West Kaua’i

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Lows around 71 near the shore to around 62 above 3000 feet. Northeast winds up to 10 mph.

Friday: Partly sunny. Isolated showers in the afternoon. Highs 84 to 90 near the shore to around 76 above 3000 feet. Light winds. Chance of rain 20 percent.

Friday Night: Mostly cloudy. Lows around 71 near the shore to around 62 above 3000 feet. Light winds becoming northeast around 10 mph after midnight.

South Kaua’i

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Isolated showers. Lows 67 to 75. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Friday: Partly sunny. Isolated showers in the morning. Highs 78 to 89. Northeast winds around 10 mph. Chance of rain 20 percent.

Friday Night: Mostly cloudy with isolated showers. Lows 68 to 75. Northeast winds around 10 mph. Chance of rain 20 percent.

Kaua’i Mountains

Tonight: Partly cloudy with scattered showers in the evening, then mostly cloudy with isolated showers after midnight. Lows around 66 in the valleys to around 58 above 4000 feet. East winds up to 10 mph. Chance of rain 40 percent.

Friday: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 75 to 82 in the valleys to around 68 above 4000 feet. Northeast winds up to 10 mph. Chance of rain 40 percent.

Friday Night: Mostly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows 64 to 69 in the valleys to around 59 above 4000 feet. Northeast winds around 10 mph. Chance of rain 40 percent.

North Kaua’i

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Isolated showers. Lows 62 to 73. East winds up to 15 mph. Chance of rain 20 percent.

Friday: Partly sunny with isolated showers. Highs 71 to 85. East winds up to 10 mph. Chance of rain 20 percent.

Friday Night: Mostly cloudy. Isolated showers in the evening. Lows 63 to 74. Northeast winds around 10 mph. Chance of rain 20 percent.

East Kaua’i

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Isolated showers. Lows 63 to 75. Northeast winds around 10 mph. Chance of rain 20 percent.

Friday: Partly sunny with isolated showers. Highs 78 to 86. Northeast winds around 10 mph. Chance of rain 20 percent.

Friday Night: Mostly cloudy. Isolated showers in the evening. Lows 64 to 75. Northeast winds around 10 mph. Chance of rain 20 percent.

Detailed Forecast

Synopsis

Light to moderate trade winds will produce a hybrid trade wind and sea breeze wind flow each day lasting into early next week. Brief passing showers will drift into the islands on the easterly trade winds, favoring windward mountain areas during the overnight to early morning hours. A more significant change to our weather pattern continues to evolve for early next week beginning sometime Monday night or Tuesday. Deeply unstable tropical moisture moving across the state will bring the potential for heavy rain and thunderstorms affecting all islands.

Discussion

Satellite and radar imagery show isolated to scattered showers moving into windward and mauka areas on the light to moderate trade wind flow this afternoon, with the best coverage over the central part of the state. In addition, showers have also developed over some leeward areas due to sea breeze activity in the lighter trade wind flow, particularly across Oahu.
The trade winds have eased to become light to moderate in response to the weakening pressure gradient as the surface high gets pushed farther northeast of the state while a low pressure system sets up to the north. These light to moderate trade winds will more or less persist through the weekend, allowing for a hybrid trade wind and land/sea breeze pattern to commence. However, with high clouds expected to move in from the west- southwest Friday through most of the weekend, sea breeze activity might be somewhat suppressed. With the weakening mid to upper level low over the state today, the instability associated with this feature has helped to support the development of afternoon showers over some leeward and interior areas as sea breezes form. However, limited available moisture and weak lifting mechanisms at the surface will continue to keep the shower activity rather limited. This mid to upper level low will lift northeastward tonight into Friday and ridging will try to nudge in from the south, limiting shower activity even further. However, deeper moisture will move over the Big Island and maybe Maui from the southeast, increasing the potential for showers along the lower south- and east-facing slopes.
Remnants of a dissipating frontal boundary extending from the low to the north may move in Saturday through Saturday night as the low dips south-southeast, bringing a slight boost in the east- northeast trade wind speeds and trade wind showers over the western end of the state as ridging tries to build in from the north. The upper level trough supporting the surface low will form a cutoff low to the northeast of the state over the weekend, driving the surface low over the same area.
Meanwhile, to the west of the state, a low will try to develop somewhere near 16N and the dateline this weekend, with the National Hurricane Center giving this feature a low (30 percent) chance of developing into a tropical cyclone over the next 7 days. Regardless of the potential tropical development, the main threat for Hawaii is the potential for heavy rain and possibly isolated thunderstorms next week as this potential low and a large convergent band of deep tropical moisture to its east get swept eastward by a passing shortwave trough and drawn across Hawaii beginning sometime Monday night or Tuesday. The threat for heavy rain may persist through much of the week as this band of deep tropical moisture lingers over the area. However, its important to note that forecast uncertainty remains moderate to high as there continues to be a fairly wide spread among the global model solutions.

Aviation

Scattered -SHRA chances continue through this afternoon with trades AOB 15kts. Winds drop to 5-10kts overnight. VFR cigs/vis prevail through the period with brief windows of MVFR with the aforementioned -SHRA.
AIRMET Sierra is in effect for mountain obscuration for north through east sections of Maui.

Marine

Easterly trade winds will be at gentle to moderate strength through Friday as surface low pressure far north of Hawaii and a nearby surface trough weaken the local pressure gradient. Trades are expected to hold at current strength and shift out of the northeast this weekend as high pressure far northwest of Hawaii pushes to low eastward. The weaker trade winds should persist Monday, but uncertainty increases on Tuesday, hinging on the potential development of a low southwest of the islands.
A small, medium-period northwest swell will arrive overnight and provide a small boost to surf along north-facing shores on Friday. A moderate, medium-period north-northwest swell will build late Saturday and peak around seasonal average on Sunday. As this swell declines on Monday, a long-period west-northwest swell will arrive, maintaining small to moderate surf into the middle of next week.
Small background level surf will prevail along south-facing shores through Friday. A small to moderate south-southwest swell is expected to boost surf to around seasonal average this weekend into early next week.
Along east-facing shores, trade-wind swell will continue to decrease through Friday, though a small, medium-period northeast swell may maintain small surf through Saturday. Aside from wrapping north-northwest swell in exposed areas, east shore surf should remain below seasonal average much of next week.

Fire weather

Trade wind speeds will continue to be suppressed through Monday as the high is pushed farther northeast while a low sets up north of the state. Passing showers will move into mainly windward and mauka areas on the trade wind flow and some showers may also be expected over leeward areas as seas breezes develop in the most wind-protected areas. Then ample tropical moisture and will move in from the west sometime beginning Monday night or Tuesday, brining a threat for heavy rain through Thursday. As a result, fire weather concerns will be low through the next several days. Inversion heights will range from 6,000 to 8,000 feet overnight into Friday.

HFO Watches/Warnings/Advisories

None.

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Data Courtesy of NOAA.gov