Trade winds will become slightly stronger Friday as a band of moisture brings an increase in rainfall to mainly windward areas. Trade winds and shower activity will diminish Saturday. An area of low pressure will develop northwest of the islands on Sunday then drift southward, causing surface 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 to gradually reduce shower chances Monday. Drier locally breezy trade winds are possible by Tuesday.
Early this morning, radar shows isolated to scattered showers moving west across the state. The highest concentration of showers is moving over southern and central Kauai. Clouds cover most of the islands, with the Big Island likely to stay mostly cloudy pretty much all day today. Winds were generally out of the east or northeast at 5 to 15 mph.
A band of enhanced moisture will finish crossing the islands today. This band marks the leading edge of increasing trade winds, supported by a strong surface high centered around 1400 miles to our north. The high will drift southeastward, pushing the band of showers to Big Island today, where the it will stall and slowly dissipate. Some showers may carry over to leeward areas, but a ridge aloft will maintain relatively stable conditions that will keep rainfall totals modest.
Guidance has come into better agreement on the upper low moving close to the islands Saturday, then stalling just to our northwest Sunday, then drifting away to the southwest Sunday night and Monday. This progression will lead to easterly trade winds weakening and rainfall diminishing Saturday. Surface winds are likely to shift out of the southeast Sunday into Monday, helping to rapidly increase moisture (1 to 1.5 standard deviations above normal). At the same time, the proximity of the upper low will help to destabilize the atmosphere increase the chance for stronger showers and even thunderstorms. These effects are likely to be felt mostly over the western half of the island.
It now appears that the heavy rain and thunderstorm risk period may last past Monday, perhaps through Wednesday. With the way models have been bouncing around concerning the evolution of this pattern, we have limited confidence in the forecast past early next week.
Showers are expected to continue Friday as easterly trade winds slightly intensify. A band of moisture will complete its trek across the state, adding to the likelihood of showers as well. Saturday should be drier before a significant rainfall period begins Sunday and lasts well into next week.
AIRMET Sierra is in effect for mountain obscurations for northern through southeastern sections of most islands due to clouds and showers moving over windward and mauka areas.
AIRMET Tango is in in effect for turbulence from 28kft to 36kft.
High pressure building north of the state will keep moderate to locally strong easterly trade winds in the forecast through Friday night. The Small Craft Advisory was extended in coverage this morning to include most windward coastal waters due to the building northerly swell expected later today. Moderate to locally strong trade winds will briefly decrease this weekend and veer from the east to southeast direction as a low pressure system develops north of Kauai and tracks southwestward. As this low 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 continue into the early part of next week. Rough conditions with large seas and heavy rain are expected across most coastal waters through Tuesday.
A moderate, medium-period north to north-northeast swell will fill in slowly today, especially this afternoon and surf heights along north facing shores will peak just below High Surf Advisory thresholds from tonight through Saturday morning.
Moderate to locally strong trade winds will keep rough and choppy surf 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 from late 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 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.
A building north to north-northeast swell today 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 Kauai Northwest Waters-Kauai Windward Waters-Kauai Channel-Oahu Windward Waters- Kaiwi Channel-Maui County Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
