Heavy Rainfall in Hurricane Milton linked to Human-Driven Climate Change, though Confidence is low 

Press Summary (First Published 2024/10/11)

• Cyclones similar to Hurricane Milton are 0-12mm/day  (0 to 20%) wetter over South Florida, while their intensity remains unchanged in other areas.

• Hurricane Milton was a largely unique event. Therefore confidence in our results is low. 

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

Event Description

Hurricane Milton formed in early October 2024 as a tropical depression in the eastern Atlantic. It quickly intensified, reaching Category 5 status by October 8, with winds peaking at 290 km/h. At this time Milton became the fifth most intense hurricane in the Atlantic Basin on record in terms of central pressure, behind Hurricane Rita (2005), and only the sixth storm in the Atlantic to have a central pressure below 900 mb.  In the process, Milton became the fastest Atlantic storm to intensify from a tropical depression to a Category 5 hurricane, with maximum sustained winds increasing from 35 mph to 160 mph in just over 48 hours. Milton maintained this strength as it moved across the Atlantic and into the Gulf of Mexico, where it eventually weakened. It made landfall in Florida on October 9 as a Category 3 storm with winds of around 160 km/h, causing severe flooding and damage before gradually decreasing in intensity to Category 1. Downpour in parts of southern Florida brought the risk of flash floods, with cities such as St. Petersburg reaching maximum values of precipitation around 18.87 inches. After the disaster caused by Hurricane Helene at the end of September in the SouthEast of the US, authorities have issued storm surge warnings and cautioned about ongoing flood risks.

The Surface Pressure Anomalies indicate a significant negative (cyclonic) anomaly of 20 hPa near the western coast of Florida, with Temperature Anomalies showing modest warm anomalies of 1.5ºC along the southern coasts and southeastern peninsula of Florida, including the Miami area. In contrast, negative temperature anomalies (-1.5ºC) are observed across the northern and northwestern parts of the region targeted by Milton (including Tampa, Clearwater, Fort Myers). Precipitation data reveal high daily rainfall amounts, exceeding 200 mm per day in northwestern areas, especially around Tampa, Clearwater, St Petersburgh and the western coast. Windspeed data suggest velocities of around 80 km/h over the Gulf of Mexico and approximately 50 km/h along Florida's central west coast, including Tampa and Clearwater.

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 Milton 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 [-85°E -80°E 25°N 28°N]. 

The Surface Pressure Changes show that cyclones similar to Hurricane Milton show anomalies of 2ºC warmer in the north of Florida in the present climate than what they would have been in the past. There are no remarkable Temperature Changes in Florida, the area targeted by Milton. Precipitation Changes show that similar events produce heavier up to 12mm/day precipitation in the present than in the past with the largest values in Southern Florida. There are no remarkable changes in Windspeed in Florida. 

We also note that Similar Past Events previously mainly occurred in October and November while in the present climate, although they are mostly happening in November, the frequencies are more alike between August and November.   

The analysis of the affected urban areas reveal that Tampa and Fort Myers see an increase in precipitation (7mm/day and 4mm/day respectively) in the present  compared to the past, while no significant changes occur in Clearwater.

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 Milton have become up to 12 mm/day (up to 20%) wetter over South Florida in the present than they have been in the past. We interpret Hurricane Milton as a largely unique event for which natural climate variability played a role. Confidence in these changes is low because of the exceptional trajectory of the cyclone.

Quotes & Contact Authors

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

- Gianmarco Mengaldo, National University of Singapore, Singapore  📨mpegim@nus.edu.sg    🗣️English, Italian, 

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