Why does Hawaii get so few hurricanes? Hans Rosendal, retired meteorologist, provides the answer:
The reasons for the lack of hurricanes in Hawaii are several. We usually don’t quite have sea surface temperatures warm enough (about 82 degrees) and deep enough warm surface waters to sustain the development of hurricanes. If we get up to 80 degrees now and then in August and September, that is about max. We therefore have to import the few hurricanes that we do get.
Secondly, the upper wind flow over our region usually features westerlies which is very detrimental to keeping a hurricane approaching from the east from being sheared off. In countless cases we have been ‘saved’ by this upper flow pattern as storms move into our region and fall apart due to it being sheared. What is needed is easterlies at both the surface and aloft so the entire column of air can move along without ventilating out the latent heat derived from the convection which in turn cause the pressure drops and wind increases.
In the case of Iniki in 1992 (an El Nino year), we had an unusual flow pattern with less vertical wind shear and other things working to sustain the health of the storm. Just like living beings, a hurricane goes through a life cycle. Since Iniki formed on this side of 140W longitude about two or three days prior to it hitting Kauai on 9-11, it was a healthy and growing youngster by the time it moved into our area. The health of storms also depend on what is going on, often far away from the storm. Let me explain.
This season with its many intense storms in the Atlantic basin has seen unusual high amplitude upper flow across North America with strong surface Highs and troughs along each coast. The pattern was so persistant that a drought was going much of the summer in the midwest. I would compare it to a flow pattern that prevailed during the early and middle 1930s across the U.S. with some areas the driest since then. Also there was a severe drought in South America with the Amazon basin being the driest in memory and lots of suffering due to lack of water for transportation. These two drought areas, to the north and to the south of the active hurricane breeding trough of the Caribbean and Gulf of Mexico, kept strong subsidence over these land areas as an equilibrium to sustain rising motion over the trough in between them and to feed a moist low level flow of air into the trough. The trough also enhanced the sea surface temperature warming over the basin.
The feed of air from the south means that it is being imported from the southern hemisphere (SH). During the past few months I have been looking at wind speed and direction at Galapagos Islands on the equator off South America at EQ and 90W. Winds there without fail have been out of the south at 10 to 15 mph flowing across the equator into the NH day in and day out. Then as the winds enter the NH, they become SW winds around 05N and flow across Panama and central America into the Caribbean where they make a left turn at 15N around the trough. This area where the winds make this left turn is where the hurricanes seem to have been forming including Wilma.
This is getting a little far away from Hawaii, but it kind of supports my own private ideas of the importance of the cross equatorial low level flow component needed, or at least being helpful, to sustain hurricane formation at low latitudes. Thus when storms threaten Hawaii it may be useful to look at what is going on at the equator and beyond to the south of us.
Your question about the effect of the mountain areas of Mauna Kea and Loa and Haleakala on hurricane motion, I prefer not to discuss here. No doubt the massive mountains have a great effect downwind, and some effect also a distance upwind during the stable stratified flow of trade winds. Thus a weak tropical depression coming in from the east and surrounded by an unstable showery air mass would likely not know the difference as it approaches the mountains. On the other hand, a more intense cyclone surrounded by subsiding ring of stable air could feel some effects. But again, I would not count on it, since at times the forcing is strong, and we have had cases of hurricanes striking the Big Island and Maui including the famous Kohala cyclone in the 19th century.
The track of Iniki northward after passing south of the Big Island was forecast quite well and the hurricane watch was issued the afternoon before on 9/10 as we realized that the storm would not pass as far west of Kauai as earlier thought. We did not get the full 36 hour lead time, we strive for. I worked on the initial issuance of the watch, so I remember it well. Courtesy of Kam Sands website kamsands.hypermart.net/maui_updates .htm