For the second wet season in a row, Hawaii is in a La Nina weather cycle, and the National Weather Service is forecasting above-average rainfall for the season.
For most of the state, the rainy season is October through April.
In a media briefing Friday, Kevin Kodama, senior hydrologist for the NWS in Honolulu, described La Nina as an “unusual or anomalous cooling” in equatorial Pacific waters. It’s the opposite of El Nino, and both have an impact on weather.
Kodama said the National Oceanic and Atmospheric Administration’s Climate Prediction Center — which declared the La Nina conditions on Thursday — is “expecting it to persist into spring of 2022.”
The intensity of the La Nina event is expected to be moderate, Kodama said.
Hawaii is just coming off its sixth-driest dry season in the past three decades, and although Kodama is predicting a wetter-than-normal rainy season, he said it would be a slow transition into the wet season for much of the state.
“So, October and November, we’re looking at continuation of below-average rainfall condition, then transitioning (to wetter weather) in December,” Kodama said. “But by the time we get into January, the model forecasts are looking at large-scale wetter-than-normal conditions across the Hawaiian Islands area, and that will last into April. So, we’re looking at the second-half of the wet season to be fairly wet.
“This is similar to the last wet season, October 2020 to April 2021, and also similar to the (La Nina) that occurred in 2011-2012.”
Regarding the Big Island, Kodama noted the irony of that prediction, considering the wet weather the windward side has experienced as of late.
“Of course, we’re doing the wet season briefing, and I’m calling for a dry start to the wet season, but Hilo just gets 10 to 11 inches (of rain) in upslope areas, so that’s the way it works sometimes,” he said. “But it should be drying out, especially the early part of next week. And the large-scale models, they’re really showing a pretty significant dry signal for October into November, and we’ll see how that all plays out.
“I think for the Kona areas, the rainfall’s going to taper off as we get farther along toward the end of the year, and it’s not going to be quite as wet as they were earlier in the summer.”
The West Hawaii coffee belt, unlike the the rest of the state, experiences its wet season in the summer — and three of the four official gauges in that area recorded above-average rainfall in September.
According to Kodama, if the La Nina proves to be moderate to strong, “what we’re looking at is the potential for heavy rainfall, mainly focused along the east-facing windward slopes.”
“And that’s due to expected higher-than-normal frequency of trade winds if we have a moderate-to-strong La Nina event,” he said. “And this also means that leeward areas of the state may end up with below-average (rainfall) totals, especially in Maui County and the Big Island.”
Kodama also presented a summary of the dry season that just occurred, saying extreme drought (D3) conditions in Maui County and severe drought (D2) on Hawaii Island have had a profound impact.
“The worst impacts … affected the agriculture section, mainly ranching,” he said. “We got a lot of impact reports from the ranchers in Maui County and the Big Island.
“On the Big Island, drought conditions contributed to the record-breaking Mana Road brush fire that happened in the latter part of summer. … The impacts of that are going to linger for quite some time because there was so much damage to the soil. It’s going to take awhile for the grasses to regrow in that area. There’s already severe erosion issues … and that’s also creating an elevated risk of flash floods in the area because of the changes to the soil.”
Email John Burnett at jburnett@hawaiitribune-herald.com.
