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Ike intensifies to Category 4 strength

By: Dr. Jeff Masters, 8:19 PM GMT on September 06, 2008

Hurricane Ike has re-intensified, and now has Category 4-strength winds, according to the latest data from the Hurricane Hunters. Both an Air Force and a NOAA aircraft recorded surface winds of 135 mph this afternoon on the northeast side of Ike. Category 4 strength winds range from 135-155 mph. Infrared satellite loops show Ike is more symmetric now, with improved upper-level outflow to the north. Shear has fallen from 25 knots this morning to 15 knots this afternoon, allowing this intensification to occur.

Track forecast for Ike
The latest 12Z (8 am EDT) computer models foresee a probable direct hit by Ike on Grand Inagua Island in the Southeast Bahamas, with the Turks and Caicos Islands also getting hit hard. The eye is about 35 miles in diameter, so a region about 50 miles wide will feel Category 3+ hurricane winds in the Southeast Bahamas. These islands can expect a storm surge of 13-18 feet, and extreme damaging winds. Ike will pass 40-80 miles north of northwestern Haiti, bringing extreme flooding rains of 6-12" to the island of Hispaniola.

All of the major models agree that Ike will hit eastern Cuba on Sunday night. After this point, the models continue to diverge. A southern camp of models, the ECMWF and UKMET, take Ike across eastern Cuba and into the western Caribbean, then across the western tip of Cuba or Yucatan Channel between Cuba and Mexico's Yucatan Peninsula. These models predict an eventual landfall near the Mexico/Texas border a week from now. This track would bring tropical storm conditions to the Cancun/Cozumel area beginning Tuesday afternoon or evening, with possible hurricane conditions by Wednesday morning.

The northern camp of models, including the GFS, NOGAPS, GFDL, and HWRF, turn Ike west-northwest over Cuba, forecasting that Ike will pop off the coast of Cuba near the Florida Keys on Tuesday, then swing north-northwest. The northward turn is delayed in the current runs, putting Ike into the central Gulf of Mexico, or several hundred miles offshore the western Florida coast. The trough of pressure that pulls Ike to the north is expected to be weak, leaving Ike in a region of weak steering currents. A similar situation occurred in 1985, when Category 3 Hurricane Elena got stranded in the Gulf and wander offshore of the Florida Panhandle for several days. So far, the GFDL has done a good job with Ike, so I will continue to lean towards that track. The GFDL tracks Ike over Cuba until the storm pops off the coast south of the Keys, and intensifies it from a borderline Category 1 or 2 hurricane to a Category 3 hurricane as it passes 50 miles southwest of Key West. The GFDL brings Category 2 winds to Key West. The model then takes Ike north-northwestward into the Gulf of Mexico to an uncertain future.

Figure 1. Expected maximum storm tide (storm surge plus adjustment for hitting at high tide) from a Category 3 hurricane moving WNW at 15 mph through the Florida Keys, hitting at high tide. This plot is an ensemble of many different hurricane tracks (shown as black lines), not just one hurricane. The maximum surge from the ensemble is plotted here. The model used is NOAA's SLOSH model. Note that ocean side surge is a foot to two feet lower than bay-side surge. Bay-side surge comes well after the storm center has passed the Keys, from the westerly winds behind the storm. Too many people have been harmed because they thought it was safe to go near the water on the bay side just after a storm has passed. Image credit: Dr. Stephen Baig, NOAA (retired).

Florida Keys are at high risk from Ike
The danger to the Keys has diminished some, thanks to the continued west-southwest motion of Ike, and the consensus model tracks taking Ike over Cuba or just south of it for a long distance. The Keys are in a Category 2 or 3 evacuation zone, and will be inundated by a direct hit from a Category 2 or 3 hurricane. It's now looking unlikely that Ike will pass through the Keys as a Category 4 hurricane. Category 3 conditions are possible in Key West and the Lower Keys, which would likely bring a maximum storm tide (storm surge plus an adjustment in case the hurricane hits at high tide) of 5-9 feet. Category 1 or 2 conditions are more likely in the Keys, though. I believe there is a 40% chance Ike will bring Category 1 strength winds or higher to Key West and the Lower Keys. The NHC Wind Probability Product forecast gives Key West a 17% chance of receiving hurricane force winds from Ike.

Intensity forecast for Ike
Sea surface temperatures (SSTs) have warmed to 29.5°C underneath Ike and will warm to 30.0°C over the Gulf of Mexico. Shear has dropped from 25 knots this morning to 15 knots this afternoon, and is forecast to drop below 10 knots tonight. Ike should continue intensifying until the eye contracts to point where it is unstable. The eyewall will collapse, and a new eyewall will form at a much larger radius from the center than before. This process is common in intense hurricanes, and is called an Eyewall Replacement Cycle (ERC). Landfall on Cuba will significantly disrupt the storm, and Ike's strength as it passes the Keys is difficult to guess at this point. The most likely strength is a Category 1, but it could easily be a Category 2 or 3, depending on how long the storm stays over Cuba, and the exact track past the Keys. Once Ike enters the Gulf of Mexico, shear is expected to be low to moderate (10-15 knots), and the ocean is warm, so additional intensification is likely.

How will Ike affect the Miami area?
Ike's continued west-southwest motion has reduced the danger to South Florida. Miami is now just outside of the cone of uncertainty, so will probably miss a direct hit by Ike. NHC's Wind Probability Product is giving Miami at 7% chance of experiencing hurricane force winds, down from 10% early this morning. If Ike follows the 5 am EDT official forecast, passing to the south of Miami along the north coast of Cuba, the expected region of tropical storm force winds will fall just south of Miami (Figure 2).

Figure 2. Radius of tropical storm force winds (green colors) and hurricane force winds (yellow colors) along the 5 am EDT Sat 9/6/08 NHC forecast track of Ike. Image was taken from our wundermap for Ike, by clicking on the "hurricane" layer and "wind radius" layer. Note that NHC does not issue a forecast of hurricane force winds for the later time periods, so no yellow colors are shown where Ike is south of Florida.

I'll have a short update by 9pm EDT tonight when the new model runs become available.

Links to follow:

Holguin, Cuba radar
Punta De Maisi, Cuba weather
Providenciales, Caicos Islands weather
Pine Cay, Caicos Islands weather

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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