Why does Hawaiʻi escape severe damage from hurricanes so often? Steve Businger of UH Mānoa put together a description of Hurricanes in Hawaiʻi, including an explanation of the shear line that is just east of the Big Island. As hurricanes approach us from the east, both the trade winds and the upper-atmosphere winds are moving in the same direction, speeding the hurricane on its way. But near the Big Island, the upper atmosphere winds flow in the opposite direction to the trades. These opposing winds tend to weaken the approaching hurricanes, so they are often downgraded to tropical storms.
However, in the cases of Iniki and Iwa and Dot, the trade winds had unfortunatley died out just at the time the hurricanes passed south of the island chain, temporarily nulling the shear effect. This means, we need to always be prepared for hurricanes, even though we have escaped so many in the past!