A stalled frontal boundary near Maui will remain over the area through Friday and bring enhanced shower activity to mainly Maui County and to a lesser extent windward portions of the Big Island. Unsettled weather resumes this weekend as moisture moves over the islands from the south, out ahead of the next approaching frontal system. The front is forecast to pass down the island chain Sunday night into Monday and bring with it a band of steady rain. Trades should return by Tuesday as a surface high passes north of the islands.
Water vapor satellite loop shows a complex of lows spinning in the north central Pacific with a shield of mid and upper level clouds streaming over Hawaii from the southwest, associated with a digging upper level trough. Down at the surface, a shallow north to south oriented surface boundary remained over Maui County today. Some pulses of energy rotating around the parent upper trough allowed for showers to reignite this afternoon with pockets of heavier showers forming along the mountainous region of eastern Molokai and lower slopes of Haleakala on Maui. RADAR estimated rain rates generally topped out in the half inch to inch an hour range. Elsewhere mid and high clouds dominated, but with little in the way of precipitation.
Tonight, the surface boundary may wobble slightly eastward, then retrograde back towards the central islands and dissipate on Friday. Thus, most shower activity over the next 12 to 18 hours should stay closer to the boundary along windward and mauka portions of Maui and to a lesser extend the Big Island. Winds will veer easterly by Friday afternoon as a transient surface high quickly passes to the north. This will allow for a brief return of scattered showers along mainly windward locations of the island chain. On Saturday, another longwave trough deepens over the region as the next cold front begins its approach. In response, light to locally moderate surface winds will veer southeasterly on Saturday, becoming southwesterly by Sunday. Both the GFS and ECMWF generate scattered shower activity this weekend, skewed towards the western half of the state, as deeper moisture gets drawn up in this southerly flow.
Sunday night into Monday, long range model push the tail end of cold front down the island chain from the northwest. A moderate to locally heavy band of rain may accompany this front, though best lift and moisture parameters remain well north of Hawaii. It is still too early to determine to what extend a flooding threat exists, but appears minimal if current trends hold. ECMWF is a few hours earlier than the GFS on the frontal passage, but both agree on Monday morning to early afternoon to be the most likely time. Post frontal, winds will be quick to veer northerly, then northeasterly as a surface high passes north of the region, allowing for the brief return of trades.
Scattered showers will continue over windward portions of Molokai and Maui. Elsewhere, limited windward showers are expected tonight into Friday morning. Brief MVFR conditions and lower ceilings are possible with any passing showers but VFR will prevail otherwise.
AIRMET Sierra may be required if showers persist with lower ceilings and resulting mountain obscuration.
SIGMET Romeo and AIRMET Tango remain in effect overnight. Numerous aircraft have reported moderate to severe turbulence between FL270 and FL420.
AIRMET Zulu remains in effect through Friday morning for light to moderate rime icing between 13,000 and 30,000 feet.
Light to moderate northeast winds will persist into Friday across most waters as a stalled frontal boundary diminishes over the eastern end of the state and high pressure builds to the north. The exception will be over the Big Island waters east of the boundary, where light east to southeast winds will prevail. Guidance indicates a progressive pattern continuing into the weekend, with a front approaching and moving through the area Sunday into Monday, followed by another front approaching by midweek. The weekend front could bring locally heavy showers to portions of the marine area, along with a period of moderate to locally fresh southwest winds.
Surf along exposed north- and west-facing shores will remain at advisory levels into Friday as overlapping west-northwest and north- northwest swells move through. As such a High Surf Advisory is in effect for north and west facing shores of Niihau, Kauai and Oahu through 6am Friday. Offshore buoy observations reflect this mixed swell regime, with north- northwest energy in the 10–12 second range. Guidance shows the swell easing late Friday into the weekend, with a small, long- period northwest swell arriving late Saturday.
Surf along east-facing shores will remain small into next week due to the lack of persistent trade winds locally and upstream.
High Surf Advisory until 6 AM HST Friday for Niihau-Kauai Leeward-Waianae Coast-Oahu North Shore-Kauai North.
