Tornado near Madison
Tornado Potential Index Explained

Abstract...
Our "Storm Center" staff have devised a new index which we believe will help our clients and folks in the general public understand the potential, or likelihood, for tornado development near them. This scale is derived from in depth analysis and research into the general mechanics in the atmosphere, which is conducive to tornadic development. It should be understood here, that this index is not in any way a forecast of where a tornado will occur! Science has not yet allowed forecasters to forecast "pinpoint" specific locations where a tornado may occur. But, this index does provide critical information as to the general area(s) where conditions in the atmosphere may support tornado development. It must be understood here, that even though a potential for tornadoes exists in a targeted area, tornadoes don't always happen, unless conditions locally become enhanced enough to trigger some spin-ups. This is where our new TPI index will enter the equation.

The TPI Index...
The TPI, or "Tornado Potential Index", is basically a pre-calculated projection of the existing degree of a threat for tornado development over a specific location (i.e. city, town, or other geographic location). The index is determined by existing conditions in the atmosphere, and the overall likelihood of tornadic development based on numerous analysis of data, short term CAM guidance, and other information. When we generate our daily convective outlooks for the coverage region, this is when a TPI index chart is prepared. This chart will illustrate those areas we're closely monitoring, with contoured values shown on the map. Any cities, towns, and other known points within a contoured zone will be under a threat for tornado development at some point. The maximum TPI index value for the current forecast period will be shown on our Storm Center page by scrolling down toward the bottom.
The TPI index is a ten (10) tier scale with values ranging from 1 to 10. Each level corresponds to a percentage value for that level. For example, let's say that Wausau Wisconsin is given a TPI of 3. This means that there is at least a 30 percent chance (out of 100%) for tornadoes, of any intensity, to develop within 50 statute miles of that city location. The higher the TPI value, the more likely a tornado may form near any given location. Please note: The TPI index is only used when conditions in the atmosphere support tornadic supercell development! Further, even though conditions may support possible tornado development within a contoured zone, tornadoes may not develop unless conditions locally become enhanced such that tornado spinups do occur.
The table below shows what the TPI index values are, and the chances for tornadoes occurring within 50 statute miles of any given location.

Tornado Potential Index (TPI)
What it means
1
There is a 10% chance out of 100% for a tornado nearby.
2
There is a 20% chance out of 100% for a tornado nearby.
3
There is a 30% chance out of 100% for a tornado nearby.
4
There is a 40% chance out of 100% for a tornado nearby.
5
There is a 50% chance out of 100% for a tornado nearby. Keep up to date on the weather!
6
There is a 60% chance out of 100% for a tornado nearby.
7
There is a 70% chance out of 100% for a tornado nearby. Please closely monitor local conditions, and stay informed.
8
There is a 80% chance out of 100% for a tornado nearby. This is a critical weather situation! Plan to seek shelter at anytime.
9
There is a 90% chance out of 100% for a tornado nearby. This is a dangerous and life-threatening situation! Plan to seek shelter at anytime.
10
There is a 100% chance for dangerous tornadoes nearby. This is a dangerous and life-threatening situation! Act quickly if necessary!

It should be emphasized here, that TPI values greater then seven (7) are used for the high end moderate or high risk areas as depicted in the current convective outlook issued. Therefore, the TPI values greaten then 7 will be used only for those higher end categorical risk areas, which are very rare in our coverage region.
If you have any questions about this new index and it's intended usage, please contact our office on the contact page.

To return to the Storm Center just follow this link.