Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

835 pm hst Sun aug 14 2022

Synopsis

Expect light to moderate trade winds as a long wave trough far north of the state breaks down the ridge and decreases the trade winds across the region. A hybrid wind pattern will develop on Monday and Tuesday with leeward sea breezes and lighter easterly trade winds lingering over windward areas. The high pressure ridge builds back in north of the islands on Wednesday with strengthening trade winds from Wednesday onward.

Discussion

Satellite imagery this evening shows a few bands of clouds drifting into each island on the trade winds. A low level trough remains roughly 300 miles southeast of the Big Island, drifting slowly westward on the trade winds. This trough may bring a slight increase in showers over the eastern islands on Tuesday as it passes south of the state. A few high cirrus clouds are drifting over the islands associated with a weak subtropical jet stream.

A long wave trough far north of the state is breaking down the subtropical ridge north of the islands and decreasing the trade wind flow over the next three days. As these large scale winds weaken, local scale land and sea breezes will strengthen from Monday into Wednesday. This trough will weaken and drift further eastward by Wednesday, allowing the ridge north of the islands to build back in and cause the trade winds to strengthen across the region. This translates to a hybrid wind pattern for the next three days with land and sea breezes over leeward areas and light trade winds over the windward zones. Clouds and a few showers will tend to favor afternoons along convergence areas between the trade winds and sea breezes over leeward areas and island interior sections. Brief showers are also possible in this hybrid wind pattern over windward zones in the overnight to early morning hours. A minor wrinkle to this forecast is a passing low level trough drifting south of the state on Tuesday and Wednesday. Moisture around the eastern side of this trough may slightly increase clouds and shower activity over the Big Island and Maui. Chances for thunderstorms pick up over the higher elevations of the Big Island from Wednesday afternoon to evening as surface heating may trigger deeper convection or thunderstorms in the unstable layer above the trade wind inversion height.

We transition back to a trade wind pattern on Wednesday as the high pressure ridge begins to build back in north of the region. Expect moderate to locally breezy trade winds from Thursday through Sunday with diminishing land and sea breezes. Clouds and brief passing showers will trend back towards the windward and mountain sections of each island.

Aviation

Light to locally moderate trade winds will persist through twenty-four hours and beyond. Ragged bands of stratocumulus and light showers will favor east and northeast facing slopes and coasts. A small eddy just west of the Big Island may keep leeward Big Island shrouded overnight in VFR cloudiness and promote afternoon sea breezes along the Kona Coast.

A trade wind inversion around 08 kft will inhibit daytime shower development over island interiors, but brief moderate to heavy showers will still be possible over interior Big Island in the late afternoon and early evening.

Localized MVFR CIG or VIS possible in showers, otherwise VFR conditions will prevail all areas. No AIRMETS are currently in effect and none are expected through the forecast period.

Marine

Moderate to fresh easterly trades will hold tonight, then ease into the gentle to moderate category Monday through Wednesday, as the ridge north of the state weakens. This pattern will translate to localized overnight land breezes and daytime sea breezes near the coasts beginning by Monday afternoon. As typically observed with these patterns, pockets of fresh breezes near some coasts can lead to choppy conditions due to a combination of daytime heating and terrain-induced accelerations. A return of fresh to strong easterly trades is expected through the second half of the week as the ridge builds to the north, which may lead to Small Craft Advisory conditions for the windier waters and channels over the eastern end of the state.

After a quiet spell through the first half of August, a more active pattern unfolding within Hawaii's swell window around New Zealand will lead to an upward trend along south facing shores beginning next weekend. Until then, expect small conditions to persist with mainly a combination of background south and southeasterly energy moving through.

Surf along east facing shores will remain small through the first half of the week as the aforementioned trades ease. A return of choppy/short-period surf is expected by the end of the week as the trades fill back in from east to west.

Surf along north facing shores will trend up slightly late tonight into Monday at spots that favor northerly swells due to a recent batch of northerly winds associated with low pressure near/over the Aleutians aimed at us. This will be short-lived, but may return to similar levels Wednesday night through Thursday as a similar scenario unfolds tonight through Monday far north of the area. Otherwise, the flat summertime conditions will prevail.

Large daily tide cycles associated with our typically observed King Tides for this time of year will continue into Monday, then steadily lower through the week.

Watches, Warnings, Advisories

None.

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