A front will bring a round of showers to Kauai tonight and Oahu early Thursday before stalling over Maui County during the day. Limited rainfall is then expected through Saturday before a stronger front brings heavy rainfall potential to the western end of the state Sunday through Tuesday.
Dearth of moisture over the islands this afternoon as evidenced by clear to partly cloudy skies over area waters and very dry soundings out of both Lihue and Hilo. At press time, strongly stable conditions prevail with an inversion around 5kft. Visible satellite indicates a decaying cold front advancing toward Kauai in response to a baroclinic frontal wave deepening well north of the islands. Satellite observations put upstream shower tops around 11kft which closely matches the modeled moisture depth as this shower band moves into the island chain tonight.
Tonight. With the parent trough already departing to the northeast, this feature will have no upper support and will instead move into the forecast area against a background of mid-level height rises and general large scale subsidence. A few leeward showers will be possible within southerly flow ahead of the front, but the convergent frontal axis itself and trailing NW winds will serve as the focus for the bulk of rainfall which along exposed slopes late this evening (Kauai) through early tomorrow morning (Oahu). Stout NW flow will rapidly weaken as it veers to NNE during Thursday. Consequently, showers may hang up along the northern slopes of Kauai for a time, but the diminishing gradient combined with crashing post- frontal dewpoints in the 50s should inhibit potential for any stream rises there as shower intensity and coverage behind the front quickly wind down.
Thursday through Saturday night. Lack of large scale forcing causes the front to stall over Maui County on Thursday while the rapidly diminishing pressure gradient allows potential for a transition from windward showers to interior showers as afternoon sea breezes develop, mainly over interior Maui. A few afternoon showers will also be possible over Oahu since the punch of dry air will struggle to push that far east. Overnight land breezes bring drying to all islands on Thursday night. The remainder of this time period will generally be dry, though moderate southerlies threaten to push the occasional shower over leeward zones of Kauai through Maui. By Saturday night, isentropic ascent associated with the approach of a much deeper upper trough will gradually saturate the mid-levels, though soundings are modeled to be initially disjointed courtesy of a stable dry layer anchored around 10kft. This will favor limited shallow leeward showers coupled with a developing layer of stratiform rain aloft, most likely centered between Kauai and Maui. Any rainfall through Saturday night is forecast to be modest.
Sunday through Tuesday. This period represents a transition to a more dynamic pattern. A second remnant front, not dissimilar from the current one, will advance into the western portion of the state during Sunday providing a boost to low-level moisture and possibly allowing for some coupling with the aforementioned layer of deepening mid-level moisture by Sunday afternoon. Outstanding right entrance jet support associated with a SWly 135kt jet then spreads over the area late Sunday as the upper trough approaches. This in turn deepens the existing cold front as healthy fgen develops through the column. The resulting mature jet-front system will be accompanied by a narrow corridor of strong uplift that will support a band of heavy rain somewhere over the western end of the state Sunday night through Monday night. The parent trough remains rather progressive suggesting a 24-36 hour type of event. Upon the departure of supporting upper dynamics on Tuesday, the orphaned frontal moisture band will once again stall over the area through mid-week in maintenance of lingering showers over the area. Southerly winds prevail throughout this time keeping leeward zones in the game for rainfall for at least the next week.
Forecast reasoning for the extended remains unchanged. Blocking high pressure over the Aleutian Island chain maintains a southward- displaced Pacific jet stream that is modeled by the EC, GFS, and machine learning guidance to settle into the low latitudes (20-25N) by the end of the forecast period. This ensures at least some potential for more active weather continuing for the foreseeable future.
South to southwest winds have strengthened across the state this afternoon ahead of a cold front approaching from the northwest. VFR conditions will continue to prevail into the early evening hours as limited clouds and showers move in with the southerly flow. The cold front is expected to reach Kauai early this evening, Oahu early Thursday morning, and then slowly diminish over Maui County. Along the front, showers will bring periods of MVFR and even brief IFR conditions will be possible. Behind the front, light to moderate northerly winds and VFR conditions are expected.
No AIRMETs are in effect, but AIRMET Sierra for mountain obscuration may be needed this evening through tomorrow morning as the front moves from west to east over the smaller islands.
Light south southwesterly winds will be in place this afternoon for all islands ahead of an approaching front from the northwest. The front will enter the Kauai waters late tonight and move east across the state through Thursday before stalling and dissipating around Maui County Thursday night. An increase of showers are expected along and ahead of the front with moderate northerly winds following behind it. Another stronger front looks to approach the islands from the northwest on Saturday afternoon through Sunday and bring moderate to fresh south southwesterly winds and increased rainfall over the western islands. Current guidance suggests this front could stall near Kauai and linger through next week.
A moderate, long period northwest swell (320 degree) will hold before reinforcing north- northwest swell (330 degree) fills in tonight and peaks Thursday morning. These swells will keep surf elevated, therefore a High Surf Advisory for select north and west facing shores remains in effect through Thursday. These overlapping swells will slowly decline Thursday night into the weekend. Another moderate, long period northwest swell (320 degree) should gradually fill in on Sunday and could produce surf near the advisory threshold by Sunday afternoon. A combination of small north swells and small west-northwest swells then look to arrive next week.
East shore surf will remain small through the forecast period due to the absence of trade winds. Expect minimal background surf for south facing shores, with a minor south- southwest pulse expected today into Thursday. South facing shores could receive small to moderate choppy windswell Saturday through next week from the southwest winds. This would depend on the location and strength of the front, likely largest for southwest shores of Kauai.
High Surf Advisory until 6 PM HST Thursday for Niihau-Kauai Leeward-Waianae Coast-Oahu North Shore-Maui Windward West-Kauai North-Molokai Windward-Molokai North-Molokai West-Maui Central Valley North-Windward Haleakala.
