Quantitative assessment of the probability forecast for the geomagnetic storm occurrence

[1] As a practical example of space weather forecasts, an attempt was made to provide monthly occurrence probabilities in advance for intense geomagnetic storms (Dst < −100 nT). A simple formula for evaluating the probabilities was developed, such that transformation of the cumulative distribution for the waiting time of neighboring storms is taken into account as a function of the time elapsed since the last event at the time of forecasting. The waiting time distribution is based on past observational data applied directly and not approximated by any exponential or other parametric distributions. The correspondence of forecast probabilities with past event frequencies is validated by the standard measures, showing that the present forecast, given its superior accuracy and resolution compared to the climatological estimation, can potentially be used as a baseline for assessing the skill of a future model.