Precipitation extremes in 2023

Precipitation extremes in 2023 | Nature Reviews Earth & Environment

2023 saw a multitude of extreme precipitation events across the globe, causing flash flooding, countless fatalities and huge economic losses. Fuelled by a combination of a strong El Niño, record ocean warmth and anthropogenic warming, these events highlight the ongoing risks posed by extreme precipitation in a warming climate.

Key points

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A blocked wave pattern in September caused slow-moving cut-off lows that drenched Europe; Storm Daniel deposited >1,000 mm rainfall in Thessaly, Greece, over a 24-hour period.
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Tropical cyclones drove extreme rainfall across many global regions; Tropical Cyclone Freddy inundated parts of Mozambique with 400–800 mm rainfall in 24 hours.
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A high number of severe convective storms caused flash flooding in many locations; Hebei Province, China, recorded rainfall totals >1,000 mm in 3 days.

Extreme precipitation arises from many atmospheric phenomena, including atmospheric rivers, tropical and extratropical cyclones and convective storms. Its impacts are often catastrophic, with resulting flash floods causing substantial economic damage (which indiscriminately affects developed and developing nations 1) and countless fatalities. For instance, monsoon-related extreme precipitation across India and Bangladesh in May–June 2022 affected and displaced ~10 million and 250,000 people, respectively; Raigad district in Maharashtra, India, recorded almost 500 mm in 72 h. Such extreme and often record-breaking precipitation events have increased globally 2, largely in line with thermodynamic forcing 3— enhanced atmospheric moisture at approximately the Clausius–Clapeyron rate of 6–7% C–1. Climate models project ongoing rainfall intensification 4, albeit with regional variability governed by local warming rates 3, local scale storm dynamics 5 and large-scale circulation variability 6, 7.

Here, we examine the key characteristics of extreme precipitation in 2023, focusing on specific record-breaking events associated with blocking systems, cyclones, convective systems and atmospheric rivers.

Blocking and low-pressure systems

Several notable precipitation events in 2023 were associated with a blocked (omega) wave pattern and corresponding cut-off lows. Such a blocking pattern arises from a reduced temperature gradient between the equator and poles, weakening the mid-latitude jet and causing weather systems to become stationary in an increasingly meandering jet. Storm Dana and Storm Daniel are two such examples of cut-off lows linked to omega blocking over central Europe. Storm Dana affected Spain in early September. In Mas de Barberans, Catalonia, 243.4 mm rain fell in 24 hours, the highest 24-h total in 23 years. Other regions of the country also received substantial rainfall totals, causing extensive flooding.

Storm Daniel followed a couple of days later, affecting much of the Mediterranean from 5th–12th September. The storm started in the Balkans, depositing 1,092 mm rain in the Greek region of Thessaly on 5th September (Fig. 1a), with more following on the 6th September to produce ~3 years’ worth of rainfall in 2 days. A total of 750 km2 of agricultural land was flooded, causing damage exceeding US $2 billion. Bulgaria was also affected at the same time, with 311 mm of rainfall falling over 14 hours on 5th September, four times the normal monthly average. The rain event caused 4 fatalities and huge infrastructural damage. The storm subsequently moved offshore on the 9th September, transitioning into a rare Mediterranean tropical cyclone — a Medicane. The cyclone hit Libya on 10th September, producing ~440 mm rainfall in 6 h that broke two dams above the city of Derna. The resulting 7-m flood wave caused tens of thousands of fatalities and damage totalling an estimated US $19 billion. Anthropogenic climate change made the extreme rainfall 50 times more likely 7.

a, Rainfall rate across Thessaly, Greece, associated with Storm Daniel, an example of a cut-off low-pressure system. Rainfall rate is from IMERG-GPM 10 and represents the average of a 9-pixel region (approximately 1000 km2). b, As in a, but for Tuscany, Italy, associated with Storm Ciaran, an example of a severe convective storm. c, As in a, but for Guam, U.S. Territory, associated with Typhoon Mawar. d, As in a, but for the Los Angeles region, California, associated with several atmospheric rivers. Various atmospheric processes contribute to extreme precipitation across the globe, including cut off lows, convective storms, tropical and extratropical cyclones, and atmospheric rivers.

Low-pressure systems not associated with blocking also caused extreme rainfall events. For example, Emilia-Romagna, Northern Italy, experienced heavy rainfall associated with three distinct low-pressure systems situated over the Tyrrhenian Sea on the 2nd, 10th and 16th May. In 2 weeks, the region received the normal amount of rainfall for 7 months, including 366.4 mm in Modigliana between 8–21 May, with a return period of ~200 years. The rainfall caused the overflow of twenty-three rivers across the region and resulted in 8 fatalities. Anthropogenic climate change had a limited influence on this heavy spring rainfall event.

A low-pressure system associated with remnants of Typhoon Haikui also brought record-breaking rainfall in Hong Kong on 7th September. The region received >600 mm rain in 24 h, over a quarter of annual rainfall. Hourly precipitation peaked at 158.1 mm (corresponding to a return period of >500 years), which broke the previous record of 145.5 mm from 2008. The event resulted in disruption to transport, including flooded subway stations, the closure of the Hong Kong Stock Exchange and the evacuation of 110 people.

In Japan, the remnants of tropical storm Yun-yeung brought similar extreme rainfall on 8th September. A total of 391.5 mm rainfall fell in 24 h in Moraba City, Chiba Prefecture, smashing the previous record of 272 mm set in October 2013. Rainfall records were also broken in Iwaki City, Fukushima Prefecture, Japan with 131.5 mm of rain recorded in just 3 hours on 8th September. Extensive flash flooding ensued, resulting in over 1,200 houses being flooded or damaged, and >200 landslides.

Unusual numbers of severe convective storms

2023 brought an unusually high number of severe convective storms with characteristic extreme rainfall. For example, the atmospheric moisture remnants of Storm Hilary contributed to convective storms in August. After causing severe flooding in western Mexico and southern California, 60 mm rainfall fell in 2 h in parts of Arizona. A state of emergency was declared in West Virginia, USA, on 28th August after heavy rainfall caused flash flooding; 105.92 mm rainfall fell in Kanawha in just 6 h, and 158.75 mm in 72 h. A notable convective storm in Tuscany, Italy, on 2nd November also delivered over 200 mm in 24 h (Fig. 1b), including 98.6 mm falling in just 4 h at Pontedera. This event caused extensive flash flooding and landslides, with 3 fatalities.

Many low-pressure systems also contained a large convective component, with subsequent flash flooding over widespread areas. In the USA, a billion-dollar storm hit Fort Lauderdale on 12th April 8, accumulating 658 mm rain at Hollywood International Airport. This total eclipsed the previous 24-hour record of 371 mm set on April 25, 1979, and was roughly equivalent to a third of the city’s annual rainfall and seven times the typical April total. Beijing also experienced its heaviest rainfall for 140 years in August, accumulating 744.8 mm in 5 days. More broadly, Hebei province recorded 1,003 mm in 3 days, equivalent to a year and a half of rainfall. Flooding from this event affected 1.3 million people, damaging 147,000 homes.

Supercharged tropical and extratropical cyclones

Tropical cyclones (TC) commonly bring intense rainfall, and as TC Freddy, TC Mocha, and Hurricane Otis highlight, those in 2023 were no exception. TC Freddy traversed the southern Indian Ocean to impact Madagascar, Mozambique and Malawi in February and March 2023. It was the longest-lived TC on record (35 days); holds the record for the highest accumulated cyclone energy (87.01 units), equivalent to that of an average full North Atlantic hurricane season; and underwent the most instances of rapid intensification (7). TC Freddy made landfall twice in Mozambique, bringing record rainfall of ~400–800 mm in southern coastal regions from 24th February to 2nd March and ~400 mm in inland regions 2 weeks later. These high rainfall accumulations were correlated to slow storm speed, allowing some of the storm’s circulation to remain over the warm waters of the Mozambique Channel and rainbands to continuously form and move inland. There were over 1434 collective fatalities, most in Malawi but also in Mozambique, Madagascar, Zimbabwe and Mauritius.

In May, TC Mocha — one of the most intense TCs observed in the Bay of Bengal — hit Myanmar and Bangladesh. In Myanmar, the towns of Chauk and Sinphyukyun, Rakhine State, reported record 24-h rainfall totals of 218 mm and 177 mm, respectively, well above the previous record of ~ 100 mm. The TC killed over 500 hundred people in Myanmar and displaced hundreds of thousands.

At the end of October, Hurricane Otis hit near Acapulco, Mexico. It was the strongest landfalling hurricane in the eastern Pacific of the satellite era, undergoing explosive intensification that took it from a tropical storm to a Category 5 hurricane overnight. It produced ~400 mm rainfall in Acapulco in 24 h. Although much of the damage was caused by winds, storm surge, flooding and mudslides also contributed to damage to buildings, power and communication outages and loss of life.

Various other TCs also contributed notable extreme precipitation globally. Cyclone Gabrielle brought record rainfall (300–400 mm) to parts of New Zealand between 12th and 14th February 2023; Super Typhoon Mawar battered Guam, the Philippines, Taiwan and Japan in May and June, with most of Guam receiving 450–600 mm over 72 hours (a maximum of 714.2 mm) (Fig. 1c) and Iosashimizu City, Japan, recording 358.5 mm in 12 hours; and Tropical Storm Hilary inundated California, causing downtown Los Angeles to break daily (63 mm) and monthly (76 mm) records in August.

Extratropical cyclones also contributed to record-breaking rainfall. In Northern Europe, for example, the winter storm season started early and was unusually vigorous, with a record number of named storms. Of those, Storm Ciaran in November was particularly impactful. It underwent explosive cyclogenesis, passing through Ireland, the UK, France, Spain, the Czech Republic and Italy. It brought ~200 mm of rain in 4 hours to the Tuscany region (more than the typical November total), resulting in the loss of lives (Fig. 1b).

Atmospheric rivers

In addition to convective storms, atmospheric rivers also produced several extreme rainfall events across the globe. California, for example, experienced a record number of atmospheric rivers throughout January to March 2023 (Fig. 1d for Los Angeles region), collectively contributing to a billion-dollar flood event 8 and causing average snowpack across the western USA to exceed levels not seen on 1st April for 40 years. In the 2 weeks from 26 December 2022, large swaths of coastal California received more than 650 mm rain, with isolated areas receiving >1,000 mm. In addition, other atmospheric rivers deposited 415.04 mm over a 48-h period in January 2023 and 117.09 mm over 24 h in February. One of these atmospheric rivers even spawned a bomb cyclone that produced the strongest March storm ever recorded in California’s Bay area. It deposited more than 150 mm rain in 24 hours in Lytle Creek, San Gabriel Mountains, and ~40 mm in downtown Los Angeles. Oregon and Washington also witnessed an atmospheric river on 4th December 2023. Record rainfall fell, with 24-h totals reaching 220.22 and 119.13 mm in Olympic National Park and Forks, WA, respectively. The Forks rainfall was more than double the previous daily rainfall record set in 1971. The floods caused 2 fatalities.

Chile was also inundated by a series of atmospheric rivers. In June, San José de Maipo in Santiago Metropolitan Region recorded 126.4 mm of rain, while Retiro in the Maule Region recorded 156.6 mm. A second serious atmospheric river occurred on 19th–21st August, with the Maule region recording 137 mm rainfall, causing >30,000 homes to be evacuated; Termas de Chillan saw 371.9 mm in the same 3-day period.

Summary

Many extreme rainfall events in 2023 were unprecedented and record-shattering. While many of these events were enhanced by anthropogenic warming, as demonstrated by attribution analysis, natural variability also had a role. For instance, many extreme precipitation events were likely fuelled by additional El Niño-related heat from June onwards. The El Niño resulted in mid-Pacific warming, which combined with a strong sub-tropical jet extending NE from the tropical Pacific to the NE Atlantic, likely providing optimal conditions for global rainfall extremes, particularly the strong atmospheric rivers in the NW Europe winter storm season 9.

2023 provided a glimpse into the future of precipitation extremes, which will likely only get worse with further warming. These events highlight the urgent need for increased investment into measures such as improved early warning systems as well as for longer-term climate adaptation to flooding.
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Acknowledgements

H.J.F. was supported by the Co-Centre for Climate + Biodiversity + Water funded by UKRI (NE/Y006496/1) and NSFGEO-NERC: HUrricane Risk Amplification and Changing North Atlantic Natural disasters (Huracan) funded by NERC (NE/W009587/1); H.J.F. and A.G. were supported by IMPETUS4CHANGE funded by HORIZON-CL5-2022-D1-02 (Grant agreement ID: 101081555) and the UKRI Horizon Europe Guarantee (10047737).

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Fowler, H.J., Blenkinsop, S., Green, A. et al. Precipitation extremes in 2023. Nat Rev Earth Environ (2024). https://doi.org/10.1038/s43017-024-00547-9
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DOI: https://doi.org/10.1038/s43017-024-00547-9