Category 2 Ike is larger and more powerful than Katrina
Hurricane Ike's winds remain at Category 2 strength, but Ike is a freak storm with extreme destructive storm surge potential. Ike's pressure fell rapidly last night to 944 mb, but the hurricane did not respond to the pressure change by increasing its maximum winds in the eyewall. Instead, Ike responded by increasing the velocity of its winds away from the eyewall, over a huge stretch of the Gulf of Mexico. Another very unusual feature of Ike is the fact that the surface winds are much slower than the winds being measured aloft by the Hurricane Hunters. Winds at the surface may only be at Category 1 strength, even though Ike has a central pressure characteristic of a Category 3 or 4 storm. This very unusual structure makes forecasting the future intensity of Ike nearly impossible. The possibilities range from a Category 1 storm at landfall--as predicted by the HWRF model--to a Category 4 storm at landfall, as predicted by the GFDL.
Ike is now larger than Katrina was, both in its radius of tropical storm force winds--275 miles--and in it radius of hurricane force winds--115 miles. For comparison, Katrina's tropical storm and hurricane force winds extended out 230 and 105 miles, respectively. Ike's huge wind field has put an extraordinarily large volume of ocean water in motion. When this swirling column of water hits the shallow waters of the Continental Shelf, it will be be forced up into a large storm surge which will probably rival the massive storm surge of Hurricane Carla of 1961. Carla was a Category 4 hurricane with 145 mph winds at landfall, and drove a 10 foot or higher storm surge to a 180-mile stretch of Texas coast. A maximum storm surge of 22 feet was recorded at Port Lavaca, Texas. Despite the fact that the center of Carla hit over 120 miles southwest of Houston, the hurricane drove a 15-foot storm surge into the bays along the south side of the city. I don't expect Ike will reach Category 4 strength, thus its maximum surge is not likely to reach the extreme values above 20 feet seen in Hurricane Carla. Like Carla, though, Ike will probably inundate a 180-mile stretch of Texas coast from Port O'Connor to just north of Galveston with a 10-15 foot storm surge. This will occur even if Ike is a Category 1 storm at landfall. The latest experimental storm surge forecast From NOAA's SLOSH model (Figure 1) shows a 10% chance that Ike's storm surge will exceed 15-21 feet at Galveston. The Galveston sea wall is 17 feet high, so it may get overtopped.
Figure 1. Experimental storm surge heights for Ike. There is a 10% chance the storm surge from Ike will exceed these values. Data courtesy of NOAA.
The amount of water Ike has put in motion is about 50% greater than what Katrina did, and thus we can expect Ike's storm surge damage will be similar to or greater than Katrina's. The way we can estimate this damage potential is to compute the total energy of Ike's surface winds (kinetic energy). To do this, we must look at how strong the winds are, and factor in the areal coverage of these winds. Thus, we compute the Integrated Kinetic Energy (IKE) by squaring the velocity of the wind and summing over all regions of the hurricane with tropical storm force winds or higher. This "Integrated Kinetic Energy" was recently proposed by Dr. Mark Powell of NOAA's Hurricane Research Division as a better measure of the destructive power of a hurricane's storm surge than the usual Category 1-5 Saffir-Simpson scale. For example, Hurricane Katrina hit Mississippi as a strong Category 3 hurricane, yet its storm surge was more characteristic of a Category 5 storm. Dr. Powell came up with a new scale to rate potential storm surge damage based on IKE (not to be confused with Hurricane Ike!) The new scale ranges from 1-6. Katrina and Wilma at their peaks both earned a 5.1 on this scale (Figure 2). At 9:30am EDT this morning, Ike earned a 5.6 on this scale, the highest kinetic energy of any Atlantic storm in the past 40 years.
Figure 2. Comparison of the potential damage from storm surge and waves on a scale of 1 to 6 (left scale, and corresponding to little "x" marks on the plot), as a function of total Integrated Kinetic Energy in Tera-joules (IKE, on the right scale, corresponding to the little squares on the plot). Hurricane Ike at 12:30pm EDT had an IKE of 134, 10% higher than the value of 122 Katrina had at landfall in Mississippi. Ike's amount of wind energy can generate storm surge and wave damage rated at 5.2 on a scale of 1 to 6, worse than Katrina's 5.1 at landfall. Image credit:"Tropical Cyclone Destructive Potential by Integrated Kinetic Energy" by Mark Powell and Timothy Reinhold.
All this energy is also going into the waves in the Gulf of Mexico, and the offshore oil rigs can expect to receive a terrific battering. At 8:50am CDT, waves at the Buoy 42001 180nm south of Louisiana were 28 feet and growing. NOAA's Wavewatch III model is predicting wave heights up to 13 meters from Ike on Friday. For comparison, Hurricane Ivan of 2004 generated 27 meter high waves in the Gulf of Mexico. Surf heights of 15 feet have been reported at beaches along the Florida Panhandle, and tides are also running extremely high. Tides are 2-4 feet above normal along the Gulf Coast from Louisiana to the west coast of Florida, and one foot above normal in Galveston. The water level will continue to rise as Ike approaches Texas, and NOAA's experimental storm surge forecast (Figure 1) is calling for a 10% chance that the storm tide from Ike will reach 24-27 feet on the south and east sides of Houston.
What should Texas residents do?
We must assume Ike will intensify to a Category 3 hurricane by landfall, which would likely do $20-$30 billion in damage. The chances of hundreds of people being killed in this storm is high if people do not heed evacuation orders to leave low-lying areas threatened by high storm surges. Ike's storm surge is going to be affect a huge area and be tremendously destructive.
I recommend Texas residents consult NHC's wind probability product to determine their odds of getting hurricane force winds.
For storm surge evacuation zone information, consult the Texas Division of Emergency Management.
For storm surge heights, consult our Storm surge risk for the Texas coast page.
Links to follow
Tide gauges along the Gulf Coast
Mid Gulf Buoy 180 nm South of Southwest Pass, LA (42001)
I'll have much more in this afternoon's blog, including a look at the rest of the tropics. We've got a new area of disturbed weather to watch, Invest 91L.