FORECAST DISCUSSION

Synopsis

Tropical Depression Darby will continue to bring unsettled weather to western portions of the state this morning as the system tracks northwestward and away from the Aloha state. Improving conditions are then expected to spread across the island chain from east to west through tonight. More typical trade wind weather will then return for the remainder of the work week as high pressure builds well northeast of the island chain.

Discussion

The 5 am HST advisory from the Central Pacific Hurricane Center (CPHC), indicated that Darby had weakened into a Tropical Depression, and was located around 55 miles west-northwest of Barking Sands. Infrared satellite imagery shows mostly cloudy skies in place across the state, with radar imagery showing moderate to heavy showers continuing to stream northward over the island of Oahu. Main short term concern for today revolves around the potential for heavy rainfall and flash flooding. Today... The latest track from CPHC continues to show Darby tracking northwestward and away from the island chain. Deep southerly flow on the eastern side of the weakening system will continue to transport tropical moisture with precipitable water values in excess of 2.0 inches across Oahu and Kauai this morning before a slightly drier and more stable airmass moves in this afternoon. As a result, we will continue the Flash Flood Watch for Oahu, Kauai and Niihau through noon. Across Maui county and the Big Island, rainfall coverage is not expected to be as great, with mainly diurnally driven convection expected. Overall, the worst weather associated with Darby has already occurred, with steadily improving conditions expected through the day. Tonight through Friday night... More typical trade wind weather will settle over the island chain as Darby or its remnants continue to track northwestward and away from the Aloha state. The muggy conditions will take some time to depart in the wake of Darby, but we should see a noticeable decrease in humidity for the middle to latter part of the work week. Overall, clouds and showers are expected to focus primarily over windward and mauka locales through the period. Next Weekend... Model guidance is in good agreement showing the trade wind flow moistening up in response to a lowering of upper level heights over the state. There is also some indication that moisture associated with the remnants of what is now Hurricane Georgette in the eastern Pacific, will get caught up in the trade wind flow. As a result, there should be an increase in trade wind showers across the state, with the majority of the activity impacting windward and mauka locales.

Aviation

AIRMET Sierra is in effect for mountain obscurations for Kauai, Oahu and portions of Maui County due to showers and isolated thunderstorms on the southern end of Tropical Depression Darby. This AIRMET is likely to remain this morning, possibly later, until most of the showers and thunderstorms follow Darby to the northwest. Expect improving visibility and ceilings east to west over the next 24 hours as trade winds build in. AIRMET Tango and Zulu is in effect for tempo moderate turbulence and icing across Kauai and Oahu. More significant thunderstorm coverage persists north of Kauai and Oahu. A SIGMET is in effect for embedded thunderstorms.

Marine

All Tropical Storm Warnings have been cancelled as the latest advisory from the CPHC indicates that Darby has weakened into a Tropical Depression. Winds are expected to remain below headline thresholds today as Darby tracks northwestward and further away from the island chain. High pressure will build northeast of the state tonight and through much of the work week, increasing the trade wind flow across the marine area. Small Craft Advisories (SCAs) may become necessary as early as tonight for the Alenuihaha Channel. The SCA conditions will then likely expand to additional marine zones through the week as the trade winds strengthen. Surf heights are expected to remain below advisory levels through the work week.

HFO Watches/Warnings/Advisories

Flash Flood Watch until noon HST today for Niihau, Kauai, Oahu.

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DISCUSSION/MARINE...Jelsema AVIATION...Foster

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