Challenges in Predicting the Intensity of Storms
Published: August 27, 2011
Irene may be the first hurricane to hit the East Coast in several years, but in one respect it is like all the others that have come and gone before it: forecasters have had difficulty predicting its strength.
The storm made landfall as a Category 1 storm in North Carolina, and New York City prepared for its arrival.
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Officials with the National Hurricane Center warned Saturday that the storm was still capable of inflicting heavy damage, particularly from flooding, as it slogged toward New Jersey and New York. But they said it had decreased in intensity, with sustained wind speeds of about 80 miles an hour, down 15 miles an hour from 12 hours before. And they acknowledged that they did not know precisely why it had weakened.
“There’s some internal dynamics of the storm that we don’t completely understand,” said Todd Kimberlain, a hurricane specialist at the center in Miami.
Mr. Kimberlain said one reason for the weakening may be that the storm had never completed a typical hurricane cycle in which the innermost band of spinning clouds, called the inner eye wall, dissipates and is replaced with an outer band that contracts.
“Some hurricanes get through this process and afterward will strengthen,” he said. “But we don’t know what has to go on internally.”
By never completing the cycle, Irene has become less organized and has lower peak winds, although it is still a very wide storm.
Hurricanes also tend to strengthen over water that is warm and deep, and Irene may have passed over areas that are a bit shallower. “There are some peculiar aspects to the water in that part of the Atlantic,” Mr. Kimberlain said. But it is very hard to know how the water may have affected Irene “because we don’t have observations everywhere.”
Mr. Kimberlain said that despite the uncertainty about the storm’s strength, he was especially concerned about the potential for heavy rainfall, especially in parts of New York and New Jersey that have received much rain in the past few weeks.
He also said the storm was still capable of producing a surge of four to eight feet over normal tides in New Jersey and New York. Storm surges are only partly related to maximum wind speed; the size of the storm and its overall speed are important as well. Irene has winds over 39 miles an hour over an area about 500 miles wide, which would tend to create higher surges, but is moving relatively slowly at about 15 miles an hour, which would tend to lessen them.
The problems in predicting Irene’s strength are typical, scientists say. Hurricane forecasting is far better at estimating where a storm will go.
“We’ve had a wonderful history of improving tracking forecasts,” said Clifford Mass, an atmospheric scientist at the University of Washington who works on numerical modeling of storms. A hurricane, he said, is essentially like a top, and it is relatively easy to gauge the steering winds and other forces that will move it.
“But we have not gotten good in intensity forecasts,” Dr. Mass said. “To get the intensity right, we have to get the innards of the storm right.”
The problem is a lack of observational data; it is very difficult to get information from the heart of a hurricane. Aircraft that fly into them do so at about 10,000 feet, far above the most intense winds and conditions. They carry radar that can gauge some conditions far below, and they also drop sensors on parachutes to measure wind speeds, air pressure and water temperatures. But it is not enough data to plug into a numerical model and yield a forecast that has a high degree of certainty, Dr. Mass said.
Mr. Kimberlain said the difficulty in gauging a storm’s intensity led the National Hurricane Center to be cautious when updating its forecasts, as it has been doing every several hours in the case of Irene.
“We’re slow to make changes to the forecast,” he said. “We’d rather be a little high than a little low.”