A short calm can be enough
A forecast doesn't need fair weather for days; it needs to find the narrow gap between two storms. A window of about a day and a half was judged just enough to launch a vast attack by sea and air.
Past patterns aren't real forecasting
Assuming the weather repeats old patterns, so you can predict it by finding a matching past case, can look like science without being it. Forecasters who read the physical state of the atmosphere rejected this method as nonsense, even in the middle of high-stakes planning.
Forecasts become command decisions
In war, a weather forecast is not background information; it decides when the operation goes. A commander moved a whole invasion by a day because of one forecaster's analysis, so disputes between forecasters became disputes over the operation itself.