A significant change in the weather pattern is expected early next week. Moderate to breezy east-southeast trade winds and mostly stable conditions will persist through the weekend. Beginning late Monday into Monday night, a deep low pressure system evolving northwest of the islands will shift winds southerly and draw deep tropical moisture northward across the state. This will bring increasing chances for heavy rainfall, thunderstorms, and strong winds starting over the western islands late Monday, then spreading statewide through midweek. Unsettled weather may persist through much of next week due to the slow- moving nature of the system.
A notable shift in the large-scale pattern is expected to unfold across the Hawaiian Islands during the upcoming week. The current dry and fairly stable regime, characterized by moderate to locally breezy east-southeast trade flow, will hold in place through the weekend. Conditions will then begin to change late Monday into Monday night as a deep upper trough develops and amplifies northwest of the islands, bringing a multi-faceted weather event to the state that could include heavy rainfall with flash flooding potential, strong to severe storms, and strong Kona winds.
Model guidance continues to indicate a strong jet streak, with core winds around 120 to 140 kt at 250 mb, digging southeast along the western flank of the evolving trough. As this occurs, the trough is expected to acquire a more negative tilt and expand toward the island chain. The islands will increasingly come under the influence of the jet streak’s left-exit region, promoting enhanced upper-level divergence and difluent flow aloft. This large-scale ascent, combined with smaller shortwave disturbances rotating around the base of the trough, could generate periods of organized convection through much of next week, potentially beginning as early as Tuesday.
At the surface, global and ensemble guidance are fairly consistent in depicting a broad cyclonic circulation developing northwest of the state by early next week in response to strong upper height falls. Central pressures near 985 to 990 mb are possible with this feature remaining well northwest of the islands. The resulting pressure pattern will support strengthening south to southeast winds across the state, allowing a plume of deep tropical moisture to surge northward into the region. Precipitable water values are expected to climb into the 1.7 to 2 inch range by Tuesday over the western islands, then spread across the remainder of the state by midweek.
The evolving thermodynamic and kinematic environment, including upper height falls, cooler air aloft, and increasing deep-layer shear, will favor episodes of heavy rainfall with embedded thunderstorms. Combined with broad confluent south-southeast flow and island terrain effects, this pattern could support training and anchored convection, particularly along favored terrain. This raises the potential for significant flooding concerns if heavier rainfall bands persist over the same areas. In addition to the heavy rainfall threat, some storms could become strong, and occasionally severe, by the middle of next week with locally damaging winds possible.
Impacts may begin to develop across the western islands as early as Monday night as the leading edge of the moisture plume reaches the state. The risk for heavy rain and thunderstorms will likely expand eastward across the remainder of the island chain Tuesday into Wednesday as deeper moisture and stronger forcing for ascent overspread the region.
While details regarding the exact timing and location of the heaviest rainfall remain uncertain, confidence continues to increase that a prolonged period of unsettled and potentially impactful weather will affect the islands next week. Flooding impacts could become more pronounced over time as soils become saturated and stream levels and reservoirs become elevated. Residents and interests across the state should continue to monitor forecasts through the weekend as this system comes into better focus and details regarding the timing and location of impacts become clearer.
Trades are predominately expected through the period, with VFR at most sites save for windward sites that may see intermittent MVFR due to lower cigs and shower activity. No AIRMETs are currently in effect for the period, but low to moderate turbulence is possible during the day tomorrow for leeward sites as trades increase in strength.
Strong high pressure far to the northeast of the state will maintain fresh to strong trade winds through the weekend, with the strongest winds over the typically windy areas around Maui County and the Big Island. A Small Craft Advisory remains in effect from the windward Oahu waters and Kaiwi Channel eastward through 6 am Sunday, with the exception of Maalea Bay. This SCA will likely need to be extended for portions of the marine area through at least Sunday. A front approaching from the west will ease the trades and shift them southeasterly on Monday, with the winds then becoming southerly and increasing to fresh to strong levels Tuesday and Wednesday.
Surf along east facing shores will remain elevated and choppy through Monday, followed by a downward trend Tuesday through late next week.
A series of west-northwest swells will keep some small surf in place along north and west facing shores during the next 7 days, but surf will remain well below advisory levels. A small to moderate sized north swell is possible around Thursday and Friday of next week.
Surf along south-facing shores will remain small through early next week, with the exception of areas exposed to trade wind swell wrap. Rough and choppy conditions appear to develop Tuesday and continue through late next week as southerly winds increase in advance of a front. A series of small long period south swells will also move through Monday through late next week.
Small Craft Advisory until 6 AM HST Sunday for Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-Maui County Leeward Waters-Pailolo Channel-Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward Waters-Big Island Southeast Waters.
