What is the Mariner's 1-2-3 rule?

The Mariner's 1-2-3 rule, also referred to as the Danger Rule, is an important guideline mariners follow to keep out of a tropical storm or hurricane's path. It refers to the rounded long-term National Hurricane Center (NHC) forecast errors of 100-200-300 nautical miles at 24-48-72 hours, respectively.

The danger area to avoid is constructed by accounting for those errors and then broadened further to reflect the maximum tropical storm force (34 knot) wind radii forecast at each of those times by the NHC.