Hanna still wandering; Ike nearing hurricane strength

Tropical Storm Hanna has weathered the worst of the wind shear affecting it, but continues to struggle. The strong upper-level winds from the north have weakened, and wind shear has fallen from 30 knots yesterday to 15 knots today. There is a large amount of dry continental air to the northwest of Hanna, and this is also interfering with the storm. Satellite loops show that Hanna is poorly organized, with heavy thunderstorms limited to the east side of the storm. Fortunately, these thunderstorms have moved away from northern Haiti, where flooding rains from Hanna killed 21 people yesterday. Satellite estimates suggest Hanna has dumped up to six inches of rain on northern Haiti and the northern Dominican Republic.

The track forecast for Hanna
Hanna has been moving erratically over the past day, and has moved considerably farther east than most of the models expected. This decreases the threat to the western Bahama Islands, Florida, and Georgia, since Hanna will be starting further east when it makes its expected turn to the northwest. A landfall location near the South Carolina/North Carolina border is more likely, which would occur Friday night. On Saturday, Hanna will be racing north and then northeast along the U.S. East Coast, bringing tropical storm conditions to the mid-Atlantic and New England states.

The intensity forecast for Hanna
The wind shear is forecast to remain at its current level, 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, with a Tropical Cyclone Heat Potential (TCHP) of 40-70, just below the value of 80 typically associated with rapid intensification. The GFDL model intensifies Hanna to a Category 1 hurricane, but the HWRF and SHIPS model keep it a tropical storm. I expect Hanna will have top winds between 60 mph and 80 mph at landfall in North or South Carolina, making it a strong tropical storm or weak Category 1 hurricane.

Ike
Tropical Storm Ike continues getting organized over the middle Atlantic, and has the potential to become a large and dangerous Cape Verdes-type hurricane by Monday, when it is expected to be in the southeastern Bahama Islands. Visible satellite loops show that heavy thunderstorm activity is starting to wrap around the core of the storm, and Ike has about 50% of an eyewall built. Upper-level outflow is good, and Ike is in a very favorable upper-level wind environment, with an upper-level anticyclone overhead, and wind shear less than 10 knots. Ike has moistened the atmosphere around it enough to wall off a large amount of dry air that surrounds it. Rather cool sea surface temperatures (SSTs) of 27.5°C. SSTs will quickly warm to 29°C over the next two days, but the shear is forecast to increase to 20-35 knots Thursday through Friday. The SHIPS model responds by strengthening Ike only to a Category 1 hurricane today, then weakening it to a tropical storm during the higher shear, then strengthening it again to a Category 1 hurricane by Saturday. The HWRF and GFDL models do not predict the shear will affect Ike as much Thursday and Friday, and intensify the storm into a Category 2 or higher hurricane by Sunday. The HWRF makes Ike a Category 4 hurricane in the eastern Bahama Islands on Monday, and the GFDL has Ike hitting eastern Cuba as a Category 2 or 3 hurricane on Monday. I expect Ike to be a hurricane by Thursday morning, and a Category 3 or higher hurricane by Monday.

The longer term fate of Ike is highly uncertain. The ECMWF and GFS models both forecast that Hanna will be strong enough to create a weakness in the ridge of high pressure steering Ike to the west. Ike will then follow Hanna's path, recurving northwards. the timing of this recurvature is critical, as the GFS shows that Ike will miss the U.S., while the ECMWF forecasts a strike in South Florida on Tuesday, then another landfall in North Carolina later in the week. If Hanna is not as strong or is faster-moving than these models expect, Ike may not recurve. Instead, Ike will cross Cuba or move through the Florida Straits, eventually emerging into the Gulf of Mexico to cause havoc there. This is my current expectation.

Josephine
Tropical Storm Josphine is a long way out to sea, and it will be at least a week before it may threaten any land areas.

Elsewhere in the tropics
The GFS model is forecasting that at least two or three more tropical waves will move off the coast of Africa over the next ten days and develop into tropical storms. The NOGAPS model is predicting possible development near the Yucatan Peninsula 4-6 days from now, either in the western Caribbean or the Gulf of Mexico's Bay of Campeche.

Gustav's march of destruction through the Caribbean
A brief summary of Gustav's path:

Haiti
Gustav hit Haiti as a category 1 hurricane, dropping up to a foot of rain on that country's southern peninsula. On 31 August, official government figures reported 76 deaths and four people missing, 8,789 people in emergency shelters, 2,121 houses destroyed, and 8,155 houses damaged. Numbers are likely to rise. With hurricane season less than half over and Hurricane Ike expected to pass close by, the misery for Haiti may only be beginning.

Dominican Republic
Damage was limited in the Dominican Republic, but a rain-triggered landslide killed eight people.

Jamaica
In Jamaica, Gustav killed 12 people and did at least $110 million in damage to roads and bridges. The banana crop was hit hard, and there was extensive damage to the power and water infrastructure. The tourist industry was relatively unaffected.

Cayman Islands
Gustav passed 25 miles south-southwest of Little Cayman Island at 10 pm on Friday, as a strengthening Category 1 hurricane. Almost every building on the island suffered roof damage, and every dock on the island was destroyed or severely damaged, according to caycompass.com. Nearby Cayman Brac Island was less severely affected, and Grand Cayman island was not seriously affected.

Cuba
Cuba took bad beating, but Gustav just missed hitting the capital city of Havana. Damage is likely to be in the hundreds of millions, and not in the billions, as originally feared. No one died in the storm. Gustav destroyed 2,000 buildings and damaged 150,000. Power was lost over the entire western end of the island, including the 2.2 million residents of Havana. Gustav's eye passed over the Isle of Youth, where 87% of all the homes were damaged or destroyed. Officials measured gusts of 212 mph (340 kph) in the western town of Paso Real del San Diego, a new national record for maximum wind speed, according to the Cuban Institute of Meteorology.


Figure 1. Radar estimated rainfall from Gustav

The U.S.
Gustav killed nine people in the U.S., Eqecat Inc., predicts insured losses will be between $3 billion and $7 billion. AIR Worldwide projects a lower number, $1.8-$4.4 billion. Using the usual rule of thumb that total losses are double insured losses, Gustav's price tag will be in the $4-$14 billion range, ranking it between 19th and 5th place on the list of costliest U.S. hurricanes. Louisiana took the brunt of the wind damage, particularly in the coastal areas west of New Orleans where the eye came ashore. Flooding from heavy rains in excess of 15 inches (Figure 1) is also responsible for significant damage in Louisiana, and a damaging storm surge of 10-12 feet affected the New Orleans region. Losses were in the tens of millions along the coast of Misssissippi, where the storm surge ranged from 11 feet in Waveland at the Louisiana border to 6.5 feet at the eastern end of the coast. The small community of Pearlington, nearest the Louisiana line in Hancock County, reported a 19-foot surge that flooded around 100 homes, according to the Mississippi Emergency Management Agency. Wind damage in Mississippi was light, since sustained winds reached only 40 mph. Damage to the offshore oil and gas facilities in the Gulf of Mexico was about $1-$3 billion.

Hearty congratulations go to the people and officials in New Orleans, who staged a very successful evacuation. Congratulations also go to the Army Corps of Engineers, whose improvements to the levee system held against storm surges similar to what Katrina brought.

When hurricanes collide
I'm getting this question a lot--can hurricane collide to form a super hurricane? No, 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 Fujiwara interaction in 1995. Iris then began interacting with a third storm, Tropical Storm Karen, which orbited and later merged with the more intense Iris.

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.

More info coming this afternoon at the usual time.
Jeff Masters