July 29th, 2015

2015 Atlantic Hurricane Season - What Are We Preparing for Anyway?

Posted at 1:00 AM ET

Any hurricane can produce wind, surge and inland flood impacts. The severity and scope of impacts is not always consistent with rating on the Saffir-Simpson scale, particularly for surge as we have seen with Katrina (2005) and Sandy (2012).

  • Wind: For a typical wood frame structure, damage usually starts from the top of the structure and most often with the roof (trees notwithstanding). These effects can become noticeable with sustained wind speeds as low as 40 mph. For more severe wind events, wind damage will affect the walls, and in extreme cases such as Andrew (1992) or Charley (2004), many structures will be barely recognizable following the event. Downed trees and powerlines are commonly found with any tropical cyclone.
  • Storm surge: Storm surge is related to many factors including wind speed over water, the area of water affected by wind, bathymetry and coastline shape. Water intrusion will ruin the interior of any coastal property. Water velocity and particularly wave activity can cause severe to complete structural damage, since water weighs about one ton per cubic yard. Water damage usually begins at the bottom of a structure and becomes more severe with increasing water levels and wave height. With excessive water velocity or wave activity, the foundation itself can be dislodged, resulting in structural failure. In extreme cases the property can be scoured from the foundation such as Katrina (2005) in the Mississippi Gulf coast area. Our most recent reminder of U.S. surge impacts is from Sandy (2012). While Sandy was a post-tropical cyclone at landfall, the size of the wind field and angle of landfall near Brigantine, New Jersey drove a historic surge event for the area as far north as Massachusetts. The severity of surge impacts was equivalent to a typical Category 3 hurricane, yet the wind speeds alone did not suggest the potential for such damage.
  • Freshwater flooding: Freshwater flooding is affected by factors such as excessive rainfall, storm motion, the capacity of local storm-water management infrastructure and local geography. The freshwater impacts of Irene (2011) and Fay (2008) were quite severe in the New England and North Florida areas, respectively. Floodwaters can ruin any structure they affect, and can even cause structural damage if water velocity is sufficient. Flood damage starts at the bottom of the structure and increases in severity with increasing inundation height.

Preparation for each of these impacts and the resulting disruption to infrastructure is an ongoing and essential process for homeowners, businesses, government agencies including NOAA and FEMA, and of course the (re)insurance industry. The landfall of one or two hurricanes cannot be ruled out for any season.

Conclusion

Hurricane activity for the Atlantic Basin is projected by seasonal outlook providers to be below average for the 2015 season, but these providers all stress preparation for a landfalling hurricane, as with any season. Cool sea-surface temperatures (SSTs) and a probable El Niño would indeed indicate reduced basin activity, although a closer look implies that these suppressing effects will be confined to the East Atlantic and deep tropics. Warm SSTs in the northern Caribbean and Gulf of Mexico warrant some caution. Hurricane landfall is a threat for any hurricane season, irrespective of basin activity, as history has shown more than once.

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