Moderate trade winds will focus showers over windward and mauka areas into Thursday. A dissipating front will move across the islands Thursday night into Friday, leading to an increase in rainfall and stronger trade winds. Drier trade wind weather is favored on Saturday, giving way to a potential increase in showers and southeast winds Sunday and Monday.
A stable and moderate trade wind flow continues to produce typical windward rainfall. A slow-moving and weak front about 100 miles north of Kauai sits between Hawaii and a somewhat strong 1035 mb surface high centered far north near 47N164W. This is resulting in a slightly looser local pressure gradient and trade winds at only moderate strength. The high clouds that covered much of the island chain overnight are clearing to the east as an upper level trough flattens overhead and pushes the subtropical jet stream south of the islands. A mid level ridge is maintaining stable conditions, and showers in the trade wind flow have produced less than a tenth of an inch of rainfall over windward areas and a few showers over the Kona slopes of the Big Island today. Expect little change into New Year's Day as the weakening front drifts southward toward the islands.
The dissipating front will move down the island chain Thursday night and Friday, leading to increased showers and strengthening trade winds. The shallow band of moisture along the feature will reach Kauai around sundown on New Year's Day, sag southward to Oahu and Maui County overnight, then reach Big Island on Friday. The remnant front will quite shallow, and with a weak mid level ridge holding, there will be no upper level support for heavier showers. As a result, expect wet conditions windward, and as the surface high the surface high to the north strengthens, breezy trades will likely carrying brief periods of showers over leeward areas from Kauai to Maui. Breezy trade winds should push much of the moisture clear of the islands on Saturday.
Uncertainty remains somewhat high for Saturday night into Monday, though the latest operational runs of the ECMWF and GFS have come into better agreement. An amplified ridge aloft north of Hawaii will break down late Saturday as a portion of a sharp upper level trough near the Dateline moves westward toward the state. This upper trough should induce surface low pressure several hundred miles west of Kauai, causing local winds to increase and veer southeasterly Saturday night and Sunday. As the upper level trough moves near then over the state later Sunday and Monday, layered high clouds should spread over the area with heavy showers, and possibly thunderstorms, developing near or just west of the islands. There is still quite a bit of uncertainty on the details, but chances for unsettled weather are increasing.
A moderate to locally breezy easterly trade wind regime will continue into tomorrow. Embedded trade wind showers will primarily affect windward and mauka areas. Enhanced showers are also possible starting tomorrow evening as a moisture band moves down the island chain from the north. Primarily MVFR conditions are expected within these showers, while VFR prevails elsewhere.
AIRMET Sierra for mountain obscurations is in effect for N through E sections of Oahu. This will likely be cancelled this evening due to a lack of upstream clouds and showers. No other AIRMETs are in effect.
Moderate to locally strong easterly trade winds will continue through Saturday as high pressure builds north of the islands. The Small Craft Advisory was extended through Thursday for the typically windy waters around Maui County and the Big Island. Model trends indicate winds veering more out of the southeast for Sunday into next week as an area of low pressure develops northwest of the state.
A medium-period north-northwest (340-350 degree) swell is now beginning to lower, with reported surf heights now just below advisory thresholds. Therefore, the High Surf Advisory has been cancelled for north and west facing shores. This swell will continue to lower through the remainder of the week. A moderate, medium-period north-northeast (360-020 degree) swell will move into Hawaiian waters by Friday, possibly producing moderate surges in north facing harbors, as well as boosting surf along north facing shores to near advisory levels.
Choppy east shore surf will continue into the weekend as locally breezy trade winds persist. Surf along south facing shores will remain tiny to small through the forecast period.
Peak astronomical monthly tides may result in minor coastal flooding from Thursday through early next week, and may be enhanced along north facing shores later this week due to the next north swell. A Coastal Flood Statement has been issued to highlight this flooding potential through Friday night.
Small Craft Advisory until 6 PM HST Thursday for Pailolo Channel- Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
