Gustav deepens explosively into a Category 3 hurricane

Hurricane Gustav is deepening explosively, and is now a major hurricane. At 8:20 am EDT, the Hurricane Hunters estimated surface winds of 120 mph, and a surface pressure of 954 mb. This is a 34 mb drop in the past 24 hours, since the storm left Jamaica. In Jamaica, Gustav may have killed 11 people, and left millions in damage. Earlier this week, Gustav killed 59 people on Haiti, and 8 in the Dominican Republic.


Figure 1. Current satellite image of Gustav.

Visible satellite loops show a well-organized and intensifying major hurricane. Upper-level outflow is well-established. Gustav has a well-formed eye and Central Dense Overcast (CDO) of high cirrus clouds, characteristic of a major hurricane. Radar from Cuba's Isle of Youth shows impressive spiral banding and a solid 35-mile diameter eyewall.

The latest computer models
The latest 00Z/06Z (8 pm/2 am EDT) model runs still offer two solutions. The main solution, offered by the GFDL, GFS, and NOGAPS models, and adopted by NHC, takes Gustav to the central Louisiana coast between Monday afternoon and early Tuesday morning. The GFDL forecasts a Category 4 storm at landfall, with Category 1 winds affecting New Orleans. It is the fastest model, bringing Gustav ashore Monday afternoon. The other solution, offered by the UKMET and ECMWF models, is to turn Gustav westwards towards Texas just before landfall in Central Louisiana Monday afternoon. The HWRF is in between, taking Gustav to the coast of western Louisiana as a Category 3 storm, then turning the storm southwestward along the Texas coast as a tropical storm. While the official NHC forecast taking Gustav ashore over central Louisiana is the most probable one, a significant chance exists of a landfall farther west, perhaps even as far west as Corpus Christi, Texas.


Figure 2. Tropical Cyclone Heat Potential (TCHP, in kJ/cm^2) for August 28, 2008. Values of TCHP greater than 80 are commonly associated with rapid intensification of hurricanes. The forecast points from the NHC 5 am Saturday forecast are overlaid. Gustav is expected to cross over a portion of the Loop Current with extremely high value of TCHP of 120 after crossing Cuba. However, Gustav will then cross over a cold eddy, and will miss crossing the warm Loop Current eddy that broke off in July. Image credit: NOAA/AOML.

The intensity forecast for Gustav
Wind shear has increased over Gustav, and is now moderate (10-15 knots). However, the storm is under an upper-level anticyclone that aids intensification, is over the highest heat content waters of the Atlantic, and has no dry air to interfere with it. I expect Gustav will continue to intensify until landfall in Cuba. Interaction with the flat land area of Cuba will probably knock down Gustav's intensity by 10-20 mph or so. However, Gustav should regain its lost strength and more once it enters the Gulf of Mexico. A region of exceptionally high oceanic heat content, associated with the warm Loop Current, lies just north of Cuba along Gustav's track (Figure 2). However, Gustav will encounter a cool eddy in ocean after this, and will miss passing over the warm Loop Current Eddy that broke off in July. As Gustav approaches landfall, the total heat in the ocean will continue to decrease, and wind shear is expected to increase to 15-20 knots, as forecast by the SHIPS model. Thus, some weakening is to be expected as Gustav approaches Louisiana.

How big will Gustav get, and what will its storm surge be like?
Gustav has expanded significantly in size, and will be a large and dangerous hurricane the remainder of its life. But will it rival Katrina in size, bringing an enormous Katrina-like storm surge to the coast? Well, our ability to predict size changes in hurricanes is poor. We do know that as storms move further from the Equator, they grow in size. This is because the Coriolis force increases as you move away from the Equator. An increased Coriolis force provides more spin to the storm, and the hurricane responds by growing in size. Thus, expect Gustav to grow in size as it approaches landfall along the Gulf coast. Hurricanes also tend to grow in size as they intensify. These two factors are taken into account when NHC makes a wind radius forecast. NHC is forecasting that Gustav's current diameter of tropical storm force (about 300 miles across) will remain about the same at landfall. This is about 70% as wide as the 440 mile-wide region of tropical storm force winds Hurricane Katrina had at landfall (5 am August 29, 2005).

A wind field of this size in a Category 4 hurricane traveling NW over the Gulf of Mexico is capable of carrying a 13-15 foot high storm tide to the coast in central Louisiana. NNW-moving storms bring the highest surge to this region of coast. Storm tide is the storm surge, adjusted upwards by 2 feet in case the storm hits at high tide.

Time to leave New Orleans
I've been criticized by some for recommending people evacuate New Orleans, since that's not my job, and for saying "it's not natural" to live in a city that lies partially below sea level. I apologize for my remarks, they could have been phrased better. We had to build New Orleans where it is, and it is a great city that needs to be protected. The fact remains that New Orleans is highly vulnerable to storms like Gustav. Gustav is capable of bringing a storm surge to the city that will overwhelm the levees. Pre-Katrina wisdom suggested that the city needed 72 hours to evacuate. With the population about half of the pre-Katrina population, that lead time is about 60 hours. With Gustav likely to bring tropical storm force winds to the city by Monday afternoon, that means it's time to leave. I'm not an emergency manager, but I am a hurricane scientist. I understand the danger this storm poses better than most. The risk of staying in New Orleans is unacceptable. This is a huge and dangerous storm that has already killed a lot of people. The projected track and strength of Gustav is very close to that of Hurricane Betsy of 1965, the Category 3 hurricane that overwhelmed New Orleans' levees, and killed 76 people. It's time to get out of New Orleans.

Why did Katrina get so huge?
We really don't understand why Katrina got so huge, though an interesting theory was provided by Pat Fitzpatrick of Mississippi State University at a recent hurricane conference. Here's the technical gist:

Katrina nearly doubled in size on 27 August, and by the end of that day tropical storm-force winds extended up to about 140 n mi from the center. A cursory examination of satellite imagery shows the possible influence of a trough or confluence zone to the north that may have contributed angular momentum to the intensifying cyclone.

Although the rapid intensification of Katrina was noteworthy, the expansion of the tropical storm-force winds is the key forecast issue. The devastation wrought by this storm upon landfall is attributable more to its size rather than its intensity, as it landed as a Category 3 hurricane. This large hurricane caused a record storm surge and exposed the coastal regions of Louisiana and Mississippi to hurricane-force winds for an extensive period of time.

Observations, as well as a Weather Research Forecast model simulation, suggest that an influx of vorticity associated with a remnant front near north Florida contributed to the wind field expansion.

Basically, Dr. Fitzpatrick is saying that satellite observations and computer modeling studies suggest that Katrina got extra spin that helped it grow in size by ingesting a portion of an old front that had stalled out over northern Florida. As Gustav approaches the U.S., we should be on the lookout for similar clumps of clouds with some extra spin that the hurricane could use to help grow in size.

Our interactive Wundermap is a good way to study the predicted size changes of a hurricane. Click on the "Hurricane" layer, then check the "Wind Radius" box to see the predicted extent of hurricane force (65 kt), storm force (50 kt) and tropical storm force (34 kt) winds, for the four quadrants of the storm.

Links to follow
Punta del Este, Cuba radar
Punta del Este, Cuba weather
Nueva Gerona, Cuba weather

Hanna and the rest of the tropics
I'll talk about Hanna and the rest of the tropics in my next blog, which will be this afternoon between 3-5 pm.

Jeff Masters