When people in Tampa Bay went to bed Sunday night, forecasters were calling for a wet Monday.
What they weren't calling for was Tropical Storm Emily.
But by the time alarm clocks were blaring, the storm had formed in the Gulf of Mexico just 60 miles west of the Tampa Bay area. And as morning commuters slogged their way along wet and flooded roads, forecasters announced the depression had grown into a storm with a name.
Though the forces at work in Emily's rapid development are familiar, experts said they came together in a way that fooled the most highly regarded computer forecast model and highlighted how unpredictable Mother Nature can be.
"What made this a potentially dangerous situation was the combination of three factors," said state climatologist David Zierden, who pointed to the storm's rapid development, proximity to a populated coastal area and growth overnight, while most people were asleep.
"That really raised the stakes," he said.
Emily began as a so-called cold-core frontal low, a type of weather front familiar this time of year, said Stacy Stewart, senior hurricane specialist at the National Hurricane Center in Miami.
These kinds of fronts typically span many hundreds of miles and have a large circulation and wind radius. That means it takes days for the heat generated by thunderstorms to warm up the front's inner core, which causes the pressure to drop and wind speeds to increase. When this happens, the front can transition to a so-called warm-core tropical low, which can develop into much stronger storms — including cyclones.
The frontal low that spawned Emily was unusually small, about 150 miles in diameter, meaning it required relatively few thunderstorms to warm up its core, Stewart said.
"We get one or two storms a year that form like this, but not this quickly," Stewart said. "It took less than 24 hours to go from a low pressure system with no thunderstorm activity associated with it to a warm-core tropical low that did have thunderstorms."
Forecasters usually need to see about 12 hours of persistent thunderstorm activity to declare a tropical cyclone, Stewart said. By Sunday afternoon, forecasters were still not seeing that. One of the world's premier computer models from the European Center for Medium-Range Weather Forecasting — "the one that most people go gaga over," as Stewart put it — gave the system no chance of development in 48 hours.
Stewart still opted to send out a forecast giving the system a 20 percent chance of development over 48 hours and 30 percent over five days.
Less than 12 hours later, by about 2 a.m. Monday, the persistent thunderstorm activity made it clear to forecasters they had a tropical cyclone on their hands. By 5 a.m., a couple of buoys in the gulf were showing wind speeds of 24 to 38 miles per hour, the range for a tropical depression, said Stephen Shiveley, a forecaster for the National Weather Service in Ruskin.
By then, the storm had moved into the range of the weather service's Doppler radar in Ruskin — a big advantage, Shiveley said, because forecasters could get a more accurate view of the storm's structure.
About an hour later, when the center of the storm was about 60 miles southwest of Tampa, the hurricane center issued its first special advisory announcing that Tropical Depression Six had formed.
Then, at 8 a.m., the center announced that the depression had become Emily. The advisory noted that the system was producing average surface wind speeds of about 40 knots, or 46 mph. Tropical storm force winds range from 39 to 73 mph.
It is unusual for such fast-growing storms to develop in the gulf, where conditions in the upper levels of the atmosphere tend to be less favorable than those in the open Atlantic Ocean, Stewart said. In Emily's case, the upper-level winds were weak enough to allow the storm to develop but strong enough to prevent it from becoming more powerful.
"(This) should highlight to people the importance of having their hurricane kits and plans ready," Stewart said. Storms, he noted, don't always take days to form and strengthen.
Zierden, the state climatologist, agreed. Though the climatic forces at work might not be the same, Tropical Storm Emily raised the specter of past storms that formed quickly and caught people off guard.
Hurricane Gaston in 2004 was forecast to remain a tropical storm but quickly developed into a Category 1 hurricane just before it made landfall near Charleston, S.C., where Zierden's sister lives.
"She went to bed thinking it would be a sloppy tropical storm," he said, "and woke up with hurricane conditions outside her front door."
Contact Tony Marrero at email@example.com or (813) 226-3374. Follow @tmarrerotimes.