How did Eta sneak up on us? The two-week-old storm was hard to forecast.

Tropical Storm Eta was expected to pass Tampa Bay comfortably to its west. Instead, the region faced tropical-storm conditions on Wednesday. Here’s why forecasters had so much trouble projecting Eta’s path.
High winds and water levels as rain increases due to Tropical Storm Eta, along Bayshore Blvd on Wednesday, Nov. 11, 2020 in Tampa.
High winds and water levels as rain increases due to Tropical Storm Eta, along Bayshore Blvd on Wednesday, Nov. 11, 2020 in Tampa. [ MARTHA ASENCIO RHINE | Times ]
Published Nov. 11, 2020|Updated Nov. 12, 2020

Tropical Storm Eta snuck up on Tampa Bay on Wednesday morning.

Many woke to the news that they were under a tropical storm warning — just hours after falling asleep with the understanding that Eta and its 60 mph winds would remain well west of Tampa Bay. Eta’s forecast had changed again, however, putting Tampa Bay square in the storm’s path.

What the heck happened?

Eta has been anything but a typical storm since it formed as a tropical depression Oct. 31 in the southern Caribbean Sea.

In the past week, it moved painstakingly slow — not moving at all at one point over Cuba, and less than 10 mph until Tuesday afternoon. It also had an irregular center. These two factors, along with Eta’s prolonged period as just a tropical storm, made it a hard storm to forecast, said Stephen Shiveley, a meteorologist with the National Weather Service’s Ruskin office.

Shiveley said Eta’s projected path and intensity changed so much the past week because weaker and slower storms — like Eta was — are harder to forecast.

"It’s ironic, but a Category 5 hurricane is way easier to forecast than a tropical storm,” said Shiveley. “It’s just the way it is — the upper-level support just isn’t there and the models have a hard time grabbing tropical storms.”

Related: Tropical Storm Eta approaches Tampa Bay with fierce winds, heavy rain

Eta, despite a 3½-hour spell as a hurricane Wednesday, had been a tropical storm since Saturday. Before that, the storm was a Category 4 hurricane that killed at least 64 in Central America, likely many more.

Colorado State University research scientist and hurricane forecaster Philip Klotzbach said Wednesday that Eta had an incredibly unusual track. He suggested it was fitting that the storm remained hard to project even two weeks after its formation.

Eta, in theory, shouldn’t have been able to survive the mountains of Central America after making landfall there Nov. 4, but it did. The storm also found a way to strengthen overnight Tuesday, despite facing large amounts of wind shear in the Gulf of Mexico, Klotzbach said.

“Slow-moving storms are tricky to forecast,” Klotzbach said. “And it’s always hard, too, when you have a lot of shear nearby that can erode the storm very quickly like there was (Tuesday). The models had a hard time knowing if it would weaken it — but it didn’t (Tuesday) night and the storm strengthened and shifted east."

Related: Latest Updates: Florida in path of Tropical Storm Eta

According to Klotzbach, the strongest winds of Eta were not right outside of its eyewall Wednesday like most tropical systems. Instead, Eta’s strongest winds were east of its center, which is why Tampa and St. Petersburg faced tropical-storm level winds despite Eta’s center still being in the Gulf of Mexico. It was another irregularity that made Eta hard to predict.

Other meteorologists made the same observation.

“On satellite, Eta did not look like a classic hurricane, nor did aircraft reconnaissance encounter strong winds in an eyewall wrapped about Eta’s center," wrote Matthew Cappucci, a meteorologist for the Washington Post. “In fact, Eta’s satellite presentation appeared degraded around sunrise Wednesday (hours before strengthening into a hurricane)."

As if its shape and speed weren’t enough — the time of year Eta formed was yet another irregularity. The storm will be the latest calendar year named storm to make landfall in Florida since Tropical Storm Gordon in 1994 if it makes landfall Thursday, as expected.