Estimation of lightning hazard of an approaching thunderstorm

Lightning location data from Finland has been analysed to provide information about the usability of the 30/30 rule, widely used in the personal lightning safety. The criteria are formalized so that a flash within 10 km of a given location launches the alarm. The storm is defined to produce a critical danger situation when a flash hits within 1 km of the location. With these definitions, the probability of false alarms is relatively high, up to 97%. On the other hand, in about 10% of the cases the 30-30 rule does not provide any pre-warning (i.e. the very first strike in the storm is 1 km away). In the cases where a prewarning is given, average lead times from first warning to danger are as much as 20 minutes. Although these results are preliminary, they suggest that the 30-30 rule may not be optimal in the case of a country with very low flash rates and densities, such as Finland.