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Posted at 10:30 AM ET, 04/21/2009

Hurricane Scale Faces Revisions

By Jason Samenow

* Late-Day Storms? Full Forecast | NatCast *

The Saffir-Simpson Hurricane Scale. Courtesy National Weather Service.

The Saffir-Simpson Hurricane Scale, which classifies hurricanes in categories between 1 and 5 based on intensity, is under revision. Popularly considered a "catch-all" scale for the potential impact of a storm, hurricane forecasters at the National Hurricane Center (NHC) have become increasingly bothered by a number of the scale's shortcomings which they fear may mislead the public. At last week's Bahamas Weather Conference, NHC officials spoke of an effort underway to re-tool the scale to focus solely on predicting wind impacts -- which they feel the scale is best suited to characterize.

The major problem forecasters have identified with the current scale (as shown above) is that, in some instances, it poorly characterizes the storm surge, a rise in water ahead of an approaching hurricane. Of all hurricane impacts, the surge causes the greatest losses to life and property.

Keep reading for more on some of the issues with the Saffir-Simpson scale and proposed NHC revisions to better communicate hurricane risks...

The surge is primarily driven by the storm's winds, but is also impacted by the size of the storm and the geographic profile of the coast it is inundating, as the National Hurricane Center describes:

The level of surge in a particular area is also determined by the slope of the continental shelf. A shallow slope off the coast will allow a greater surge to inundate coastal communities. Communities with a steeper continental shelf will not see as much surge inundation, although large breaking waves can still present major problems.

For example, a hurricane striking the mid-Atlantic would have a smaller surge than the same storm along the coast of the western Gulf of Mexico which has a fairly flat coastal profile. But the Saffir-Simpson scale does not take the slope of the coast into account.

The scale also fails to take into the account the physical size of a hurricane. Generally, the larger the storm in areal extent, the bigger the surge. A very intense but compact storm may have a smaller surge than a weaker but more expansive storm. For example, the powerful, but tiny category 4 Charley (2004) -- had just a 7 foot surge when it washed ashore southwestern Florida whereas the somewhat less intense (category 3) but much more massive Katrina produced a devastating 28-foot surge along a portion of the Louisiana coast.

Other scale shortcomings:

  • Rainfall: The Saffir-Simpson category assigned to a hurricane has little bearing on its rainfall potential. The amount of rain a storm produces depends mostly on a storm's forward speed, rather than its intensity. So, a slow moving category 1 hurricane would likely cause much greater flooding risks than a fast moving category 5 one.
  • Tornadoes: The Saffir-Simpson category level does not communicate tornado risk, which depends on a range of factors not incorporated into the scale.

In light of all of these shortcomings, in a presentation given at the Bahamas conference, NHC Science and Operations Officer Chris Landsea indicated the NHC is close to finalizing the following changes to the scale:

  • Removing all the surge and flooding values and descriptions from the scale
  • Revising the wind-impact wording
  • Renaming the revised scale to: The Saffir-Simpson Hurricane Wind Scale

According to Landsea and NHC Director Bill Read, the move to re-frame the Saffir-Simpson Scale as a wind scale is motivated by the recognition that the smorgasbord of storm hazards can't be packaged into a single index. Rather, each hazard must be appropriately communicated based on the characteristics of a given storm and the area it is threatening.

Although Landsea indicates that the NHC is still finalizing the changes to Saffir-Simpson scale, some of the changes he described seem to have already made it onto NHC's Web site. At its page describing the Saffir-Simpson scale, storm surge values for the different categories are no longer given. Instead, the NHC writes: "Wind speed is the determining factor in the scale, as storm surge values are highly dependent on the slope of the continental shelf and the shape of the coastline, in the landfall region."

So with the Saffir-Simpson scale focused on wind, how will the NHC now communicate vital storm surge information? Although hurricane researchers have developed a wonkish metric called Integrated Kinetic Energy (IKE) -- which captures the aggregate effect of the wind, surge, and waves, Landsea recommends decoupling surge from wind using a scale we're all familiar with: "It's called feet. ... That's the way to depict it."

Related videos from the Bahamas Weather Conference:

Disclaimer: The author attended the Bahamas Weather Conference as a guest of the Bahamas Ministry of Tourism, which paid some of his expenses.

By Jason Samenow  | April 21, 2009; 10:30 AM ET
Categories:  Tropical Weather  
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My chief issue with both the Saffir-Simpson scale and the Fujita/Enhanced Fujita scale is that neither is open-ended like the Richter scale used for earthquakes.

For instance, the Tri-State tornado in the 1920's was so large and intense that a mere F5 or EF5 wouldn't do it justice. That tornado was probably an "EF6" on an open-ended scale.

Storm surge is a phenomenon which is difficult to quantify. One has to take into account the topography of the shoreline as well as the strength of the wind. A category 1 hurricane striking at the right point could generate an enormous storm surge in a place such as the Bay of Fundy.

Posted by: Bombo47jea | April 21, 2009 2:32 PM | Report abuse

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