Power and Impact Scale 7 days ago

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 In modern times it has been replaced by the Saffir-Simpson Hurricane Wind Scale, which only gives information about the estimated maximum sustained winds.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.

Original Saffir-Simpson Scale
Strength Maximum Sustained Wind Speed (kt) Maximum Sustained Wind Speed (mph) Pressure (mb)
Category 1 64-82 74-95 > 980
Category 2 83-95 96-110 965-979
Category 3 96-113 111-130 945-964
Category 4 114-135 131-155 920-944
Category 5 >135 >155 < 919

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.

Pressure Tendency Factor
Decrease in Pressure +1 Level
Increase in Pressure -1 Level

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.

Radius of Sustained Winds
Strength Hurricane Force Winds Tropical Storm Force Winds
Level 1 < 50 miles < 100 miles
Level 2 51-100 miles 101-200 miles
Level 3 101-150 miles 201-300 miles
Level 4 151-200 miles 301-400 miles
Level 5 > 200 miles > 400 miles

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).

Hurricane Charley (2004)
Variable Estimate P&I Level
Maximum Sustained Winds 150 mph 4
Radius of Hurricane Force Winds 20 nm 0.5
Radius of Tropical Storm Force Winds 75 nm 0.5
Minimum Pressure 941 mb 4
Pressure Tendency at Landfall -39 mb/24 hours +1
Rating 2.5 +1 3.5

Hurricane Carla (1961)
Variable Estimate P&I Level
Maximum Sustained Winds 145 mph 4
Radius of Hurricane Force Winds 300 nm 5
Radius of Tropical Storm Force Winds 500 nm 5
Minimum Pressure 927 mb 4
Pressure Tendency at Landfall steady -
Rating 4.5 + 0 4.5

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.

Hurricane Sandy (2012) at Landfall
Estimate P&I Level
Maximum Sustained Winds 85 mph 1
Radius of Hurricane Force Winds 150 nm 4
Radius of Tropical Storm Force Winds 400 nm 5
Minimum Pressure 948 mb 3.75
Pressure Tendency at Landfall -4 mb/6 hours -
Rating 3.5 + 0 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