097 FXHW60 PHFO 130637 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 837 PM HST Sun Sep 12 2021 .SYNOPSIS... A high pressure ridge north of the islands will keep moderate trade winds in the forecast through Monday. An upper level low northeast of the islands will drift slowly west through the week weakening the trade winds from Tuesday through early Thursday morning. Localized sea breezes will develop on Tuesday and Wednesday along the western slopes of each island. Instability associated with the upper low will keep periods of showers in the weather forecast for all islands through Thursday. Drying trends return from Friday onward as trade winds strengthen across the region. && .DISCUSSION... The satellite picture this evening shows an unstable upper level low, roughly 600 miles northeast of the state, slowly drifting towards the islands on a westward track. This low is drifting slowly westward bringing periods of enhanced showers to all islands as it approaches the state. Bands of low clouds are moving through the region in a moderate trade wind weather pattern. Radar imagery this evening shows scattered showers across the area with periods of numerous shower activity mainly along the windward slopes of each island. A weak high pressure ridge north of the islands will continue to produce moderate trade winds on Monday. This ridge will weaken a bit with light to moderate trade winds forecast from Tuesday into Thursday as the upper level low passes just north of the island chain. Lighter large scale winds will allow a sea breeze pattern to set up on the terrain sheltered western slopes of each island from Wednesday into Thursday. Stronger trade winds will return after the low passes by the islands from late Thursday through next weekend. Expect continued periods of enhanced showers through Thursday as a weakened ridge north of the state and the approaching upper level low will increase the trade wind inversion heights into the 7,000 to 10,000 foot range. The higher rainfall amount totals will favor windward and mountain areas with some of the stronger showers pushing into typically drier leeward areas, especially in the overnight to early morning time periods. On Wednesday, forecast guidance continues to show the upper low passing just north of the state with increasing potential for heavy showers and isolated thunderstorms over the eastern islands from Wednesday into Thursday. Much of these weather impacts will depend on the strength and track of the approaching upper low. By Thursday, the low will pass to the west of the islands and we will transition to stronger trade winds, with drying trends from Friday into the weekend as a high pressure system builds just north of the state. && .AVIATION... High pressure northeast of the the state will allow for moderate northeasterly trades across the islands tonight. Scattered shower activity will mainly be confined to the windward coasts and slopes though a few showers may pass over on leeward areas. Expect brief periods of MVFR ceilings and visibility associated with this activity. There are currently no AIRMETs in effect. However, AIRMET SIERRA may need to be issued later tonight if cloud cover along the windward slopes increases. && .MARINE... High pressure far north of the state will drive moderate to fresh trade winds through Tuesday, with winds flirting with Small Craft Advisory (SCA) levels across the typically windy waters around Maui and the Big Island. The trades will ease slightly on Wednesday as surface trough slides by to the north of the islands. As this trough exits to the northwest of the state Wednesday night and Thursday, the trades will ramp up to locally strong levels, with winds reaching SCA thresholds over the typically windy Maui and Big Island waters. The strong SCA level trades are expected to expand to most if not all waters Friday through next weekend. Long period south swell will maintain surf at or slightly above the September average tonight, then decline on Monday. A southwest swell originating from the Tasman Sea will arrive late Monday and hold through Wednesday, leading to inconsistent surf near seasonal levels for September. Small surf will prevail on north facing shores, with a small bump possible next weekend. East shore surf will remain well below normal through Wednesday. As trade winds ramp up Thursday through next weekend, surf along east facing shores will rise back to seasonal levels and will be quite rough Friday through the weekend. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Bohlin AVIATION...JT MARINE...Jelsema