The National Hurricane Center has issued its official Tropical Cyclone Reports on several of 2005’s storms, including Hurricane Katrina.

I’ll get to the nerdy stuff in a second, but first the question that even non-nerds want to know the answer to: How strong was Katrina in New Orleans? The answer, as expected, is unsettling:

While the intensity of Katrina was Category 3 as the center of the eye made its closest approach (about 20 n mi) to the east of downtown New Orleans, the strongest winds corresponding to that intensity were likely present only over water to the east of the eye. The sustained winds over all of metropolitan New Orleans and Lake Pontchartrain likely remained weaker than Category 3 strength. The strongest sustained wind in New Orleans is subject to speculation since observations are sparse, due in part to the power failures that disabled ASOS stations in the area before peak wind conditions occurred. However, the NASA Michoud Assembly Facility in eastern New Orleans measured a 1-minute sustained wind of 84 kt (at an elevation of about 12 m) near 1100 UTC 29 August. Also, a few instrumented towers placed in various locations in the metropolitan area by the Florida Coastal Monitoring Program (FCMP) and by Texas Tech University measured sustained winds in the range of 61-68 kt. The Mid-Lake Pontchartrain NWS site (16 m elevation), located along the Lake Pontchartrain Causeway about 8 n mi north of the south shore of the lake, also measured a one-minute sustained wind of 68 kt. Even though these various sites likely did not experience the maximum wind in the area, the Mid-Lake Pontchartrain site had open marine exposure, unlike most locations in the city of New Orleans. It appears likely that most of the city experienced sustained surface winds of Category 1 or Category 2 strength. It is important to note, however, that winds in a hurricane generally increase from the ground upward to a few hundred meters in altitude, and the sustained winds experienced on upper floors of high-rise buildings were likely stronger than the winds at the same location near the ground. For example, on average the 25th story of a building would experience a sustained wind corresponding to one Saffir-Simpson category stronger than that experienced at the standard observing height of 10 m.

For those who still think New Orleans experienced its worst-case scenario this past August, read that again: KATRINA’S WINDS WERE ONLY CAT. 1-2 IN THE CITY!!! And the ridiculous 27-foot storm surges didn’t occur in Louisiana, they occurred in Mississippi. New Orleans was submerged by a hurricane with Category 1, maybe 2, winds in the city, and a Category 2, maybe 3, surge in the city. All the heavy Cat. 3-4 stuff was elsewhere, so don’t be fooled by the big numbers. New Orleans was, meteorologically speaking, spared the brunt of it.

Good Lord, imagine how much worse it would have been, if the track had been just slightly different, bringing the winds and surge that hit Mississippi over New Orleans. I know I’ve said this before, but now there’s more hard data backing it up, and it really needs to be emphasized. You think those predictions of 100,000 dead were exaggerated? Think again, buster. It almost happened. And if you’re a New Orleans resident considering moving back home, my personal advice would be: don’t do it until they’ve got a Category Five-strength levee in place. I know Ray Nagin and Kathleen Blanco and George Bush don’t want me to tell you that, but it’s the truth. Your city dodged a meteorological bullet on August 29 — and it was still destroyed. Granted, shoddily designed levees were the proximate cause of its destruction, but the fact remains that even those reinforced Cat. 3-strength levees they’re promising by next June won’t protect you from a storm that does what Katrina almost did. And if such a storm ever hits — check that: when it hits — the city will be f***ed unless there’s a levee system much, much stronger than anything that’s ever been there before. Is it worth the risk of creating a whole new life in a city that’s just asking to be destroyed again, when the hammer stroke could fall at any time? Just my personal opinion, but no way, I wouldn’t live in New Orleans till they’ve got that new levee up. (So I guess you can consider this my “stay the hell out” post.)

Not that Katrina wasn’t plenty bad enough without resorting to even more horrifying worst-case scenarios. Indeed, reading the NHC’s dry recitation in the “Casualty and Damage Statistics” section of the storm’s devastating toll — not just on New Orelans, but on the whole central Gulf Coast — leaves one almost dumbfounded:

The extent, magnitude, and impacts of the damage caused by Katrina are staggering and are well beyond the scope of this report to fully describe. Thousands of homes and businesses throughout entire neighborhoods in the New Orleans metropolitan area were destroyed by flood. Strong winds also caused damage in the New Orleans area, including downtown where windows in some high rise buildings were blown out and the roof of the Louisiana Superdome was partially peeled away. The storm surge of Katrina struck the Mississippi coastline with such ferocity that entire coastal communities were obliterated, some left with little more than the foundations upon which homes, businesses, government facilities, and other historical buildings once stood. Despite being more distant from the eye of Katrina, the storm surge over Dauphin Island, Alabama destroyed or damaged dozens of beachfront homes and cut a new canal through the island’s western end. Many of the most severely impacted areas along the northern Gulf coast could take years to completely rebuild. Katrina’s heavy rains in southern Florida flooded some neighborhoods, primarily in Miami-Dade County. Many other structures from Florida and Georgia westward to Louisiana that avoided surge or fresh water floods, including some areas well inland, were damaged by strong winds and tornadoes. Considerable damage to some homes and agricultural facilities was caused by several tornadoes in Georgia. Strong winds caused significant tree damage throughout much of Mississippi and Alabama. Combining all of the areas it impacted, Katrina left about three million people without electricity, some for several weeks.

The economic and environmental ramifications of Katrina have been widespread and could in some respects be long-lasting, due to impacts on large population and tourism centers, the oil and gas industry, and transportation. The hurricane severely impacted or destroyed workplaces in New Orleans and other heavily populated areas of the northern Gulf coast, resulting in thousands of lost jobs and millions of dollars in lost tax revenues for the impacted communities and states. Along the Mississippi coast, several large casinos on floating barges were damaged or destroyed when the surge pushed them onshore. Large numbers of evacuees have not returned home, producing a shortage of workers for those businesses that have reopened. Major beach erosion occurred along the tourism-dependent coasts of Mississippi and Alabama. A significant percentage of United States oil refining capacity was disrupted after the storm due to flooded refineries, crippled pipelines, and several oil rigs and platforms damaged, adrift or capsized. An oil rig under construction along the Mobile River in Alabama was dislodged, floated 1.5 miles northward, and struck the Cochrane Bridge just north of downtown Mobile. An offshore oil rig washed up near the beach of Dauphin Island, Alabama. Several million gallons of oil were spilled from damaged facilities scattered throughout southeastern Louisiana. While several facilities have since resumed operations, as of this writing oil and natural gas production and refining capacity in the northern Gulf of Mexico region remains less than that prior to Katrina. Key transportation arteries were disrupted or cut off by the hurricane. Traffic along the Mississippi River was below normal capacity for at least two weeks following the storm. Major highways into and through New Orleans were blocked by floods. Major bridges along the northern Gulf coast were destroyed, including several in Mississippi and the Interstate 10 Twin Span Bridge connecting New Orleans and Slidell, Louisiana.

Estimates of the insured property losses caused by Katrina vary considerably and range between about $20 billion and $60 billion. The American Insurance Services Group (AISG) estimates that Katrina is responsible for $38.1 billion of insured losses in the United States. A preliminary estimate of the total damage cost of Katrina is assumed to be roughly twice the insured losses (using the AISG estimate), or about $75 billion. This figure would make Katrina far and away the costliest hurricane in United States history. Even after adjusting for inflation, the estimated total damage cost of Katrina is roughly double that of Hurricane Andrew (1992). Normalizing for inflation and for increases in population and wealth, only the 1926 hurricane that struck southern Florida surpasses Katrina in terms of damage cost. However, this would not be the case if the values on the higher end of the range of Katrina estimates are later found to be the most accurate. The Insurance Information Institute reports that, mostly due to Katrina but combined with significant impacts from the other hurricanes striking the United States this year, 2005 was by a large margin the costliest year ever for insured catastrophe losses in this country.

Data provided by the Federal Emergency Management Agency (FEMA) indicate that over 1.2 million people along the northern Gulf coast from southeastern Louisiana to Alabama were under some type of evacuation order, but it is not clear how many people actually evacuated. Media reports indicate that many displaced residents have moved either temporarily or permanently to other areas in the United States. A large number of these people might never return to live in their pre-Katrina homes or cities. Thousands of people are still living in hotels and temporary shelters as of this writing. Some people remain separated from other family members and/or are unable to determine if their family member(s) survived the storm.

Wow.

Okay, on to the nerdy stuff. The report answers, or at least attempts to answer, several questions that have been bothering me for months. One is: how could the winds have been so (relatively) weak at landfall, when the pressure was so low?

The estimated Buras landfall intensity of 110 kt, just beneath the threshold of Category 4, is quite low relative to many other hurricanes with a comparable minimum central pressure. In fact, the central pressure of 920 mb is now the lowest on record in the Atlantic basin for an intensity of 110 kt, surpassing Hurricane Floyd (1999) that at one point had a central pressure of 930 mb with an intensity of 110 kt. The 920 mb pressure is also the third lowest at U. S. landfall on record, behind only Hurricane Camille in 1969 (909 mb) and the 1935 Labor Day hurricane that struck the Florida Keys (892 mb). The relatively weak winds in Katrina for such a low pressure are the result of the broadening pressure field on 29 August that spread the pressure gradient over a much larger than average distance from the center, as confirmed by both surface and aircraft observations. The generally weakening convection likely also reduced momentum mixing down to the surface, contributing to surface winds being less than what the usual 90% adjustment from flight level winds would dictate. Katrina exemplifies that there is not simply a direct one-to-one relationship between the central pressure and the maximum sustained winds in a hurricane.

Question number two: why was the storm surge so ridiculously high — as high as, if not higher than, Cat. 5 Camille — if Katrina was only a weakening Cat. 3 at landfall in Mississippi?

The massive storm surge produced by Katrina, even though it had weakened from Category 5 intensity the previous day to Category 3 at landfall in Louisiana, can be generally explained by the huge size of the storm. Katrina had on 29 August a large (about 25-30 n mi) radius of maximum winds and a very wide swath of hurricane force winds that extended at least 75 n mi to the east from the center. Even though Hurricane Camille (1969) was more intense than Katrina at landfall while following a similar track, Camille was far more compact and produced comparably high storm surge values along a much narrower swath. Also, Katrina had already generated large northward-propagating swells, leading to substantial wave setup along the northern Gulf coast, when it was at Category 4 and 5 strength during the 24 hours or so before landfall. In fact, buoy 42040, operated by the National Data Buoy Center (NDBC) and located about 64 n mi south of Dauphin Island, Alabama, reported a significant wave height (defined as the average of the one-third highest waves) of 30 feet as early as 0000 UTC 29 August. This buoy later measured a peak significant wave height of 55 feet at 1100 UTC that matches the largest significant wave height ever measured by a NDBC buoy. Overall, Katrina’s
very high water levels are attributable to a large Category 3 hurricane’s storm surge being enhanced by waves generated not long before by a Category 5 strength storm.

And what about Katrina’s rapid weakening in the final hours before landfall? What the hell?

The new eyewall that formed late on 27 August and contracted early on 28 August began to erode on its southern side very late on 28 August, while another outer ring of convection consolidated. These structural changes likely contributed to the rapid weakening that was observed prior to final landfall. Katrina turned northward, toward the northern Gulf coast, around the ridge over Florida early on 29 August. The hurricane then made landfall, at the upper end of Category 3 intensity with estimated maximum sustained winds of 110 kt, near Buras, Louisiana at 1110 UTC 29 August. Katrina continued northward and made its final landfall near the mouth of the Pearl River at the Louisiana/Mississippi border, still as a Category 3 hurricane with an estimated intensity of 105 kt. The rapid weakening of Katrina, from its peak intensity of 150 kt to 110 kt during the last 18 h or so leading up to the first Gulf landfall, appears to have been primarily due to internal structural changes, specifically the deterioration of the inner eyewall without the complete formation of a new outer eyewall. However, Katrina remained very large as it weakened, and the extent of tropical storm-force and hurricane-force winds was nearly the same at final landfall on 29 August as it had been late on 28 August. The weakening could have been aided by entrainment of dry air that was seen eroding the deep convection over the western semicircle while Katrina approached the coast. Gradually increasing wind shear, slightly lower ocean temperatures, and (following the first Gulf landfall) interaction with land each could also have played a role. Without extensive investigation, however, it is not possible to assess the relative roles played by these various factors. The weakening of major hurricanes as they approach the northern Gulf coast has occurred on several occasions in the past when one or more of these factors have been in place. Indeed, an unpublished study by the National Hurricane Center (NHC) reveals that, during the past 20 years, all 11 hurricanes having a central pressure less than 973 mb 12 h before landfall in the northern Gulf of Mexico weakened during these last 12 h.

There’s more about this issue too — an even lengthier, nerdier discussion, later in the report — but I won’t go into it here. If you can handle the weather-nerd stuff, though, I definitely recommend reading the whole thing (PDF).

Oh, and last but not least, the NHC gives itself a well-deserved pat on the back for its extraordinarily accurate forecasting of Katrina’s track, which established it as, apparently, the only competent government agency in America:

Average official forecast track errors in nautical miles (n mi)… for Katrina were 24… 42… 64… 96… 174… 213… and 244… for the 12, 24, 36, 48, 72, 96, and 120 h forecasts respectively. These forecast errors were considerably less than the average official Atlantic track errors for the 10-year period 1995-2004 of 42, 75, 107, 138, 202, 236, 310 for the 12, 24, 36, 48, 72, 96, and 120 h forecasts, respectively (Table 4). … The 12-48 h official forecasts during Katrina represented significant improvements of 43%, 44%, 40%, and 31%, respectively, over the corresponding 10-year averages. The relatively small errors at 12-48 hours greatly helped in the issuance of generally accurate and timely coastal watches and warnings (Table 5). The 72, 96, and 120 h official track forecast errors were still less than the long-term averages, but only by 14%, 10%, and 25%, respectively. The errors at these longer lead times can be partially attributed to the difficult forecast scenario associated with Katrina’s west-southwestward motion across the southern Florida peninsula and eastern Gulf of Mexico. …

The official track forecasts for Katrina issued within about two and a half days of landfall of the center in Louisiana were exceptionally accurate and consistent. Every official forecast that was issued beginning at 2100 UTC 26 August showed a track crossing the coast of Mississippi and/or southeastern Louisiana. The NHC does not explicitly issue forecasts for the precise location or timing of landfall. However, it is instructive to examine the forecasts for Katrina verifying at 1200 UTC 29 August, less than an hour following the initial Louisiana landfall. The official track forecasts issued 12, 24, 36, and 48 hours prior to 1200 UTC 29 August were in error by only 19, 24, 32, and 56 n mi, respectively. These errors are less than half the magnitude of the corresponding 10-year averages among all Atlantic basin forecasts. Importantly, they were all issued with a hurricane watch or warning in effect for the northern Gulf coast. Much of the credit for these very small errors must be given to the guidance provided by several dynamical models during this time frame.

Proving once again, as if it needed proving, that Rick Santorum is an idiot.

The NHC goes on to point out that the intensity forecasts were less accurate than usual, thanks to Katrina’s two rapid-intensification cycles. “Obviously, accurately forecasting the timing and magnitude of such events remains an operational challenge, in part because the available intensity guidance models generally have little or no skill in forecasting rapid intensity changes.” However, “every official forecast within about three days of landfall in Louisiana correctly anticipated that Katrina would be a major hurricane (at least Category 3) at landfall on the northern Gulf coast.”

P.S. This is scary:

The total number of fatalities known, as of this writing, to be either directly or indirectly related to Katrina is 1336, based on reports to date from state and local officials in five states: 1090 fatalities in Louisiana, 228 in Mississippi, 14 in Florida, 2 in Georgia, and 2 in Alabama. The total number of fatalities directly related to the forces of Katrina is estimated to be about 1200 spread across four states, with about 1000 of these in Louisiana, about 200 in Mississippi, 6 in Florida, and one in Georgia. Especially for Louisiana and Mississippi, the number of direct fatalities is highly uncertain and the true number might not ever be known. This uncertainty exists because complete statistics on causes of death are only available from some areas. As of this writing, bodies are still being found, many of those found have not yet been identified, and the causes of many deaths remain under investigation. More than four thousand persons are still reported missing, so it is possible the death toll could grow beyond current estimates.

One hopes, fervently, that most of those are “phantom” missing persons, along the lines of the many supposed 9/11 victims who turned out months later to be a-okay.