CLEARWATER — The gradual slope of the continental shelf in the Gulf of Mexico. Raging winds pushing water onto shore. The low elevation of waterfront — and many upland — communities in a county surrounded by water on three sides.
Add them all together and emergency management officials have a good idea of the outcome: massive flooding caused by storm surge that could inundate about 42 percent of Pinellas County homes in a Category 3 hurricane.
Roughly 60 percent of the county's homes likely would experience surge flooding in a Category 5.
Think of people cutting and kicking their way out of attics and onto rooftops during Hurricane Katrina in August 2005. More than 1,800 died. Several hundred have never been found.
Or think about Hurricane Ike, which rolled ashore near Galveston, Texas, in September 2008. A surge estimated at 20 feet breached sea walls and knocked down buildings. At least 20 people died in Texas, Louisiana and Arkansas, according to the National Hurricane Center. Most drowned. More than a dozen people have never been found.
But even those frightening scenarios may not be enough to convince Tampa Bay residents that it is not wind, but storm surge — a wall of water pushed onshore by a hurricane — that they should fear most.
"Wind is not the killer that storm surge is," Pinellas County emergency management spokesman Tom Iovino said.
The county's five evacuation zones are determined by height of predicted storm surge, not wind speeds. Evacuations are ordered to get people out of the way of those walls of water — to save lives. Yet Pinellas officials estimate that just 50 percent of residents will heed evacuation orders as a major storm approaches.
"That means the potential for a lot of casualties," said Sally Bishop, Pinellas' emergency management director.
The vast majority of Pinellas residents weren't alive when the last big one hit. The October 1921 Category 3 hurricane formed in the Gulf of Mexico and made landfall near Tarpon Springs, sending a storm surge about 10 feet high onto land. The storm washed away part of what is now Gulf Boulevard in Indian Rocks Beach, flooded the St. Petersburg downtown waterfront and sent water spilling over the sea wall along Bayshore Boulevard in Tampa.
Last month, a hurricane expert with the Weather Channel listed the Tampa Bay area as one of the five areas most overdue for a major storm.
It's something local officials are well aware of, and it's exactly why they want residents to become more educated about the dangers of surge. While boarding up your home might help it escape damage in a severe wind event, that won't protect you from the danger of drowning in rushing water.
Residents of properties that front the gulf or the bay are at obvious risk, but officials also are worried about low-lying areas in Pinellas Park, eastern Largo and north St. Petersburg. Major thoroughfares, some used for evacuation routes, crisscross those low areas and could flood out.
Many residents don't even know the elevation of their homes. Pinellas officials hope to create a searchable database in the near future that will allow residents to put in their address and learn the elevation of their property. In the meantime, Iovino said, homeowners can find that information on their property surveys. If a survey isn't available, he suggests checking Google Earth, a free downloadable program that reports elevation data.
But when a storm surge warning is issued, Iovino said residents shouldn't rely too heavily on their own calculations of whether their property has enough elevation to stay dry. Instead, he said, they should use evacuation orders as their guide, because emergency management officials will have more information about the variables that can change surge projections.
"Let's say we're calling for a 20-foot storm surge. That could climb as high as 24 feet," Iovino explained. "I wouldn't want you to bank on 'It's 20 feet and I'm at a 21-foot elevation.' "
Even if a home in that situation had only a few feet of flooding, residents might be surrounded by much deeper water, impeding escape and making it difficult for rescue teams to assist.
In a worst-case scenario — a large Category 5 storm moving slowly from south to north and making landfall at the upper reaches of Pinellas County — the storm surge in Pinellas could be roughly 30 feet, according to data from the National Hurricane Center. At that height, a one-story home at an elevation of 20 feet would have water to its rooftop.
Officials with the National Hurricane Center said they are working on a plan to put out more specific, targeted storm surge warnings, hoping to help residents better understand how a major hurricane would affect them.
And Pinellas is considering installing "surge markers" around the county to demonstrate visually how high the water would rise in different areas. Hillsborough County officials erected similar markers at 30 sites throughout that county last spring.
Surge markers are not widely used, but may be effective visual cues to alert people to the potential dangers of flooding, said Jamie Rhome, a National Hurricane Center storm surge specialist.
"I think anything that raises attention or awareness of storm surge can only help," he said. "It is one of the most misunderstood phenomenons out there, yet so much of our coastal population is at risk."
The variables that affect surge are vast and complicated, including changes in storm intensity, speed, size, angle of approach and coastal features like bays and estuaries, Rhome said.
"It's way too much to expect people to fully understand. … That's my job," Rhome said.
The best way to understand the risk facing you, Rhome said, is to know your evacuation zone.
And the best way to protect yourself and your family? Get out when officials tell you to, he said.
Rita Farlow can be reached at firstname.lastname@example.org or (727) 445-4157.