Communicating uncertainty in weather forecasts: the role of forecast changes

Skilful weather forecasts help users make sound decisions when faced with potentially hazardous climatic conditions. However, this beneficial result may be reduced or negated in the absence of an effective communication of forecast uncertainty. On average, forecast skill improves for shorter lead times, which implies that we expect differences between successive forecasts. While there is a vast literature on the communication and visualisation of weather forecast uncertainty, little attention has been dedicated to communicating forecast changes to non-specialist audiences. Nonetheless, this is a key dimension of forecast uncertainty, and there are several user cases in which providing information about possible future changes in weather forecasts can improve their use.

An illustrative example is the situation in which a user has to decide whether to act now or wait for the next forecast. This can be as simple as a professional deciding whether to drive or not to a client on a day for which extremely heavy rainfall is forecasted, potentially leading to flash flooding. Cancelling well-ahead of time makes rescheduling easier, yet the forecast has a larger chance of being wrong. Cancelling on short notice minimises the chance of a false alarm, but poses greater logistical challenges for both the professional and the client. Something as simple as knowing how often the later forecast is better – for example knowing that 9 times out of 10 a heavy rainfall forecast issued one day ahead is better than one issued 5 days ahead – can qualitatively help the non-specialist users in this fictitious example to make a more informed decision.

In this contribution, we consider a variety of cases in which information on forecast changes may be of value. We then present a set of easily interpretable metrics making information on such changes accessible to non-specialist users.