With 13.26 inches (33cm) of rain falling in Long Island just over 9 hours, Wednesday 13 August, it may seem like it was no ordinary storm. The massive amounts of rainfall and ensuing floods that struck much of eastern and northeastern USA between 12 and 14 August may have been unprecedented. But it wasn’t unexpected, at least not by US climate scientists. The intensity of extreme precipitation seen in the USA this week is exactly how the National Climate Assessment had predicted.
The National Climate Assessment (NCA) report of May this year said:
Heavy downpours are increasing nationally, especially over the last three to five decades. Largest increases are in the Midwest and Northeast. Increases in the frequency and intensity of extreme precipitation events are projected for all U.S. regions.
The report goes on to demonstrate that rainfall totals in the heaviest rain events in the Northeast have increased by 71% since 1958.
The NCA report is in no doubt that the increase in rainfall intensity is down to a changing climate.
Warmer air can contain more water vapor than cooler air. Global analyses show that the amount of water vapor in the atmosphere has in fact increased over both land and oceans. Climate change also alters dynamical characteristics of the atmosphere that in turn affect weather patterns and storms. In the mid-latitudes, where most of the continental U.S. is located, there is an upward trend in extreme precipitation in the vicinity of fronts associated with mid-latitude storms. Locally, natural variations can also be important.
Last week North and South Carolina, then Michigan, saw record rainfall and floods. This week Maryland, New York, New Jersey, Rhode Island, Connecticut and Maine all saw astonishing amounts of rain fall in just a few hours, matching perfectly the National Climate Assessment’s predictions and the pattern of climate change.
The headlines in some US newspapers called it Wet Wednesday. Perhaps Global Warming Wednesday would be more appropriate.