The Mariner's 1-2-3 Rule is the guideline mariners follow to keep out of a tropical storm or hurricane's path

Mariner's 1-2-3 Rule

Example of the Mariner's 1-2-3 Rule where the shaded region represents the danger area of the tropical cyclone.

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.

For more information:

Mariner's Guide For Hurricane Awareness
in the North Atlantic Basin

National Hurricane Center Experimental
Replacement Chart

National Hurricane Center