Hanna heads to South Carolina; Ike a significant threat to the Florida Keys

Tropical Storm Hanna is becoming better organized as it approaches landfall in South Carolina early Saturday morning. The latest 7:11 am EDT center fix from the Hurricane Hunters found a central pressure of 980 mb, down 5 mb from the 5 am reading. Peak winds were mostly in the 55-65 mph range. Radar animations from the Melbourne radar show that Hanna has built about 1/3 of an eyewall. There has been a slight increase in the organization of Hanna's spiral bands, and the amount and intensity of the precipitation has increased. The outermost spiral bands of Hanna are spreading intermittent heavy rains along the east coast of Florida, and these rains will spread into Georgia, South Carolina, and North Carolina later today. Wind shear of 20 knots continues to interfere with Hanna's organization, and there is also plenty of dry air interfering.


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. Both North Carolina and South Carolina will see winds of 50-60 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.

The wind shear is forecast to remain in the moderate to marginal range, 15-20 knots, over the remainder of Hanna's life. There is a large amount of dry continental air lying between Hanna and South Carolina, which will continue to cause problems for the storm. However, sea surface temperatures are a warm 29°C. None of the intensity models forecast Hanna will become a hurricane, but given the recent increase in the storm's organization, I forecast a 40% chance that Hanna will be a Category 1 hurricane at landfall. Rapid intensification of Hanna is very unlikely, and the strongest I can see this storm getting is a Category 1 storm with top winds of 80 mph. The most likely top winds at landfall will be 70 mph, just below hurricane strength.

Links to follow
Melbourne, FL radar
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.

Ike
Hurricane Ike remains a large and dangerous Category 3 hurricane today, despite the presence of about 25 knots of hostile wind shear. Infrared satellite loops show that the cloud tops are not warming over the core of the hurricane, but the storm has a squashed appearance. Ike 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 deeper into the core of the storm, and the shear and dry air have made it into the inner core and are now disrupting the eyewall (Figure 2).


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
Ike has begun a west-southwest motion in recent hours, which increases the probability that the hurricane will enter the Southeast Bahama Islands on Sunday. The computer models which called for this more southerly path include the GFDL and HWRF models. With its latest run (06Z, 2am EDT) the GFDL takes Ike through the Southeast Bahamas and Turks and Caicos Islands as a Category 3 hurricane early Sunday morning. The HWRF has the same track, but makes Ike a Category 4. The two models then diverge, with the GFDL taking Ike into eastern Cuba as a Category 3 hurricane on Monday, and along the length of Cuba into the Florida Keys as a Category 2 hurricane on Wednesday morning. The HWRF has Ike skirting the northern coast of Cuba, arriving at Key Largo, Florida as a Category 4 hurricane on Tuesday night.

Considerable uncertainty surrounds the path of Ike once the storm reaches the vicinity of South Florida, since a trough of low pressure capable of turning Ike to the north will be passing to the north. A turn to the north over South Florida, or just on either side of the state is possible. One possible track, similar to the NOGAPS model forecast, takes Ike near or over Miami, then northwards towards North Carolina. This is a track similar to Hurricane Floyd of 1999. Another feasible track, similar to the HWRF solution, is like 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, resulting in a n eventual landfall on the Gulf Coast between the Florida Panhandle and Texas. This is the forecast of the ECMWF and GFS models. My current thinking is along these lines:

20% chance Ike will hit the east coast of Florida.
30% chance Ike will hit the Florida Keys.
30% chance Ike will hit Cuba. If this happens, there is 30% chance it would miss Florida and head into the Gulf of Mexico.
10% chance that Ike will miss Florida, but hit further north along the U.S. coast.
10% chance Ike will curve north out to sea and not hit the U.S.

Overall, I'd give the Gulf Coast a 70% chance of getting hit (including the west coast of Florida).

Florida Keys are at high risk
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, a 48-72 hours are required to evacuate the Keys. Tropical storm force winds can be expected in the Keys on Tuesday afternoon, which means officials in the Keys may need to start ordering evacuations on Saturday. This would likely begin as an evacuation of visitors and tourists on Saturday afternoon or Sunday morning.


Figure 3. 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 29.0°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. The shear has eaten its way into the northwestern eyewall, as seen in recent microwave imagery (Figure 2). Re-intensification is likely beginning on Saturday afternoon, when the shear is forecast to fall below 15 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.

If Ike enters the central and western Bahamas, the oceanic heat content reaches a maximum--80 kJ/cm^2, similar to the levels of heat content that fueled Gustav's rapid intensification south of Cuba. Ike may also be moving under an upper-level anticyclone then, which would provide very favorable conditions for intensification. If Ike hits Florida, it will probably be at Category 3 strength or higher, assuming the storm misses Cuba. If Ike hits Cuba first, Florida can expect a Category 1 or 2 hurricane.

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.

Josephine
Tropical Storm Josephine 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.

Why hurricanes recurve
The prevailing winds over the U.S. are from west to east, but in the tropics, they blow east to west. This pattern arises because we live on the surface of a spinning sphere that is heated unequally at the poles and equator. When a hurricane forms in the tropics, it moves east to west with the prevailing winds. However, if the storm gets far enough north, it will suddenly encounter a flow of air moving the opposite direction. This will force the hurricane to move northwards and then eastwards, as the storm gets caught up in the west to east flow of air. The boundary between these two air flow regimes fluctuates, depending upon the position of the jet stream. When a low pressure system moves across the U.S., the jet stream dips to the south, bringing the prevailing west to east winds over the U.S. closer to the tropics. Thus, hurricanes are more prone to recurve to the north when there is an approaching low pressure system passing to their north.

My next blog entry will be this afternoon. I've gotten several complaints that I'm only updating the blog once per day; I assure you that I've been doing twice daily updates for several weeks, and these will continue for the foreseeable future. Those experiencing problems may have browser refresh issues on their computer.

Jeff Masters