Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

330 pm hst Wed aug 12 2020

Synopsis

A nearby upper low to the northwest of the state will bring a slight chance of thunderstorms for areas around Kauai through tonight. Otherwise, trade wind weather is expected into early next week with a slight weakening of the wind speeds Thursday and Friday. Clouds and passing showers will tend to favor windward and mountain areas, but localized sea breezes in protected areas may bring a few afternoon clouds as the background winds weaken.

Discussion

No significant changes to the forecast with the afternoon package. Made some tweaks to the PoPs and related fields for the first 36 hours of the forecast package, to tap down the showers in several locations based on the latest guidance and in light of today's shower activity. Have left the slight chance of thunderstorm area the same through tonight. There has been some thunderstorm activity just outside the coastal waters today, but otherwise things have remained pretty quiet.

The upper level low to the northwest was producing quite a bit of thunderstorm activity this morning just northwest of the coastal waters, but even the activity there has died down there since midday. The afternoon sounding from Lihue shows 500 mb temperatures colder than normal, with the models showing colder temperatures over the coastal waters to the northwest and west of Kauai. Precipitable water (PW) values remain near normal around, with slightly higher values over the coastal waters. With little change in the upper level low, have left the area for the slight chance of thunderstorms the same near Kauai through tonight. Have not extended the area into tomorrow, but will need to see how things pan out tonight.

Otherwise, the high pressure system far north of the area will dominate our weather maintaining a trade wind pattern with clouds and shower focused over the windward slopes. The high is expected to weaken over the next couple of days, with a new high building northwest of the area this weekend. As the current high weakens and new high settles in, we could see the winds weaken bit, which may allow for some localized land breezes in the afternoon. This could lead to some interior clouds and shower development.

Aviation

A weak upper level low northwest of Kauai, with associated cold and unstable air aloft, will allow for increased shower and isolated thunderstorm activity this evening and into tonight over Kauai and surrounding waters. Elsewhere, persistent high pressure north of the state will bring moderate trades to the islands. This pattern will concentrate most isolated to scattered shower activity along the windward coasts and slopes. Brief periods of MVFR ceilings and visibility may occur with this activity tonight.

No AIRMETs are currently in effect.

Marine

A large and strong surface high far north of the area will slowly weaken through Friday. A new high will build northwest of the area over the weekend then move southeast through the middle of next week. These highs will maintain moderate to fresh northeast to east trade winds over Hawaiian waters through at least early next week. Winds over the coastal waters are expected to remain below Small Craft Advisory (SCA) speeds. A low aloft will bring a slight chance of thunderstorms to waters around Kauai through tonight, and some of the trade wind showers there may be heavier than normal.

Surf will remain below advisory levels along all shores into next week. A NE swell expected Thursday and Friday will boost surf along east facing shores and may also reach some north shore locations. A small long-period swell from Hurricane Elida is possible over the weekend. Small surf along south facing shores will build starting over the weekend and continuing into next week.

Watches, Warnings, Advisories

None.

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