2023/09/29 New York floods

Heavy precipitation in New York mostly strengthened by human-driven climate change 

Press Summary (First published 2023/10/02, Updated 2023/11/23)

Event Description

On September 29th, an exceptionally violent storm has struck the East Coast, particularly in the states of New York and New Jersey. The dynamics leading to these torrential rains are primarily linked to a convergence of cold air masses descending from Canada and very warm, humid air rising from the Gulf of Mexico. In New York City, where a state of emergency has been declared, up to 200 mm of rain has fallen mostly on the 29th. Brooklyn and Manhattan have been submerged under one and a half meters of water, with restaurants, shops, and especially subways all flooded. While no fatalities have been reported, scenes of chaos emerged as residents waded through knee-deep water in search for higher ground and drivers abandoned their vehicles on overwhelmed highways. The deluge wreaked havoc on the country's biggest transportation network, as heavy rains inundated certain subway stations in New York City and caused significant disruptions on various regional train lines. JFK Airport experienced about 200 mm of rain, making it the wettest day recorded there since 1948, while LaGuardia Airport was similarly submerged with nearly 120 mm of rainfall, leading to flight cancellations, according to reports. Even the Central Park Zoo found itself underwater, prompting a temporary escape by a sea lion from its overflowing enclosure.

The Surface Pressure Anomalies reveal a weather pattern characterized by a low-pressure system off the coast of North Carolina and a high-pressure system to the north. This unusual atmospheric setup led to a significant influx of rainfall directed toward the city. Precipitation Data indicate that the New York City area experienced rainfall amounts surpassing 100 mm per day, which, while substantial, was less than the measurements recorded at select weather stations where rainfall reached as high as 200 mm per day.

Climate and Data Background for the Analysis

The IPCC AR6 WG1 report states that the frequency and intensity of heavy precipitation events are projected to increase almost everywhere in the USA. In the Eastern North America region observed increase in extreme precipitation events have been already detected in the past and anthropogenically attributed with medium confidence. With global temperature increases due to human-induced climate change, New York may face more frequent and severe heatwaves, heavy precipitation leading to urban flooding, elevated storm surge risks, and chronic flooding from sea-level rise. The extent of these impacts depends on factors such as the global warming, local geography, and regional climate patterns. 

Our analysis approach rests on looking for weather situations similar to those of the event of interest observed in the past. For the New York floods we have medium-high confidence in the robustness of our approach given the available climate data, as the event is similar to other past events in the data record.

ClimaMeter Analysis

We analyse here (see Methodology for more details) how events similar to the low pressure systems leading to the recent New York floods have changed in the present (2001–2022) compared to what they would have looked like if they had occurred in the past (1979–2000) in the region [-80°E -70°E 35°N 45°N]. The Surface Pressure Changes show that low pressure systems have not significantly changed their intensity compared to the past. Precipitation Changes show that similar events produce larger (0-3 mm/day) amounts of precipitation in New York area. However, they also tend to produce the same amount of precipitation amount in other areas, such as Southern part of the region. Considering the affected urban areas, New York, Brooklyn and Hoboken see an increase in precipitation in the present (0-3 mm/day). We also find that Similar Past Events have become more frequent in October, by doubling their frequency of occurrence, and less frequent in August and September. 

Finally, we find that sources of natural climate variability may have partly influenced the event. This suggests that the changes we see in the event compared to the past may be mostly due to human driven climate change, with a secondary contribution from natural variability.


Based on the above, we conclude that low pressure systems leading to New York floods similar to that observed in September 2023 are 0-3 mm/day wetter than they would have been. We interpret the New York floods as an event whose characteristics can mostly be ascribed to human driven climate change.

Contact Authors

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

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

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

Additional Information : Complete Output of the Analysis

The figure shows the average of surface pressure anomaly (msl) (a), average 2-meter temperatures anomalies (t2m) (e), cumulated total precipitation (tp) (i),  and average wind-speed (wspd) in the period of the event. Average of the surface pressure analogs found in the counterfactual [1979-2000] (b) and factual periods [2001-2022] (c), along with corresponding 2-meter temperatures (f, g),  cumulated precipitation (j, k), and wind speed (n, o).  Changes between present and past analogues are presented for surface pressure ∆slp (d),  2 meter temperatures ∆t2m (h), total precipitation ∆tp (i), and windspeed ∆wspd (p): color-filled areas indicate significant anomalies with respect to the bootstrap procedure. Violin plots for past (blue) and present (orange) periods for Quality Q analogs (q), Predictability Index D (r), Persistence Index Θ (s), and distribution of analogs in each month (t). Violin plots for past (blue) and present (orange) periods for ENSO (u), AMO (v) and PDO (w).  Number of the Analogues occurring in each subperiod (blue) and linear trend (black).  Values for the peak day of the extreme event are marked by a blue dot. Horizontal bars in panels (q,r,s,u,v,w) correspond to the mean (black) and median (red) of the distributions.