Tropical Storm Idalia could rapidly intensify. How and why?

Tropical Storm Idalia is forecast to enter a record-hot Gulf of Mexico, prompting concerns the system may rapidly strengthen before striking Florida’s west coast later this week.

“There’s a notable risk of rapid intensification while the system moves across the record warm eastern and northeastern Gulf of Mexico,” the National Hurricane Center cautioned Sunday about Tropical Storm Idalia, pronounced “ee-DAL-ya,” which was expected to come ashore between Tampa Bay and the Florida Panhandle as at least a Category 1 hurricane.

Rapid intensification, when a storm’s top wind speeds rise by at least 35 mph in a single day, can be difficult to predict — so when a storm suddenly strengthens near the shore, coastal communities have little time to prepare or evacuate.

There are three main factors that lead to rapid intensification: warm waters, stable atmospheric conditions and high humidity in the middle layers of the atmosphere. All three are exacerbated by climate change.

‘Hurricanes feed off heat’

Earlier this month, federal ocean scientists increased their hurricane season prediction to include more storms. One of the main reasons: Record-breaking Atlantic ocean temperatures are counterbalancing the ongoing El Niño, which typically weakens hurricanes.

Hurricanes gather their strength from the heat stored in ocean waters. The warmer the surface temperature of the sea, the more fuel storms have to feed on.

There’s a general understanding in hurricane science that wind speeds are expected to increase about 5% for every 1 degree Celsius increase in tropical ocean temperature, Jeff Masters, a hurricane scientist formerly with the National Oceanic and Atmospheric Administration, previously told the Tampa Bay Times. Each degree increase equates to about 50% more storm damage from heightened storm surge, higher wind and rain destruction, Masters said.

“When waters are hot enough to feel like a hot tub,” he said, “it’s a concern for hurricane season — because hurricanes feed off of that heat.”

Oceans have absorbed about 90% of the warming that has occurred in recent decades due to increasing greenhouse gases, and the top few meters of the ocean store as much heat as Earth’s entire atmosphere, according to NASA.

During July, average water surface temperatures in the Gulf of Mexico ran nearly 2 degrees Fahrenheit above normal. That’s the highest on record, according to an analysis provided by Matthew Rosencrans, the lead hurricane season outlook forecaster at the National Oceanic and Atmospheric Administration.

Some parts of the gulf are running hotter than that. Offshore of Tampa Bay and the Louisiana coast, for instance, sea surface temperatures are more than 5 degrees hotter than normal.

Early studies suggest climate change has already made rapid intensification more common. A 2021 IPCC report found that the global frequency of tropical cyclone rapid intensification events has likely increased over the past four decades” and added that researchers have “medium confidence” that “none of these changes can be explained by natural variability alone.”

The possible track of Tropical Storm Idalia. [ National Hurricane Center ]

‘Notoriously hard to predict’

Before Hurricane Ian slammed into Florida’s southwest coast with 155 mph winds, it went through two separate bursts of so-called rapid intensification.

This process took Ian from a tropical storm with 60 mph winds to a Category 3 hurricane with 125 mph winds, National Hurricane Center bulletins show. After ravaging western Cuba, Ian rapidly intensified again over a particularly warm pocket of the Gulf of Mexico.

Hurricane Ian’s intensity forecasts were worse than the average of the last five years, according to the hurricane center. Researchers said this was largely because Ian’s rapid strengthening made intensity forecasting more difficult.

Earlier this summer, the National Oceanic and Atmospheric Administration unveiled a new system they hope will help them better anticipate rapid intensification, which is notoriously hard to predict.

The newly launched Hurricane Analysis and Forecast System, which became operational in late June, has been shown to improve forecasting by 10 to 15%.

A sign of the new model’s capabilities: It was the first to anticipate Ian would rapidly intensify for a second time as it moved off the coast of Cuba toward Florida, the National Oceanic and Atmospheric Administration said.