Safety First Hurricane Planning, Inc. Hurricane Hazards

H u r r i c a n e H a z a r d s

A hurricane is a severe type of storm, or cyclone, that typically forms in the tropics. The cyclone has deep low pressure at its center, bands of thunderstorms, and a counterclockwise circulation of intense winds near the earth's surface (rather than aloft) blowing at 74 mph or more. Tropical storms are less severe (but just as menacing) tropical cyclones with winds from 39 mph to 73 mph. While winds are used to classify hurricanes and tropical storms, they are only one of its many life-threatening hazards.

HIGH WINDS

Saffir-Simpson Hurricane Damage Potential Scale
This commonly used scale rates hurricanes from 1 to 5 based on current intensity. Developed by structural engineer Herbert Saffir and former National Hurricane Center director Robert Simpson, it is used to estimate potential damage and coastal flooding from a landfalling hurricane. Storm surge values are for Pinellas County and Hillsborough County, Florida.
Category 1	Winds 74-95 mph, storm surge 5-7 feet.
Category 2	Winds 96-110 mph, storm surge 8-12 feet.
Category 3	Winds 110-130 mph, storm surge 13-17 feet.
Category 4	Winds 131-155 mph, storm surge 18-24 feet.
Category 5	Winds faster than 155 mph, storm surge higher than 24 feet.

By far a hurricane’s most wide reaching effect at and near landfall is damaging wind. Air spirals faster and faster into a hurricane to try to neutralize the difference between very low pressure in its calm center, or eye, and higher pressure outside the storm. Hurricane winds blow fastest in the ring of thunderstorms surrounding the eye, called the eyewall. Sustained wind speeds mentioned in advisories issued by the National Hurricane Center are the highest 1-minute averages, termed “sustained” winds, estimated to be in the hurricane’s eyewall over open water. Gusts are estimates of three-second winds, and can be as much as 50% higher than the sustained winds. Sustained winds in Hurricane Charley’s eyewall were estimated to be 150 mph at their peak, and gust speeds were thought to be 170-180 mph. In stark contrast, the steady winds and gusts in Frances and Jeanne across the Tampa Bay area were 100 mph slower than what Punta Gorda and Port Charlotte experienced in Charley.

STORM SURGE

Winds blowing inward on all sides of a hurricane pile water up around its middle in a dome shape, which then moves along with the storm. As a hurricane reaches the coastline, this dome of water surges against the shore near and to the right of the hurricane’s calm eye. Storm surge, which can range from a few feet to more than 20 feet depending on hurricane intensity, arrives on top of already high storm tides, and its height doesn’t include the battering waves on top. A medium-size hurricane with steady winds of about 120 mph – the speed Charley’s winds were forecast to be when it was expected to strike the Tampa Bay area in 2004 – would generate a storm surge 13-17 feet high on Florida’s west-central coast, inundating half of Pinellas and coastal Hillsborough counties. Storm surge has the highest potential for loss of life in a hurricane, a potential that was realized in Hurricane Katrina after more than a 35-year campaign to reduce storm surge deaths proved essentially successful. Katrina showed that catastrophic flooding can occur even in a storm that is weakening as it makes landfall. This was partly due to Katrina's mammoth size. In 2008, Hurricane Ike made it clear that hurricane size does matter when its massive circulation, covering nearly the entire Gulf of Mexico, drove a low end Category 4 storm surge of 15-20 feet across Galveston Island and into the low-lying Houston area, decimating the barrier island north of Galveston known as the Bolivar Peninsula. Ike's winds of 110 mph only made it a strong Category 2 hurricane at landfall.

HEAVY RAIN & FLOODING

Hurricanes and tropical storms carry huge amounts of heat and humidity around that feed tropical downpours. Bands of torrential rain spiraling into an average-size hurricane moving ashore at 12 mph can easily drop 6-10 inches of rain in less than half a day. That’s often enough to overflow creeks, streams and rivers, creating flooding. Add to that rising water along the coast, and rivers will back up, worsening the flooding. River flooding occurs in addition to the widespread flooding of streets and low-lying and poorly drained areas that can’t handle such a high volume of rain.

TORNADOES

As if the collective hazards of a hurricane aren’t bad enough, tropical cyclones almost always spin up tornadoes as they move ashore. Most form in the outer bands of rain and can quickly create dangerous conditions well away from the hurricane’s main area of high winds and heavy rain. Tornadoes in hurricanes aren’t typically as ferocious as their Midwest cousins. However, they can be just as deadly and often spin up with almost no advance warning.