A weakened Gustav still presents a grave danger to New Orleans

Gustav roared over Cuba as a Category 4 hurricane with 150 mph winds last night, but Cuba exacted a heavy toll on the storm. Gustav weakened to a Category 3 storm after passage over the island, and is still weakening, according to the latest Hurricane Hunter reports. At 9:48 am EDT, the Hurricane Hunters reported a pressure of 962 mb, up 2 mb from their previous pass through the eye at 7 am. Surface and flight level winds measured by the Hurricane Hunters suggest that Gustav may only be at Category 2 strength. Top winds seen by the plane's SFMR instrument so far this morning have been about 105 mph. The top flight level winds of 103 knots at the flight altitude of 10,000 feet also support a surface wind of 105 mph. NHC has elected to keep Gustav at Category 3 120 mph strength with their 11 am advisory, probably because the satellite appearance still supports a Cat 3. The boundary between Category 2 and Category 3 is at 115 mph.

Visible satellite loops show a ragged and lopsided looking hurricane. Upper level winds from the south are creating 10-15 knots of shear over Gustav, and restricting the upper-level outflow on the south side. The 28-mile wide eye is not very distinct, and the Hurricane Hunters reported that the eyewall was missing a chunk on the south side. Long range radar from Key West also shows the missing southern portion of the eyewall.

The latest track forecast
The latest 00Z (8 pm EDT) model runs have united around a strike in central or southeast Louisiana late Monday morning or early afternoon. 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 be correct. 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've been worse. None of the models are currently forecasting that Gustav will drift southwestward back over the Gulf of Mexico after landfall.

The intensity forecast for Gustav
Wind shear has remained in the moderate range (10-15 knots) the past day, and is forecast to increase to marginal late tonight (15-20 knots). Moderate to marginal shear will still allow Gustav to intensify. Gustav is moving further away from the upper-level anticyclone that helped it intensify as it approached Cuba. Overall, the upper level wind environment is favorable for intensification, but not as favorable as during yesterday's rapid intensification. Gustav is currently over the Loop Current, containing the highest heat content waters of the Atlantic (Figure 1). Both the GFDL and HWRF models forecast that these high heat content waters should result in a 15-20 mph increase in Gustav's winds today. By this evening, Gustav will be passing over a cold eddy. The heat content of the Gulf will decrease as Gustav approaches the coast. As seen in a simulation done yesterday using the GFDL model (Figure 2), the relatively shallow depth of warm water near the coast will allow Gustav to upwell large amounts of cold water from the depths. This will chill the surface waters down by up to 5°C (9°F), which should weaken Gustav's winds by about 15 mph. This cooling effect does not occur for Gustav's path over the southern Gulf of Mexico, due to the great depth of warm waters there. Both the GFDL and HWRF models respond to the lower heat content waters near Louisiana by weakening Gustav to a Category 3 hurricane with 115-120 mph winds at landfall. These models are the only ones that incorporate detailed depictions of the thermal structure of the Gulf of Mexico into their runs.


Figure 1. 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 currently crossing over a portion of the Loop Current with extremely high value of TCHP of 120. However, Gustav will then cross over a cold eddy, and will miss crossing the warm Loop Current eddy that broke off in July. Note that this forecast is old, and the newer forecasts bring Gustav much closer to New Orleans. Image credit: NOAA/AOML.


Figure 2. Forecast track and sea surface temperature response to the passage of Gustav, as simulated by the GFDL model at 8 am EDT Saturday 8/30/08. Passage of Gustav over the relatively shallow depth of warm water near the coast will allow Gustav to upwell large amounts of cold water from the depths. This will chill the surface waters down by up to 5°C (9°F). Note that this forecast is old, and the newer forecasts bring Gustav much closer to New Orleans. Image credit: Isaac Ginis, University of Rhode Island.

Gustav's storm surge may breach the New Orleans levees
NHC's current storm surge forecast calls for a storm surge of 12-16 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 to 4 hurricane. The levee system of New Orleans is designed to withstand a storm surge characteristic of a Category 3 storm. If the NHC storm surge forecast verifies, there is a significant threat of multiple levee failures in the New Orleans levee system resulting in flooding of portions of the city. The latest 06Z (2 am EDT) model runs have shifted their landfall points a bit further west, slightly reducing the odds of a Category 4 storm surge in New Orleans. My best guess is that New Orleans will suffer a Category 3-level storm surge. Let's hope that the Army Corps of Engineers' assertion that the levee system can withstand a Category 3-level storm surge is correct.

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 3). Betsy brought a storm surge of up to 15 feet to Louisiana (Figure 4). 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 a future hurricane of Besty's strength--but no stronger.


Figure 3. Track of Hurricane Betsy of 1965.


Figure 4. Simulated maximum storm surge from Hurricane Betsy of 1965, as modeled using the ADCIRC model. Image credit: ADCIRC Development Group.

Links to follow:
Key West radar
New Orleans weather

I'll have a new blog entry later today that will provide an update on Gustav. I'll also cover Hanna, which may hit the U.S. East Coast late this week.

Some prayers this morning for New Orleans would be in order!

Jeff Masters