La Nina could bring above-average rainfall to state … or not

The strength of the event, if it comes, will determine which areas will get wet

With conditions “leaning” toward a La Nina event, above-average rainfall is forecast this winter in Hawaii, the National Weather Service said Tuesday in its “Wet Season Rainfall Outlook.”

But the strength of the La Nina if it comes this fall — and there is a 55 to 60 percent chance — will determine whether the entire island gets rain or just the windward sides, said Kevin Kodama, National Weather Service hydrologist, Tuesday. A strong La Nina would mean rainfall mostly on windward sides and a weaker event would spread precipitation over leeward sides as well, he said.

When asked if the La Nina, which involves cooler-than-normal waters near the equator, will be weak or strong, Kodama said “that is hard to say” because the models are conflicting. La Nina’s counterpart, El Nino, involves warmer waters and often leads to increased tropical cyclones.

“There still is uncertainty” about whether current neutral conditions will turn into a La Nina, but “we are pretty close already” to the wet season, which strengthens the prediction, he indicated.

“Probabilities favor above-average rainfall through the wet season,” the outlook for October through April said. “Wet seasons during recent moderate to strong La Nina events have had wet conditions over the windward slopes but dry conditions over leeward areas.

“Weaker La Nina events had more rainfall make it to leeward areas.”

The La Nina event last winter season was “weak and short-lived,” which led to “an erratic wet season” with a heavy rain month followed by a dry month, Kodama said. Rain in April and some in May put the island in decent shape heading into the dry season but July through September “was pretty dry,” he said.

Kodama said he believes Upcountry, which has been under a Stage 1 water shortage declaration since July 18, “might be in good shape” this winter, especially with the possibility of rain in the East Maui watershed. With the approval of Mayor Alan Arakawa, Department of Water Supply Director Dave Taylor declared a Stage 1 drought when anticipated demand was expected to exceed supply by 1 to 15 percent.

Stage 1 calls for voluntary water conservation. Stage 2 and 3 declarations impose increased water rates.

Taylor noted Tuesday that the ongoing Stage 1 declaration “is not reducing usage as we had hoped.” The department would like to reduce the advance notification time for a declaration from 90 days to 30 days, which “would allow us to be more precise with our messaging and, hopefully, get a better response from users,” he said.

The measure is currently before the Maui County Council.

“We are aware of the forecast and are hopeful that we will receive ongoing rain through the winter, hopefully without heavy flooding,” Taylor said. “We constantly monitor the weather and our water storage situation. We will all breathe a sigh of relief when we start getting constant rain. We are always hoping to avoid a higher stage shortage.”

If a stronger La Nina prevails, there is a chance that some areas still will be under drought conditions after the wet season, Kodama said. The areas affected could cover the leeward Haleakala slopes including Kihei, Keokea, Kahikinui and Kaupo, he said. In the West Maui Mountains, Lahaina, Ukumehame and Olowalu could be affected.

“Existing drought may persist or worsen in some of the leeward areas, especially on the Big Island and Maui County,” the outlook said. Those areas could be in a more precarious position after the wet season, Kodama explained, because instead of recovering during the winter, those dry areas would be “getting worse.”

There also is the danger of brush fires, he added.

The May-to-September dry season “had near to below average rainfall,” the outlook said. Drought on the Big Island spread to the other counties and intensified during the summer. Unlike 2015 and 2016, cyclone activity was down, which meant Hawaii did not get a boost of rainfall during the summer months.

“Nobody in our office is complaining,” Kodama said about the lack of hurricane activity so far this hurricane season, which ends Nov. 30. “2015 was crazy” with a record 15 named tropical systems. Luckily, there were only close calls.

* Lee Imada can be reached at leeimada@mauinews.com.

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