Why Were Irene’s Intensity Predictions So Off?

Hurricane path prediction has enormously improved. Forecasters knew days before it made landfall that Irene would hit the Carolinas and move up the East Coast, reaching New York and New England.

“There have been tremendous improvements in hurricane track forecasts over the past 20 years,” says Gerald Heymsfield, a research meteorologist at NASA’s Goddard Space Flight Center. Information from aircrafts flown by NOAA and the Air Force provided data, as well as NOAA radars on the ground along the East Coast. “This was an ideal situation compared to storms forming over the ocean or around the islands,” he adds.

The Associated Press points out that better computer models and better data for the models have led to drastically improved predictions of hurricanes’ paths:

By Monday night, five days before Irene first hit the East Coast, the hurricane center figured the storm would come ashore around the North Carolina-South Carolina border. By Tuesday night, they predicted it would rake the coast. And on Friday morning—24 hours before landfall—they had the storm’s next day location to within 10 miles or so.

Twenty years ago, 24-hour forecasts were lucky if they got it right within 100 miles and the average 36-hour forecast within 146 miles. With Irene, that was about the accuracy of the five-day forecast.

While path prediction has steadily improved over the decades, forecasting the intensity of storms still proves tricky. Irene’s expected monster intensity—much to the nation’s relief—was far less as she weakened a day or so after reaching land. “What made Irene especially difficult for the forecasting models was that she had three landfalls and followed the coastline,” says Heymsfield. “We need a lot more research to understand how to better model this land interaction.”

Others point to the unusual way Irene’s “eye wall”—the inner core of storms surrounding the hurricane’s eye—behaved. New York Times reports:

Forecasters had expected that a spinning band of clouds near its center, called the inner eyewall, would collapse and be replaced by an outer band that would then slowly contract. Such “eyewall replacement cycles” have been known to cause hurricanes to strengthen.

While its eyewall did collapse, Irene never completed the cycle, [James Franklin, chief of the hurricane specialist unit at the National Hurricane Center in Miami] said. “There were a lot of rain bands competing for the same energy,” he said. “So when the eyewall collapsed, there were winds over a large area.”

Rather than the intensifying “outer ring” storms that normally replace the collapsed inner band, the hurricane became diffused, with weaker winds spreading out over a larger area. So instead of the predicted Category 2 or 3 hurricane with 100 mph winds, Irene reached the Outer Banks as a milder, Category 1 storm.

Other factors that weakened Irene include moving over land, passing over shallow or cooler waters, and encountering unrelated wind, according to a follow-up article.

With new tracking technology and NASA’s upcoming Hurricane and Severe Storm Sentinel mission, scientists hope to better understand hurricane formation and improve intensity predictions in the next few years.