Above-average rainfall expected for Hawaiʻi’s wet season, October – April

The May through September dry season was exceptionally dry for the Big Island with extreme drought conditions starting early, by mid-May, and causing significant brush fires.

But while the state’s overall outlook for the upcoming wet season from October through April is for above-average rainfall, there is a possibility of drought continuing on the leeward areas of the Big Island, according to Kevin Kodama, senior service hydrologist with the National Weather Service in Honolulu.

This is the third year in a row that Hawai’i will be in La Niña, an oceanic and atmospheric phenomenon marked by cooler-than-average ocean water in the central Pacific Ocean. It is the colder counterpart of El Niño, as part of the broader El Niño–Southern Oscillation (ENSO) climate pattern

Two years is a row for La Niña is usual, three consecutive years is rare, Kodama said during a press briefing on Wednesday.

This is the first “triple dip” La Niña of the century, according to an update from the United Nations’ World Meteorological Organization. Since 1950, the longest La Niña was 37 months, from the spring of 1973 through the spring of 1976. 

The WMO El Niño/La Niña Update predicts the continuation of the current La Niña over the next five months, with a 70% chance in October through November but gradually decreasing to 55% in December through February.  La Niña started in September 2020.

La Niña conditions in the tropical Pacific strengthened during mid-July to mid-August 2022, as trade winds intensified. It affected temperature and precipitation patterns and exacerbated drought and flooding in some areas around the world, including Hawaiʻi.

La Niña’s “cooling influence is temporarily slowing the rise in global temperatures – but it will not halt or reverse the long-term warming trend,” said the World Meteorlogical Organization’s secretary-general, Petteri Taalas, in a news release.

Kodama said La Niña is a major factor in the mild hurricane season that was just experienced in the central Pacific.

For the state, climate model consensus favors large scale above average rainfall for the wet
season, especially from December 2022 through April 2023.

Kodama said rainfall distribution can be influenced by the strength of La Niña.

• Stronger La Niña events can have a higher than normal trade wind frequency which will focus rainfall on windward areas.

• Weaker La Niña events tend to have more weather systems that produce significant leeward rainfall.

Unfortunately for the Big Island, a weak to moderate La Niña event is favored. Thus, the reason the drought on the leeward areas of the county may continue.

In June and August there were several sites on the BIg Island with record dryness. Due to heavy rains in the Kona area in September, Kona slopes had only a short period of drought and have since improved.

For the upcoming wet season, Kodama said heavy rainfall may be focused primarily on the windward slopes, with flash floods and other flooding.

“In any wet season, whether or not we are projecting wetter-than-average wet season, we can have flooding even in a drought,” Kodama said. “In Hawaiʻi, these events can happen so quickly. You can go from bright sunny skies to thunderstorms within an hour. So, you should always be on the lookout and stay tuned to what our forecasters are saying.”

Kodama said motorists should be aware of flooded roads and realize that “1 to 2 feet of fast-flowing water is enough to sweep most vehicles off the road.”

Kodama said hikers should also be aware.

“If you see a flooded stream, itʻs better to wait it out and donʻt cross,” he said. “Our streams going up very quickly in Hawaiʻi but they also tend to recede very quickly. So it’s much better to wait it out if you are stuck on the wrong side.”

Kodama said extreme weather can happen during the wet season. Thunderstoms are rare, but they do happen and can bring large hail, strong out-flow winds, lightning and tornadoes. While lightning strikes do not occur often in Hawaiʻi, he said there have been fatalities in the state caused by lightning.

“So if there is a thunderstorm out there, itʻs best to get indoors and stay safe,” he said. “Just be on the lookout for rapidly changing conditions.”

You can sign up for Wireless Emergency Alerts (WEA) on mobile phones. They provide notifications when you’re in a significant, life-threatening flash flood warning area.

Other sources of weather information:

• NOAA National Weather Service Honolulu HI: https://www.weather.gov/hfo/
• NOAA Weather Ready Nation: https://www.weather.gov/wrn/
• NOAA Climate Prediction Center: https://www.cpc.ncep.noaa.gov/
• FEMA Flood Preparedness Information: https://www.ready.gov/floods
• Hawaiʻi Emergency Management Agency: http://dod.hawaii.gov/hiema/
• State of Hawaiʻi-DLNR National Flood Insurance Page: https://dlnreng.hawaii.gov/nfip/
• U.S. Drought Monitor: https://droughtmonitor.unl.edu/