FORECAST DISCUSSION

Synopsis

Trade winds will continue a gradual decline, with showers, some locally heavy, remaining focused over windward slopes tonight. Widespread afternoon sea breezes will trigger isolated heavy downpours on Wednesday, mainly over interior and mauka sections, as the atmosphere becomes increasingly unstable. A more stable but humid, hybrid trade wind and weak afternoon convective pattern will prevail on Thursday, followed by light and variable winds and a land and sea breeze pattern over most islands heading into the weekend.

Discussion

Trade winds have continued a downward trend today as the surface ridge far north of the state has been eroded by a deepening trough to the northwest of Kauai. Windward rainfall has been modest, with the exception of Kauai, where a pocket of moisture moving along the trade wind flow has been generating some enhanced rainfall along northern and interior slopes. The afternoon Lihue sounding showed this moisture and the easterly trade wind flow confined to the lowest 13000 ft of the atmosphere. This moisture is expected to linger into the evening, then move off to the west. Elsewhere, the decreasing trade winds gave way to leeward sea breezes near high terrain, and with the inversion elevated and weakening, isolated showers, some locally heavy, have been flaring on mainly leeward Big Island and Maui slopes. This activity should diminish toward sunset. The atmosphere will become increasingly unstable tonight as a rather strong upper level trough approaches from the northwest. The highest chances for showers will be on windward slopes, especially across the eastern end of the state, and as easterly trade winds shift out of the east-southeast to southeast in the lowest 10000 ft, conditions will likely become conducive for heavy shower development along windward slopes of the Koolau Range on Oahu overnight. As trades weaken further on Wednesday and instability is maximized, afternoon sea breezes will trigger convection over island terrain and leeward areas. The GFS and, to a lesser degree, the ECMWF continue to hold the deepest moisture off to the northeast of the state, but there should be ample moisture to generate localized heavy showers over interior and mauka areas. The upper level trough will drop 500 mb temperatures over the islands to -10 to -11C, and given the shower activity we have seen today, a slight chance of thunderstorms has been added for some interior areas. A hybrid trade wind / convective pattern will prevail on Thursday, 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 holiday 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 during the weekend.

Aviation

Clouds and showers will continue to favor windward and mountain areas under a veering light to moderate trade wind regime. Meanwhile along the leeward coasts, light sea breezes are driving the development of convective showers over the southwest slopes of the big island and maui. Windward showers are expected to increase or persist throughout the night, but leeward slopes and coasts should experience partial clearing overnight. The air mass over the islands is especially moist today so isolated MVFR conditions are expected in locally heavy showers. The air mass over the state is also becoming increasingly unstable in response to the upper level trough gradually approaching from the northwest. Shower activity tonight is expected to be greatest just to the north of the smaller islands and over windward slopes. An AIRMET for mountain obscuration may become necessary on some islands overnight.

Marine

Today will be the last true trade wind day through the holiday weekend. The are no marine advisories in effect, and we do not anticipate seeing a Small Craft Advisory (SCA) through the week. Trade winds will shift easterly tonight, and they are expected to remain out of the east to east-southeast through Thursday. During that time, winds could be accelerated to near SCA strength over waters north and east of Maui and the Big Island. Lighter winds are due friday through the holiday weekend. Small surf will dominate on most shores this week. The one exception is a potentially 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 the 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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