Heavy precipitation and strong winds in Hurricane Helene mostly strengthened by human-driven climate change 

Contact Authors

-Davide Faranda, IPSL-CNRS, France  📨davide.faranda@lsce.ipsl.fr   🗣️French, Italian, English

-Carmen Alvarez-Castro, University Pablo de Olavide, Spain  📨mcalvcas@upo.es    🗣️Spanish, French, Italian, English

-Flavio Pons, IPSL, France  📨 flavio.pons@lsce.ipsl.fr  🗣️ Italian, English, French

-Tommaso Alberti, INGV, Italy  📨tommaso.alberti@ingv.it 🗣️Italian, English

-Erika Coppola , ICTP 📨coppolae@ictp.it 🗣️Italian, English

Press Summary (First Published 2024/09/30)

• Cyclones similar to hurricane Helene are up to 17 mm/day (up to 20%) wetter over the South East of the United States  and locally up to 5 km/h (up to 7%) windier in Florida's Gulf in the present than they have been in the past.

• Hurricane Helene was a largely unique event. 

• Natural climate variability likely played a role in driving the pressure pattern and the associated increase in precipitation and wind-speed linked to Hurricane Helene, but human-driven climate change has also contributed.

Event Description

After forming on September 22nd as a broad low-pressure area over the western Caribbean, Helene became the eighth named storm of the 2024 season. On September 25th, it was upgraded to hurricane status by the National Hurricane Center as it was entering the Gulf of Mexico, becoming Hurricane Helene. The storm moved to the North while undergoing a rapid intensification, and hit Florida's Big Bend as a category 4 hurricane, making landfall near the Aucilla River on September 26th with maximum sustained winds up to 220 km/h and a storm surge up to more than 9 feet (about 3 meters). After landfall, Helene quickly moved towards the North across the South-Eastern US, and interacted with a low-pressure system located between Mississippi and Tennessee. Despite losing its hurricane features, Helene produced extensive damage and disruption well after its landfall, between late September 26th and September 27th. Several tornadoes were spawned by supercells embedded in its eastern bands over the Carolinas. However, the heaviest impacts were caused by extreme precipitation that affected Florida, the Carolinas, Georgia, Tennessee and Virginia, with a provisional recorded maximum accumulation of 751 mm in Busick, North Carolina. This extreme precipitation event was caused by the interaction of an off-the-charts atmospheric river driven by Helene from the Gulf of Mexico with the Appalachian mountain range, where orographic lifting exacerbated the already extreme weather conditions. The exceptional rainfall total was also due to precipitation that fell before Helene’s landfall, a phenomenon known as “predecessor rainfall event”. Although focused studies are needed to confirm this, it is likely that the unusually high heat content in the Gulf of Mexico has contributed to both Helen’s rapid intensification and the exceptional atmospheric river that caused extreme floods over the South-Eastern US. As of September 30, Helene claimed at least 95 lives across 6 states, mainly due to unprecedented floodings.

The Surface Pressure Anomalies show a large negative (cyclonic) anomaly of 10hPa over the South East of the United States with Temperature Anomalies displaying warm anomalies over a large part of the South Eastern domain (Virginia, North and South Carolina, Kentucky and Tennessee states) while negative anomalies are observed over Mississippi, Arkansas and Louisiana states. Precipitation data show high daily amounts of precipitation over the south east of the country reaching more than 100 mm per day in states such as North and South Carolina, Georgia and the North of Florida. Windspeed Data indicate up to 60 km/h in the coasts of Carolinas and the Northwestern coast of Florida .

We remind you that our analysis is based on MSWX data. This product does integrate some station observations especially for rain data. The values reported here can be different from those observed at single weather stations. 

Climate and Data Background for the Analysis

According to the IPCC report (IPCC AR6 WGI FR - Page 205), anthropogenic climate change has increased observed precipitation, winds, and storm surge associated with some tropical cyclones, and It is likely that there is an increase in the annual global proportion of Category 4 or 5 tropical cyclones and the frequency of rapid intensification events have increased globally  in recent decades. Additionally, it is likely that TC translation speed has slowed over the USA since 1900. (IPCC AR6 WGI Chapter 11) Tropical cyclones, severe wind, and dust storms in North America are becoming more extreme, with a stronger trend for the high intensity one rather than increased frequency, but specific regional patterns remain uncertain (medium confidence). There have been recent observations of slower tropical cyclone translation speeds and higher rainfall totals over the North Atlantic, influenced by substantial natural variability. Projections show low confidence in changes to the overall number of tropical cyclones in the North Atlantic but medium confidence in the likelihood of more intense storms with higher winds, precipitation, and storm surge along Mexico, the US Gulf, and Atlantic coasts. (IPCC AR6 WGI Chapter 12). Regarding the economic damages caused by individual extreme events, the report (IPCC AR6 WGII FR - Page 1990) states that formal attribution to anthropogenic climate change has been limited, but climate change could account for a substantial fraction of the damages.  In summary, the IPCC report suggests that climate change has had an impact on hurricane intensity and associated hazards, but the confidence level for these findings varies. The IPCC report also highlights the potential economic damages caused by individual extreme events, and recent studies have used a variety of approaches for attributing these damages to climate change.

Our analysis approach rests on looking for weather situations similar to those of the event of interest having been observed in the past. For this event, we have low confidence in the robustness of our approach given the available climate data, as the event is largely unique in the data record. Moreover, the analogues approach does not guarantee that the identified past events do actually correspond to tropical cyclones.

ClimaMeter Analysis 

We analyse here (see Methodology for more details) how events similar to the low pressure system leading to Hurricane Helene have changed in the present (2001–2023) compared to what they would have looked like if they had occurred in the past (1979–2001) in the region [-90°E -75°E 29°N 40°N]. 

The Surface Pressure Changes show that cyclones similar to Hurricane Helene  show no significant differences in the present climate than what they would have been in the past in the southeastern coast of North America. Temperature Changes show that similar events produce temperatures in the present climate that are between 1 ºC and 2 ºC warmer than what they would have been in the past, over the eastern coast of the region analyzed. The Precipitation Changes show up to 17 mm/day (up to 20%) wetter conditions over a large area affected by the cyclone. Windspeed Changes indicate up to 5 km/h windier conditions over the Northwestern coast of Florida (Big Bend coast region). We also note that Similar Past Events previously mainly occurred in November, while in the present climate they are occurring in September and October.  The analysis of the affected urban areas reveal that Charlotte (North Carolina), Atlanta (Georgia) and Knoxville (Tennessee), are up to 12 mm/day wetter in the present than in the past and they also experience slightly more windy conditions in the present than in the past. 

Finally, we find that sources of natural climate variability, notably the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation, may have influenced the event. This suggests that the changes we see in the event compared to the past may be partly due to human driven climate change, with a contribution from natural variability.

Conclusion

Based on the above, we conclude that Hurricanes similar to Helene have become up to 17 mm/day (up to 20%) wetter over the South East of the United States  and locally up to 5 km/h (up to 7%)  windier in the Florida’s Gulf coast in the present than they have been in the past. We interpret Hurricane Helene as a largely unique event for which natural climate variability played a role.