Assess Thunderstorm Warning System performance

Under the IEC 62305-2:2024 standard, the use of a Thunderstorm Warning System (TWS) compliant with the IEC 62793:2020 standard can be taken into account when calculating the lightning risk of a site.

When carrying out the risk assessment or ARF, the effectiveness of the warning system must be specified through a performance indicator: PTWS.

Our Lightning Alert service complies with the IEC 62793:2020 standard.

Through this PTWS: FTWR and PODx Assessment service, we provide the two standardised performance indicators for the service, FTWR and PODx, calculated from lightning data detected over the last 10 years at the location of the site under study.

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 last 10 years according to a standard configuration of the Lightning Alert service:

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:

95 warnings were issued at least 15 minutes in advance.

The POD15 is therefore 95%.

For subscribers to our Lightning Alert services, the indicators are calculated based on the operational configuration currently deployed at the protected site and are directly available in their customer area.

For any other requirements, please contact our sales team.