FORECAST DISCUSSION

Synopsis

Trade winds will gradually decrease over the next couple of days, with afternoon sea breezes becoming widespread over leeward areas by Wednesday. Moderate showers will continue to favor windward slopes into Tuesday, and chances for heavier showers will increase on Wednesday, especially over interior sections, as the atmosphere becomes unstable. A more stable, hybrid trade wind and weak afternoon convective pattern will prevail on Thursday and Friday, followed by light and variable winds and a land and sea breeze pattern over most islands this weekend.

Discussion

A weakening and somewhat stable trade wind flow will focus clouds and showers over windward slopes into Tuesday. Surface high pressure centered roughly 1100 miles northeast of the state is being eroded and pushed eastward by an approaching front. As a result, winds at most local observing sites are down compared to yesterday and will continue a slow decline during the next couple of days. Low level ridging is keeping the atmosphere somewhat stable, even though weak upper level troughing overhead is inhibiting a well-established inversion from forming. Expect moderate showers to continue to be focused over windward slopes, with pockets of increased moisture riding through. On late Tuesday night and Wednesday, unstable conditions will develop as a somewhat strong mid to upper level trough swings over and north of the islands. As this feature approaches Tuesday night, trade winds will become easterly and may shift out of the east-southeast, possibly setting up conditions for some heavy showers along windward slopes of the Koolau Range on Oahu. As trades weaken further on Wednesday and instability is maximized, afternoon sea breezes will likely trigger spotty convection over leeward and interior areas of the islands. According to the GFS and ECMWF, the deepest moisture will be held off to the northeast, but we have added a chance for heavy showers in the official forecast. If models trend toward the current GFS solution, we will have to consider introducing a slight chance of thunderstorms as well. A hybrid trade wind / convective pattern will continue on Thursday and Friday, though conditions are expected to be more stable as the upper level trough departs. The weak trade winds will produce modest showers over windward slopes. Meanwhile, afternoon sea breezes will generate clouds and a few afternoon showers over leeward and interior sections, followed by clearing at night as land breezes develop. Heading into the weekend, trade winds are expected to be cutoff over most islands. While southeast winds may hang on over the Big Island, most islands should experience a humid land and sea breeze regime, featuring afternoon clouds and spotty interior showers with partial clearing at night. Details remain unclear at this time, but volcanic emissions may be drawn up over the smaller islands. Uncertainty increases for early next week. Odds are high that trade winds will remain suppressed and that a land and sea breeze regime will continue. The GFS and ECMWF are hinting at another upper level trough dropping toward the state and pushing a weakening front near the islands. This could enhance convection again on the Memorial Day holiday.

Aviation

Trade wind flow will focus patchy clouds and showers along the windward slopes and coasts of all islands. Isolated MVFR ceilings and visibilities are expected in passing showers. Weak sea breezes and abundant moisture will combine along the Kona coast of the big island to encourage afternoon and evening convective build-ups over inland areas. Partial clearing is expected across leeward big island after midnight. VFR conditions are expected to prevail through at least early Tuesday. No AIRMETS in effect or anticipated at this time.

Marine

Trade winds will be gradually declining during the next couple of days. We have canceled the Small Craft Advisory (SCA) for the typically windy waters around the Big Island and Maui, and there is high confidence that the SCA flag will remain down into early next week. In fact, we could see variable winds develop near the smaller islands during the holiday weekend. Small surf will dominate on most shores this week. The one exception is a small to moderate southwest swell that will build on Thursday, peak on Friday, then decline during the weekend. This swell will originate from the Tasman Sea, and given the large amount of south Pacific island blocking on swells from this direction, surf locally tends to be inconsistent. See the recently issued Collaborative Nearshore Swell and Wind Forecast (SRDHFO) for more details.

HFO Watches/Warnings/Advisories

None.

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Public and Marine...Wroe Aviation...Bedal

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