Moderate trades briefly weaken on Thursday then transition to cool and dry northeasterly trades for the weekend.
The islands reside at the inflection point between expansive deep ridging to the west and upper troughing to the east at press time. Stable, moderate trades prevail beneath this pattern, though the inversion does slope upward from 6kft at Lihue to 8kft at Hilo owing to the latter's closer proximity to upper troughing. In this area, showers are somewhat deeper and more numerous relative to the western end of the state, but overall a very typical stable trade wind pattern remains established through today.
Existing upper trough then serves as a conduit for additional mid- latitude energy to dig southward, the associated surface pressure falls eradicating the pressure gradient by tomorrow. Consequently, a land and sea breeze pattern with pockets of interior afternoon showers is expected for Thursday. Friday into Saturday represents a transition period to noticeably cooler weather as additional upper energy digs southward over the islands. Meaningful forcing remains largely north and east of the area during this time as ridging remains firmly in control. This wave will, however, force a shallow cold front through the area late Friday through Saturday. The inversion remains stout for the duration of the frontal passage limiting rainfall potential. The exception may be the high elevations of the Big Island and potentially Maui where a brief window for thunderstorms will exist Friday into Friday night. The front itself will hit with a brief round of shallow showers followed by winds veering to the northwest Friday night and eventually transitioning to breezy NEly trades by late Saturday.
This weekend will be characterized by dry weather and dewpoints falling through the 60s into the 50s bringing a seasonable chill to air. Trades persist through this time with strong post-frontal ridging bringing very high stability and significantly reducing shower coverage and intensity.
Strong consensus exists among the extended guidance that another upper wave will dig toward the islands during the middle of next week causing trades to weaken once more. This likely precedes another cold front pushing into the forecast area.
Moderate trades will gradually weaken through tomorrow as a cold front approaches from the northwest. Clouds and showers will generally favor windward and mauka areas, especially overnight into the early morning hours. However, with the trades weakening, sea breezes may develop later today and bring clouds and few showers to leeward interior areas this afternoon and land breezes will likely help to clear out leeward areas overnight. Brief MVFR conditions can be expected in showers, but otherwise, VFR should prevail.
No AIRMETs are in effect.
Trade winds will ease through tonight then become gentle and variable on Thursday as a cold front approaches. The front, currently about 600 miles northwest of Kauai, will push the subtropical ridge toward the islands over the next 24 hours, causing the moderate to fresh trade winds to drop considerably this afternoon and tonight. A weak prefrontal trough could develop near Maui on Thursday and serve as the boundary between gentle west to northwest winds over most islands and gentle southerlies around the Big Island. The shallow front will reach Kauai Thursday night, then bring gentle to moderate northwest winds as it moves down the island chain Friday and Friday night. Thunderstorms will be possible near Maui along the trough Thursday night, with chances spreading to Big Island waters Friday. High pressure will build north of the region during the weekend, leading to increased fresh to strong north to northeast winds. A Small Craft Advisory (SCA) will likely be needed.
The current northwest swell will decline through Thursday, followed by a larger swell Friday. PacIOOS nearshore buoys are showing the declining northwest swell at 5 to 6 feet 13 seconds this morning, and a continued slow decline will occur through tomorrow. A deep, slow-moving, northwest Pacific storm has been generating a large swell that will pass through the area into the weekend. Long period forerunners will arrive Thursday night, and the swell will produce High Surf Warning conditions across most north and west facing shores during the peak Friday into early Saturday. An SCA will be needed for waters exposed to this swell. The swell will gradually decline later Saturday into Monday, followed by a slightly smaller northwest swell next Wednesday.
An out of season period of south-southwest swell is due Thursday through the weekend. An increase in small inconsistent swell is expected Thursday, followed by a larger pulse arriving Friday. South shore surf is expected to peak at minimum around summertime average during the peak of the swell Friday and Saturday, then gradually decline Sunday and Monday.
East shore surf is already below seasonal average and will decline over the next day or two as trade winds become disrupted. Rough east shore surf will rebuild Saturday night and Sunday as northeasterly trade winds strengthen, then decline again early next week as winds ease.
None.
