Background

The biggest difference between our Hurricane analysis and others is the proprietary Power and Impact (P&I) Scale, which has been developed over a decade. The P&I Scale gives a more complete picture of the impacts that a hurricane will produce, most appropriately at landfall. Because of the tremendous complexities of tropical cyclones, it is unlikely that any scale can quantify a storm's impacts 100 percent of the time. We are attempting to improve describing a storm's total effects, but maximum effects within a storm and their uncertainty should be evaluated as part of any threat assessment.

The original Saffir-Simpson scale took into consideration a storm's pressure and wind speed. In modern times it has been replaced by the Saffir-Simpson Hurricane Wind Scale, which only gives information about the estimated maximum sustained winds. This leaves out important information about the impacts of the storm, often leading to confusing messaging from both experts and emergency managers. The P&I Scale incorporates additional factors, including subjective tweaks that can be made to the scale in real-time, most notably with the pressure tendency.

The Basics

The P&I Scale takes into account the following factors:

• Maximum Wind Speed
  • Radius of Hurricane Force Winds
  • Radius of Tropical Storm Force Winds
• Minimum Central Pressure
  • Pressure Tendency

The P&I Scale builds on the original Saffir-Simpson Scale, using the Sea Level Pressure and Maximum Sustained Winds.

Pressure tendency is also an element of the P&I Scale, as rapidly deepening storms will have their P&I category increased. With a weakening storm, it's the opposite. The reason for this is rapidly deepening storms are likely tightening up quickly with almost perfect alignment between the lower and middle levels of the atmosphere, promoting the transfer of the strongest winds from just above the surface down to the surface. Weakening storms are the opposite, as a weakening storm likely has competing processes occurring.

The new variables added last year were radius of Hurricane and Tropical Storm force winds. The reason for this is intuitive as the greater the diameter of strong winds the more energy is available to increase wave action and enhance storm surge.

Example 1

Both Hurricane Charley (2004) and Hurricane Carla (1961) were Saffir-Simpson Category 4s, but there was a huge difference in the nature of the two. Carla was a monster bulldozer while Charley was a fist of fury over a small area. On the P&I Scale, there is a difference between the two, as Charley had Level 4 winds and pressure (as Carla did) but Carla also has a Level 5 wind diameter (Charley was a Level 0.5 in both).

Example 2

Hurricane Sandy had Level 1 winds and only slight filling (944 mb to 948 mb) in the last 6 hours before landfall (no influence). Its pressure was a strong Level 3 (assigned a Level 3.75). Hurricane-force winds were in a 150-200-mile diameter, which was like a Category 4 while tropical storm force winds were 400-500 miles out (Level 5). Overall Sandy's was a major impact hurricane with a P&I - 3.5.

Notes

A storm like Sandy, with weaker winds at its core but more spread out, would be closer to Charley than Charley was to Carla. even though both of the latter two were Category 4s on the Saffir-Simpon Scale.

Other impacts, mainly having to do with rainfall, not wind, are separate issues. The slower a storm moves, the greater the rain totals, but this scale was developed to add additional parameters to describe more precisely the total power of the storm, separate from the rain.

Developed by Joe Bastardi

Written by Thomas E. Downs, V

Special Contributors: Antonio Riggi and Steve Hallett

Last Updated on March 17, 2021