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Ike has an eyewall, beginning to strengthen

By: Dr. Jeff Masters, 12:59 AM GMT on September 10, 2008

Hurricane Ike has taken advantage of the warm Gulf of Mexico waters it is over, and has already built an eyewall. At 7:02 m EDT, the Hurricane Hunters found a complete eyewall, which can also be seen on infrared satellite loops and Key West radar. The infrared satellite imagery also shows a rapid cooling of the cloud tops in Ike's eyewall and some of the spiral bands, indicating that the thunderstorms are penetrating higher into the atmosphere--a sign of strengthening. The latest data from a NOAA Hurricane Hunter aircraft indicates that the pressure has begun to fall, but Ike's winds remain at minimal hurricane force, 75 mph. All indications are that Ike will intensify into a very dangerous major hurricane that will hit the Texas coast Friday night or Saturday.

Figure 1. Current Key West radar image.

Track forecast for Ike
A trough of low pressure is currently passing to the north of Ike, and this trough has been able to turn Ike north of due west. Ike is now moving west-northwest, and this motion is expected to continue today. By Wednesday, Ike is expected to take a more westerly motion again, as high pressure to the north builds in. As Ike approaches Texas on Friday, a new trough of low pressure is expected to pass to the north, potentially turning Ike to the northwest.

The latest 18Z (2pm EDT) computer models that have come in so far--the GFS, GFDL, and NOGAPS--point to a landfall near Corpus Christi. All of the major models foresee a landfall between Corpus Christi and Galveston. Landfall would occur late Friday night or early Saturday morning, and tropical storm force winds would arrive at the coast on Friday morning. Given the inability of the models to agree until now, this landfall is certainly not a "sure thing", and the cone of uncertainty covers the entire coast of Texas. Data from the NOAA jet will go into tonight's 00Z (8 pm EDT) model runs, which will be available first thing Wednesday morning. That set of model runs should give us a pretty good idea of where Ike will go. I'm sure emergency managers are not eager to call for an evacuation of Houston, after the debacle of the evacuation for Hurricane Rita in 2005. Over 110 people died in the evacuation--far more than died in the storm. Still, there is a significant chance that an evacuation of large stretches of the Texas coast--possibly including portions of Houston--will have to be ordered on Wednesday or Thursday.

Figure 2. The inside of Ike's eye, as photographed from a NOAA P-3 Hurricane Hunter aircraft on Sunday, September 7, 2008. Image credit: captcosmic.

Intensity forecast for Ike
Ike survived the passage of Cuba well, and remains a large and well-organized hurricane. Significant strengthening is ready to occur, now that Ike has built a new eyewall. I expect Ike will be a Category 2 or 3 hurricane by Wednesday night, and Ike has the potential to become a Category 4 hurricane by Thursday, as forecast by the HWRF and GFDL models. Water temperatures are a warm 29.5°C in the Gulf of Mexico, and wind shear is expected to be modest, 10-15 knots, for the remainder of Ike's life. Ike will be crossing over two regions of high heat content associated with the Loop Current and a Loop Current eddy (Figure 3). There is much higher oceanic heat content off the Texas coast than was present off the Louisiana coast for Gustav. Thus, it is more likely that Ike will be able to maintain major hurricane status as it approaches the coast. The GFDL model predicts landfall near Corpus Christi as a Category 3 hurricane Friday night. The SHIPS model is less aggressive, and foresees a strong Category 1 hurricane at landfall. Given the impressive appearance of Ike on satellite imagery, and the forecasts of high heat content and low shear along its path, I would be surprised if Ike hit as anything weaker than a Category 2 hurricane with 100 mph winds. Here's my rough probability break-down for Ike's strength at landfall, I forecast a 50% chance Ike will be a major hurricane at landfall:

Category 1 or weaker: 20%
Category 2: 30%
Category 3: 30%
Category 4 or 5: 20%

Figure 3. Projected path of Ike overlaid on the current map of Tropical Cyclone Heat Potential (TCHP). TCHP is a measure of total ocean heat content, and TCHP values in excess of 80 kJ/cm^2 (yellow colors) are frequently associated with rapid intensification of hurricanes. Ike will be passing over two regions of high heat content--one associated with the Loop Current, and another associated with an eddy that broke off from the Loop Current in July. Note that heat content stays relatively high all the way to the coast of Texas, in contrast to what Gustav experienced as it approached the coast of Louisiana. Image credit: NOAA/AOML.

Texas is highly vulnerable to storm surge
The Texas coast is highly vulnerable to large storm surges, due to the long expanse of shallow Continental Shelf waters offshore. The shallow depths allow large the swirling winds of the hurricane to pile up huge mounds of water, which then sweep inland when the hurricane makes landfall. Even Category 1 hurricanes are capable of generating 15 foot storm surges along some sections of the Texas coast. For example, the August 29, 1942 hurricane hit near Port O'Connor, Texas as a Category 1 storm with 80 mph winds. However, this hurricane had been a Category 3 storm with 115 mph winds the day before landfall, allowing it to pile up a large storm surge over the Continental Shelf just offshore the central Texas coast. The storm weakened suddenly in the 12 hours before landfall, but brought a storm surge characteristic of a Category 2 or 3 hurricane to shore, since the high angular momentum of the swirling storm surge waters did not have time to decrease much. A 10-15 foot storm surge came ashore over a 100-mile stretch of coast between Port O'Connor and Freeport (Figure 4). Actually, looking at these storm surge values, I wouldn't be surprised if the 1942 storm was stronger both at landfall and before landfall than the official HURDAT database advertises. This storm came before the era of satellites and Hurricane Hunter aircraft.

Figure 4. High water marks from the August 29, 1942 hurricane along the Texas coast. Image credit: "Characteristics of the Hurricane Storm Surge", by D. Lee Harris, U.S. Weather Bureau, 1963.

A realistic worse-case scenario for Texas
There is a significant chance that Ike will be the worst hurricane to hit Texas in over 40 years. The latest run of the HWRF and GFDL models paint a realistic worst-case scenario for Texas. These models bring Ike to the coast as a Category 4 hurricane (which I give a 20% probability of happening). The HWRF predicts a 170-mile stretch of coast will receive hurricane force winds of 74 mph or greater. A 100-mile stretch of coast will receive winds of Category 3 strength and higher, 115 mph. Hurricane force winds will push inland up to 50 miles, along a 50-mile wide region where the eyewall makes landfall. A 100-mile stretch of Texas coast will receive a storm surge of 10-15 feet, with bays just to the right of where the eye makes landfall receiving a 20-25 foot storm surge. This is what Hurricane Carla of 1961 did to Texas. 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.

If you live in Texas, what are your chances of getting hit?
I recommend Texas residents consult NHC's wind probability product to determine their odds of getting hurricane force winds. At present, NHC is calling for these odds of getting hurricane force winds at various Texas cities:

Brownsville: 8%
Corpus Christi: 8%
Freeport: 10%
Galveston: 9%
Houston: 5%

I think the odds are roughly double what NHC is advertising for the above cities.

For storm surge evacuation zone information, consult the Texas Division of Emergency Management. I'll be posting some more detailed storm surge info Wednesday morning.

Elsewhere in the tropics
A area of disturbed weather near 10N, 21W, about 300 miles south of the Cape Verdes Islands, has changed little today. This morning's QuikSCAT pass showed a sloppy circulation and some 25 mph winds in heavy thunderstorms to the south. The region is currently under about 20 knots of shear, but shear is expected to decline over the disturbance as it moves west-northwest at 10-15 mph this week. No models currently predict development of this disturbance, but it is worth keeping an eye on. The disturbance will be near the northern Lesser Antilles Islands 7-8 days from now.

Tonight, at 9pm EDT, I'll make my annual appearance on the Internet Partnership Radio program, "Center of Circulation". You can listen in at http://www.ipr365.com/. I'm usually on for at least 45 minutes.

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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