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

As Hurricane Michael sped northwards on October 9 towards a catastrophic landfall on Florida’s Panhandle, the mighty hurricane put on a 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.

Figure 1. Hurricane Harvey hit Texas as a Category 4 hurricane with 130 mph winds on August 25, 2017, then stalled over Houston for three days, dumping over 50” of rain. Harvey caused $125 billion in damage, and killed 89 people. Above: People walk down a flooded street as they evacuate their homes after the area was inundated with flooding from Hurricane Harvey on August 28, 2017 in Houston, Texas. Image credit: Joe Raedle/Getty Images.

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)

Poor intensity forecasts make us vulnerable

While track forecasts of hurricanes have improved by more than a factor of two over the past 20 years, intensity forecasts have shown little improvement. Dr. Emanuel gives four reasons for this:

1) Very high resolution computer models are needed (1 km resolution or better), which are beyond the capability of modern computers to run economically.

2) We have poor understanding of and models of the processes in the lowest few hundred meters of the atmosphere (the boundary layer).

3) We have difficulty modeling how the top few hundred meters of the ocean responds to a storm.

4) The process of collecting observations that show a dramatic variation over short distances and correctly initializing a hurricane model with these observations is difficult.

Figure 2. Hurricane Patricia as seen by the MODIS instrument on NASA's Terra spacecraft at 1:30 pm EDT (17:30 UTC) October 23, 2015. At the time, Patricia had 205 mph sustained surface winds and a central pressure of 878 mb. Patricia had peaked at 215 mph sustained winds and a central pressure of 872 mb six hours previously. Image credit: NASA.

The worrying case of Category 5 Hurricane Patricia’s rapid intensification in 2015

Dr. Emanuel gave another troubling example of a rapid intensification evert that was poorly forecasted: Hurricane Patricia of October 2015, which hit a relatively unpopulated portion of the Pacific coast of Mexico as a Category 4 storm with 150 mph winds after topping out as the strongest tropical cyclone ever measured, with 215 mph sustained winds. During a 24-hour period from 6Z October 22 to 6Z October 23, Patricia intensified by an astonishing 120 mph—from an 85 mph Category 1 storm to a 205 mph Category 5 storm (this was very close to the maximum change in intensity that theory says can happen: 125 mph in 24 hours). During this same period, the National Hurricane Center predicted an intensification by only 35 mph. Dr. Emanuel noted, “Had the storm made landfall at the end of this period of rapid intensification, the result could have been catastrophic given the poor anticipation of the magnitude of the event.” As I wrote in my 2016 blog post, Hurricane Patricia's 215 mph Winds: A Warning Shot Across Our Bow, we need to be prepared for global warming to bring us more hurricanes like Patricia, which theory and computer modeling predict will happen more often in a warming climate.

Bottom line: time to spend more money on hurricane research

With increasing coastal populations, limited skill in intensity forecasting, and steadily increasing sea levels, this potential increase in rapidly intensifying hurricanes results in the “risk of an increased frequency of poorly anticipated high-intensity landfalls leading to higher rates of injury and death,” wrote Dr. Emanuel. He recommended that “greater emphasis be placed on improving hurricane intensity prediction and on preparing populations to respond to high intensity landfalling hurricanes at short notice.”

In other words, we need to fund hurricane research at a much higher level than we do now. As I discussed back in 2006, the National Science Board, in a report issued in the wake of Hurricane Katrina, called for the establishment of a National Hurricane Research Initiative (NHRI), which would "provide urgently needed hurricane science and engineering research and education". They recommended an increase of $300 million per year in hurricane research funding—a whopping boost from the $20 - $50 million per year we’ve been funding hurricane research at over the past 15 years. The report concluded:

Can we as a Nation continue to remain vulnerable to hurricanes that are an inevitable part of our future, that have demonstrated the capacity to inflict catastrophic damage to our economy, and that kill hundreds of our citizens? The hurricane warning for our Nation has been issued and we must act vigorously and without delay.

Hear, hear!