Ike intensifies; Texas at high risk

By: Dr. Jeff Masters , 3:29 PM GMT on September 10, 2008

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Hurricane Ike is steadily intensifying over the warm Gulf of Mexico waters as it heads west-northwest towards the Texas coast. At 10 am EDT, the Hurricane Hunters found that the pressure has continued to fall, and now stands at 957 mb--a 10 mb drop in the past 12 hours. Ike's eyewall was missing a chunk earlier this morning, but the storm his since put together a complete eyewall. Visible satellite loops show that Ike has become better organized in recent hours, with a more symmetrical appearance, and respectable upper-level outflow channels open to the north and the south. Outflow and cloud cover is restricted on the storm's west side, where dry air and wind shear of 10-15 knots is affecting the storm. Water vapor loops show that some of this dry air may be getting wrapped into the circulation, and this could slow Ike's intensification today, until it can build a stronger eyewall. Ike continues to bring very heavy rains to western Cuba, southwest Florida, and the Florida Keys, where more than five inches has fallen (Figure 1). 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.


Figure 1. Current radar-estimated precipitation from Ike.

Track forecast for Ike
Ike is moving west-northwest under the influence of a blocking ridge of high pressure to its north. As Ike approaches Texas on Friday, a trough of low pressure is expected to pass to the north, potentially turning Ike more to the northwest. 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 remain closed most of Saturday.

The latest 00Z/06Z (8pm/2am EDT)) computer models have begun to zero in on Corpus Christi to Freeport as the most likely landfall location. However, with a trough of low pressure expected to turn Ike close to landfall time, a slight variation in timing of this trough could put Ike ashore farther north, near Galveston. There is also a chance the ridge pushing Ike west Thursday could be stronger than expected, forcing Ike more to the west towards a Brownsville landfall. However, I believe that this is lower probability, and that Galveston is more likely to get hit than Brownsville. The cone of uncertainty still covers the entire Texas coast. If Ike hits Corpus Christi, it will miss most of the oil platforms in the Gulf of Mexico (Figure 2), but a hit closer to Galveston would seriously disrupt the oil and gas industry.


Figure 2. Location of Gulf of Mexico oil platforms.

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 are too low, and should be bumped up by 5-10%.

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 4 hurricane Friday night. The SHIPS model has gotten more aggressive, and now foresees a strong Category 2 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.

A comparison to other severe Texas hurricanes
There is a significant chance that Ike will be the worst hurricane to hit Texas since Hurricane Celia of 1970. Ike has the potential to be worse than both Hurricane Alicia of 1983 and Hurricane Beulah of 1967, which hit as Category 3 hurricanes. A good benchmark for comparison is probably Hurricane Celia of 1970. Celia followed a similar track to Ike, and intensified steadily right up to landfall, which came over Corpus Christi as a Category 3 storm with 125 mph winds. Celia inflicted severe damage on Corpus Christi and Post Aranasas, with 80% of all buildings in Corpus Christi damaged or destroyed. Damages totaled to $453.8 million, 15 people were killed, and 466 others were injured, mostly by glass shards from shattered windows. Curiously, the storm surge did little damage. According to the NHC post-storm report, "In the entire area of Corpus Christi Bay, Port Aransas, Aransas Pass, and Copano Bay there was no evidence of major damage due to storm surge alone. Not a single house was washed off its foundation. At Port Aransas many modern expensive homes built on pilings 10' or 12' off the ground were a total loss, but not due to water." The high water marks for Celia were 9.2' and 9.0', measured at Port Aransas Beach and Port Aransas jetty, respectively. The relatively low storm surge and light storm surge damage is not likely to be repeated for Ike, since Ike is at least 50% larger than Celia was, and will generate a higher storm surge spread out over a larger region of coast.


Figure 4. Boats blown ashore at Aransas Pass by Hurricane Celia in 1970. Image credit: NOAA Photo Library.

Storm surge risk
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 of over ten feet to a 100 mile long stretch of Texas coast from the eye northwards along the Texas coast. This is what the August 29, 1942 hurricane did when it hit near Port O'Connor, Texas as a Category 1 storm with 80 mph winds. This was a large hurricane that 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. The high angular momentum of the swirling cylinder of ocean water did not have time to decrease much, and 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, and I'm guessing too low an intensity was assigned to this storm.


Figure 5. 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.

The latest run of the HWRF and GFDL models paint a realistic scenario of what could happen to Texas from Ike. These models intensify Ike right up until landfall, hitting between Corpus Christi and Port O'Connor as a Category 3 or 4 hurricane. The HWRF predicts a 120-mile stretch of coast will receive hurricane force winds of 74 mph or greater. An 80-mile stretch of coast will receive winds of Category 3 strength and higher, 115 mph. Hurricane force winds will push inland up to 30 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 15-20 foot storm surge. As seen in the maximum storm tide risk map for the Texas coast (Figure 6), a worst-case Category 3 hurricane hitting at high tide will bring a 15-foot storm surge to Corpus Christi, Port O'Connor, or Galveston. Maximum surge values will be higher at the heads of inland estuaries that act to funnel the storm surge as it rushes inland. Ike is already generating tides 2-4 feet above normal along the Gulf Coast from Louisiana to the Florida Panhandle.

The scenario above is a little less extreme than what the worst hurricane in Texas history wrought. Hurricane Carla of 1961 was a massive Category 4 hurricane that filled the entire Gulf of Mexico, and brought 145 mph winds to the coast near Port Lavaca. Carla 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 doubt Ike will measure up to Carla, but it could (5% chance).


Figure 6. The maximum storm tide (storm surge plus an adjustment for hitting at high tide) expected from any Category 3 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 3 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 will only occur 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.

For storm surge evacuation zone information, consult the Texas Division of Emergency Management. I'll be posting a full set of storm surge maps for the Texas coast this afternoon. High tide along the Texas coast is Saturday morning at about 2am local time. Tidal range between low and high tide is about two feet.

Elsewhere in the tropics
There are no threat areas to discuss, and none of the reliable computer models are prediction tropical storm formation in the Atlantic over the next seven days. Today marks the halfway point of hurricane season, but I'm expecting that we've already seen about 2/3 of the action we're going to get this year.

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

Jeff Masters

Hurricane Ike washes out Simonton St (BilgeH2OMgt)
Hurricane Ike flooding on the south side of Key West. This shot is the south end of Simonton Street, next to the
Hurricane Ike washes out Simonton St

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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.