Why Tropical Cyclone Marcia is forecast to make landfall in Queensland, Australia as a category 5 storm after originally being tagged by Australia’s Bureau of Meteorology as a Category 2 just 24 hours earlier?
The culprit is “rapid intensification,” a meteorological term that remains a moving target for catastrophe models, forecasters and insurers and reinsurers.
“These are rare events and they are very hard to signal,” says John Kaplan, a researcher with the NOAA’s Atlantic Oceanographic and Meteorological Laboratory Hurricane Research Division. “What makes it so difficult to forecast rapid intensification is that there are so many factors that go into it, some of which we understand and others that we don’t.”
Broadly speaking, there are different definitions of rapid intensification of storms in different regions.
In the U.S., rapid intensification is defined when there is an increase in the maximum sustained winds of a cyclone of at least 35 knots in a 24-hour period, Kaplan says. Research has show that 60 percent of all hurricanes will rapidly intensify at least one point in 7 day lifecycle.
Kaplan explains there are broader environmental factors that forecasters can use to predict rapid intensification, such as how organized the system is, whether there is deep warm water below the storm to act as fuel and how strong the vertical windshear is between 40,000 and 50,000 feet .
But more nuanced indicators, such as the number of thunderstorms located closer to the eye wall of a cyclone are more difficult to determine. “You need all these factors to work together in a very short period of time,” Kaplan says.
For catastrophe modelers, rapid intensification remains a hurdle for accurate loss estimates.
“There are various examples of storms that have been expected to rapidly intensify but never did, and storms that intensified but were forecast to remain weak,” says Neena Saith, director of Model Solutions at RMS in London. “The ultimate impact rapid intensification, however, is that forecast losses can be more serious than anticipated.”
Saith explains that forecasting models find anticipating rapid intensification a “very difficult proposal.” .
“If a storm tracks over one of the warm eddies of the Gulf of Mexico, that could trigger rapid intensification,” Saith says. “But the storm moves just 50 to 100 kilometers of that track it will never increase in strength.”