Wednesday, August 29, 2012

Weather Whys Wednesday: Isaac & The Saffir-Simpson Scale


With Isaac making landfall yesterday and again earlier today in Louisiana, it seems like a good time to talk about what I call the intensity conundrum with hurricanes.

Most of us are familiar with the Saffir Simpson Scale, which measures a hurricane's intensity. The scale was developed to measure the maximum sustained winds of a storm. Herbert Saffir was an engineer and Bob Simpson (turning 100 years old this year) was director of the National Hurricane Center (NHC) when the scale was developed. Saffir did work for the UN to develop a scale to measure the effects of a hurricane on low cost housing in areas threatened by hurricanes. It was a scale of 1-5 based on wind speed, and when Saffir gave the numbers to the NHC, Simpson added storm surge effects. The scale was overhauled in 2009 to eliminate the impacts of storm surge and pressure. This made sense because storms like Katrina, Ike, Charley, and probably now Isaac did not produce storm surge that would have been expected with their wind speed intensities.

An "all in one" hurricane rating scale is extremely difficult. Let's look right at Isaac. If you plotted maximum sustained winds vs. minimum central pressure of tropical systems, you generally get a nice linear relationship. Isaac, while certainly not the only storm to ever do this, was very much an outlier. The "typical" range for a Category One hurricane's minimum central pressure would fall somewhere above 980 mb. Isaac was at least a 967 mb hurricane, rated as a Category One at the time. Its barometric pressure was more along the lines of what is typical for a strong Cat 2 or even a weak Cat 3 storm.

So for Isaac...
Winds were clearly Cat 1 sustained with Cat 2 gusts.
Pressure was more like a strong Cat 2.

Isaac's Track vs. Katrina's. Credit: NOAA
So what about storm surge? Well, we've already heard about (especially in Plaquemines Parish, LA) about flooding worse than Katrina in 2005 in a handful of spots. Much of this is because of the angle of approach of Isaac. When Katrina hit in 2005, it made landfall only about 20-30 miles or so northwest of where Isaac brushed the Mississippi Delta yesterday. But, it was travelling due north and accelerating. Isaac made landfall while travelling west, wobbling, and slowing down. Therefore, the angle of the surge was more more prone to funneling water up the Mississippi River in that area, while the strong winds created waves on the river to help overtop the levies in those areas. In New Orleans, the shear force of the surge/wind/waves was enough to overpower the levee system in 2005. Isaac is testing it, and while it is very powerful, it lacks the extra muster Katrina had. 

Storm surge can often be best predicted by taking a combination of the strength of the winds, combined with the radius of those winds. While Isaac may have lacked hurricane force winds until yesterday, the radius of its tropical storm force winds was at times 200-250 miles from the center of circulation. That's a huge chunk of real estate. When Ike struck Texas in 2008, it had tropical storm force winds extend out over 250 miles from the center and hurricane force winds out over 100 miles from it. It was a large storm and it made landfall with a massive storm surge and low pressure relative to its wind speed. Isaac's storm surge certainly varied, but was probably more like a solid Cat 2 vs. a weak Cat 1 on the scale.

So...
Sustained Winds were Cat 1.
Gusts were Cat 2.
Pressure was strong Cat 2.
Surge was similar to a Cat 2.

Isaac before strengthening on Tuesday.
Credit:  NASA GOES Project 
All tropical systems also have heavy rainfall, and this will dump 12-20" of rain over a pretty large area, which will exacerbate problems.

Additionally, Isaac was strengthening at landfall...not weakening as many stronger Northern Gulf storms do. A storm on the upswing often seems to carry more punch than one on the downswing. Of course, in instances like Katrina, Rita, and Ike...all weakening on their final approaches, they were able to build up enough strength in surge over the water for it not to matter. So while monster storms are monster storms....weak storms, strengthening at landfall, tend to be extremely formidable...moreso than many people expect. And I guarantee you'll be hearing a lot of "we were really surprised how strong this was" from people that rode it out once Isaac departs.

So the bottom line is: As of yesterday Isaac was being considered by a lot of folks as formidable, but...not THAT bad. In reality, Isaac had more properties of it that might be associated with a solid category 2 hurricane than that of a strong tropical storm to weak category one storm.

To editorialize for a moment: While the Saffir-Simpson scale is a nice gauge, people focus way too often and way too much on that single number. Whether this is the fault of meteorologists, the media, or just the brain in general (needing to rationalize something by a simple number), is up for debate. In cases like Ike, Isaac, or even Irene and Katrina, the intensity of the sustained winds matters much less than the other impacts they bring. People tend to forget than category one hurricanes are still hurricanes. They pack a mean punch. We've probably been numbed by what we saw in the mid 2000s with Isabel in 2003, the Florida Four in 2004, Katrina, Rita, Wilma in 2005, Ike in 2008, among others. Just because it doesn't reach an arbitrary number, doesn't mean it's not big or dangerous. I think the bottom line is that we have a problem (similar to the warning issue we have with sirens and tornadoes in the Midwest, Southeast, and Plains). Arbitrary numbers don't work all the time, and I think the next test of the meteorological community will be to determine what might be a better way to express the true intensity of tropical systems to the general public.