Category 5 Ice Storm? A New Index Rates Ice Storm Impacts

http://www.weather.com/news/weather-winter/rating-ice-storms-damage-sperry-piltz-20131202

If you hear a forecast of a foot of snow, you know it will mess up your commute and could shut down schools, businesses, perhaps even the local airport. You're prepared for the impacts and will likely adjust your schedule accordingly.
What about an ice storm? Do you know what impacts a half inch of ice accumulation typically produces in your area?
A new index hopes to more clearly communicate the risk from ice storms.
Developed by Sid Sperry, director of public relations, communications and research for the Oklahoma Association of Electric Cooperatives, and Steven Piltz, meteorologist-in-charge at the National Weather Service in Tulsa, Okla., the Sperry-Piltz Ice Accumulation Index (SPIA) uses forecast information to rate an upcoming ice storm's impact from 0 (little impact) to 5 (catastrophic damage to exposed utility systems). Think of this as the Enhanced-Fujita scale for ice storms, issued ahead of the storm.
(MORE: Ice Storm Impacts and Damage)

Sample SPIA index forecast for Dec. 15-18, 2010. SPIA index values are color-shaded, with legend below the map. (Copyright: Sidney Sperry/spia-index.com; used with permission)

Specifically, SPIA incorporates not only forecast ice accumulation, but also forecast winds and temperatures. Ice storms producing the most severe impacts are those with strong winds accompanying heavy ice accumulation, both exerting stress on power lines and tree limbs.
"The public underestimates the power of wind in an ice storm," said Sperry, who has worked in the electric utilities industry for 34 years and is President/CEO of SPIDI Technologies, LLC.
Sperry described a phenomenon on power lines called galloping, which can occur with relatively little ice accumulation, but high winds. Providing lift similar to that over an aircraft's wing, power lines in this scenario can rise and fall 6 to 8 feet, leading to outages.
Temperature also plays a role in an ice storm's destructive power, but not simply by determining who will be too warm for ice.