On September 20, 2017, Hurricane Maria hit the southeast coast of Puerto Rico as a category 4 hurricane with 155 mph winds, making it the strongest hurricane to make landfall on the island since 1928. Maria dumped up to 37.90 inches (962.7 mm) of rain, resulting in unprecedented flooding and mudslides. One study estimated the excess death toll at close to 5,000 (Kishore et al., 2018), whereas an independent assessment produced by George Washington University and the University of Puerto Rico calculated what would become the official excess death toll as 2,975. Both studies had large uncertainty ranges.

A 2019 study by Keellings and Ayala, Extreme Rainfall Associated With Hurricane Maria Over Puerto Rico and Its Connections to Climate Variability and Change, concluded that Hurricane Maria brought the largest maximum rainfall event of any of the 129 tropical cyclones to pass within 500 km of the island between 1956 and 2017 (Figure 1), and also the greatest island-averaged rainfall event (not shown).

Figure 1. Maximum single-location rainfall (mm) in Puerto Rico for 129 tropical cyclones to pass within 500 km, 1956-2017. These numbers, which are estimated from an interpolated analysis, differ from the actual observed local maxima shown in the NOAA list below. Note that what NOAA considers the wettest tropical cyclone in the island’s history, Tropical Depression 19 of 1970, is not highlighted in the figure above, but was used in the analysis, which used only a 4-day accumulation period compared to NOAA’s 9-day period. (Image credit: Keellings and Ayala, 2019, Extreme Rainfall Associated With Hurricane Maria Over Puerto Rico and Its Connections to Climate Variability and Change, Geophysical Research Letters, https://doi.org/10.1029/2019GL082077)

Tropical cyclones (and precursors) with the largest single-location storm-total rainfall amounts observed in Puerto Rico history (according to NOAA):

41.68” (1058.7 mm) Tropical Depression 19, 1970
37.90” (962.7 mm) Hurricane Maria, 2017
34.12” (868.7 mm) Hurricane Fiona, 2022
33.29” (845.6 mm) Tropical Depression Eloise, 1975
31.67” (804.4 mm) Tropical wave that became Tropical Storm Isabel, 1985
30.51” (775.0 mm) Hurricane Georges, 1998

Figure 2. Total rainfall in Puerto Rico on October 2-10, 1970, from the wettest tropical cyclone in the island’s history: Tropical Depression 19. (Image credit: NOAA)

Fiona breaks Maria’s island-wide precipitation record

In addition to looking at the highest storm totals at individual spots, as featured above, we can also examine the highest island-wide averages produced by various storms. Just five years after Maria, we have now seen an island-wide event, Hurricane Fiona, that exceeds all others examined in the Keellings and Ayala study. The NOAA/NWS Weather Prediction Center has estimated (see tweet above) that the island-averaged storm-total rainfall from Fiona was 15.80 inches (401.3 mm). This figure is higher than the island-wide average for all of the events studied in the Keellings and Alaya paper, including Hurricane Maria (which they estimated as 14.72 inches, or 374 mm).

Figure 3. Radar loop of Hurricane Fiona drenching Puerto Rico, September 17-19, 2022. Note how the storm’s outer bands delivered most of the rain to the island. (Image credit: National Weather Service and Brian McNoldy)

The 2019 Keellings and Ayala study concluded that the return period of an island-wide precipitation event like Maria brought in 2017 had decreased from approximately a 1-in-300-year event to a 1-in-115-year event, and that the probability of a precipitation event of Maria’s magnitude has increased by a factor of 4.85 – both as a result of long-term climate trends.

In an email, one of the paper’s authors, Jose Hernandez Ayala, said, “Hurricane Fiona was definitely a historical rainfall event that confirms the findings of our 2019 paper that suggested that extreme precipitation events like this would be occurring in shorter return periods in Puerto Rico because of climate change.”

Climate change and extreme precipitation: a strong link

One of the more confident predictions for hurricanes in the future is that they will dump more rain. Global warming increases the rate at which ocean water evaporates into the air, and increases the amount of water vapor the atmosphere contains when fully saturated. This result is about 7% more water vapor in saturated air for every one degree Celsius of ocean warming. This increase in atmospheric water vapor can cause a much larger increase in hurricane rainfall than one might think: The reason — water vapor retains the extra heat energy required to evaporate the water, and when the water vapor condenses into rain, this “latent heat” is released. The extra heat helps power the hurricane, making it larger and more intense, allowing it to pull in water vapor from an even larger area and thus dump more rain.

A growing body of literature shows that heavy precipitation events of all kinds – including those from tropical cyclones (which include all hurricanes, tropical storms, and tropical depressions) – have already grown more common. In fact, the three highest-volume U.S. precipitation events in the past 73 years have occurred since 2016.

Higher-than-predicted tropical cyclone rainfall being observed

A 2021 paper by Guzman and Jiang, Global increase in tropical cyclone rain rate, found a 21% global increase in tropical cyclone rainfall rates from 1998-2016 (1.3% per year) using satellite observations. The increase was primarily driven by increases in outer rainband rainfall in the Northern Hemisphere, and not inner-core rainfall (which showed a decline). This observational data is concerning, as the increase in rainfall observed is far in excess of theoretical and model-produced estimates of what is expected in a warming climate. The authors noted the following: “The differences in the order of magnitude between the expected (modeled) vs. observed rainfall rates in our period of study are substantial; while modeling averages projects a +14% for a 2°C warming increase, our result for the 19-year time series is close to +21% for a variation of 0.21°C only (with more presence in the northern hemisphere).”

The extreme rainfall of Fiona in Puerto Rico was primarily from the storm’s outer rainbands, not the core region – supporting the results of the 2021 Guzman and Jiang paper (see tweet below). More research is needed to confirm the results of this worrisome result and determine where the discrepancy lies between observations and theory/modeling.

Does tropical cyclone translation speed slowdown cause more precipitation?

Part of the reason for Fiona’s record rains lies in the slow forward speed of the hurricane as it passed Puerto Rico: 8-10 mph. Slower-moving tropical cyclones can drop larger amounts of rain over a given location (Hall and Kossin 2019), causing more flooding issues.

It is uncertain if global warming is causing tropical cyclones to move more slowly and thus dump more rain. There is some evidence from Kossin (2019) and Hall and Kossin (2019) for a slowing of tropical cyclone movement over the continental U.S. over the past century or in near-U.S. coastal regions between 1948 and 2017, which can plausibly be linked to climate change. But more research is needed to make this connection (see, for example, Zhang et al. 2020)

Please donate to help hurricane victims

To help out with Fiona recovery efforts in Puerto Rico, please join us in donating to one of these worthy causes:

The Partnership for Inclusive Disaster Strategies (Formerly Portlight.org), helping disabled people in Puerto Rico recover from the hurricane.

The Fiona Community Response Fund, a coalition of community-led organizations working on immediate response to fulfill needs over the short- and long-term.

Red Cross

Website visitors can comment on “Eye on the Storm” posts (see comments policy below). Sign up to receive notices of new postings here.

Jeff Masters

Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...

Bob Henson

Bob Henson is a meteorologist and journalist based in Boulder, Colorado. He has written on weather and climate for the National Center for Atmospheric Research, Weather Underground, and many freelance...