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Gustav rips through Cuba at Category 4 strength

By: Dr. Jeff Masters, 8:28 PM GMT on August 30, 2008

Gustav roared over Cuba's Isle of Youth at 2 pm today, and is now clobbering the western tip of Cuba with 145 mph Cat 4 winds and a storm surge estimated at 18-23 feet. This will be a massive disaster for Cuba, as portions of the stretch of coast to the right of where Gustav will make landfall are heavily populated (Figure 2). Furthermore, Gustav's hurricane-force winds extend out 60 miles from the eye, meaning that the western reaches of the capital city of Havana will receive a formidable blow. Havana's construction is fairly dilapidated, and Category 1 hurricane winds will do heavy damage. Adding to the catastrophe will be rainfall amounts of up to 25 inches, causing dangerous fresh-water flooding. Gustav is likely to be one of the five most damaging hurricanes in Cuban history. Fortunately, Cuba has a top-notch hurricane civil defense operation, and I'm confident they have gotten all of the population at risk out of harm's way.

Figure 1. Visible satellite image of Hurricane Gustav at landfall in Cuba, 3:02 pm EDT 8/30/08. At the time, Gustav was a Category 4 hurricane with 145 mph winds. Tropical Storm Hanna is visible at the right. Image credit: NASA.

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 27-mile diameter eyewall. The latest Hurricane Hunter report at 4:09 pm EDT put Gustav's pressure at 942 mb, with 145 mph surface winds. Winds at flight level of 10,000 feet were a little higher than in the eye penetration completed at 2 pm, so Gustav is still intensifying, although at a slower rate than earlier today.

Figure 2. Population density map of Cuba. The Isle of Youth, where the western eyewall of Gustav passed over, is lightly populated. However, the portion of the coast to the right of where Gustav is making its second landfall is highly populated. Image credit: University of Texas.

The latest computer models
Louisiana, here comes Gustav. The latest 12Z (8 am EDT) model runs have united around a strike in central Louisiana on Monday afternoon or evening. We should not be surprised if the center comes ashore as far east as Mississippi, or as far west as Galveston, though, given the current boundaries of the cone of uncertainty. A landfall on the western side of the cone of uncertainty, in Texas, is more likely than one on the eastern side, in Mississippi. Once Gustav makes landfall, it will slow down, and pose a significant rainfall/flooding threat to Louisiana and Texas. Portions of this region are under moderate to severe drought, so the flooding could've been worse. Several of the models are forecasting that Gustav will drift southwestward back over the Gulf of Mexico after landfall, and this is a distinct possibility. The motion of Gustav in the longer term may be influenced by Hanna, which I will talk about in the Hanna section.

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. Furthermore, the storm has armored itself with a formidable Cat 4 eyewall which will resist the effects of wind shear, thanks to its high level of angular momentum. Interaction with the flat land area of western Cuba will probably knock down Gustav's intensity by 10-20 mph. 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 3). 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 modest weakening is to be expected as Gustav approaches Louisiana. Still, Gustav will likely be a Category 3 or 4 hurricane at landfall in Louisiana, as forecast by the HWRF and GFDL models.

Figure 3. 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.

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 320 miles across) will grow to about 350 miles across at landfall in Louisiana. This is about 80% 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
It should go without saying that it's time to leave New Orleans. But, I'll say it anyway: It's Time to Leave New Orleans! 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. Get out now.

Links to follow:
Key West radar

Tropical Storm Hanna
Tropical Storm Hanna continues to struggle. The upper level low to Hanna's west has weakened some, but is still creating substantial wind shear over Hanna that is keeping the storm from putting its center of circulation underneath its heaviest thunderstorm activity. Visible satellite loops show the exposed surface circulation, with the heavy thunderstorm activity blown to the northeast side of the storm. The upper low is also pumping in plenty of dry air into Hanna's core, as seen on water vapor satellite loops. However, Hanna is a large storm, and appears to making headway against the shear in recent hours. Heavy thunderstorm have managed to wrap around the north side of the center. Hanna has two prominent upper-level outflow channels visible, on the north and east sides. This morning's QuikSCAT pass saw winds of 40-50 mph at the surface. The first Hurricane Hunter mission for Hanna is scheduled for Sunday afternoon.

The track forecast for Hanna
Steering currents imparted by the counter-clockwise flow around the upper-level low to its west will keep Hanna moving west to west-northwest, towards the Bahamas. As this upper low weakens and moves off, Hanna may begin a period of erratic motion three or so days from now. By 4-5 days from now, a strong ridge of high pressure is expected to build in to the north of Hanna, forcing it west or even southwest. The UKMET model pushes Hanna across Cuba into the western Caribbean by Friday, but the rest of the models foresee that Hanna will loop back to the northwest, eventually threatening Florida, Georgia, or South Carolina by Friday. With such a divergent set of models, it is impossible to predict where and when Hanna will eventually make landfall.

The tropical disturbance behind Hanna, near 22N 45W, is under too much wind shear to develop, and will not influence Hanna's track over the next 2-3 days. If Gustav stalls over the western Gulf of Mexico, as some models are predicting, Hanna's track may be influenced by the presence of another hurricane so close to it. The FujiWhara Effect occurs when two hurricanes approach within 900 miles of each other (roughly the diameter of the Gulf of Mexico). The two storms tend to rotate counter-clockwise around a common center. This effect would tend to send Gustav southwest, over Texas, and Hanna to the northwest. Since Hanna and Gustav are forecast to be roughly 1000 miles apart, it is unlikely that this effect will be a major player in the track.

The intensity forecast for Hanna
Hanna will not be able to intensify until the upper low to its west weakens and moves away. The computer models have, without much success, been predicting this will happen, and continue to do so. Assuming this does really happen, Hanna is large and well-organized enough that it could develop into a Category 1 hurricane fairly quickly once the upper low finally does go away. However, strong upper-level winds from an approaching trough of low pressure are expected to bring 20-30 knots of shear over Hanna beginning Tuesday, and this should substantially weaken the storm, perhaps even to a minimal tropical storm. Oddly, the GFDL model is forecasting Hanna to intensify to a Category 3 hurricane during this high wind shear, which is rather improbable.

Elsewhere in the tropics
An area of disturbed weather located near 22N, 45W, in the mid-Atlantic Ocean, is under 30-40 knots of wind shear. This is too high to allow development to occur, and the shear is expected to remain too high for development for at least 2-3 more days. NHC is giving this system a low (<20% chance) of developing into a tropical depression by Monday.

Cape Verdes Islands disturbance, 97L
A low pressure system (97L) with a large circulation and plenty of spin is located near the Cape Verde Islands off the coast of Africa. This morning's QuikSCAT pass missed 97L. This system has developed a significant amount of heavy thunderstorms, and appears poised to become a tropical depression in the next two days. NHC has given this system a high (>50%) chance of developing into a tropical depression by Monday. The models have been very aggressive developing this system, and both the GFDL and HWRF models predict 97L will be a hurricane five days from now. The storm is expected to pass to the north of the Lesser Antilles Islands, and it is too early to speculate whether this storm might end up recurving out to sea or not.

There are two other impressive African tropical waves lined up behind 97L that are also likely to be a threat to develop once they move offshore Africa next week. The long-range GFS model develops both of these waves.

My next blog will be Sunday morning, unless there's some significant development before then.

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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