textproduct: Honolulu

SYNOPSIS

A weather pattern featuring light winds, mostly cloudy skies, and scattered to numerous showers will continue today and tonight, with the greatest moisture residing over portions of Maui County and the Big Island. The weather will once again become more active beginning as early as Thursday as a new low develops, bringing renewed chances for heavy rain and moderate kona winds, especially for Friday into the weekend. Given the high soil saturation from the recent kona storm, even moderate rainfall rates could pose a risk for rapid runoff and flooding, especially during the latter portion of the week.

DISCUSSION

Early this morning, light to occasionally moderate showers continue across portions of the Big Island as a low-level trough moves overhead. Automated rainfall gauges across the Big Island show that accumulations have been relatively light at most locations, with very few locations measuring over 0.5 inch since midnight. Meanwhile, widespread cloud cover continues to stream over primarily the eastern end of the state as well. Elsewhere, conditions have remained mostly dry throughout the night in the wake of the trough. An overnight advanced scatterometer pass show light winds prevailing across the region. At the summits of the Big Island, however, latest observations continue to show very strong winds well in excess of 65 mph for the last several hours. Therefore, the High Wind Warning for Big Island summits will remain in effect through 6 PM HST this evening.

The state continues to reside beneath a large upper-level troughing pattern, which continues to transport moisture-rich air into the region. The 12z upper air soundings from Lihue and Hilo continue to show a moisture-rich atmosphere over the islands, which will support keeping scattered to numerous showers in the forecast for today and tonight. Despite this abundance of moisture in the region, today is still likely to be the driest portion of the forecast for the rest of this week, ahead of the next low pressure system that is forecast to form nearby. Winds will remain relatively light and variable through tonight.

From Thursday onward, the upper-level winds will become stronger and a trough will deepen into a low just west of the state. Once again, a rich plume of deep tropical moisture will be drawn over the islands, especially the eastern half of the state over Maui County and the Big Island, and will mark the return of south to southwesterly surface flow (kona winds). As a result, shower chances will increase significantly again beginning for some areas on Thursday and lasting into the weekend, including the potential for heavy rain and flooding. The latest model guidance continues to support an additional several inches of rainfall possible over portions of the state during this period. While this next system is not anticipated to be as intense as the recent kona storm event, it is important to note that it will impact an already highly saturated environment.

Given the antecedent conditions, including elevated streams and saturated soils, even moderate rainfall rates may quickly lead to runoff issues. The threat for flash flooding will become elevated statewide by Friday, with the potential for more significant impacts persisting through the weekend. We will continue to refine the forecast over the next couple of days as details become more clear.

AVIATION

A low level trough passing through the Big Island will continue to bring periods of rain and MVFR conditions into the Big Island this morning. These MVFR conditions will decrease in coverage through the afternoon.

AIRMET Sierra remains in effect for mountain obscuration over the Big Island at or above 020. No other AIRMETS are currently in effect.

MARINE

Light to locally moderate east to southeast winds will slowly strengthen to moderate speeds by Thursday as a ridge builds to the north of the state. Winds across the coastal waters will veer to the south by Thursday into the weekend. By this weekend, a weak low approaching from the west traverses across the state and begins to lift north on Saturday, which appears likely to maintain light to moderate southerly winds for the area.

Offshore NDBC buoy northwest of the islands continues to rise as this growing north-northwest swell continues to exceed guidance. Therefore, the High Surf Advisory has been extended through the afternoon for north facing shores of Kauai, Oahu, Molokai and Maui. Surf will gradually decline through Friday as the swell shifts out of the north and north- northeast. A new small to moderate long period west- northwest swell is expected to arrive late Saturday and hold through early next week.

Surf along south shores will see moderate surf just below High Surf Advisory (HSA) criteria as the long period south swell continues to fill in this morning and will hold through Thursday. Surf along east facing shores will remain well below average due to the lack of trades locally and upstream. The exception will be for east facing shores exposed to north- northeast swells Thursday through the weekend.

HFO WATCHES/WARNINGS/ADVISORIES

High Surf Advisory until 6 PM HST this evening for north facing shores of Niihau, Kauai, Oahu, Molokai, and Maui.

High Wind Warning until 6 PM HST this evening for Big Island Summits.