from Florida’s west coast. The buoy was a NOAA buoy 42,036 and located 208 km WNW of Tampa, Florida.

Fatalities in the United States Mainland from Hurricane Helene and Ex Hurricane Helene

From Ref. [4], Florida had 18 direct fatalities and 16 indirect fatalities, totalling 34; Georgia had 28 direct fatalities and 9 indirect fatalities, totalling 37; South Carolina had 26 direct fatalities and 24 indirect fatalities, totalling 50; North Carolina had 86 direct fatalities and 21 indirect fatalities, totalling 107; Tennessee had 15 direct fatalities and 3 indirect fatalities, totalling 18; Virginia had 2 direct fatalities and 1 indirect fatality, totalling 3; Indiana had 1 direct fatality and nationwide totalled 176 direct fatalities and 74 indirect fatalities, resulting in 250 fatalities.

Helene is the most devastating natural disaster in western North Carolina’s history and Coast Guard personnel rescued at least 865 people in western North Carolina, mostly from floodwaters. Media reports indicate that the storm damaged or destroyed more than 125,000 housing units across western North Carolina. The North Carolina Forest Service estimated 822,000 acres of damaged timberland, which resulted in $214 million in damages to North Carolina forests. In extreme examples, trees on entire mountainsides were blown down in the higher elevations of North Carolina.

2. Wind Structure Associated with Extreme Inland Freshwater Flooding Around the Globe

Below the following analyses is from Ref. [5] detailing disastrous events (fatalities 10 or more) which were caused by winds turning in an anti-cyclonic sense with height between 850 hPa and 500 hPa levels with the wind data obtained from radiosonde data. For brevity we call this wind structure Warm Air Advection (WAA) due to it having similar turning with height characteristics of Quasi Geostrophic warm air advection [6]. The exception being the Hurricane Mitch case where the wind data was obtained from reanalyses data. Studies showed that the stronger the winds resulted in heavier rainfall and the anti-cyclonic turning of the winds were between 850 hPa and 500 hPa, the low-level winds had a high moisture content, and the turning angle was approximately 90° or less. For high rainfall amounts, the optimum turning angle between the 850 hPa to 500 hPa level should be 90° or less. For example, 850 hPa winds coming from SSE (160°) should be turning to winds from 240° (WSW) at the 500 hPa level which is a turning angle of 80°. In the Northern hemisphere, clockwise flow is in an anticyclonic direction with the opposite in the Southern hemisphere.

2.1. Examples of Extreme Rainfall

We will show that extreme rainfall in inland Carolina from Hurricane Helena had this wind structure and here is the list of disastrous rainfall events mentioned above.

Mumbai (Santacruz) recorded 944.mm with 481.2 mm in the 4 h to 1300 UTC 26 July 2005 and then 708.6 mm in the following 7 h. This extreme rainfall caused 445 fatalities.

Hurricane Mitch slowly moved over land dropping historic amounts of rainfall in Honduras, Guatemala and Nicaragua during October 1998 causing 19,325 deaths.

• Seoul 26–28 July 2011 69 fatalities.
• Jeddah Saudi Arabia 25 November 2009 122 fatalities and 350 missing.
• Zixing China Typhoon Bilis 15 July 2006 843 fatalities.
• Chennai India 1 December 2015 280 fatalities.
• Chuuk Island Pacific Ocean 2 July 2002 791 fatalities.
• Hue and Danang Vietnam 1–4 November 1999 48 fatalities.
• Kagoshima Japan 6 August 1993 48 fatalities.
• Rapid City South Dakota US 9–10 June 1972 238 fatalities.
• Big Thomson River Colorado US 31 August–1 July 1976 145 fatalities.
• Athens Greece 2 November 1977 36 fatalities.
• Nimes France 3 October 1988 9 fatalities.
• Nimes France 8 September 2002 37 fatalities.
• Lisbon Portugal 19 November 1983 10 fatalities.
• Casablanca Morocco 29–30 November 2010 32 fatalities.
• Samsun Turkey 3 July 2012 12 fatalities.
• Beijing China 21 July 2012 79 fatalities.
• Shanghai and Hangzhou 6–8 October 2013 12 fatalities.
• Wenjiang China 10 July 2013 73 fatalities 180 missing.
• Hyderabad India 23–24 August 1980 140 fatalities.
• Hyderabad India 8–9 August 2008 30 fatalities.
• Hanoi 30–32 October 2008 66 fatalities.

2.2. Other Examples of Extreme Rainfall

Hurricane Ida (2021) caused catastrophic flooding in New York City and New Jersey where the hourly rainfall record was broken at Central Park Manhatton. The GFS winds at 00Z 2 September 2021 over Central Park in New York were 850 hPa 160/50 knots, 700 hPa 180/60 knots and 500 hPa 195/60 knots, which is of course extraordinarily strong turning in an anti-cyclonic sense. The nearest radiosonde station at Upton on Long Island at 0000 UTC 2 September 2021 had the same vertical turning structure but from the model data further to the east of the maximum winds near Central Park on Manhattan:

850 hPa 175/26 knots 700 hPa 190/48 knots 220/52 knots.

Central Park recorded rainfall of 80.01 mm in 1 h to 0151 UTC 2 September 2021. Forty-five people drowned in the floods generated by this extreme rainfall.

Below are references to other extreme rainfall events in Australia and the United States associated with this wind structure.

3. Methods, Analysis of Vertical Wind Structure, and Buoyancy

We have investigated winds turning anticyclonic (anticlockwise in the Southern Hemisphere or clockwise in the Northern Hemisphere) from the 850 hPa level up to the 500 hPa level. This phenomenon appears to be entirely published by our work and references to this wind structure are from citing our studies. This wind structure is associated with tropical cyclone intensification in the tropics and extreme rainfall both in the tropics and in higher latitudes. Examples of this wind structure causing extreme rainfall and being associated with tropical cyclone intensification can be found in Refs. [5,7,8,9,10,11,12,13,14,15,16,17]. Such WAA has a wind structure that produces streamwise vorticity [18] encouraging rotating updrafts which separate updrafts from the destructive effects of downdrafts. Shallower WAA to 700 hPa in tropical air masses can also produce extreme rainfall and flooding from enhanced thunderstorms [5,9].

The common summer wind pattern off the Northeast Australian Coast is the opposite to the WAA pattern and we refer to it as cold air advection (CAA) and this wind structure, which contributes to convective suppression. This was illustrated in Figure 2 in the paper of Ref. [14] which shows the winds turning clockwise (cyclonic SH) with height over the area on average through January, February, and March. The rainfall associated with this pattern is light rain with the heavy rain located further north in the monsoon trough across the Gulf of Carpentaria and Cape York. The author spent the active La Nina 1973/1974 summer on Willis Island Meteorological station in the Coral Sea and with monotonous regularity when south of the monsoon trough, the radar balloon flight showed a CAA structure with low level southeast winds turning clockwise with height through southerly winds up to south-westerly at 500 hPa. During this time only light rainfall was observed. It was only when a vortex developed, or an upper trough system extended up into the tropics (see example Appendix I [14]) that a pattern conducive to heavy rainfall was observed. In those cases, because the most common 850 to 500 hPa vertical wind shear was westerly, a dipole structure was produced with WAA and ascent in the east and CAA and descent in the west.

In the Queensland Severe Weather Section of the Bureau of Meteorology a diagnostic for WAA and heavy rain and TC intensification was developed using 850 hPa, 700 hPa, and 500 hPa winds from the European Forecasting Model. This became possible from the 1990s when computer forecasting models became freely available. This clearly showed to us the relationship between WAA and heavy rain and TC intensification. This relationship was first posted in Ref. [8] and the paper used data from Ref. [19] to show that intensifying tropical cyclones in the Australian/southwest Pacific region had an asymmetric convective structure associated with WAA when considering the climatological winds at 850 hPa and 500 hPa and moisture data.

Previous studies examined winds associated with extreme rainfall in both the tropics and the mid-latitudes of Australia [7,8,9]. Further studies found this to apply in many cases around the globe [5]. Theoretical arguments suggest, assuming gradient wind balance, that isentropic uplift is likely to be associated with winds that turn anticyclonically with height in most heavy rain-bearing systems, including the tropics and subtropics [7]. Two Australian studies examined extreme rainfall and major flooding events in coastal catchments and more broadly over southeastern Australia [8,9]. Using radiosonde and reanalysis data they examined the vertical structure of these systems in the period for which upper wind data became available and found WAA wind patterns were associated with extreme rainfall.

The two forcing terms in the Quasi Geostrophic Omega equation are the vertical derivative of cyclonic vorticity advection and the thermal advection term. The vorticity terms tend to dominate except when there is a slope to the system under investigation and then thermal advection dominates in forcing up motion [20]. This can be understood considering a vertical stacked system would consist of unidirectional winds and have no turning winds with height and when the system slopes the winds that begin to turn anticyclonic with height on one side of the system producing the WAA structure. Ref. [6] came to similar results when he analysed winds in a sloping 3-layered model where the thickness pattern lagged the wind pattern such that there is a slope westward in the 700 hPa circulation.

4. Results

4.1. The Track of Hurricane Helene

The track of Helene showing its intensity at various intervals is shown in Figure 1 indicating it lost hurricane intensity as it passed Savanah (Georgia) and weakened further as it crossed the Appalachians despite this region suffering the most damage from the storm. Hurricane Helene reached peak intensity with a minimum barometric pressure of 938 hectopascals at 3:10 UTC on 27 September as it made landfall around 16 m west-southwest of Perry Florida. Figure 2 shows the eye just after it began moving over Perry with the mean sea level pressure there reading 948.5 hPa. Helene then moved towards the north northeast and then northwards into Georga, claiming 37 lives and causing severe damage. However, this study focuses on the impact of Helene on Northwest Carolina and nearby eastern Tennessee.

In Figure 3, the centre at 0800 UTC 27 September 2024 was located near the town of Eastman in Georgia where the mean sea level pressure was recorded as 967.8 hPa. The surface dewpoint isopleths are plotted, and high moisture (23 °C) reached Greenville in South Carolina. Overall tropical moisture can be seen over Georgia, Carolina, and Northern Florida. An hour later Helene was moving through Northern Georgia with the tropical moisture creeping towards the Blue Ridge Mountains (Figure 4). By 1200 UTC 27 September (Figure 5), the storm begins to move over the Blue Ridge Mountains and south southwest of Asheville dragging the tropical moisture with it.

4.2. Rainfall and Flooding in the Asheville Area

The heavy rainfall in the Asheville area is highlighted in Figure 6. Asheville is the centre of Buncombe County and as of October 27, major damage has been reported to 601 residential and 241 commercial buildings in Buncombe County. Another 294 residential and 153 commercial buildings in the county were destroyed. Located 30.6 km northeast of Asheville is Mount Mitchell. This is the highest peak of the Appalachian Mountains (Elevation: 2037 m Prominence: 1856 m) and the highest peak in mainland North America east of the Mississippi River. Wind gusts reached 92 knots (47.4 m/s) on Mount Mitchell which was the second-highest wind gust from Hurricane Helene. The highest wind gust was slightly more at 93 knots (47.8 m/s) in the United States recorded during Helene, which was measured by a buoy offshore from Florida’s west coast.

The heaviest rainfall recorded during Helene was from Busick on the east-southeast slopes of Mount Mitchell. Water from the Mount Mitchell area drained southwards into the Swannanoa River and northwards into the Nolichucky River. The location of Mt Mitchell is denoted in Figure 6 to show how it splits the flow southward into the Swannanoa River and northwards into the Nolichucky River. South of Asheville in Figure 6, heavy rain can be seen to have been recorded, which drained into the French Broad River, which met the flow from the Swannanoa River at Asheville. This confluence of major floods in these two rivers produced catastrophic conditions at Asheville. During the height of the storm, the French Broad River crested (from gauge data) at 24.67 ft (7.52 m), and the Swannanoa River reached 26.1 ft (8.0 m), both higher than the all-time records set by the Flood of 1916.

The highest observed rainfall total was in Busick, North Carolina, where 30.78 inches (781.8 mm) was recorded from 1200 UTC 25 September to 1200 UTC 28 September. From Ref. [4], an NWS Cooperative Observer Program (COOP) observer near Celo, North Carolina, measured 26.65 inches (676.9 mm) of rain. Both of those sites are in Yancey County. Farther south, a rainfall total of 29.98 inches (761.5 mm) was measured in Transylvania County, a short distance north of the South Carolina border. A Hydrometeorological Automated Data System (HADS) site at Sunfish Mountain in Greenville County, South Carolina, measured 21.66 inches (550.2 m) of rain. Available observations indicate that rainfall amounts of more than eighteen inches (457.2 mm) occurred across portions of Transylvania, Henderson, Buncombe, Polk, McDowell, Yancey, Mitchell, Burke, Avery, and Watauga Counties in North Carolina, as well as in northern Pickens and northern Greenville Counties in South Carolina.

A large area stretching from northwestern South Carolina into western North Carolina and southwestern Virginia received 3-day rainfall totals that had less than a 1 in 1000 (<0.1%) chance of occurring in any given year.

A few other notable rainfall maxima occurred with Helene. Amounts as high as 12 to 13 inches were observed within the Atlanta metro area, which has just a 0.2% to 0.5% chance of occurring in any given year. Atlanta Airport recorded 105 mm from 1800 UTC 26 September to 0600 UTC 27 September and 82.1 mm 0600 UTC to 1200 UTC 27 September. The radiosonde data in the vicinity of Atlanta (station identifier FFC) shows a WAA pattern 0000 UTC 26 September (Figure 7 below) and at 0000 UTC 27 September (Figure 8).

Figure 9 records the fatalities in the counties in Northwest Carolina and note that they are mostly clustered around the Asheville area in Buncombe. The heavy rain from the Mount Mitchell region as mentioned above flowed into the Nolichucky River, which surged into Tennessee, and from Figure 10, the highest number of fatalities there were at Erwin on the Nolichucky River. The French Broad River and Nolichucky Rivers merged and became the Tennessee River eventually flowing into the Mississippi. The Catawba and Broad Rivers marked in Figure 6 eventually flowed into South Carolina and Figure 11 shows the fatalities in counties in that state. They resulted mostly from the rivers draining out of Northwest North Carolina (Saluda, Broad and Catawba Rivers). The Source of the Saluda River was near Rocky Bottom in Pickens County (see Figure 11) which recorded 21.66 inches (550 mm) during Hurricane Helene. Rocky Bottom lies just inside the border of South Carolina south of Transylvania County in North Carolina. Various cities and towns in Pickens County received the highest level of rainfall in South Carolina. Table Rock experienced a total of 16.51 (419 mm) inches of rain and Liberty saw 13.12 inches (333 mm).

4.3. Vertical Wind Structure and Extreme Rainfall

Figure 7 shows the reanalysis charts at 0000 UTC 26 September 2024 with wind corresponding observations from radiosonde data and the station identification highlighted in red indicate ascent from the winds turning clockwise (anticyclonic) in direction from 850 hPa up to 500 hPa. The rainfall station of Busick (maximum known rainfall) is highlighted in white. Reanalysis wind plot near Busick show the wind turning from southerlies at 850 hPa to south-southwesterlies at 700 hPa to a more south-westerly direction at 500 hPa, which indicated ascent in the vicinity of Busick. This ascent region, the position of Helene, is denoted by a black X. From Figure 12, the rain began at Busick around 7 pm 25 September 2024 (2300 UTC 25 September 2024), which is close to the time in Figure 7, so the ascent at Busick is consistent with this heavy rain onset.

Figure 13 shows the reanalysis charts at 1200 UTC 26 September 2024, with wind corresponding to observations from radiosonde data, and the station identification highlighted in red indicates ascent from the winds turning clockwise (anticyclonic) in direction from 850 hPa up to 500 hPa. The reanalysis wind plot near Busick show weakened ascent winds with contrary turning from 850 hPa to 700 hPa, and nearby radio stations indicate lack of an ascent structure. This occurred during the lowest rainfall at Busick, and from Figure 11, the lowest rainfall intensity occurred around this time with 1.81 ins (45.9 mm) in the 6 h to 1017 UTC 26 September 2024 and 0.18 ins (4.6 mm) on 26 September from 1317 UTC to 1417 UTC. Further south, radiosonde stations indicated ascent, as did the reanalysis winds.

From Figure 8, the ascent type wind structure at 0000 UTC 27 September 2024 resumed in the reanalysis winds at Busick (southeast winds turning to south-southeast to southerly winds from 850 hPa up to 500 hPa). The surrounding radiosonde stations also indicated ascent all the way south to Florida. From the hourly rainfall data at Busick, heavy rain there was recorded as 5.73 ins (145.5 mm) in 9 h for 0717 UTC 27 September 2024.

From Figure 14, the ascent type wind structure at 1200 UTC 27 September 2024 continued in the reanalysis winds at Busick (southeast winds turning to south-southeast to southerly winds from 850 hPa up to 500 hPa). The surrounding radiosonde stations with ascent were now located further north. From the hourly rainfall data, this was the heaviest rainfall at Busick 10.31 inches (261.9 mm) in 6 h to 1417 UTC 27 September 2024. From Figure 15, images of 85–92 GHz Polarization-Corrected Brightness Temperature an area of intense convection (extreme rainfall in the green to yellow to red area) can be seen to move up to Busick coincident with the heaviest rainfall.

5. Discussion and Conclusions

It was shown in this study how the greatest impact from Hurricane Helene occurred well after landfall and in an area well removed from the landfall site. As Helene tracked up to Northwest Carolina, it developed a very strong deep cyclonic circulation up to the 500 hPa level, with intense warm air advection from the radiosonde data at RNK (Blacksburg Viginia), GSO (Greensboro North Caroline), and MHX (Newport North Caroline) and from the reanalysis data in Busick, North Carolina. This can be seen in Figure 7 and Figure 8, and in 15, satellite data show that a large area of intense convection moves up towards Northwest Carolina. As this occurred from Figure 3, Figure 4 and Figure 5, tropical air (dewpoints to 23 °C) advanced up towards Ashville and the southern Appalachians. Therefore, the extremely unusual weather pattern associated with Helene brought extraordinarily strong winds, which extended up to middle levels of the troposphere and had a wind structure exhibiting a warm air advection pattern conducive to extreme rainfall particularly as the air mass developed tropical characteristics as it moved northwards. The devastating inland flooding was associated with strong features including high moisture being advected from the Atlantic Ocean into the Ashville area where there was strong ascent indicated by the turning wind structure. This led to some useful warnings as from the National Hurricane Centre Report on Helene [4]. Forecasters upgraded to a high-risk warning around 0800 UTC 25 September (48 h of lead time) as it continued to become more apparent that the rainfall from ahead of the core of Helene would increase as the core came closer, increasing confidence of a catastrophic event. This is the 3rd longest high-risk lead time for a tropical cyclone since 2017. Wording was enhanced even further in the 2100 UTC Public Advisory on 25 September (34 h of lead time): “This rainfall will likely result in catastrophic and potentially life-threatening flash and urban flooding, along with significant river flooding”.