Long periods of strong southward magnetic fields are known to be the primary
cause of intense geomagnetic storms. The majority of such events are caused by
the passage over Earth of a magnetic ejecta. Irrespective of the interplanetary
cause, fast-forward shocks often precede such strong southward B$_{z}$ periods.
Here, we first look at all long periods of strong southward magnetic fields as
well as fast-forward shocks measured by the \textit{Wind} spacecraft in a
22.4-year span. We find that 76{\%} of strong southward B$_{z}$ periods are
preceded within 48 hours by at least a fast-forward shock but only about 23{\%}
of all shocks are followed within 48 hours by strong southward B$_{z}$ periods.
Then, we devise a threshold-based probabilistic forecasting method based on the
shock properties and the pre-shock near-Earth solar wind plasma and
interplanetary magnetic field characteristics adopting a `superposed epoch
analysis'-like approach. Our analysis shows that the solar wind conditions in
the 30 minutes interval around the arrival of fast-forward shocks have a
significant contribution to the prediction of long-duration southward B$_{z}$
periods. This probabilistic model may provide on average a 14-hour warning time
for an intense and long-duration southward B$_{z}$ period. Evaluating the
forecast capability of the model through a statistical and skill score-based
approach reveals that it outperforms a coin-flipping forecast. By using the
information provided by the arrival of a fast-forward shock at L1, this model
represents a marked improvement over similar forecasting methods. We outline a
number of future potential improvements.
