AbstractLong 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 Bz periods. Here we first look at all long periods of strong southward magnetic fields as well as fast‐forward shocks measured by the Wind spacecraft in a 22.4‐year span. We find that 76% of strong southward Bz periods are preceded within 48 hr by at least a fast‐forward shock, but only about 23% of all shocks are followed within 48 hr by strong southward Bz 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‐min interval around the arrival of fast‐forward shocks have a significant contribution to the prediction of long‐duration southward Bz periods. This probabilistic model may provide on average a 14‐hr warning time for an intense and long‐duration southward Bz 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.