You’ve been hearing the phrase “Rapid Intensification” (RI) in connection with hurricanes a lot lately. What does it mean? Here’s some notes and a discussion with respect to Hurricane Helene.
There is no globally accepted definition of RI. The US National Hurricane Center defines it as an increase of the one minute sustained winds 30 knots (35mph) in a 24 hour period. The thing is, using that definition, around 1/3 of hurricanes undergo RI at some point, so it’s not really uncommon. So just because you see that phrase used it doesn’t mean the storm is unusually dangerous. You can read a bit more about RI on the Wikipedia page, which seems pretty good.
Although we sort of know some of the factors that causes RI, how it all works isn’t well understood, and forecasting it doesn’t have a great track record due to the complexity of hurricanes and the difficulty of forecasting their environment. Deep warm water is one requirement; that’s easy because we can see it from satellites and it doesn’t really change that fast. Wind shear and divergence at the top of the strengthening storm are also factors – and harder to forecast.
What is a concern is if a storm rapidly intensifies and it’s unexpected, especially if it happens before landfall. That happened with Hurricane John a few days ago, it was forecast to only be a tropical storm when the warnings went out, but it rapidly intensified and was a Cat 3 at landfall. That’s a very nasty surprise for anyone in the hazard zone. That’s not the case with Hurricane Helene – NHC has been saying the ingredients for the storm to intensify over the Gulf since before it even formed.
Helene should have low shear, at least for now, and will be over very warm water. So intensification, including RI, won’t be a surprise. The dedicated hurricane models like the new HFS are pretty enthusiastic and intensify the storm to at Category Five before landfall. However, there are other factors like the approaching frontal system that may cause enough shear to stop those trends. This is a big unknown right now which will prevail, but don’t be surprised if Helene is a Cat 4 – or a Cat 2 – at landfall. Even with all the satellites, computer models, and other tools, intensity forecasts are the most difficult part of hurricane forecasting. That’s why NHC often shades things on the high side.
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