Little change to Hanna; threat grows from Ike

By: Dr. Jeff Masters , 8:55 PM GMT on September 04, 2008

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Tropical Storm Hanna has not changed much, and remains on track to hit North or South Carolina as a tropical storm or Category 1 hurricane Friday night. Hanna has characteristics of a subtropical storm. A subtropical storm tends not to have any heavy thunderstorm activity near its center. Instead, the heaviest rain is located in a band 100 or more miles from the center. The difference is not important as far as the winds are concerned, since both types of storms generate similar winds. Satellite loops show that Hanna has one spot of heavy thunderstorm activity to the northwest of the center, and there is also a band of heavy thunderstorms several hundred miles north of the center. This band will hit the Carolinas well before the storm's center arrives.

Wind shear of 20 knots continues to interfere with Hanna's organization, and there is also plenty of dry air to its west that is causing the storm trouble. The amount and intensity of Hanna's thunderstorms has not changed much in the past day, and the central pressure has remained in the 985-990 mb range. The latest Hurricane Hunter report found a central pressure of 988 mb, with surface winds of 50-60 mph, at 3:17 pm EDT.


Figure 1. Latest satellite image of Hanna.

The track forecast for Hanna
The computer models continue to come into better agreement on Hanna's track. Hanna will come ashore in North Carolina or northern South Carolina. The exact landfall location is relatively unimportant in this case, since 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 intensity forecast for Hanna
The wind shear is forecast to remain in the moderate to marginal range, 15-25 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. The main intensity models--HWRF, GFDL, and SHIPS--all forecast that Hanna will not become a hurricane. Given the rather subtropical appearance of Hanna, with the heaviest thunderstorms well removed from the center, plus the rather high wind shear over the storm, rapid intensification is very unlikely. A tropical storm needs to have its heaviest thunderstorms close to the center in order to undergo significant strengthening. Hanna should intensify slowly, if at all, and be no stronger than a Category 1 hurricane with 80 mph winds at landfall in the U.S.

Ike
Hurricane Ike remains a large and dangerous Category 4 hurricane today, despite the presence of about 20 knots of hostile wind shear. Satellite estimates of Ike's intensity remain unchanged from early this morning, but infrared satellite loops show that the cloud tops are warming over the core of the hurricane, signifying weakening. The storm has a squashed appearance, and is flattened on the northwest side, where there is no upper-level outflow. This is due to 20 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.

Track forecast for Ike
The computer models were in two distinct camps this morning, but are now in better agreement on a more southerly track for Ike. This increases the danger for the Bahamas, Haiti, Cuba, and Florida. A southward component of motion is now forecast by all of the computer models except the UKMET, making it very likely that Ike will move into the Bahamas by Sunday. The GFDL is the furthest south, projecting a landfall in Cuba as a Category 3 or 4 hurricane on Sunday night. The UKMET is the furthest north, projecting that Ike will miss the Bahamas, but hit South Florida. All of the models bring Ike within 200 miles of Miami by Tuesday. The HWRF brings Ike to a point 50 miles from Miami on Tuesday, as a Category 4 hurricane.


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 black line is what the operational version of the GFS model predicted in its previous cycle (6 hours before the ensemble plotted here). Forecast points are not 12 hours apart, as stated.

Ike's long-term fate has two main possibilities:

1) Ike may hit eastern Cuba, as forecast by the latest (12Z, 8am EDT) runs of the GFDL and ECMWF models, and a number of ensemble members of the latest 12Z GFS model (Figure 2). A hit on Cuba would severely disrupt the storm, weakening it to a Category 1 or 2. Ike could then move on into the Gulf of Mexico and re-intensify, as forecast by the ECMWF model.

2) Ike may plow through the Bahamas and come very close to South Florida (the consensus of the HWRF, NOGAPS, and GFS models). A trough of low pressure may then pull Ike to north. This turn to the north might occur over Florida, or over the western Bahamas, within 200 miles of the Florida coast. In the latter case, North Carolina might be at risk. The recent model trend has been to depict a weaker trough, resulting in Ike getting stranded, like Fay and Gustav did. Ike would resume a slow motion to the west as ridge of pressure builds in, potentially crossing Florida into the Gulf of Mexico.

There is a third possibility--Ike may recurve before hitting the U.S., and move harmlessly out to sea. That possibility appears lower probability than cases 1 and 2 above, at this point.

Intensity forecast for Ike
Sea surface temperatures (SSTs) have warmed to 28.5°C and will warm an extra degree over the next two days. However, the shear is forecast to be unfavorable for intensification through Friday night: 20-30 knots. The high shear may be able to disrupt the inner core of Ike, reducing it to a Category 2 storm by Friday night. Re-intensification is likely beginning on Saturday morning, when the shear is forecast to fall below 10 knots. The shear is forecast to remain below 10 knots Saturday through Tuesday. On Sunday, when Ike may be approaching the southeastern Bahamas, it should encounter cooler waters stirred up by Hurricane Hanna. This may limit the re-intensification. Once Ike crosses Hanna's wake and 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.

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 90 people in Haiti, and the situation there is being described as desperate.

The first Hurricane Hunter mission into Ike is scheduled for Friday morning.

Josephine
Tropical Storm Josephine has been severely damaged by the twin effects of wind shear and dry air. The storm is a low-level swirl with a llittle thunderstorm activity to the north. The storm is forecast to dissipate by several of the models. However, most of the models predict it will hang in there and eventually eventually track north of the Lesser Antilles Islands.

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.

Gustav
With two other storms to be concerned with, my day quickly runs out of time to talk about the aftermath of Gustav. Louisiana, Mississippi, and Arkansas continue to grapple with the damage and flooding wrought by Gustav. Gustav has dropped up to 20 inches of rain on central Louisiana (Figure 3), and spawned 50 tornadoes. Gustav took out the power to huge sections of the state (Figure 4).


Figure 3. Total rainfall from Gustav. Figures were not available for Texas and extreme western Louisiana. Image credit: NOAA/HPC.


Figure 4. Power outage status for Louisiana, as of 9am Wednesday.

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.

When hurricanes collide
Many readers have asked if Hanna and Ike could collide and make a super hurricane. Well, hurricanes cannot collide to make a bigger hurricane. When hurricanes get within about 900 miles of each, they begin to interact. There are three possible outcomes:

1) The larger storm will destroy the smaller one. The larger storm's upper-level outflow will bring hostile wind shear over the smaller storm, and the larger storm may steal the smaller storm's moisture. This occurred in 2005, when Hurricane Wilma destroyed Tropical Storm Alpha over Hispaniola.

2) Both hurricanes will compete for the same energy, resulting in weakening of both storms.

3) The storms will rotate around a common center of rotation (the Fujiwhara Effect), before going on their separate ways. Hurricane Humberto and Hurricane Iris took part in a brief Fujiwhara interaction in 1995. Iris then began interacting with a third storm, Tropical Storm Karen, which orbited and later merged with the more intense Iris. In
cases, the two storms will merge, such as occurred in 1997 in the Pacific with Typhoon
Yule and TD 16W
.

Sometimes, a recurving hurricane will leave behind an enhanced trough of low pressure that will act to help recurve the storm behind it along the same path. This is possible this week with Ike and Hanna.

My next blog entry will be Friday morning.

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.