Trade winds will become breezy tonight and Friday as a band of moisture brings an increase in rainfall to mainly windward areas. Trade winds and shower activity will diminish on Saturday. An area of low pressure will develop northwest of the islands on Sunday then drift southward, causing winds to shift out of the southeast and bringing cloudy conditions with increasing chances of showers and thunderstorms. Strong and gusty easterly winds are expected gradually reduce shower chances on Monday. Drier locally breezy trade winds are possible by Tuesday.
Increasing trade winds will push a band of enhanced moisture across the islands tonight and Friday. For much of the day, the narrow band has been focused on Kauai, where more than a quarter of an inch of rainfall has fallen over windward slopes, and the feature has been starting to move onto Oahu and Maui County this afternoon. A strong surface high centered 1400 miles north of the state is producing breezy trade winds along and north of the moisture band. The high will drift southeastward, pushing showers over windward areas from Kauai to Maui tonight and eventually to Big Island on Friday, where the band will stall and slowly dissipate. Breezy trade winds along and north of the band will periodically carry some showers over leeward areas, but a ridge aloft will maintain stable conditions that will keep rainfall totals modest.
Model guidance begins to diverge on Saturday, but overall, expect easterly trade winds to ease and rainfall to diminish as high clouds increase. The official forecast leans toward the ECMWF, which is hinting at moisture hanging up on windward Big Island and keeping shower chances higher there.
Uncertainty in the details remains high later in the weekend, but probability of unsettled weather with locally heavy showers is increasing, especially over the western end of the island chain. Guidance continues to show an upper level low developing several hundred miles north of Kauai Saturday night and drifting west of Kauai late Sunday. The current forecast is based on the ECMWF and UKMET solutions of a weak surface low or trough developing under the upper low, instead of the GFS, which appears to deepen the feature excessively and produce a strong surface low. Under the favored ECMWF and UKMET solutions, winds will shift out of the southeast Saturday night and focus showers over windward and southeast slopes as high clouds thicken. The low at the surface and aloft is expected to pass to the west of the state Sunday and Sunday night, and broad convergence on its eastern flank will produce higher shower chances Sunday and Sunday night, especially over Kauai and Oahu. Thunderstorms have been introduced around Kauai and on the Big Island interior for this forecast package, and we will continue to assess the flood threat over the next couple of days.
Longer range guidance leans toward decreased rainfall chances and the development of strong and gusty east to southeast winds as the low drifts away to the west on Monday. Drier locally breezy trade winds are possible by Tuesday.
A band of moisture moving in overnight from the north could provide enhanced trade wind showers over windward and mauka areas with some spillover into leeward areas across the state. Primarily MVFR conditions are expected within these showers, and VFR will prevail elsewhere. Trade winds will remain moderate to locally breezy tonight, with potential to slightly strengthen tomorrow after the band of moisture passes.
AIRMET Sierra is in effect for mountain obscurations for Kauai and Oahu due to clouds and showers moving over windward and mauka areas. This will likely be adjusted through the night as the moisture band progresses through the island chain.
AIRMET Tango is not currently in effect, but will likely be needed tonight for upper level moderate turbulence. This is associated with the subtropical jet moving closer to the islands from the south.
High pressure building north of the state will slightly strengthen the trade winds this evening with moderate to locally strong easterly trades expected to persist through Friday night. The Small Craft Advisory has been extended and may need to be expanded to other coastal waters tonight depending on the ASCAT observations. The moderate to locally strong trade winds should decrease this weekend and veer toward the southeast as a trough of low pressure develops north of Kauai and tracks southwestward. As this trough passes just west of the state, strong to near gale force easterly trade winds are expected to impact the Hawaiian Waters by Sunday evening and likely continuing into the early part of next week. Rough conditions with large seas and heavy rain is expected across many coastal water zones early next week.
Current medium period north-northwest swell will continue to decline tonight and shift to a more northerly direction. Another moderate, medium-period north to north-northeast swell will fill in on Friday especially during the afternoon and peak Friday night into Saturday morning.
With a slight increase of trade winds tonight, a subtle increase of rough and choppy surf is expected along east facing shores. A much stronger fetch of strong to near gale force trade winds will bring a significant increase of easterly wind swell Sunday into early next week, which should exceed advisory thresholds for east facing shores. Surf along south facing shores will remain tiny through the forecast period.
Peak astronomical monthly tides with water levels running roughly 0.5 ft above normal should produce minor coastal flooding through this weekend, and may be enhanced along north facing shores Friday and Saturday due to the incoming north swell. A Coastal Flood Statement has been issued to highlight this flooding potential, which should persist through this weekend with peak high tides expected late tonight and late Friday night.
A building north to north-northeast swell on Friday will also produce moderate surges for north facing harbors, especially for Kahului and Hilo. A Marine Weather Statement remains in effect for maritime interests to watch for these impending harbor surges.
Small Craft Advisory until 6 AM HST Saturday for Pailolo Channel- Maalaea Bay-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
