Forecast Discussion

Synopsis

29 Apr 2025 19:57:00 GMT: A moist unstable airmass will hold through today. Light to gentle winds will allow for sea breezes to develop in the afternoon and land breezes overnight. A cold upper level low east of the state will increase the chance of locally heavy showers and a few thunderstorms this afternoon. Land breezes will keep most shower activity over the coastal waters or near the coast at night. Trade winds are expected to return Wednesday, reaching breezy levels Friday into early next week, along with a more typical windward and mauka shower pattern.

Discussion

29 Apr 2025 19:57:00 GMT: A weak surface trough continues to meander between Kauai and Oahu, while high pressure is centered far northeast of the islands. This is keeping winds light across the region. Satellite and radar imagery continues to show clouds and scattered moderate to locally heavy showers mostly over the coastal waters surrounding Oahu this morning. A deep upper low just east of the state will help to increase our instability, particularly east of Kauai. Heavy showers and thunderstorms will be possible during the afternoon hours as sea breezes become more developed and favor interior and leeward areas. Land breezes will develop shortly after sunset that could help push shower activity out over the coastal waters or areas near the immediate coast overnight. Wednesday will serve as a transition day where gentle to moderate trades will gradually build back as the surface trough dissipates west of the state allowing drier air to slowly filter in from the east. The airmass will also gradually become more stable as the upper level low shifts eastward and opens up to a trough, allowing a mid level ridge to build over the state. Moderate to locally breezy trades are expected to return Friday then hold through early next week as a pair of strong high pressure systems move north of the state. Expect showers to favor windward and mauka areas, particularly during the overnight and early morning hours. Volcanic emissions and light background east southeast flow will keep vog in place over most of the state through today. The returning trades should clear the vog out of the smaller islands by late Wednesday.

Aviation

29 Apr 2025 19:57:00 GMT: A weak trough of low pressure slowly passing over the islands will keep the background wind flow light and variable through tomorrow afternoon. This flow pattern will lead to land breezes in the morning hours and sea breezes in the afternoon and early evening hours. Diurnal heating, combined with instability aloft, will combine to generate heavier showers and isolated thunderstorms over inland areas later today and possibly tomorrow. Brief periods of MVFR/IFR conditions may occur with the more robust showers and storms. No AIRMETs are currently in effect, but mountain obscuration could become a concern late this afternoon or early evening. An upper level low northeast of the Islands is generating persistent thunderstorm activity along the major flight routes between Hawaii and California. SIGMET November remains in effect. Conditions are expected to persist through this evening and possibly beyond.

Marine

29 Apr 2025 19:57:00 GMT: A trough of low pressure near Kauai will linger through the day and continue to weaken the high pressure ridge over the Hawaiian Islands, maintaining light and variable winds across the region. This boundary combined with an upper level disturbance moving down the island chain will increase the potential for showers and thunderstorms over the coastal and offshore waters. Trades will build back in from Wednesday onward, strengthening to Small Craft Advisory levels for the typical windier coastal waters around Maui and the Big Island by Thursday. A small northwest swell will slowly fade today, allowing surf along north and west facing shores to remain near or below average levels for this time of year (6 feet for the month of May). By Thursday, a moderate long period north-northwest swell is expected to arrive, producing surf above the seasonal average. This north-northwest swell will slowly decline over the weekend. Multiple small south swells will maintain small background southerly energy today. A bump up in south-southwest swell energy will arrive on Wednesday, peak Thursday producing moderate surf, and then slowly fade through the weekend. However, a mix of reinforcing south-southwest swell and new south-southeast swell energy will arrive this weekend and should help maintain surf near or above the summer average (5 feet). Then another pulse of south-southwest swell energy will arrive early next week. Weak upstream trade wind flow will keep surf on the small side for east facing shores. An increase is expected by Wednesday as the trade winds return. Trade winds should strengthen through Friday with rough and choppy surf expected along east facing shores through the weekend.