The band of heavy rainfall that produced significant impacts over Kauai on Thursday through Thursday night and Oahu today is shifting east tonight, with Maui County and the Big Island now facing the potential for heavy rain and flash flooding overnight and into Saturday. Although conditions may briefly improve for most areas by Saturday, another disturbance is expected to renew deep tropical moisture across the state late Saturday night into early next week, bringing the potential for another round of heavy rainfall and renewed flooding concerns statewide. Light and variable winds are expected to prevail through the weekend and much of the upcoming week.
Preliminary peak storm total rainfall amounts across the western end of the state from Thursday into Thursday night across Kauai have generally ranged from 12 to 18 inches, while today’s event over Oahu has produced widespread totals of 4 to 8 inches over the past 12 hours. These amounts have resulted in substantial runoff impacts and flooding, elevated stream levels, and saturated soils across both islands.
Through tonight, the axis of deep tropical moisture and associated band of heavy rainfall continues to shift eastward, with the primary focus now transitioning to Maui County and the Big Island. Similar impacts to those observed farther west remain possible, including periods of intense rainfall rates, isolated thunderstorms, and flash flooding into early Saturday morning.
The initial shortwave responsible for the most organized large- scale lift earlier is continuing to shift northeast away from the state, and upper-level support is expected to gradually weaken into Saturday. As a result, rainfall intensity should begin to ease somewhat by Saturday over the eastern end of the state where the axis of deep moisture will linger. Over the western end of the state, drier air is beginning to filter in from the west, which should limit rainfall chances. This improvement, however, is expected to be temporary.
Guidance continues to show another shortwave rotating through the broader trough northwest of the state Saturday night into early next week. This feature is expected to reinvigorate the deep moisture plume already draped across the islands and support another round of widespread showers with periods of heavy rainfall. Confidence continues to increase that renewed flooding impacts may develop by Sunday and persist into early next week.
Antecedent conditions are now becoming the primary concern statewide. With soils saturated, streams running elevated, and some areas already having received significant rainfall totals, flooding impacts will be much easier to realize with the next round of heavy rain. While some guidance continues to suggest an additional several inches may be possible through the duration of the next event, exact storm totals are less important than rainfall intensity and duration. Any period of intense rainfall rates could quickly lead to renewed flash flooding impacts.
Although the Flood Watch has been cancelled for Kauai with the eastward progression of the current event, it will likely need to be reissued later tonight or Saturday as the next round of heavy rainfall potential develops. This will include an extension of the watch for the remainder of the island chain.
Although the overall large-scale pattern across the region will linger, one notable change in the forecast heading into next week is the low-level wind pattern. The strong southerly winds associated with this system that we've experienced over the western end of the state are expected to diminish, with mainly light and variable winds expected from Saturday through much of next week statewide. This lighter flow regime may allow heavy showers to persist over localized areas for longer durations.
A band of showers, some heavy, were moving E over Maui County this evening and bringing widespread MVFR ceilings and visibility with isolated IFR. This activity is forecast to weaken late tonight. By Saturday, expect mostly VFR around this area with areas with MVFR in passing showers and low clouds.
AIRMET Sierra is in effect for mountain obscuration above 2000 ft for all islands.
AIRMET Zulu remains in effect for moderate icing between 140-FL200 across the eastern end of the state.
A gale force low that is currently northwest of the main Hawaiian Islands continues to draw deep moisture across the region. Moderate to locally heavy showers are continuing across the central waters this evening, focused along a trough that extends from the low. Chances for locally heavy showers and isolated thunderstorms will continue through the rest of tonight as southerly winds trend down. The trough will linger in the vicinity of the islands into early next week, keeping winds lighter than seasonal average starting Saturday. East of the trough, moderate to fresh easterly trade winds veer southeasterly, while winds west of trough will be light and variable, which may become southerly next week but remain generally light.
Decreasing southerly winds will lead to declining surf along southern shores tonight and a return to smoother, less choppy conditions. However, slightly elevated surf will still linger into Saturday with a sustained medium-period south swell. Continued pulses of southerly swell will keep surf along south facing shores at least small through mid-next week.
Along north facing shores, a moderate, short-period northwest swell generated by the gale force low to the northwest will bring a quick jump of small to moderate surf to north and west facing shores which peaks on Saturday before slowly diminishing into early next week.
Surf along east facing shores remains below seasonal average with lighter than average trade winds near and upstream of the islands forecast through next week.
Flood Watch until 6 AM HST Saturday from Oahu to the Big Island.
Winter Weather Advisory until 6 AM HST Saturday for Big Island Summits.
Wind Advisory until 6 AM HST Saturday for Big Island Summits.
