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Hanna closes in on South Carolina; Ike weaker, but a major threat to the Florida Keys

By: Dr. Jeff Masters, 8:53 PM GMT on September 05, 2008

Tropical Storm Hanna remains close to hurricane strength as it approaches landfall in South Carolina early Saturday morning. The latest 4:11 pm EDT center fix from the Hurricane Hunters found a central pressure of 984 mb, up 4 mb from the 7 am reading. Peak winds measured by the SFMR instrument were in the 65-75 mph range. Radar animations from the Charleston, SC radar show that Hanna does not have an eyewall, so this will limits its intensification potential. There has been little change in the organization of Hanna's spiral bands, and the amount and intensity of the precipitation has stayed about the same in recent hours. The outermost spiral bands of Hanna are spreading intermittent heavy rains along the east coast of Florida, Georgia, South Carolina, and North Carolina. Wind shear has fallen from 20 knots to 15 knots this afternoon, which may allow Hanna to intensify slightly before landfall. Visible satellite loops of Hanna show a much more symmetric and well-organized system, and Hanna will may start building an eyewall in the next few hours. However, it doesn't have much time to do so, and the strongest it can get is a Category 1 hurricane with 80 mph winds.

Figure 1. Latest radar image of Hanna.

The forecast for Hanna
A landfall in South Carolina is the unanimous consensus of the the computer models, and this landfall will occur near midnight tonight. Hanna is a large storm with winds well removed from the center. Coastal North Carolina and South Carolina will see winds near hurricane force (74 mph) from this storm. Given that Hanna is so large and fast-moving, the entire mid-Atlantic and New England coast should see sustained winds near tropical storm force (39 mph) this weekend as Hanna zooms northeast.

Links to follow
Charleston, SC weather
Myrtle Beach, SC weather

The U.S. is getting pummeled this hurricane season
With hurricane season less than half over, it's clear that the U.S. is taking an unusually harsh beating this year. Already, we've had two Category 2 hurricanes (Dolly and Gustav), two strong tropical storms (Edouard and Fay), and now a third strong tropical storm or Category 1 hurricane (Hanna). Hanna is the fifth consecutive named storm to hit the U.S., tying a record. The other years this happened (kudos to NOAA's Ryan Sharp for compiling this):

2004 (Frances, Gaston, Hermine, Ivan, and Jeanne)
2002 (Edouard, Fay, Gustav, Hanna, and Isidore)
1985 (Gloria, Henri, Isabel, Juan, and Kate)
1979 (Bob, Claudette, David, Elena, and Frederic)
1971 (Doria, Edith, Fern, Ginger, Heidi)

There's good chance that Ike will make it six consecutive strikes.

Hurricane Ike has taken a beating from wind shear, and its top winds have decreased to borderline Category 2/Category 3 strength, about 115 mph, according to data this afternoon taken by the Hurricane Hunters. Infrared satellite loops show that the cloud tops are warming over the core of the hurricane, and the amount and intensity of heavy thunderstorm activity continue to decrease. The eye has disappeared on satellite imagery, and the Hurricane Hunters noticed a gap in the northwest side of the eyewall. Ike is not symmetric, and is flattened on the northwest side, where there is no upper-level outflow. This is due to 25 knots of wind shear impacting the storm, thanks to strong upper-level winds out of the northwest. The shear is enabling dry air to penetrate deep into the core of the storm, disrupting it.

Figure 2. Microwave satellite image of Ike from 7:48 am EDT 9/5/08. Ike's eye is getting tough to pick out, thanks to wind shear eroding away the northwest past of the eyewall. The eye is the blue dot at center; most of Ike's heavy thunderstorms (red colors) are on the southeast side of the hurricane. Image credit: Navy Research Lab, Monterey.

Track forecast for Ike
The latest 12Z (8am EDT) computer models increase the risk for a direct hit by Ike on the Bahamas, Cuba, Florida Keys, and South Florida. The models are grouped more tightly now around a track through the Southeast Bahama Islands, then west-northwest, over or just north of Cuba towards South Florida and the Keys. None of the major models are expecting Ike to recurve and miss the U.S. It now appears likely that Ike will have three landfalls--one in the Southeast Bahamas, one in northern Cuba or the Florida Keys/South Florida, and one on the Gulf Coast. The trough of low pressure expected to turn Ike to the north is, in general, weaker and slower moving than originally forecast, resulting in a delayed turn by Ike to the north. Several models--the UKMET, ECMWF, and Canadian--forecast the trough will not pull Ike to the north at all, and the storm will track west-northwest into the middle of the Gulf of Mexico by the middle of next week. The other models--NOGAPS, GFS, HWRF, and GFDL--all foresee a turn to the north, but this turn is delayed until Ike reaches the Keys. All of these scenarios look bad for the Florida Keys, and there is a high probability the Key will have to be evacuated.

The furthest south models continue to be the GFDL and ECMWF, which take Ike into northeastern Cuba Sunday. The GFDL forecasts Ike will be a Category 3 hurricane when it hits Cuba, then weaken to a Category 2 when it pops off the coast of Cuba on Tuesday and passes through the Upper Keys. This currently appears to the best-case scenario for the Keys. If Ike misses Cuba, as the other models predict, the Keys can expect a major Category 3 or higher hurricane. Tropical storm force winds can be expected in the Keys as early as Monday night.

Once the storm reaches the Keys, we have three models that turn Ike to the north, resulting in a Gulf Coast landfall along the west coast of Florida. Ike's path and intensity could well imitate those of Hurricane Donna of 1960, which blasted through the Keys as a Category 4 hurricane, then up the west coast of Florida.

It is also possible that the trough of low pressure will not be strong enough to turn Ike to the north, and that the storm will enter the Gulf of Mexico. A second trough of low pressure would then turn Ike north later in the week, resulting in an eventual landfall on the Gulf Coast between the Florida Panhandle and Texas. This is the forecast of the ECMWF, UKMET, and Canadian models. It is too early to speculate where on the Gulf Coast Ike would hit.

Florida Keys are at high risk from Ike
The Florida Keys are highly vulnerable to hurricanes, and are at great risk from Ike. With only one road connecting the Keys to the mainland, 48-72 hours are required to evacuate the Keys. Tropical storm force winds can be expected in the Keys as early as Monday night, which means officials in the Keys may need to start ordering evacuations on Saturday morning. This would likely begin as an evacuation of visitors and tourists.

Figure 2. Forecast tracks for Ike from the latest run of the GFS model ensemble. The ensemble is generated by initializing the GFS model with 21 slightly different initial conditions, then plotting where Ike goes with each model run. There are a lot of possibilities! The white line is what the operational version of the GFS model predicts. The GFS ensemble is not available as quickly as the regular operation version of the GFS.

Intensity forecast for Ike
Sea surface temperatures (SSTs) have warmed to 29.2°C and will warm an extra 0.5°C over the next three days. However, the shear is forecast to be unfavorable for intensification through Saturday morning: 20-30 knots. The high shear may be able to reduce Ike to a Category 1 storm by Saturday morning. Re-intensification is likely beginning on Saturday night, when the shear is forecast to fall below 5 knots. It should take Ike at least a day to recover to Category 3 strength, meaning that the Southeast Bahamas may be spared a major hurricane. The shear is forecast to remain below 15 knots Saturday through Tuesday. The low shear and warm waters favor re-intensification of Ike into a major hurricane by Monday.

Ike's projected damage
The primary danger in the Bahama Islands will be wind damage, since the storm surge typically flows around small islands. However, a Category 4 or 5 hurricane is capable of generating a maximum storm surge of 13 feet in the Turks and Caicos Islands on the right side of the eye, if it makes a direct hit on an island.

On Haiti and in Cuba, there is the potential for very heavy rains of 4-8 inches or higher, beginning on Saturday night. These rains could be particularly devastating for northern Haiti, where the ground is already saturated due to Hurricane Hanna's rains. Hanna killed at least 137 people in Haiti, and over 600,000 people need assistance there. Heavy rains of 3-6 inches are possible in the Dominican Republic.

There's been little change to Tropical Storm Josephine today, which continues to struggle against the twin effects of wind shear and dry air. The models are split on whether Josephine will survive. If it does, the storm may be a threat to Bermuda in a week or so.

Elsewhere in the tropics
The GFS model has considerably toned down its forecasts of tropical waves coming off the coast of Africa that develop. It now forecasts that just one new tropical storm will form over the next two weeks.

I'll be available on a chat forum at 5:15 pm EDT today at http://news-press.com/masterschat2

I'll also be on the radio on the "Science Fantastic with Dr. Michio Kaku" show Saturday at 5:20 pm on http://talkradionetwork.com/. It's a call in show, so you can ask questions.

Record rate of Arctic sea ice loss in August
In climate news that has implications for our children and grandchildren that will be living in Hurricane Alley, the National Snow and Ice Data Center has reported that the rate of sea ice loss in the Arctic was the greatest on record during August. Sea ice continues to decline, and we may break the record for least sea ice coverage, set just last year. This summer's decline in sea ice reinforces the possibility that significant melting of the Greenland Ice Sheet may occur in the coming decades, raising sea levels on the order of 1-3 feet by the end of the century. This will greatly increase the damage potential from hurricane storm surges.

My next blog entry will be Saturday morning.

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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