West Kaua’i

Tonight: Partly cloudy. Lows around 69 near the shore to around 59 above 3000 feet. Light winds.

Sunday: Mostly sunny. Highs 79 to 84 near the shore to around 72 above 3000 feet. Southwest winds around 10 mph.

Sunday Night: Isolated showers early in the evening, then numerous showers in the late evening and early morning. Occasional showers early in the morning. Lows around 68 near the shore to around 59 above 3000 feet. West winds 10 to 15 mph. Chance of rain near 100 percent.

South Kaua’i

Tonight: Partly cloudy. Lows around 69. Light winds.

Sunday: Mostly sunny. Highs 78 to 84. Southwest winds around 10 mph.

Sunday Night: Mostly cloudy. Isolated showers early in the evening, then numerous showers in the late evening and early morning. Scattered showers early in the morning. Lows around 69. West winds around 10 mph. Chance of rain 40 percent.

Kaua’i Mountains

Tonight: Mostly cloudy in the evening then becoming partly cloudy. Lows 61 to 67 in the valleys to around 57 above 4000 feet. Light winds.

Sunday: Mostly sunny. Isolated showers in the afternoon. Highs 73 to 80 in the valleys to around 65 above 4000 feet. West winds up to 10 mph shifting to the southwest in the afternoon. Chance of rain 20 percent.

Sunday Night: Numerous showers in the late evening and early morning. Occasional showers early in the morning. Lows 61 to 67 in the valleys to around 57 above 4000 feet. West winds 10 to 15 mph. Chance of rain near 100 percent.

North Kaua’i

Tonight: Partly cloudy. Lows 62 to 70. Light winds.

Sunday: Mostly sunny. Highs 75 to 82. West winds up to 10 mph.

Sunday Night: Numerous showers late in the evening. Occasional showers after midnight. Lows 62 to 69. West winds 10 to 15 mph. Chance of rain near 100 percent.

East Kaua’i

Tonight: Partly cloudy. Lows 62 to 72. Light winds.

Sunday: Mostly sunny. Isolated showers in the afternoon. Highs 73 to 83. Southwest winds up to 10 mph. Chance of rain 20 percent.

Sunday Night: Mostly cloudy. Numerous showers in the late evening and early morning. Scattered showers early in the morning. Lows 62 to 72. Southwest winds 10 to 15 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

Stable and mostly dry conditions are accompanied by light winds through Sunday. A quick-moving cold front then brings a quick hit of showers to the islands late Sunday night into Monday. A second front will bring another round of showers Wednesday night into Thursday with increased potential for heavy rainfall around Kauai during Wednesday.

Discussion

Positively tilted ridge will amplify over the state through tonight allowing surface high pressure and moderate SE winds to build over the eastern end of the state for Sunday. The gradient over Oahu and Kauai transitions to light SW but likely remains light enough to support another day of expanding sea breezes and a few afternoon showers over the interior.
A shallow cold front with cloud tops generally 10kft or less is noted on visible imagery over and east of Midway Atoll. Ahead of it, low and mid-level moisture lingers in the form of disorganized showery clouds. An upper wave only just beginning to deepen over the North Pacific west of the Dateline is seen on water vapor and is handled well by the guidance. This wave will undergo substantial amplification through Monday, the attendant upper jet coupling with the aforementioned front as deep layer ascent increases. Combined with an influx of prefrontal moisture, the frontal circulation will deepen considerably as cloud tops grow to at least 15-20kft supporting heavier shower development. The mature jet-front system reaches Kauai late Sunday night bringing a quick hit of showers, some of which may be heavy along west and north facing slopes. The front then rapidly weakens and shallows as dynamics depart to the NE. Intensity will trend steadily downward as height falls and strong NW flow ushers it eastward across the state during Monday. Leading SW flow will steer the occasional shower over leeward portions of the smaller islands prior to the frontal passage. Dry and strongly subsident NW flow fills in behind the front resulting in a rapid transition to a high stability post-frontal environment as dewpoints crash well into the 50s. Lows over the western end of the state likely fall into the low 60s with sheltered and interior locales dipping into the 50s by Tuesday morning. Light to moderate trades and dry/low dewpoint weather prevails on Tuesday.
By Tuesday night, the next wave in the sequence approaches the forecast area. Veering winds will cause the remnants of Monday's front to lift northward providing substantially more prefrontal moisture this time around. Leeward locations will likely see a greater coverage of showers within prevailing SW flow late Tuesday night and potentially through Wednesday. Of note, the EC and GFS both allow frontal remnants to organize within convergent, prefrontal SW flow over Kauai for all of Wednesday. This period will bear watching as a flood threat perhaps 12-18 hours in duration could manifest for Kauai during this time. The front itself arrives Wednesday night and is almost indistinguishable from Monday's frontal passage in that it will arrive as a strong, dynamically coupled feature that will lose support and weaken as it advances eastward. Similarly, dewpoints once again fall into the 50s in its wake as breezy post-frontal NW winds veer to trades by Thursday night.

Aviation

Light background winds have resulted in daytime sea breezes across the island chain today, increasing cloud cover and producing a few showers over the islands. Any remaining showers should diminish this evening with the loss of diurnal heating and the development of the offshore land breezes.
Little change is expected into Sunday, with daytime sea breezes developing and increasing clouds over the islands once again. VFR flight conditions are anticipated to prevail over most locations throughout the next 24 hours.
A front will arrive by Sunday night, and showers are expected from Sunday night through Monday along the frontal boundary as it moves down the island chain. Ahead of the front, light southerly winds will prevail, but as the front passes, winds will switch to become northerly and increase to moderate levels.
There are no AIRMETs currently in effect, and none are anticipated through tonight.

Marine

A weak high pressure ridge will remain nearly stationary over the central waters this weekend. A cold front will sweep from west to east down the island chain on Monday, stalling near the Big Island by Monday night. An even stronger cold front will quickly move through the islands, spreading strong to near gale force winds across the northwestern coastal waters late next week from Wednesday into Thursday. A Small Craft Advisory is in effect for Hawaiian Coastal Waters exposed to the large northwest swell, these elevated seas above 10 feet are forecast to last in Sunday for waters and channels exposed to the northwest swell.
A High Surf Advisory (HSA) is in effect through Sunday for exposed north and west facing shores. Near shore buoys show a a height and period upward spike as the new swell arrived. This swell will peak tonight and hold above HSA thresholds into Monday as another much larger overlapping northwest (320-330 degree) swell swiftly builds surf to High Surf Warning levels potentially from Monday through Tuesday. Yet another XL swell will build into the islands by late Thursday, potentially producing another round of warning level surf along exposed north and west facing shores.
Surf along east facing shores will remain small through the weekend as winds over and upwind of Hawaii remain weak. Some increase in east shore surf is possible early next week as a cold front moves down the island chain. Along south shores, surf will remain very small through the forecast period.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 PM HST Sunday for Niihau, Waianae Coast, Oahu North Shore, Maui Windward West, Kauai North, Kauai East, Molokai Windward, Molokai North, Molokai West, Maui Central Valley North, Windward Haleakala.
Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Kaiwi Channel, Maui County Windward Waters, Alenuihaha Channel, Big Island Windward Waters.

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Data Courtesy of NOAA.gov