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A year after Long Island's historic deluge, scientists study forecasting models

Sunrise Highway remains closed in both directions due

Sunrise Highway remains closed in both directions due to heavy rainfall Wednesday afternoon at Route 111, exit 45, in Islip, N.Y., on August 13, 2014. Photo Credit: James Carbone

A year ago Thursday morning, Nicholas Leonardo, 27, was astonished to find more than 5 inches of rainwater in his then Lindenhurst basement. That was nothing compared to what greeted those just a little farther east, where 9.7 inches had fallen from 5 to 7 a.m.

Lawns and roadways also flooded, leading to road closures, transportation delays and rescues of stranded drivers. Dropped jaws were also elicited from forecasters watching the complex, hard-to-predict, micro-scale event deliver a historic 13.57 inches of rain at Long Island MacArthur Airport over 24 hours.

Assessing his basement, Leonardo said his first thought was, "there goes my washer and dryer." His next thought: "How did this happen?"

His fascination with high-impact weather led him to start tinkering earlier this year with a small-scale numerical forecasting model to try to simulate what the Northeast Regional Climate Center called a 200-year storm event.

A doctoral student in atmospheric science at Stony Brook University, Leonardo presented his case-study findings last week at an American Meteorological Society conference in Boston, showing that when run at high resolution with certain assumptions, such as geographic parameters, the Weather Research and Forecasting Model is capable of simulating the event.

Still, the project is a work in progress, he said, as more needs to be done to address various uncertainties.

Dissecting such extreme rain events and seeking ways to better forecast them become of heightened importance, as "a warmer climate holds more moisture," and climate models are indicating an increase in such episodes, said Brian Colle, professor in Stony Brook University's School of Marine and Atmospheric Sciences and Leonardo's adviser.

The event was born in the convergence of an upper-level disturbance, a low-pressure area at the surface and a very moist environment. The National Weather Service issued flash flood watches and a warning, indicating up to a possible 4 inches of rain in localized areas.

Along with moderate-to-heavy rain from the surface low, precipitation on the morning of Aug. 13 reached deluge proportions as a localized wind flow from the north-northeast set up and -- right over southern Suffolk County and the line of heaviest rain -- collided with winds that were coming from the southeast. This forced air to rise significantly for a two- to three-hour period, "wringing out all the water in the air," said David Stark, weather service meteorologist in Upton.

When it comes to such scenarios, forecasting models do not have the capacity "to capture important small scale details associated with heavy rain," said Lance F. Bosart, professor in the Atmospheric and Environmental Sciences department of the University at Albany.


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