Yesterday at this time, we weren’t expecting then Tropical Storm Michael to become a hurricane until Friday at the earliest. The storm was forecast to have maximum winds of 60 mph. Last evening the forecast was a little higher, having Michael become a 75 mph hurricane by this morning. Well, Michael had other plans and went through a process we call rapid intensification, RI for short. RI is normally defined as an increase of more than 30 mph in a 24-hour period. Our Michael increased from 50 mph at 5 a.m. yesterday to 115 mph at 5 a.m. today -- a huge 65 mph increase! Fortunately Michael is at sea and only a threat to shipping.
But what if Michael was in the Gulf of Mexico rather than the open Atlantic? Pretend it is 5 a.m. on a summer morning and we have a 50 mph tropical storm centered southeast of Galveston, moving northwest at 10 mph, and forecast to be a 60 mph Tropical Storm at landfall the next day on our coast. The community makes preparations and accounts for it maybe being 15 to 20 mph stronger, just in case. That says you are prepared for a 75 to 80 mph hurricane. Few if any communities require an evacuation for this storm. That evening as the outer rand bands come ashore the forecast now calls for 75-80 mph winds at landfall so you assume you have done the right thing. For the next 12 hours, your worst nightmare begins as our storm goes through an unforecast RI and is now coming ashore as a 115 mph hurricane, catching most people in surge prone low lying areas unprepared.
This was the scenario that would keep me awake at night when I was director at the National Hurricane Center. Our science has greatly improved our ability to forecast the track of tropical cyclones, reducing the error by two thirds since I started in this science 45 years ago. Unfortunately, the same cannot be said for forecasting rapid change in intensity.
You say, well fine, but isn’t this an incredibly unlikely occurrence? In 1932, just such a rapid intensification occurred off Galveston. The eye wall came through Freeport and West Columbia with gusts to 130 mph after being a 65 mph storm just 24 hours earlier. Want more proof? Hurricane Charley in 2004, Opal in 1995, Audrey in 1957 were also notable Gulf storms that went through unforecast RI just a day before landfall.
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