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Unstable air mass prompts flash flood, winter storm watches

The Big Island, as well as the entire state, remains under a flash flood watch until 6 a.m. Saturday.

According to the National Weather Service in Honolulu, chances of heavy rain in Hilo are 90% today and 80% tonight, with scattered showers in the forecast for Saturday. There’s also a chance of thundershowers.

“There’s an upper-level disturbance,” said Kevin Kodama, NWS Honolulu senior hydrologist on Thursday afternoon. “It’s weakening, but as it weakens, it’s going to move out over the state to the northeast. It’s going to bring in a band of instability, and as that’s happening, there’s a pocket of moisture that’s coming in from the east. If the two coincide as they move over the state, they can cause some bouts of heavy rain.”

Kodama said the heavy rains and possible thundershowers, if they occur, will be “probably starting off on the east half of the state then moving off to the west as the band of enhanced moisture I was talking about moves westward.”

“There’s a chance of thunderstorms even into Saturday,” he said. “But probably the highest chances are going to be … through (tonight).”

Temperatures have cooled a bit the past few days. Hilo saw a run of five record-tying temperatures recorded at the airport throughout a two-week period, but the last day that occurred was on Monday. Kodama said the spring heat wave was “because our winds have been real light and it’s been stable.”

“We’ve had some pretty stable weather with hardly any clouds and the trades have been week, so we’re not getting the usual trade wind showers, so the temperatures have been coming up,” he said.

If flooding occurs, roads and other low lying areas could be closed because of elevated runoff and overflowing streams.

A winter storm watch also was posted for summit areas of Maunakea and Mauna Loa above 12,000 feet in elevation until 11 a.m. today, also a potential product of the weather conditions described by Kodama.

When the watch was originally posted Thursday morning, forecasters were predicting up to 6 inches of snow at the upper elevations. An update at mid-afternoon Thursday scaled back that prediction to total snow accumulation of up to 3 inches.

As of about 6:30 p.m. Thursday, there was no snow on the summits, but the Maunakea Access Road was closed.

Email John Burnett at
Source: Hawaii Tribune Herald

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