08.29.12

Hurricane Isaac Brings Back More Memories Of Irene

Posted in Storm Track, Commentary, Storm History, Storm Facts, Tracking the Tropics at 9:37 pm by gmachos

Latest Gulf Storm Shares Similarities With Memorable Storm From 2011

During the course of the day on Wednesday, I listened to the Weather Channel, and heard a storm surge analyst from the National Hurricane Center say that no two storms are the same. However, two storms can have some parallels. Isaac has shown that it is not your typical Category One Hurricane on the Saffir-Simpson Scale. While it may only have had winds of minimal hurricane force, its pressure, size, and duration have made it a very memorable storm for those who are dealing with it in the Northern Gulf states. Moreover, this latest storm of 2012 shares some interesting commonalities with a memorable storm from 2011.

Isaac has brought back some recollections of Hurricane Irene, a vast storm that was the first storm to make landfall in New Jersey since 1903 a year ago this week. Both storms were very vast storms with large wind fields. Isaac grew to have hurricane force winds extend some 60 miles from the eye while tropical storm force winds reached out some 185 miles. Irene was actually larger with hurricane force winds extending 125 miles while tropical storm force winds stretched out some 240 miles.

These large circulations produced a good deal of surge and wave action. Isaac’s was much bigger because of the shallow coastline that exists in the Northern Gulf. Storm surge amounts with Isaac have been as high as 11 feet above normal in places just outside of New Orleans. The surge has been reported to be worse to the southeast in Plaquemines Parish, where parish President Billy Nungesser indicated that the flooding there was worse than during Hurricane Katrina.

Another similarity shared by these two storms is that they were both Category One storms with very low pressures at landfall. When Irene came through New Jersey last summer, its pressure was as low as 970 millibars or 28.63 inches of Hg. Prior to making landfall, Isaac bottomed out at 968 millibars, or 28.58 inches of Hg. These pressures were much lower than the usual threshold for a minimal Category One Hurricane. Usually pressures for minimal Cat One storms are about 985 to 988 millibars. The central pressures of Isaac and Irene were more characteristic of a Category Two storm.

The problem that these two storms had was the fact that they were so vast that they had a tough time trying to tighten up and get stronger. Isaac dealt with this problem throughout almost all of its storm life while Irene was plagued by it after it had gone through the Bahamas and trekked north toward the Carolinas and the rest of the Mid-Atlantic. A big difference between these two storms was that Isaac was like a marathon runner that sprinted his way to the finish line while Irene was one that limped her way to the finish. Another issue that both of these storms had to contend with was dry air entrainment.

Isaac battled dry air for just about its entire life while Irene began dealing with the dry air a little while after it moved through the Bahamas. Irene had indicated signs of strengthen after moving out of the Northern Bahamas, and emerging into the Gulf stream, but then the eye faded, and gradually, dry air began to take the life out of the storm, which was the saving grace for the East Coast. The Northern Gulf were also spared a worse fate from Isaac thanks to the dry air. Without ample water vapor in the atmosphere around it, Issac struggled to generate the thunderstorms needed for better organization and strengthening.

In addition to these similarities, and the obvious common trait they share as the I named storm for their respective years, they were both storms that made double landfalls in a state. Irene made two landfalls in New Jersey at Cape May and Little Egg Harbor while Isaac made two landfalls in Louisiana at Grand Isle and just west of Port Fourchon. There are differences though between these two storms. Irene was a major hurricane at one point in its lifetime having winds as high as 120 miles per hour while Isaac never got as strong as 80 miles per hour. Although both produced a lot of rain, Isaac generated much higher rainfall totals since its forward motion was much slower than Irene.

Isaac could end up putting down some 20 to 30 inches of rain in some parts of Louisiana and Mississippi. In terms of its size and duration, Isaac is quite similar to Hurricane Frances, which grinded its way through Florida back in 2004. Frances was a much bigger wind producer though.

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