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| How tropical storm Isaac matches up against hurricane Katrina | How tropical storm Isaac matches up against hurricane Katrina |
| (about 1 month later) | |
| UPDATE: As of the National Hurricane Center's 5pm report, tropical storm Isaac's maximum sustained winds are at 70mph. The storm is forecast to become a low-end category 2 storm by landfall, with 100mph winds, slightly greater the previously expected 90mph. But that is still 20mph less than Katrina's wind speed. [ends] | UPDATE: As of the National Hurricane Center's 5pm report, tropical storm Isaac's maximum sustained winds are at 70mph. The storm is forecast to become a low-end category 2 storm by landfall, with 100mph winds, slightly greater the previously expected 90mph. But that is still 20mph less than Katrina's wind speed. [ends] |
| Tropical storm Isaac has left minimal damage in southern Florida, but is churning on a path towards New Orleans. Landfall is expected Tuesday night into Wednesday morning. Not surprisingly, the allusions to 2005's hurricane Katrina have already begun, but let's talk about some critical differences it has with Katrina right now, and the forecast for Isaac going forward. | Tropical storm Isaac has left minimal damage in southern Florida, but is churning on a path towards New Orleans. Landfall is expected Tuesday night into Wednesday morning. Not surprisingly, the allusions to 2005's hurricane Katrina have already begun, but let's talk about some critical differences it has with Katrina right now, and the forecast for Isaac going forward. |
| 1. Strength | 1. Strength |
| As of 2pm Monday, Isaac is about 280 miles southeast of the mouth of the Mississippi river. The minimum pressure is 984 millibars. That's not particularly high; I've tracked many winter storms with lower central pressure. | As of 2pm Monday, Isaac is about 280 miles southeast of the mouth of the Mississippi river. The minimum pressure is 984 millibars. That's not particularly high; I've tracked many winter storms with lower central pressure. |
| At this same point in Katrina's run, minimum central pressure of 935 millibars. The relative difference between 984 and 935 is greater than what you would normally see between a high-pressure and low-pressure system. Katrina's dropped even further upon landfall on the Louisiana/Mississippi border, to 920 millibars. | At this same point in Katrina's run, minimum central pressure of 935 millibars. The relative difference between 984 and 935 is greater than what you would normally see between a high-pressure and low-pressure system. Katrina's dropped even further upon landfall on the Louisiana/Mississippi border, to 920 millibars. |
| You can expect Isaac's pressure to drop as well as it approaches Louisiana, but it will almost certainly never come close to 935 millibars, let alone 920. There is too much mid-level dry air being introduced into the system. Dry air may fuel fires, but it kills hurricanes. | You can expect Isaac's pressure to drop as well as it approaches Louisiana, but it will almost certainly never come close to 935 millibars, let alone 920. There is too much mid-level dry air being introduced into the system. Dry air may fuel fires, but it kills hurricanes. |
| 2. Wind speed | 2. Wind speed |
| Isaac's maximum sustained winds are 65mph. That's nothing to scoff at, but it'll need to pick up by 10mph in order to earn designation as a category 1 hurricane. | Isaac's maximum sustained winds are 65mph. That's nothing to scoff at, but it'll need to pick up by 10mph in order to earn designation as a category 1 hurricane. |
| Katrina had category 4 wind speeds of 145mph at this point in its journey. Just the differential between Isaac's and Katrina's wind speed, 80mph, is equivalent to the power of a category 1 hurricane. | Katrina had category 4 wind speeds of 145mph at this point in its journey. Just the differential between Isaac's and Katrina's wind speed, 80mph, is equivalent to the power of a category 1 hurricane. |
| Katrina's winds did drop to a somewhat weaker category 3 storm, with 120mph winds upon landfall. Isaac's are expected to strengthen to about 90mph (a high-end category 1). This 30mph difference between Katrina and what's predicted for Isaac is very significant. | Katrina's winds did drop to a somewhat weaker category 3 storm, with 120mph winds upon landfall. Isaac's are expected to strengthen to about 90mph (a high-end category 1). This 30mph difference between Katrina and what's predicted for Isaac is very significant. |
| Here's how the official Saffir-Simpson scale defines a category 1: "very dangerous winds will produce some damage … [including] damage to roof, shingles, vinyl siding and gutters … power outages that could last a few to several days." The definition of a category 3 is much more dire: "devastating damage will occur … many trees will be snapped or uprooted, blocking numerous roads. Electricity and water will be unavailable for several days to weeks after the storm passes." | Here's how the official Saffir-Simpson scale defines a category 1: "very dangerous winds will produce some damage … [including] damage to roof, shingles, vinyl siding and gutters … power outages that could last a few to several days." The definition of a category 3 is much more dire: "devastating damage will occur … many trees will be snapped or uprooted, blocking numerous roads. Electricity and water will be unavailable for several days to weeks after the storm passes." |
| One facet of Isaac that could worsen the damage is that the storm is expected to move relatively slowly. Isaac's wind field is also wider than what we normally see, so that will also create longer periods of moderately high winds. I should also note that wind strength is one of the more difficult elements to predict of a tropical storm. | One facet of Isaac that could worsen the damage is that the storm is expected to move relatively slowly. Isaac's wind field is also wider than what we normally see, so that will also create longer periods of moderately high winds. I should also note that wind strength is one of the more difficult elements to predict of a tropical storm. |
| 3. Rainfall | 3. Rainfall |
| The slow forward movement of Isaac is also likely to produce rain in quantities comparable to Katrina. We're looking at rainfall totals generally between 6 and 12 inches, but maximum amounts are likely to exceed that considerably. | The slow forward movement of Isaac is also likely to produce rain in quantities comparable to Katrina. We're looking at rainfall totals generally between 6 and 12 inches, but maximum amounts are likely to exceed that considerably. |
| If we're talking about 15 inches or more over this wide area, then that will lead to a lot of flooding. New Orleans may escape the brunt of it, but the seaside towns of Alabama and Mississippi will not be so lucky. | If we're talking about 15 inches or more over this wide area, then that will lead to a lot of flooding. New Orleans may escape the brunt of it, but the seaside towns of Alabama and Mississippi will not be so lucky. |
| A look at Katrina finds the maximum rainfall was very similar to Isaac's, but the peak quantities were, if anything, slightly lower. Katrina's maximum rainfall was also slightly to the west. | A look at Katrina finds the maximum rainfall was very similar to Isaac's, but the peak quantities were, if anything, slightly lower. Katrina's maximum rainfall was also slightly to the west. |
| 4. Storm surge | 4. Storm surge |
| If it were only rain, Hurricane Katrina might have joined a list of memorable storms but not once-in-a-lifetime events. What wreaked havoc on New Orleans was the breach of levees due to high storm surge aided by strong winds. | If it were only rain, Hurricane Katrina might have joined a list of memorable storms but not once-in-a-lifetime events. What wreaked havoc on New Orleans was the breach of levees due to high storm surge aided by strong winds. |
| Storm surge for Isaac is expected to be a maximum of 11-13ft above normal tide level, with most of the central Gulf coast dealing with 5-10ft. That's certainly nothing to be complacent about, but it doesn't hold a candle to Katrina. | Storm surge for Isaac is expected to be a maximum of 11-13ft above normal tide level, with most of the central Gulf coast dealing with 5-10ft. That's certainly nothing to be complacent about, but it doesn't hold a candle to Katrina. |
| The maximum storm surge for Katrina was 25-28ft above normal along the Mississippi coast, with 10-20ft along the Louisiana coast. Part of the blame for New Orleans' levee failure during Katrina was because of negligent upkeep. Everything we hear out of Louisiana is that the levees are better-designed and ready for Isaac. | The maximum storm surge for Katrina was 25-28ft above normal along the Mississippi coast, with 10-20ft along the Louisiana coast. Part of the blame for New Orleans' levee failure during Katrina was because of negligent upkeep. Everything we hear out of Louisiana is that the levees are better-designed and ready for Isaac. |
| Conclusion | Conclusion |
| The obvious comparisons between Isaac and Katrina have been made by many. While the storms are bound to affect the same general area, there are critical differences. Isaac is a weaker storm that should have less wind force and storm surge than Katrina, and about equivalent rainfall totals. | The obvious comparisons between Isaac and Katrina have been made by many. While the storms are bound to affect the same general area, there are critical differences. Isaac is a weaker storm that should have less wind force and storm surge than Katrina, and about equivalent rainfall totals. |
| Isaac's expected category 1 status at landfall is not apocalyptic, but strong enough to carry a significant risk of damage, and tropical storm force winds exceeding 39mph are expected to begin by early Tuesday morning in New Orleans. We should also expect tornadoes on the outer bands of Isaac. Preparations for the storm should be made now if they haven't been already. | Isaac's expected category 1 status at landfall is not apocalyptic, but strong enough to carry a significant risk of damage, and tropical storm force winds exceeding 39mph are expected to begin by early Tuesday morning in New Orleans. We should also expect tornadoes on the outer bands of Isaac. Preparations for the storm should be made now if they haven't been already. |
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