166 FXHW60 PHFO 060131 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 331 PM HST Sun Sep 5 2021 .SYNOPSIS... Trade winds will decline to gentle to moderate strength tonight and Labor Day. While showers will continue to favor windward areas, a nearby upper level low will produce locally enhanced showers and allow a few showers to develop over leeward areas, especially on the Big Island during afternoon hours. Trade winds will strengthen during midweek as a more stable shower pattern returns. && .DISCUSSION... The atmosphere has become slightly unstable today as trade winds slowly declined. Trades are easing as surface high pressure far to the north weakens and an upper level low centered about 150 miles northeast of Maui helps relax the local pressure gradient. The upper low is also weakening the local inversion and causing showers to be heavier than yesterday. This is more noticeable within a batch of low level moisture affecting the eastern half of the island chain. As a result, windward areas of the Big Island and Maui have experienced more rainfall today, and spotty heavy showers have flared over leeward Big Island slopes north of Kailua-Kona this afternoon. The batch of moisture will clear late tonight, but expect windward areas of the Big Island and Maui to remain somewhat wet, with heavy showers on leeward Big Island decreasing during the evening. Trades will decline to gentle to moderate strength for Labor Day as the ridge to the north further weakens, and the lingering upper low will maintain slightly unstable conditions. Low level moisture within the trade wind flow will likely be below normal, which suggests that showers will not be extensive over windward slopes. However, sea breezes over sheltered leeward areas may produce spotty afternoon showers, especially on the Big Island. Trade winds will gradually strengthen Tuesday and Wednesday and may become locally breezy next weekend. The upper level low will be kicked away from the region by a ridge aloft, leading to a stable weather pattern that will focus showers over mainly typical windward areas. && .AVIATION... Lighter winds are expected over the next day or two due to the weakening ridge north of the area. The lighter winds could lead to some afternoon clouds and showers forming over some leeward areas tomorrow afternoon. An upper level low northeast of the islands is helping to enhance trade wind showers over the next day or so. The afternoon sounding from Hilo shows the inversion has lifted to near 13K feet. This is aiding in showers reaching interior and leeward areas. Anticipate pockets of MVFR conditions mainly over windward and mountain area, although brief MVFR conditions can't be ruled out over leeward areas. AIRMET SIERRA was recently expanded to include all of the Big Island, and the Haleakala slops of Maui. MVFR and isolated pockets of IFR have been observed on the windward side of the Big Island, while MVFR conditions are more likely over the remainder of the AIRMET areas. Expect the AIRMET to continue into the evening, with the leeward Big Island and Maui improving after sunset. && .MARINE... A ridge of high pressure far north of the state will weaken tonight in response to a front passing north of the area. This will result in the trades gradually weakening through Monday, then remaining at moderate speeds with locally fresh breezes through most of the week. Winds are expected to remain below Small Craft Advisory thresholds through the week. A series of small long period south swells will maintain surf heights along south facing shores near the seasonal average during the next few days, then gradually decline as we approach the middle of the week. Surf heights along northeast and east facing shores will trend lower over the next few days as the trade wind speeds decrease, lowering wave heights across the region. No significant swells are expected through the forecast period. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Wroe AVIATION...M Ballard MARINE...Kino