Why are Hurricanes Rare in Hawaii?

Today, tropical storm Iselle is hitting the big island of Hawaii and the much stronger Hurricane Julio is right behind (see infrared satellite image below).  Julio, the more eastern storm, has a far more defined structure, and possesses a nice eye and substantial spiral rainbands.

Tropical storms are relatively rare in Hawaii, although direct hits have occurred, such as the devastating landfall of Hurricane Iniki on Kauai in 1992.  FAR more hurricanes occur at similar latitudes in the Caribbean.

A plot of historical hurricanes tracks shows the story clearly (see image).  Hurricanes move way north in the Caribbean and eastern Atlantic.  The same for the western Pacific.  But over the eastern Pacific they die quickly as they approach Hawaii.

A blow-up image illustrates this better:

What is it about Hawaii that works against their getting strong tropical storms and hurricanes?

The main reasons:  the  water temperatures are generally too cool and there is often too much vertical wind shear, the change in horizontal winds with height.

Cool sea surface temperatures

Anyone who has taken a vacation in Hawaii knows the story.  The waters of the Pacific are relatively cool and you know it when you snorkel or swim.   70s are typical, ten degrees or more cooler than the Caribbean for example.

The minimum temperature generally conducive to hurricane formation is 26C (79F), and often the waters around Hawaii during the tropical cyclone season (mid summer to early fall) are cooler than
that.   During the past few months the sea surface temperature (SST) has been above normal in the eastern Pacific around Hawaii, something that is illustrated by the SST anomalies (differences from climatology) shown below.  This has pushed the 26C sea surface temperature north of Hawaii.

 Why do hurricanes care about the temperature of the ocean surface?  Because the heat and moisture (evaporation) from the ocean surface is the fuel for hurricane development and maintenance.

Vertical Wind Shear

The change in horizontal wind with height is called vertical wind shear.  Such shear is bad for tropical storms, since it tends to distort the vertical structures necessary for the development of these systems.There is often a lot of shear near Hawaii, with easterly trade winds at low levels and westerly  winds aloft.
        Here is a plot of the vertical wind shear over the eastern Pacific and western U.S. yesterday.  Reds are high shear. The two red dots indicate Iselle and Julio.   You will notice that the shear was relatively low over their locations...that allowed these storms to develop and to retain key structures.  The vertical wind shear over them is less than normal.

So the combination of warmer than normal sea surface temperatures and reduced wind shear has helped tropical storms to develop and approach Hawaii.

    Tropical Storm Iselle is dumping heavy precipitation on some of the Hawaiian islands, but it now appears that Julio will move north of the island chain, reducing its impacts substantially (see graphics from the National Hurricane Center)

One place that will NEVER get tropical storms is the Pacific Northwest.  Our 50F water is a barrier that will never let them approach our coast.