Ike grows in size and strength

By: Dr. Jeff Masters , 8:17 PM GMT on September 10, 2008

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Hurricane Ike has grown into a very large and powerful Category 2 hurricane. At 2 pm EDT, the Hurricane Hunters found maximum winds had increased to 100 mph. This increase in winds was a reaction to the 10 mb drop in pressure noted in the past 12 hours. The most recent pressure measured--958 mb at 3:09 pm EDT--was actually a 1 mb increase from the 10 am reading, indicating that Ike's intensity has likely leveled off for now. Visible satellite loops show that Ike has ingested some dry air from the west, which is visible as a spiral dark streak that wraps into the core of the storm. The small 11-mile diameter eye occasionally pops into view, and is exhibiting the unusual behavior of orbiting around in a large circle within the hurricane. Hurricane Wilma of 2005--the strongest hurricane on record--exhibited this behavior during its intensification phase, as well. However, Wilma was not sucking in dry air at the time, and Ike is not likely to approach Wilma's ferocity.

A large spiral band surrounding Ike's inner eye is attempting to close off and form a new outer eyewall with a diameter of 100 miles. The power struggle between the small inner eyewall and the large outer spiral band will likely go on until Thursday, resulting in little intensification of Ike this evening. By Thursday, the power struggle will likely be over, and Ike will probably resume intensification. If the small eyewall wins, Ike could intensify rapidly to a Category 4 hurricane; if the large spiral band takes over as the new eyewall and the inner eyewall crumbles, we can expect more gradual intensification to a Category 3 hurricane.

Ike continues to grow in size, and its tropical storm force winds extend out almost as far as Katrina's did. This large wind field is already starting to pile up a formidable storm surge. Tides are already running 2-4 feet above normal along the Gulf Coast from Louisiana to the west coast of Florida. Visible satellite loops show that Ike has good upper-level outflow channels open to the north and the south. Outflow and cloud cover are restricted on the storm's west side, where dry air and wind shear of 10-15 knots are affecting the storm. All indications are that Ike will intensify into a major hurricane that will bring widespread destruction to a large stretch of the Texas coast. I expect Ike will generate a 10-15 foot storm surge along a 100-mile stretch of Texas coast from the eye landfall location, northwards. I urge Texas residents to take this storm very seriously and heed any evacuation orders given. Most of you living along the coast have never experienced a major hurricane, and Ike is capable of causing high loss of life in storm surge-prone areas. Tropical storm force winds will spread over the Texas coast beginning Friday afternoon, and evacuations must be completed by Friday morning. All airports in eastern Texas will be forced to close Friday night, and will probably remain closed most of Saturday. Ike has a good chance of becoming the most destructive hurricane in Texas history--though not the most powerful.


Figure 1. The maximum storm tide (storm surge plus an adjustment for hitting at high tide) expected from a mid-strength (145 mph) Category 4 hurricane hitting anywhere along the coast of Texas at high tide. This so-called "MOM" (Maximum Of the Maximum Envelope Of Waters) is computed using NOAA's SLOSH storm surge model. The plot above IS NOT the expected storm tide everywhere along the coast from a hit by Hurricane Ike. The plot is the MAXIMUM high water for a worst-case scenario Category 4 hurricane moving at the worst possible angle at the worst possible forward speed. As such, this plot is the combination of SLOSH runs from over 50 different simulated hurricanes approaching the coast at different angles and different forward speeds. The maximums plotted here are only possible along a 20-mile stretch of the coast on the north side of Ike's eyewall. SLOSH model runs are advertised as being in error by plus or minus 20%. Image credit: NOAA.

Track forecast for Ike
The latest 12Z (8am EDT)) computer models are in even less agreement than the previous set of runs. There has been a northward shift in several models, most notably the GFDL, which now has Ike making landfall at Galveston as a strong Category 3 hurricane. With a trough of low pressure expected to turn Ike northwestward close to landfall time, slight variations in the timing of this trough among the models is causing a large spread in landfall locations. Given the recent trend in the models to take Ike farther north, I would expect more of the models in future runs may be joining the GFDL in predicting a Galveston landfall. The cone of uncertainty still covers the entire Texas coast, and residents of southwestern Louisiana are also at risk.

I recommend Texas residents consult NHC's wind probability product to determine their odds of getting hurricane force winds. At 11 am EDT, NHC called for these odds of getting hurricane force winds at various Texas cities:

Brownsville: 9%
Corpus Christi: 17%
Port O'Connor: 24%
Freeport: 23%
Galveston: 20%
Houston: 13%

As you can see, Port O'Connor is considered the most likely city in Texas to receive hurricane force winds. I believe the percentages for the cities above except Brownsville and Corpus Christi are too low, and should be bumped up by 5-10%.

Intensity forecast for Ike
The intensity forecast remains the same. 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, and Ike has the capability of intensifying right up to landfall. This is the forecast of the HWRF model, which has Ike hitting Port O'Connor as a Category 4 hurricane with 145 mph winds. The weakest I think Ike will be at landfall is Category 2 hurricane with 100 mph winds.

Storm surge risk
We've put together today a page of storm surge risks for the Texas coast. These images show the maximum storm tide (storm surge plus an adjustment for hitting at high tide) expected from a mid-strength hurricane of each Saffir-Simpson Category hitting anywhere along the coast of Texas at high tide. These so-called "MOMs" (Maximum Of the Maximum Envelope Of Waters) are computed using NOAA's SLOSH storm surge model. A sample image is shown in Figure 1 for a Category 4 hurricane affecting the Galveston area. A storm of this magnitude is expected to bring a maximum 22 foot storm tide (storm surge plus a 2-foot adjustment in case it hits at high tide) to Galveston. A maximum 28-foot storm tide could affect the built-up areas along the east side of Houston. Note that most Category 4 hurricanes making a direct hit on Galveston will bring a significantly lower storm surge than the worst-case 22-foot scenario pictured here. For example, the Great Galveston Hurricane of 1900 was a Category 4 storm that hit the city head-on, but generated a storm surge of only 15 feet. Even so, this hurricane was the deadliest disaster in American history, killing an estimated 8,000-12,000 people. Since then, Galveston has built its seawall to a height of 17 feet, which would probably withstand a direct hit by Ike at Category 4 strength.

For storm surge evacuation zone information, consult the Texas Division of Emergency Management.

Ike is a large storm, and will probably attain Category 3 or higher status over the Gulf of Mexico. This will set in motion a huge volume of water that will pile up into a large storm surge once Ike reaches the shallow Continental Shelf waters off the coast of Texas. Even if Ike weakens significantly before landfall, I am still expecting the storm to bring a storm surge 10-15 feet to a 100 mile long stretch of Texas coast from the eye northwards along the Texas coast. High tide on Saturday morning along the Texas coast is at 2am CDT. The range between low tide and high tide along the Texas coast is about 2 feet.

Links to follow
Both GOES-East and GOES-West are operating in rapid scan mode, and you can see some pretty spectacular animations of Ike at Colorado State University's CIRA/RAMMB site.

Tide gauges along the Gulf Coast

Storm surge risk for the Texas coast

I'll be speaking at hurricanecity.com tonight at 9pm EDT. I'll be posting a new blog by 9pm EDT.

Jeff Masters

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About

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.