A guide to understanding the historic rain in New York

New York City collapsed on Wednesday night (1) under record-setting exceptional rainfall and a recurrence time of centuries. The forecast was already of extraordinary rain in the region, but the storm surprised the population and even astonished the North American meteorologists. Expressions such as “epic rain” and “Bible storm” frequented the manifestations of meteorologists in New York City and local authorities.

Flooded expressway in Brooklyn, New York in the early hours of this Thursday | Ed Jones/AFP/MetSul Meteorology

New York’s Mesonet network and ConEdison power company ConEdison’s monitoring stations in New York City recorded extreme rain on all borons at 213.3 mm in Staten Island, 200.6 mm in the Bronx, 190.5 in Manhattan , 195.5 mm in Queens and 170.1 mm in Brooklin.

The volumes were exceptionally high in short periods, which ended up bringing precipitation with a recurrence time of centuries. Based on data from the official Central Park station of the National Weather Service, the accumulated rainfall in one hour of 80 mm in one hour has a recurrence of 200 years. The volume of 118.1 mm in two hours has an estimated recurrence time of 500 years. Records of 132.0 mm in three hours and 173.2 mm in six hours also have an estimated recurrence of 500 years.

The total rainfall yesterday at the Central Park historic reference station was 181.1mm while the normal for the entire month of September is 109.4mm. Weather records in New York City began in 1869. The season last week had its record rain in just one hour of 49.2mm with Storm Henri and a week later the record was broken again with 80.0 mm in just 60 mm.

Yesterday’s 118.1mm in New York made September 1, 2021 the second wettest day on record in September, only surpassed by 210.3mm on September 23, 1882. On September 21, 1966 it rained in one dia 140.7 mm. On September 16, 1999 127.5 mm fell. On September 8, 1934, it was 123.4 mm.

With yesterday’s rain, 2021 could turn out to be the wettest year in New York City history. The annual record belongs to 1983 with 2046.2 mm, followed by 2011 with 1849.3 mm, 1972 with 1702.5 mm, 2018 with 1664.9 mm and 1989 with 1653.7 mm. The accumulated rainfall from 2021 to date is less than 50 mm from surpassing 1983.

What happened?

New York City had an extraordinary rain event because it experienced an exceptional and unusual weather situation that will be the subject of many studies by Meteorology. On Sunday, category 4 hurricane Ida touched land in Louisiana with category 4, 930 hPa and sustained wind of 241 km/h with gusts of up to 277 km/h.

The tropical cyclone, as is normal when advancing over land, lost its supply of latent heat energy from the ocean waters and weakened, changing first to a tropical storm and then to a tropical depression. Yesterday, the tropical depression Ida reached the Northeast of the United States. Normally, the system would already cause intense rain in the region, but there was an aggravating factor. A jet stream (wind) south of the Great Lakes region intensified the instability. The result was extreme rain in the region between Philadelphia and Southern New England with the accumulated extremes and history observed.

New York City had the highest amount of rain in an hour in its history and the US National Weather Service issued an unprecedented flash flood emergency alert for the city | Ed Jones/AFP/MetSul Meteorology

With Ida, the New York region was receiving tropical moisture, which favors more intense rainfall. The displacement of the tropical cyclone created an atmospheric river with high levels of precipitable water from the Gulf of Mexico to the Northeast of the United States, where, in addition to rain, there were tornadoes.

The effect of climate change

It is a remarkable fact that in just a few days New York City has beaten its record for accumulated rain in an hour twice in a time series starting in 1869. Equally astonishing are the recurrence times of last night’s rain that range from 200 to 500 years for rainfall of one to six hours.

While it is not uncommon for weakened tropical systems to bring excessive rain and flooding, extreme rains have become more common since the 1960s, especially in the Northeastern United States. This is because the warming of the waters of the Atlantic Ocean along the coast of the North American Northeast is increasing storms and the warmer air is able to retain more moisture, two situations of climate change caused by humans. The frequency of the heaviest rains has increased by 50% in the Northeastern United States since the 1960s.

New York collapsed with “epic” rain on Wednesday night | Social Media/Reproduction

Physics explains the process that leads to the most frequent extreme rain events. There is a physical principle known as the Clausius-Clapeyron relationship that relates temperature, pressure and water vapor. The principle shows that warmer air can hold more water vapor with about 7% more water vapor per 1°C. The warmer the atmosphere, the more water vapor it will hold, and the more vapor means more moisture is available to precipitate as rain, which leads to higher rainfall rates. The same, paradoxically, occurs in snowstorms.

On average, the planet has warmed more than 1°C since pre-industrial times, according to the report by the Intergovernmental Panel on Climate Change released last month. The IPCC document in its projections warned of more extreme rain in the North East of the United States. In terrestrial areas, there has been further warming, and particularly in the eastern United States, which has led to a noticeable increase in heavy rains that bring flash floods, according to the most recent National Climate Assessment by the US government.

The report explains that rising temperatures control “changes in water vapor through increased evaporation and the atmosphere’s water-holding capacity.” On a global scale, the water vapor content “increases approximately following the Clausius-Clapeyron (CC) ratio, with an increase of approximately 7% per 1°C of warming”.

“Climate model projections show that rising water vapor leads to robust increases in precipitation extremes everywhere, with a magnitude ranging between 4% and 8% per degree Celsius of surface warming,” says the IPCC . However, there are “several cases indicating that very extreme precipitation can increase at a rate greater than the CC rate,” the report notes. Climate warming can also affect “dynamic” processes in the atmosphere, according to the IPCC, such as the frequency and intensity of cyclones, fronts and convective systems:

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The number of studies on hourly extreme rainfall projections “is limited,” notes the UN’s Intergovernmental Panel on Climate Change. Simulations by regional climate models, which are capable of simulating convective processes known as convective permission models, are “limited and only available in some regions because of the high processing costs per computation”. The report, however, highlights that “most of the available convection permit simulations project increases in the intensities of extreme subday precipitation events by an amount similar to or greater than the CC scale rate”.