FORECAST DISCUSSION

Synopsis

Locally breezy trade winds will continue through early next week as high pressure persists north of the state. Passing showers will favor windward and mauka areas, with showers more active tonight through Sunday night. By the middle of next week, we may begin to feel impacts from Tropical Storm Madeline.

Discussion

Water vapor imagery shows an upper level ridge north of the state, with a low far to the northwest. At the surface, high pressure is located far to the north, with a ridge extending to the south- southwest. 00z soundings from Lihue and Hilo show inversions just under 9kft and precipitable waters of 1.25 and 1.34 inches respectively. Early afternoon MIMIC total precipitable water imagery shows an area of moisture near Maui County/Big Island extending 200 miles to the northeast, with drier air across Kauai/Oahu. A wedge of drier air is also located between 50 and 150 miles east of the Big Island, with one surge of tropical moisture centered near 150W and a larger one near 140W associated with Madeline. Trade winds will be the dominant feature for the next few days. Even though the high is far north of the state, a nearly stationary low to our south will help to enhance the pressure gradient across the islands. This will lead to locally breezy conditions, especially from tonight into Monday. Rainfall has been greatest across Oahu/Maui County with the wettest gages receiving a quarter to a half inch of rain over the past 12 hours. The least rainfall occurred over Kauai, where gages only measured a couple hundredths of an inch. An area of showery clouds is located within the pocket of higher moisture northeast of the state. Showers have already started to increase across Maui County, and will spread across the rest of the state tonight. High-resolution model guidance shows a surge in trade winds with this feature, and forecast soundings/cross sections show an increase in moisture depth. Rainfall chances will be highest across windward sections, but given the higher inversions and locally breezy trades we will see an increase in showers across leeward sections as well. Behind this feature, moisture depth starts to decrease Sunday night, and we may be drier than normal by Monday. Another surge of moisture to enhance trade showers is possible on Tuesday, but that is starting to get into the more uncertain period of the forecast. The National Hurricane Center in Miami is currently issuing bulletins on Tropical Storm Madeline, which is located just east of 140W. Madeline is expected to move northwest for the next few days before turning west, and may impact the state during the middle of the week. There has been a lack of consistency in model guidance with this system, leading to greater than normal uncertainty in the track and intensity forecast. This may result in significant changes to the types of impacts we could see across the state. Be sure to stay up to date with the latest forecasts, which are issued every six hours. When Madeline crosses 140W into the Central Pacific Basin this evening, the Central Pacific Hurricane Center in Honolulu will begin issuing advisories. In addition, Hurricane Lester is located far to the east, but may affect the state over the upcoming holiday weekend. Given it's more extreme distance, there is even greater uncertainty as to possible impacts to the state from Lester.

Aviation

Tradewind moisture will affect mainly windward and mountain areas of the islands occasionally this weekend. Some MVFR ceilings and visibilities are expected with the clouds and showers passing through. But overall...VFR conditions will prevail around the state. Cloud tops are running mostly between 9000 and 11000 feet. Currently there are no AIRMETS or SIGMETS.

Marine

A Small Craft Advisory remains in effect for the typically windier locations around Maui county and the Big Island, due to locally breezy trade winds accelerating around the island terrain. The morning ASCAT pass missed the windier areas, but otherwise showed winds on the lighter side today. Winds are expected to pick up tonight as the cloud feature northeast of the state moves through. No significant swells are expected through the middle of next week, though surf along east facing shores will be slightly elevated due to the stronger trades. Winds and seas, as well as surf along east facing shores, may increase toward the middle and end of next week due to the possible effects from Tropical Cyclones Madeline and Lester. Uncertainty in this forecast scenario is high.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

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Discussion/Marine...Bravender Aviation...Fujii

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