Kona lows, other storm events make outsize contribution to state's rainfall

Jun. 14—Kona storms are notorious for bringing powerful winds, torrential rain and flash flooding to the islands, not to mention the possibility of lightning, hail, waterspouts and more.

A team of scientists who examined 20 years of daily rainfall and storms in Hawaii have found that what usually is considered a wet or dry year on Oahu is primarily determined by the frequency and rainfall intensity of such "Kona low " events.

The study also found that while most wet-season moisture comes from normal tradewind weather, storm events such as Kona lows, cold fronts and tropical storms can account for as much as 48 % of the wet season.

The study, published recently in the American Meteorological Society's Monthly Weather Review, underscores the importance of storm events to the island's overall rain totals and suggests that climate models and other forecasting efforts must account for their contribution to the islands' overall rain budget.

The paper by researchers from the East-West Center at the University of Hawaii and the University at Albany, N.Y., also warns about a possible future with fewer cold fronts crossing the islands under the spell of climate change.

The research team, led by East-West Center research fellow Ryan Longman, created time series data that identified cold fronts, Kona lows (storms that form in the midlatitudes and move southeast toward Hawaii ), upper-tropospheric disturbances (upper-level lows ) and tropical cyclones that occurred between October 1990 and September 2010.

The researchers compared this data with the average daily rainfall for all of Oahu over the same period to help understand how these weather events fit into the rainfall totals.

The scientists counted nearly 400 storm events over the 20-year period, with January being the most active month with 21 % of a year's storms on average.

The study found that rainfall driven by weather events can account for as much as 48 % of total rainfall in a single wet season and are especially impactful to dry leeward areas. Kona lows, in particular, were found to dump on average 300 % more rainfall on Oahu than normal and up to 600 % more than normal on the island's West side.

During the wettest seasons, 70 % of the rainfall occurred on storm event days on average, according to the paper, with 40 % of the rainfall alone occurring on Kona storm days.

"If you have a lot of Kona low storms, you're going to have a wet year, " Longman said.

In addition, the study found that cold fronts—cold air masses that usually generate rainfall—are the most common type of weather disturbance, averaging 14 per year. The cold fronts that cross over the island contribute a great deal to Oahu's annual rainfall totals.

However, cold fronts that approach but do not cross the island actually result in less rainfall than normal, according to the research. That's because those storms usually end up cutting off the tradewinds while not drawing close enough to Oahu to help generate any additional rain.

"Not only are you not receiving that rainfall from the crossing front ; you're actually getting conditions that are drier than what it usually is, " Longman said.

With climate scientists warning that storm tracks are shifting poleward due to climate change, it looks like the islands could see more noncrossing cold fronts in the future, Longman said And because leeward regions are dependent on storm events for much of their rainfall, he said, it appears those areas will be even drier as climate change becomes more prevalent.

The research was initially funded by the Honolulu Board of Water Supply, with additional support coming from the U.S. Army Corps of Engineers, the National Science Foundation's Hawaii Established Program to Stimulate Competitive Research (EPSCoR ) 'Ike Wai project, and the Pacific Islands Climate Adaptation Science Center over more than five years.

The next project for Longman and his team is the creation of new high-resolution daily rainfall maps that will be updated in near-real time. This aims to help researchers look even further into the nature of disturbance-driven rainfall and help resource managers make better water management decisions.

Correction : Scientists counted nearly 400 storms over 20 years. An earlier version of this story had incorrect information about the number of storm events counted over the study period.