Millions of residents along Australia’s eastern coast are bracing for the arrival of a very slow-moving storm, the strongest tropical cyclone to threaten the region in more than 50 years. Tropical Cyclone Alfred is expected to make landfall late Friday or early Saturday with the strength of a category 1 Atlantic hurricane,

Residents are stacking sandbags to protect low-lying properties ahead of a tropical cyclone that is forecast to become the first in 51 years to hit the Australian east coast near Brisbane, the nation’s third-most populous city.

Cyclone Alfred is unusual for two reasons — it has formed way too south, and is moving slowly. Both these factors make it more dangerous. We explain why.