With Hurricane Milton reaching category 5 so quickly, do we need a 'category 6' for storms?

A report in The Independent notes that Hurricane Milton’s rapid intensification to a Category 5  is: “just the latest in a series of storms so extreme that the current hurricane scale may no longer fully capture their severity.” 

The potential “once in a lifetime” storm rapidly intensified from a Category 1 to a Category 5 hurricane in a matter of a few hours, said The Independent. In the article, AccuWeather lead hurricane expert Alex DaSilva says, “the ocean heat content is at the highest level on record for this time of year in the Gulf of Mexico,” providing Milton with the “rocket fuel” to intensify. 

The storm comes a few days after Helene where work is still ongoing to clean up the debris and wreckage caused by that storm 

Rapid intensification of storms: “is prompting some scientists to question whether another classification – Category 6 – should be introduced for the most intense hurricanes that are becoming more frequent.” 

For more than 50 years, the National Hurricane Centre has used the Saffir-Simpson Windscale to communicate the risk of tropical cyclones. 

As The Independent reports: “The last, Category 5, is open-ended. Any storm with wind speeds above 157mph falls in this category. However, several storms in recent years have been reaching increasingly higher upper limits. In a study published earlier this year, researchers said that increasingly intense hurricanes, fuelled by warming ocean waters, could warrant a Category 6, reserved for storms with sustained winds over 192 mph. Climate experts Michael Wehner and Jim Kossin have argued that hurricanes have grown more powerful in recent decades, fuelled by warmer ocean waters.” 

The Independent article concludes: “Though the National Hurricane Centre hasn’t proposed adding a Category 6, researchers believe it is important to reassess how hurricanes are categorised. Millions of people depend on the scale’s categorisation to assess the risk from storms. But experts also warn that focusing solely on wind speeds may overshadow the broader dangers of storms, such as storm surges and inland flooding, which can cause severe damage even after a storm is downgraded.” 

To this end, when we talk about a category 5 storm reducing to a category 3 due to ‘weakening’ this is based upon the perceived maximum potential of the storm - as per current forecasting methods. Notably, at the onset of the rapid intensification period when the storm was less than a hurricane - in strength - the upper band for maximum winds suggested Cat 5 156mph, and most forecasts consolidated around 140mph (still a Category 4 storm). Within hours, the storm jumped from hurricane strength to this value and quickly advanced to 165mph prompting forecasters to re-evaluate this potential… but with what data?  

The only thing possible at this time was an extrapolation of the maximum forecasts which ultimately presented a guessing game.  In fact, with such an abundance of warm water at depth, Milton, which quickly advanced to 175mph suddenly presented another challenge to forecasters, adding another 10 mph to a seemingly impossible event.  

The only tangible factor to inhibit further strengthening (note the storm did peak at 180mph) was either a proximity to land reducing the availability of warm water to ingest or its succumbing to its own fragility as a moving, spinning mass of moisture and inertia. To this extent, the storm temporality weakened (in maximum wind speed) but this was a trade-off between size of the hurricane wind field. This phenomenon (known as an ‘Eye-wall replacement cycle) is only partially understood with respect to its precursors and timing, but often occurs after a period of rapid intensification inside a hurricane.  

The outcome is perhaps a 10-20% reduction in maximum windspeed (at the centre), but the centre is now 60% larger (possibly more), and so the impact zone of this storm has increased.  

In this example, with a long fetch (ocean track) before land, this storm has ample opportunity to reconsolidate, regain a solid eye-wall structure and present the same maximum winds but this time across an additional 20-30 miles.   

At face value, the weakening is visible, but as for the impact at landfall, the devil is in the detail, the difference of just a few miles in landfall location could make the 10’s of Billions of dollars difference in economic loss. This, in an area already fragile from Helene’s impacts 10 days ago.  

Russell will be watching this one very closely.