Dry air collapsed the eyewall, diminishing its feared winds, but the vast storm pounded the East Coast with flooding rain
Computer modeling, satellite data, and high-tech instruments and sensors have taken the science of hurricane prediction to new heights, but Hurricane Irene — much anticipated as the storm of the century — confounded forecasters.
The storm proved to be much less violent than predicted. The reason: An unexpected infusion of dry air caused Irene’s wind-generating engine to sputter. Hurricanes are “full of surprises,” says Jeff Masters, director of meteorology at the website Weather Underground (www.wunderground.com), who followed the storm’s development from start to finish. This one fooled Masters, as it did most other forecasters. It turned out to be more of a rain event than a wind-and-surge event.
As Irene swirled past the northern Bahamas on Aug. 25 as a Category 3 hurricane, a small high-altitude, low-pressure system sitting atop an area of high pressure in the Gulf of Mexico “opened up, sending a sudden burst of [dry] southwest winds” pouring into the vast storm system, which by then stretched more than 500 miles across, Masters says. The burst of dry air came just as Irene was replacing its eyewall.
“Dry air penetrated to the center of the storm and the eyewall collapsed,” Masters says. This weakened Irene’s circulation — warm, moist air spiraling into the storm at sea level, rising tens of thousands of feet, cooling, dumping rain and creating low pressure at the center so that yet more moist air would be sucked in at ever-accelerating speeds, generating ever-more violent winds.
The eyewall damage was most severe on Irene’s southwest side, which became its weak side. After this blow to its moist-air intake system, “Irene never really got its act back together,” Masters says. “It never got a new eyewall.”
The low that sent that rush of dry air pouring into Irene at a critical point in its development was “relatively subtle and small-scale,” he says. “The models didn’t see it coming. This blew the forecast for intensity.”
Expected to mushroom into a monster Category 4 storm, Irene made its first U.S. landfall near Cape Lookout, N.C., as a Category 1 with winds of 85 mph and continued to weaken, making a second landfall at New Jersey’s Little Egg Inlet with 75-mph winds — barely hurricane strength. By the time it made its third U.S. landfall, over Coney Island in Brooklyn, N.Y., it had been downgraded to a 65-mph tropical storm. “We lucked out, for sure,” Masters says.
Still, the storm was so big and was carrying so much rain that it caused massive flooding along parts of the East Coast and Northeast — New Jersey and Vermont, in particular. It piled up more surge than usual for a storm of Irene’s intensity because of its sheer size and pushed that surge ashore on top of a new-moon tide, which caused forecasters to worry about downtown flooding in New York City. As it turned out, surge wasn’t Irene’s biggest threat, mainly because its wind decelerated, but also because its weak side — the west side, the side most badly compromised by wind sheer and dry air — was the side that most of the Mid-Atlantic states saw, according to Masters’ daily hurricane blog.
Even though Irene’s wind and surge proved anticlimactic, the storm lived up to and even exceeded expectations that it would unleash a deluge. “Part of the reason Irene was such a big rainmaker is because the warm waters [of the coast] were 1 to 3 degrees centigrade above average,” Masters says. This caused more moisture to evaporate off the ocean and get caught up in Irene’s circulation. He says as much as 20 inches of rain fell in some places, turning gently flowing streams into raging torrents, but 6 to 12 inches was more typical.
The overblown intensity forecast exposes one of the weaknesses of today’s hurricane forecasting: The meteorological tools for forecasting storm track are a lot more accurate than those for forecasting strength, Masters says. There’s a reason for this. The steering currents that guide a storm usually are large-scale phenomena — high-pressure ridges and low-pressure troughs — while factors influencing intensity may be more subtle and harder to spot, such as the small upper-level trough that suddenly materialized and fouled Irene’s circulation with dry air.
In his daily blog, Masters notes that hurricane track forecasts have improved more than 50 percent since 1990, from an average error of 105 miles in a 24-hour forecast to 50 miles today. Meanwhile, forecasts of intensity have improved hardly at all in 20 years, although research to remedy this problem is ongoing, he says.
This hurricane season has been a bit odd, with more than the usual number of storms developing in the Atlantic but hardly any hurricanes. Masters says an abundance of dry, stable air in the tropics this summer is one reason the eight tropical storms that were spawned in the Atlantic prior to Irene fizzled out before reaching hurricane strength. He says it’s unusual to go into late August before seeing a tropical storm develop into a hurricane.
“We had large areas of sinking air,” he says. “As air sinks from low-pressure areas up high to high-pressure areas down below, the air warms and dries. … That’s bad for hurricanes.”
Irene was born Aug. 15 as a tropical wave cast off western Africa near the Cape Verde Islands. August is the African monsoon season. Hot air rising from the super-heated land is replaced by moist air rushing in from the Atlantic, which generates heavy rains and thunderstorms that are carried out over the ocean — one after the other — on an easterly jet stream. Five days later, this wave’s circulation became organized enough — it started to spin counterclockwise and ingest moist air from the sea surface — that the National Hurricane Center designated it Tropical Storm Irene, the ninth named Atlantic storm of the season, with surface winds of 50 mph.
Forecasters at first gave it little chance of survival because of the dry air in the atmosphere and the moderate wind sheer — in this case, upper-atmosphere westerlies — that disrupted the storm’s circulation. Heading into the western Atlantic, however, Irene became embedded in an envelope of moist air, which along with toasty sea surface temperatures as high as 86 F and diminishing sheer was expected to feed the storm the moist air it needed to become a hurricane.
By Aug. 21, forecasters were beginning to see Irene as a threat to the eastern Caribbean and possibly the U.S. East Coast. The storm was expected to hit Puerto Rico, Haiti and the Dominican Republic as a possible hurricane and then arc north-northwest, steered between a low-pressure trough settling in along the U.S. East Coast and a weakening Bermuda high in the Atlantic. The interaction of this trough and the Bermuda high was critical in establishing the track Irene would take and typically is a factor in the track of most storms coming off the African coast.
“That’s usually what controls these types of storms,” Masters says. “The fight between the [trough and the Bermuda high] is the dominant steering force on these hurricanes.” The timing of the trough’s arrival and its strength determine whether the storm keeps trucking west into the Caribbean or whether it turns north, and how sharply it turns. The Bermuda high acts as a blocking force that keeps the storm from following its natural tendency to curve north and west.
A trough can weaken the effect of that high and allow the storm to turn north. This trough arrived in time and with enough strength to turn Irene sharply enough to miss Florida but not the Bahamas and put it on a north-northwest track right up the East Coast — a course from which it did not deviate.
Irene was in its element now. The sea surface was warm. Wind sheer was weak. Forecasters worried that conditions were right for Irene to become that perfect storm they had been dreading: a Category 4 with winds of 131 to 150 mph aimed at 65 million people along the Interstate 95 corridor. Fortunately, Irene weakened from a Category 3 to a Category 1 between the Bahamas and the Outer Banks, while still directly affecting “more people than any storm in history,” Masters says.
As the East Coast continued to clean up after Irene, Hurricane Katia was struggling with dry air and moderate wind sheer in the central Atlantic, its track uncertain as forecasters waited to see how the fight between the Bermuda high and two successive East Coast troughs would play out. Masters notes that September and October are the most active period of the hurricane season. NOAA has predicted 19 named storms, 10 of them hurricanes, this year. Masters says we could see nearly as many storms in 2011 as in 2005, when a record 28 occurred. He predicts 25.
“At the rate we’re going, we could have the second-most number of storms on record,” he says.
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This article originally appeared in the November 2011 issue.