BLOG: It's the water, not the wind

BLOG: It's the water, not the wind
(Source: National Hurricane Center)
(Source: National Hurricane Center)
(Source: National Hurricane Center)
(Source: National Hurricane Center)
Although elevated, this home on the North Carolina coast could not withstand the 15 ft. storm surge that accompanied Hurricane Floyd (1999) - (Source: National Hurricane Center)
Although elevated, this home on the North Carolina coast could not withstand the 15 ft. storm surge that accompanied Hurricane Floyd (1999) - (Source: National Hurricane Center)

Here's something you probably didn't know: categorizing tropical cyclones using the Saffir-Simpson Hurricane Scale (SSHS) has everything to do with wind, but nothing to do with the main threat, water.

Here's the background: Dr. Herb Saffir was an engineer and Dr. Robert Simpson was a meteorologist. As a civil engineer, Saffir wanted the focus to be on damage, which is often a direct result of wind. And so, the Saffir-Simpson Scale was developed shortly after Hurricane Camille - a "cat 5 storm" packing winds of 175 mph - flattened the Mississippi coast during the summer of 1969, killing more than 250 people.

You've heard it on the news; the scale has been widely used since 1973 as a way to communicate the strength of a hurricane. Category 1 storms are weakest, with maximum sustained winds of at least 74 mph. The highest classification in the scale, category 5, is reserved for hurricanes with winds exceeding 156 mph.

While big wind numbers may get your attention, when it comes to hurricanes, it's not wind, but rather water that's the number one killer. In fact, 90% of all fatalities in the past 50 years or so can be directly attributed to water.

Freshwater flooding, often miles inland away from the coast accounts for more than 25% of all deaths, but ocean-water drownings top the list, accounting for more than 60% of tropical cyclone fatalities, the vast majority coming from storm surge.

So what is storm surge?

According to the National Hurricane Center, storm surge is an abnormal rise of water generated by a tropical cyclone, over and above the predicted astronomical tides. Storm surge should not be confused with storm tide, which is defined as the water level rise due to the combination of storm surge and the astronomical tide (total tide).

This rise in water level can cause extreme flooding in coastal areas, particularly when storm surge coincides with normal high tide, resulting in storm tides reaching more 20 feet high in some cases.

What has forecasters most concerned is this: the forecasts are increasingly more accurate and today the lead time needed to people to prepare is the greatest it's ever been. Yet, water-related deaths have seen very little down-turn. With that in mind, new forecast graphics - solely for storm surge, experimental at first - will accompany the wind forecasts this hurricane season.

Because most coastal residents can remain in their homes (or a secure structure nearby) and be safe from a tropical cyclone's winds, evacuations are generally needed to keep people safe from storm surge. Having separate warnings for these two hazards will save lives by helping to guide decisions being made by emergency managers, and by enhancing public response to instructions from local officials.

This storm surge information is from the National Hurricane Center: http://www.nhc.noaa.gov/surge/surge_intro.pdf.

Surge Vulnerability Facts:

From 1990-2008, population density increased by 32% in Gulf coastal counties, 17% in Atlantic coastal counties, and 16% in Hawaii (U.S. Census Bureau 2010. Much of the United States' densely populated Atlantic and Gulf Coast coastlines lie less than 10 feet above mean sea level. Over half of the Nation's economic productivity is located within coastal zones.

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