Dangerous rapidly intensifying landfalling hurricanes like Michael and Harvey may become more common in a warmer world

Desdemona Despair

Dr. Jeff Masters
12 October 2018

(Weather Underground) – As Hurricane Michael sped northward on 9 October 2018 toward a catastrophic landfall on Florida’s Panhandle, the mighty hurricane put on an phenomenal display of rapid intensification. Michael’s winds increased by 45 mph in the final 24 hours before landfall, taking it from a Category 2 hurricane with 110 mph winds to an extremely dangerous high-end Category 4 storm with 155 mph winds. It was a disturbing déjà vu of what had happened just one year earlier. On August 25, 2017, Hurricane Harvey rapidly intensified by 40 mph in the 24 hours before landfall, from a Category 1 storm with 90 mph winds to a Category 4 storm with 130 mph winds.

Extreme rapid intensification rates expected to become more common

In a 2016 paper, “Will Global Warming Make Hurricane Forecasting More Difficult?” (available here from the Bulletin of the American Meteorological Society), MIT hurricane scientist Kerry Emanuel explained that not only will global warming make the strongest hurricanes stronger, it will also increase how fast they intensify. Troublingly, intensification rates don’t increase linearly as the intensity of a storm increases–they increase by the square power of the intensity. Thus, we can expect future hurricanes to intensify at unprecedented rates, and the ones that happen to perform their rapid intensification just before landfall will be extremely dangerous.

Dr. Emanuel used a computer model that generated a set of 22,000 landfalling U.S. hurricanes during the recent climate period of 1979 – 2005, then compared their intensification rates to a similar set of hurricanes generated in the climate expected at the end of the 21^st century. For the future climate, he assumed a business-as-usual approach to climate change—the path we are currently on. The analysis found that the odds of a hurricane intensifying by 70 mph or greater in the 24 hours just before landfall were about once every 100 years in the climate of the late 20th century. But in the climate of the year 2100, these odds increased to once every 5 – 10 years. What’s more, 24-hour pre-landfall intensifications of 115 mph or more—which were essentially nonexistent in the late 20th Century climate—occurred as often as once every 100 years by the year 2100. The major metropolitan areas most at risk for extreme intensification rates just before landfall included Houston, New Orleans, Tampa/St. Petersburg, and Miami.

In an email, Dr. Emanuel said: “My own work shows that rates of intensification increase more rapidly than intensity itself as the climate warms, so that rapidly intensifying storms like Michael may be expected to become more common.” Moreover, he added, “My work on Hurricane Harvey and a few other heavy rain-producing hurricanes such as Florence, together with much previous work by others on the subject, strongly suggests that hurricane-related flooding will increase as the climate continues to warm.”A dangerous scenario: a rapidly intensifying hurricane making landfall

Hurricanes like Michael and Harvey that rapidly intensify just before landfall are among the most dangerous storms there are, since they can catch forecasters and populations off guard, risking inadequate evacuation efforts and large casualties. Lack of warning and rapid intensification just before landfall were key reasons for the high death toll of the most intense hurricane on record to hit the U.S–the 1935 Labor Day hurricane in the Florida Keys. That storm intensified by 80 mph in the 24 hours before landfall, topping out as a Category 5 hurricane with 185 mph winds and an 892 mb pressure at landfall. At least 408 people were killed, making it the eighth deadliest hurricane in U.S. history. Another rapidly intensifying hurricane at landfall, Hurricane Audrey of June 1957, was the seventh deadliest U.S. hurricane, killing at least 416. Audrey’s winds increased by 35 mph in the 24 hours before landfall near the Texas/Louisiana border, when it was a Category 3 hurricane with 125 mph winds. Lack of warning and an unexpectedly intense landfall were cited as key reasons for the high death toll.

Of course, nowadays we have satellites and radar and regular hurricane hunter flights and advanced computer forecast models, so the danger of another Audrey or 1935 Labor Day hurricane taking us by surprise is lower. Or is it? All of that fancy technology didn’t help much for 2007’s Hurricane Humberto, which hit Texas as a Category 1 storm with 90 mph winds. Humberto had the most rapid increase in intensity in the 24 hours before landfall of any Atlantic hurricane since 1950: 65 mph. A mere 18 hours before landfall, the National Hurricane Center (NHC) predicted a landfall intensity of just 45 mph, increasing that estimate to 65 mph in a forecast issued 6 hours later. It’s fortunate that Humberto was not a stronger system, or the lack of warning could have led to serious loss of life.

Historical records show that since 1950, the greatest 24-hour intensification rates prior to a U.S. landfall were:

• Humberto, 2007 (65 mph increase)
• King 1950 (60 mph increase)
• Eloise 1975 (60 mph increase)
• Danny 1997 (50 mph increase)
• Michael 2018 (45 mph increase)
• Harvey 2017 (40 mph increase)
• Cindy 2005 (40 mph increase) [more]

Dangerous Rapidly Intensifying Landfalling Hurricanes Like Michael and Harvey May Grow More Common

ABSTRACT: Hurricane track forecasts have improved steadily over the past few decades, yet forecasting hurricane intensity remains challenging. Of special concern are the rare instances of tropical cyclones that intensify rapidly just before landfall, catching forecasters and populations off guard, thereby risking large casualties. Here, we review two historical examples of such events and use scaling arguments and models to show that rapid intensification just before landfall is likely to become increasingly frequent and severe as the globe warms.

Will Global Warming Make Hurricane Forecasting More Difficult?