Weather 101 - Hawaiian Hurricanes and Tropical Cyclones

I visited Hawaii in the mid 1990's, a few years after Hurricane Iniki ravaged the island of Kauai, in the western part of the Hawaiian Island chain. While much of the damage had been repaired, there were still some hotels and homes that showed the marks of the storm (out of some 14,000 that were damaged or destroyed). Some, not yet repaired, wore signage indicating the owner's displeasure with their insurance company.

But, since the islands have relatively little experience with tropical storms and hurricanes, there was little surprising.

Stephen Spielberg and his film crew were surprised, however, during the filming of Jurassic Park in 1992. So, they capitalized on the Iniki's arrival and used some storm footage in the movie.

With Felicia threatening the islands (although down to weak tropical storm status - Figure 1) in August 2009, I am again reminded that the islands are not immune to these tropical visitors.

Dr. Steven Businger, a professor at the University of Hawaii on Oahu, is among those studying Hawaiian weather and hurricanes. In 1998, working within the framework of a multi-agency cooperative agreement, he summarized much of the historical basis of these storms in Hawaii.

Climatology

Due to climatological conditions involving the location of warmest sea surface temperatures, upper level jet stream and zone of strongest trade wind easterlies, most tropical storms and hurricanes in the central Pacific Ocean pass to the south of the Hawaiian Islands. The southernmost point of the Big Island of Hawaii, the largest and geologically the newest of the island chain, lies at about 19 degrees North latitude. All the other Hawaiian Islands lie to its northwest.

The same set of factors favor storms moving primarily from east to west (or east-southeast to west-northwest) through the area. Only when the high-pressure ridge to the north of the trade winds weakens can the hurricane or tropical storm curve towards the north. This is what Iniki did in September 1992.

Nonetheless, only 5 storms in the past 6 centuries have affected the Hawaiian Islands as hurricanes (Figure 2). These were Hiki (1950), Nina (1957), Dot (1959), Iwa (1982) and Iniki (1992). A handful of tropical storms and about a dozen tropical depressions have also passed through the island chain (Figure 3).

The Central Pacific Hurricane Center has also documented storms that affected the area between the early 1800's and 1950.

Dangers

The dangers of high winds, storm surge, rip currents and heavy rainfall that we deal with in the continental U.S. are only exaggerated in the Hawaiian Islands due to its extensive coastline, its terrain and its population concentrations.

First, due to the terrain and building guidelines, the population of the islands is concentrated along the coast and in ocean-facing, steeply inclined, narrow, inland mountain valleys. This means that when heavy tropical cyclone rains arrive, people living in and near rivers and streams often see a quick gathering of rainwater and equally rapid stream rise to flood range. Other people live along coastal areas where large waves and storm surge can wreak further destruction.

Mountain rainfall is enhanced when winds blow upslope - rising air cools, its moisture condenses and rainfall amounts increase. In fact, some of the world's heaviest rainfalls occur in the Hawaiian Islands.

Conversely, as air descends on the other side of the mountains, clouds evaporate and rainfall diminishes dramatically. Hence, some of the lowest average annual rainfalls in the Pacific Ocean basin also occur in Hawaii. Honolulu, lying to the southwest of the mountains on Oahu only receives an average of about 18.29 inches of rain annually. That's less than half what New York City, NY and less than Amarillo, TX receives annually.

Puako (elevation 5 feet above sea level) on the Big Island of Hawaii only receives an average of 8.93 inches annually (period of record 13 years). Meanwhile, Mt. Waialeale (elevation of about a mile) on Kauai receives 460 inches an average each year (period of record 30 years). Depending upon measurement practices, procedures and period of record variations, this is among the highest values around the world.

Due to erosion on the steeply faced, impermeable mountainsides, valleys are often narrow, with populations having little choice but to locate within flood potential zones. The soils of the islands are quite impermeable, too. Hence, rain often runs off faster than it sinks into the ground.

While tropical cyclones may often be the culprits behind Hawaiian floods, subtropical lows, known as Kona storms, can linger to the southwest of the island chain and bring strong southerly winds, clouds and heavy rainfall into the region, especially outside of tropical cyclone season. Trust me on this one. I spent a vacation in Oahu one December and suffered through 6 out of 7 rain-filled days. Tropical systems, at least, keep moving.

That movement (more or less uniform) allows swells to build over the oceans. When the swells (general push of water in one direction by persistent winds) coupled with with-driven waves arrive, the coastline can be battered. It's not unusual for waves to crash on Hawaii's rocky beaches sending waves 10 to 15 feet or more into the air.

Hawaii's surfing community loves the storm waves and many surfers may venture forth even in dangerous conditions. Most residents and tourists will avoid the beaches (and hence waves and rip currents).

Winds tend to increase with increasing altitude and this holds for higher Hawaiian terrain. However, the channeling of winds into narrow canyons and over mountain peaks adds to higher wind speed levels. Not surprisingly, it's usually at higher elevations that trees and homes receive the full brunt of hurricane and tropical storm force winds.

You can read more about heavy rainfall events in Hawaii at http://www.prh.noaa.gov/hnl/pages/events.php and more about Hawaii's hurricane history at http://www.prh.noaa.gov/cphc/summaries/.