FORECAST DISCUSSION

Synopsis

Moderate trade winds will trend lighter over the weekend, but will continue to deliver passing clouds and showers to windward areas, mainly during nights and mornings. The lighter winds over the weekend will likely allow afternoon sea breezes to drive the formation of clouds and a few showers over leeward areas of all islands, and a weak disturbance aloft may allow a few briefly heavy showers to develop, especially on Sunday. Breezy trade winds are expected to develop early next week.

Discussion

High pressure cells remain centered far NNW and NNE of the islands, with a low and associated stalled front separating the two. The low- level gradient S of the highs is supporting trade wind flow over the islands, with speeds ranging from light in sheltered leeward locations to breezy in the more exposed areas. Soundings indicate that the island atmosphere remains capped at a height near 8 kft, which is sufficiently low to prevent significant rainfall, but also sufficiently high to allow efficient warm rain processes to produce light to moderate showers. Although recent rain gage reports indicate little in the way of shower coverage and rainfall amounts, the typical diurnal trend toward increased shower coverage is expected overnight, with showers favoring windward slopes and coasts. On water vapor imagery, a weakening low aloft about 1150 miles ENE of the islands near 27N140W is noted to be moving N while a weak trough aloft is approaching from the distant NW. Little significant change to the overall weather pattern is expected through Friday, with moderate trade winds delivering passing clouds fueling a few showers, mainly over windward areas. A trend toward weaker trade winds is anticipated Saturday into Monday as a weakness in the low-level gradient, and potentially a weak low-level trough, moves over the islands from the E. At the same time, a subtle increase in low-level moisture is expected to arrive on the trade flow. Although the low aloft to the NE is expected to dissipate soon, it leaves behind a weakness as the approaching trough aloft to the NW helps to cool the mid-levels over the islands. The combination of slowly and steadily cooling mid-level temperatures and the arrival of increased moisture is expected to maintain passing showers over windward areas. In addition, the light winds will likely allow afternoon sea breezes to drive the development of clouds and showers over leeward and interior areas, with the weakness aloft potentially allowing a few showers to become briefly heavy. Latest guidance indicates that greatest instability, although still modest, will be over Kauai and Oahu, and centered on Sunday. Light winds and slower shower movement means there will be a chance for higher localized rain totals where any showers do develop. The combination of light winds, the extreme sun angle, and dew points near 70F will likely make for sticky conditions over the weekend. Early next week, the high to the NNW will move E, and models agree that breezy ENE trades will prevail, delivering a few showers to windward areas. A ridge aloft is expected to build over the area from the NW as the trough aloft dissipates and moves SW.

Aviation

Light to locally moderate trade winds will persist through the period. Ragged clouds and scattered showers will continue to favor windward and mountain areas. Brief MVFR conditions possible in passing showers, especially at night. Otherwise VFR conditions will prevail all areas. No AIRMETs in effect or anticipated at this time.

Marine

A ridge of high pressure north of the islands will keep moderate to occasionally breezy trade winds in place through the weekend, but winds and seas are expected to remain below Small Craft Advisory (SCA) levels. The trades are expected to strengthen early next week as high pressure builds northeast of the islands, and SCAs for winds may be needed for the typically windy waters at that time. A new reinforcing SSW swell will arrive tonight, peaking near or just below advisory levels Friday. Surf along south facing shores will remain above average Saturday and Sunday, and into early next week, but remain below advisory levels. In addition to the new southerly swell, tides have been running about a half a foot above predicted levels. These elevated tide levels will increase the risk for minor coastal flooding when combined with the above average south swell. For the next few afternoons, the predicted high tides are around 2.5 feet MLLW, which does not include the extra 1/2 foot mentioned above. See the latest Special Weather Statement (SPSHFO) for additional details on the elevated tide levels and potential impacts. Small trade wind and SE swells will continue into the weekend. In the longer range, models indicate a northeast swell developing for the beginning of next week. A High Surf Advisory may eventually be needed for east facing shores if the swell comes in bigger than anticipated. For more details on the surf, please refer to the Oahu Collaborative Surf Forecast (SRDHFO) prepared by Pat Caldwell under WMO header FXHW52 PHFO.

HFO Watches/Warnings/Advisories

None.

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Discussion...Birchard Aviation...Bedal Marine...Kinel

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