Gustav is intensifying again, and headed for Louisiana

Gustav is intensifying again, and threatens to bring a destructive storm surge that will offer a significant test to New Orleans' rebuilt levees when it strikes Louisiana on Monday. Gustav tore across Cuba with 150 mph winds last night, but Cuba exacted a heavy toll on the storm. Passage over Cuba allowed wind shear to penetrate into the core of the storm, disrupting the eyewall and cutting the storm's winds by 35-45 mph. However, Gustav's day over the warm Gulf of Mexico has begun to rejuvenate the storm, and the pressure is starting to fall again. At 3:17 pm EDT, the Hurricane Hunters reported a pressure of 957 mb, down 5 mb from the eye report at 9:48 am. Winds at the surface and at the flight altitude of 10,000 feet have not changed significantly since this morning, and still support classifying Gustav as a strong Category 2 hurricane with top winds of 105 mph. However, the winds should start to increase by late tonight in response to the falling pressure.

The satellite appearance of Gustav is steadily improving. Visible satellite loops of Gustav show that the storm has recently assumed a more symmetric appearance, though the heaviest thunderstorms are still just on the south side of the eye. Upper level winds from the south are creating about 15 knots of shear over Gustav, restricting the upper-level outflow on the south side. The 35-mile wide eye is not very distinct, and the Hurricane Hunters reported that the eyewall was missing a chunk on the southeast side. However, the eye was elliptical this morning, and has now assumed a more circular, well-formed appearance.

The outer spiral bands of Gustav are now visible on New Orleans radar, and some rain bands have already affected the Mississippi River Delta region.


Figure 1. Estimated storm surge from NHC's experimental storm surge model. Note that these values will often differ from the latest official NHC forecast, and one should consult the latest text advisory for the most current storm surge information.

The latest track forecast
The latest 12Z (8 am EDT) model runs have shifted slightly west, with a central Louisiana landfall still the most popular solution. The GFDL model, which has all along insisted that Gustav would arrive at the coast a day earlier than the other models, has proven to have the correct timing. We should not be surprised if the center comes ashore as far east as New Orleans, or as far west as western Louisiana, given the current spread in the model tracks. 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 have been worse. Only the HWRF model is forecasting that Gustav will drift southwestward back over the Gulf of Mexico after landfall. I am not expecting Gustav to be reborn off the Texas coast late in the week.

The intensity forecast for Gustav
Wind shear has remained in the moderate range (about 15 knots) today, and is forecast by the SHIPS model to decrease to 10 knots tonight. This amount of shear will allow Gustav to intensify. Gustav is moving further away from the upper-level anticyclone that helped it intensify as it approached Cuba, however. Overall, the upper level wind environment is favorable for intensification, but not as favorable as during yesterday's rapid intensification. Gustav has now left the warm waters of the Loop Current, and is over waters of much lower heat content than it has had. The latest 12Z (8 am EDT) run of the GFDL model forecasts no strengthening, and brings Gustav to shore as a Category 3 hurricane with 115 mph winds. The latest SHIPS intensity model predicts the same thing. The 12Z (8 am EDT) run of the HWRF shows that Gustav will weaken to a Category 2 hurricane by landfall. Given the recent improvement in Gustav's organization, I believe that the storm has time to intensify into at least a Category 3 hurricane with 125-130 mph winds by landfall. Gustav should not intensify as rapidly as it did when approaching Cuba.

Gustav's storm surge is not likely to breach the New Orleans levees--if they perform as designed
Gustav is a very large storm. Like Katrina, Gustav may carry a larger storm surge to the coast than its wind speeds might suggest. Currently, Gustav's diameter of tropical storm force winds is 340 miles. By landfall, this number is forecast to increase to 360 miles, which would make Gustav 80% as large as Katrina was at landfall. NHC's current storm surge forecast calls for a storm surge of 10-14 feet to the right of where the center of Gustav comes ashore. The latest computer generated storm surge map shows that highest surge will be along the levee system along the east side of New Orleans. Storm surge levels of this magnitude are characteristic of a Category 3 hurricane. The levee system of New Orleans is designed to withstand a Category 3 storm surge. If Gustav intensifies more than the NHC forecast is calling for, there is a significant threat of multiple levee failures in the New Orleans levee system resulting in flooding of portions of the city. However, the latest 12Z (8 am EDT) model runs have shifted their landfall points a bit further west, reducing the odds of a Category 4 storm surge in New Orleans. My best guess is that New Orleans will suffer a Category 2 or 3-level storm surge. The levees will hold with that level of storm surge, if they perform as designed.

Comparing Gustav to "Billion-Dollar Betsy"
Gustav's track and expected intensity at landfall are similar to those of Hurricane Betsy of 1965. Betsy was a strong Category 4 hurricane as it crossed the Gulf of Mexico, which then weakened to a Category 3 at landfall, right where Gustav is predicted to make landfall (Figure 2). Betsy brought a storm surge of up to 15 feet to Louisiana (Figure 3). According to wikipedia, levees for the Mississippi River Gulf Outlet along Florida Avenue in the Lower Ninth Ward and on both sides of the Industrial Canal failed. The flood waters reached the eaves of houses in some places and over some one story roofs in the Lower Ninth Ward. These levee breaches flooded parts of Gentilly, the Upper Ninth Ward, and the Lower Ninth Ward of New Orleans as well as Arabi and Chalmette in neighboring St. Bernard Parish. Seventy-six people died in Louisiana, and "Billion-Dollar Betsy" became the first billion-dollar hurricane ($10 billion in 2008 dollars). As a result of the hurricane, the Army Corps of Engineers was tasked with the job of upgrading the New Orleans levee system to withstand "the most severe
combination of meteorological conditions reasonably expected". As of today, that means protection from a Category 3 hurricane, but no higher.


Figure 2. Track of Hurricane Betsy of 1965.


Figure 3. Simulated maximum storm surge from Hurricane Betsy of 1965, as modeled using the ADCIRC model. Image credit: ADCIRC Development Group.

Links to follow:
New Orleans radar
New Orleans weather

Tropical Storm Hanna
The Hurricane Hunters are flying their first mission into Tropical Storm Hanna, and have found a weak tropical storm with 50 mph winds and a central pressure of 997 mb, according the 1:36 pm EDT eye report. Watching satellite loops of Hanna is kind of like watching a stick caught in turbulent rapids--the atmosphere is extremely chaotic in Hanna's vicinity, and it's tough to follow what is going on. Hanna has moved underneath an upper-level low that is pumping cold, dry air in. Hanna is also experiencing wind shear from the upper-level outflow from Hurricane Gustav. These effects have combined to keep Hanna a weak tropical storm. While Hanna does have a large circulation and some respectable upper-level outflow to the north, heavy thunderstorm activity is limited and is removed from the center.

The track forecast for Hanna
The current steering flow driving Hanna to the west is expected to collapse soon, resulting in a slow, erratic motion or small loop for Hanna. By 4-5 days from now, a strong ridge of high pressure is expected to build in, forcing Hanna to the northwest. Most of the long-range models foresee a landfall along the U.S. East Coast late in the week. The most popular model solution (GFS, GFDL, and HWRF) is a landfall Friday in North Carolina, followed by a track up the East Coast. The UKMET model targets South Florida on Friday, and the NOGAPS and ECMWF models target Georgia/northern Florida on Thursday night.

The intensity forecast for Hanna
Hanna will not be able to intensify significantly over the next two days, due to upper low it is situated under, and the outflow from Hurricane Gustav. By Tuesday, both Gustav and the upper low should be weaker, potentially allowing Hanna to intensify. The GFDL, HWRF, and SHIPS intensity models all predict Hanna will be a Category 1 hurricane on Friday. However, due to the uncertain future evolution of Gustav, consider all intensity forecasts for Hanna beyond two days from now low confidence.

Middle Atlantic disturbance, 98L
An area of disturbed weather (98L) is located near 22N, 45W, in the mid-Atlantic Ocean. Visible satellite images show that this disturbance has a well-defined surface circulation. However, the disturbance has very little heavy thunderstorm activity, thanks to 20 knots of wind shear, and some dry air to the west. Wind shear should remain too high to permit development over the next two days, and is highly uncertain after that, due to the upper-atmosphere interactions occurring with Hanna and Gustav. NHC is giving this system a low (<20% chance) of developing into a tropical depression by Tuesday. None of the models develop 98L, but Bermuda should keep an eye it.

Cape Verdes Islands disturbance, 97L
A low pressure system near 16N 35W (97L), is a few hundred miles west-northwest of the Cape Verde Islands. Visible satellite loops show that 97L has a very large closed circulation, but little heavy thunderstorm activity. Dry air on the west side of the storm is getting wrapped into the circulation, interfering with development. Wind shear is a moderate 15 knots, and is expected to stay in the low to moderate range over the next four days. NHC has given this system a high (>50%) chance of developing into a tropical depression by Tuesday. Several of the models develop this system, and the storm is expected to pass close to the northern Lesser Antilles Islands on Friday.

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 this week. The long-range GFS model develops both of these waves.

I'll have a short update tonight, if there's any major developments to report.

Jeff Masters