Assess Thunderstorm Warning System performance

As part of the IEC 62305-2:2024 standard, a Thunderstorm Warning System (TWS) can be taken into account in the lightning risk calculation of a site when it complies with the IEC 62793:2020 standard.

To evaluate the effectiveness of this system, the standard relies on one value: PTWS.

The PTWS: FTWR and PODx Evaluation service provides these indicators using lightning observations collected over the past 10 years.

The indicators can be used to support lightning risk assessments and evaluate the performance of a Thunderstorm Warning System according to the methodologies defined by the standards.

What is PTWS?

PTWS is a value defined by the IEC 62305-2:2024 standard to take into account the performance of a thunderstorm warning system in a lightning risk assessment.

PTWS is calculated from the FTWR and PODx indicators.

How is PTWS calculated?

The PTWS value is calculated according to the following formula:

PTWS = maximum value between:

the Failure To Warn Ratio (FTWR)
and 1 – the Probability Of Detection with a lead time of x minutes (1 – PODx)

These performance indicators are calculated from thunderstorms observed over the past 10 years according to a standard configuration:

25 km radius,
15-minute lead time,
warning triggered on the first lightning strike,
60-minute hold time.

FTWR and PODx values directly available for lightning risk assessment studies.

Understanding key concepts

The lightning event (LRE)

The dangerous event, called a Lightning Related Event (LRE), is characterized by the presence of one or more cloud-to-ground lightning strikes in the Surrounding Area (SA), which is a geographical area surrounding and including the target (generally defined as a circle with a radius of 2 to 5 km around a site).

FTWR

The FTWR (Failure To Warn Ratio) is the ratio between:

the number of warning failures;
and the total number of situations with an LRE in the surrounding area.

It determines the effectiveness of the system through its ability to warn a site before a hazard occurs.

In other words:

Of all dangerous situations observed, how many did not trigger a warning?

Example

Out of 100 dangerous thunderstorm situations:

98 were detected;
2 were not detected.

The FTWR is therefore 2%.

PODx

The PODx (Probability Of Detection with a lead time of x minutes) is the percentage of warnings issued at least x minutes before impact in the surrounding area.

It measures operational effectiveness because it includes compliance with the lead time required to implement protective measures.

Example (for a 15-minute POD)

Out of 100 dangerous thunderstorm situations: