It’s a La Nina year, which for weather officials generally means a busier Atlantic and quieter Central Pacific hurricane season, along with a natural cooling of certain parts of the equatorial Pacific.
The National Weather Service this morning said in its 2020-2021 rainfall outlook that it also translates into wetter than average conditions across the Hawaiian islands during the wet season from October through April.
The models show that the month of October is expected to start off dryer than average, go through a transition in November, and then bring larger-scale, wetter than average conditions from December through April.
The intensity of La Nina, however, will determine the distribution of the rainfall, and how much drought relief she actually brings to the leeward side of Hawaii’s isles, according to senior service hydrologist Kevin Kodama.
A stronger La Nina event is expected to bring higher than normal trades and rains to the windward slopes, said Kodama, whereas a weaker La Nina event tends to provide more rainfall for the leeward sides.
“There’s a higher chance that drought recovery will occur in the western half of the state from Kauai to Molokai, and also on the windward slopes of Maui and Big Island,” said Kodama. “However there is a chance, especially if La Nina ends up being stronger, then drought as it exists now could persist through the wet season. We’ll probably see some improvements in drought conditions. However, they may not completely go away by the end of April when the wet season ends.”
This was the case back in the 2010-2011 wet season, according to Kodama, when extreme drought conditions persisted in much of the state. By the end of April 2011, several areas of the state still remained in drought condition.
This could happen again, he said, but chances are pretty good that drought will be eliminated for the windward areas this season.
Hawaii is heading into the wet season with below average rainfall for most locations across the state and drought in all counties.
Although most areas of the state started off drought-free due to wet conditions earlier this year, Maui County experienced a drier 2019-2020 wet season.
Parts of Maui County, including the western half of Molokai and the lower leeward slopes of Haleakala in September reached levels of extreme drought, categorized as D3 in the U.S. drought monitor map.
Severe drought, categorized as D2, meanwhile, reached the southern half of Lanai and the Upcountry region of Haleakala on Maui. Severe drought also developed in West Oahu, from Waianae to Ewa, and along the lower slopes of the South Kohala and Kau districts of Hawaii island.
On Oahu, several records were broken for the lowest September rainfall. Dry vegetation along lower leeward slopes of Oahu brought an elevated risk of brush fires.
On Sept. 24, a brush fire scorched Waahila Ridge above the University of Hawaii at Manoa campus. The dry conditions produced a higher than normal consumption of public water across parts of Oahu.
The Maui County Department of Water Supply in September began mandatory Stage 1 water restrictions for residents in Upcountry and West Maui due to the water shortage. The Honolulu Board of Water Supply earlier this month also asked Oahu residents to conserve water because rainfall has been less than 50% of the normal average for the past five months.
Kodama said this was the 11th driest season over the last 30 years, but that the intensity of the drought in August and September were in stark contrast to wetter than average conditions during the dry seasons of the past seven years.
La Nina, according to the National Oceanic and Atmospheric Administration, is a natural ocean-atmospheric phenomenon marked by cooler-than-average sea surface temperatures across the central and eastern Pacific Ocean near the equator, and the opposite of El Nino, which features warmer-than-average sea surface temperatures in that region.
The last La Nina event in Hawaii occurred in the wet season of 2017-2018, according to Kodama, which followed another La Nina in 2016. Historically, La Nina events have produced big rain events. In 2008, for instance, La Nina brought nearly 40 inches of rain to the Hilo and Puna areas.