Rainy season forecast to be wetter than last year’s
Keep that umbrella handy because we might see plenty of extra rainfall in the coming months. Or maybe it’ll be an average to near-average rainy season, according to theNational Weather Service.
Mahalo for reading the Honolulu Star-Advertiser!
You're reading a premium story. Read the full story with our Print & Digital Subscription.
Already a subscriber? Log in now to continue reading this story.
Keep that umbrella handy because we might see plenty of extra rainfall in the coming months. Or maybe it’ll be an average to near-average rainy season, according to the National Weather Service.
Whatever the case, this year’s wet season promises to have a lot more moisture than last year’s, when an unusually strong El Nino weather pattern held Hawaii in the grips of a drought that lasted through the winter months and into spring.
The National Weather Service issued its wet season rainfall outlook for the state Thursday, forecasting near-average to above-average rainfall through April.
Whether we see average or above average depends on what happens to a developing La Nina, the global weather pattern that is the polar opposite of El Nino.
The National Oceanic and Atmospheric Administration’s Climate Prediction Center issued a La Nina watch last week, saying it appears the current El Nino neutral conditions are expected to turn into the cool phase, where temperatures at the equatorial Pacific are below average.
But while a La Nina is expected to develop during the fall, it might not last long as it is only slightly favored (55 percent) to last through the winter.
“That makes it harder to predict what will happen,” said Kevin Kodama, National Weather Service hydrologist in Honolulu.
Prior wet seasons during weak La Nina events have seen near-average rainfall in Hawaii, Kodama said, while seasons with neutral conditions have tended to experience above-average rainfall here.
Looking back over the past 30 years, seven of the 10 wettest wet seasons have occurred during years with El Nino-neutral conditions, he said.
Why that is remains a mystery, he said, because La Nina used to mean rainier weather for Hawaii. But starting in about 1982 large-scale atmospheric circulation patterns changed, resulting in shifting storm paths, according to a 2015 University of Hawaii study that Kodama helped with.
Whether this year’s rainy season is average or above average, it looks like drought recovery is in store for leeward areas still enduring parched landscapes.
Thanks to a surprisingly wet dry season, only about 10 percent of the islands are experiencing drought right now, with only a couple of leeward areas — on Maui near Kihei and on Kauai near Hanapepe — experiencing what the Hawaii Drought Monitor describes as severe drought.
How wet was it during the May-through-September dry season?
Turns out it was the second-wettest dry season in the last 30 years, based on rankings from eight key sites. Only last year’s El Nino-enhanced dry season was wetter.
“It would have been a record except for last year,” Kodama said. “Last year was a crazy year.”
In 2016 monthly rainfall records were broken at
several locations in July, August and September, and the amount of moisture that fell on the leeward sides of the islands exceeded all expectations.
A busy tropical cyclone season and above-average sea surface temperatures helped boost rainfall totals, with tropical disturbances bringing rain bands to the islands with moisture from the deep tropics fueling intense rainfall.
Out of the six cyclones that entered the Central Pacific since June 1, five of them brought rainfall to the islands, Kodama said.
“But it wasn’t just that,” he said. “Even the remnants of dead tropical cyclones were felt here. Five of them affected us with remnant moisture. That’s
The biggest impact came from Tropical Storm Darby, which made landfall on Hawaii island in July before punishing
Honolulu with severe flash flooding.
In September elevated rainfall from a weak tropical disturbance fueled sudden flooding in Maui’s Iao Valley, resulting in the evacuation of residents along the Wailuku River.
Kodama said residents need to be prepared as Hawaii enters the wet season.
“With this time of year, we have a greater chance of flash flooding, especially over the leeward areas,” he said. “People should be aware we will have flooded roads, impacts on traffic and road closures. And if you live in a flood-prone area, a low-lying spot near a stream, be aware that these things can happen very quickly. So have a plan in place if you don’t already have one.”