West Kaua’i

Tonight: Mostly cloudy with a 20 percent chance of showers. Lows around 70 near the shore to around 62 above 3000 feet. Light winds becoming northeast up to 10 mph after midnight.

Sunday: Mostly cloudy. Slight chance of showers in the morning, then slight chance of showers and thunderstorms in the afternoon. Highs around 83 near the shore to around 73 above 3000 feet. East winds up to 10 mph in the morning becoming light. Chance of rain 20 percent.

Sunday Night: Mostly cloudy. Slight chance of showers and thunderstorms in the evening, then chance of showers and slight chance of thunderstorms after midnight. Lows around 69 near the shore to around 61 above 3000 feet. Northeast winds up to 10 mph. Chance of rain 40 percent.

South Kaua’i

Tonight: Mostly cloudy with a 20 percent chance of showers. Lows around 70. Northeast winds 10 to 15 mph.

Sunday: Mostly cloudy. Slight chance of showers in the morning, then slight chance of showers and thunderstorms in the afternoon. Highs 79 to 84. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Sunday Night: Mostly cloudy with chance of showers and slight chance of thunderstorms. Lows around 70. Northeast winds 10 to 15 mph. Chance of rain 40 percent.

Kaua’i Mountains

Tonight: Mostly cloudy with a 50 percent chance of showers. Lows 64 to 69 in the valleys to around 60 above 4000 feet. Light winds becoming east up to 10 mph after midnight.

Sunday: Mostly cloudy. Chance of showers in the morning, then chance of showers and slight chance of thunderstorms in the afternoon. Highs 74 to 80 in the valleys to around 66 above 4000 feet. East winds up to 10 mph. Chance of rain 50 percent.

Sunday Night: Mostly cloudy with chance of showers and slight chance of thunderstorms. Lows 63 to 69 in the valleys to around 59 above 4000 feet. Southeast winds up to 10 mph. Chance of rain 50 percent.

North Kaua’i

Tonight: Mostly cloudy with a 40 percent chance of showers. Lows 64 to 72. Southeast winds up to 10 mph.

Sunday: Mostly cloudy. Slight chance of showers in the morning, then chance of showers and slight chance of thunderstorms in the afternoon. Highs 75 to 82. East winds up to 15 mph. Chance of rain 40 percent.

Sunday Night: Mostly cloudy with chance of showers and slight chance of thunderstorms. Lows 64 to 71. Southeast winds up to 10 mph. Chance of rain 40 percent.

East Kaua’i

Tonight: Mostly cloudy with a 50 percent chance of showers. Lows 65 to 74. Northeast winds up to 10 mph.

Sunday: Mostly cloudy. Slight chance of showers in the morning, then chance of showers and slight chance of thunderstorms in the afternoon. Highs 73 to 83. East winds up to 10 mph. Chance of rain 40 percent.

Sunday Night: Mostly cloudy with chance of showers and slight chance of thunderstorms. Lows 64 to 73. East winds up to 10 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Moderate trade winds will weaken and veer out of the east- southeast across the western half of the island chain tonight as low pressure deepens several hundred miles northwest of Kauai. Shower activity should be low and focused mainly over windward areas as high clouds briefly diminish. An upper-level trough will will bring increased high clouds and renewed chances for isolated heavy showers and thunderstorms Sunday afternoon and Monday. Light winds favor chances for mainly afternoon showers Tuesday and Wednesday, followed by rebuilding trade winds Thursday and Friday.

Discussion

An upper-level trough passing overhead has brought extensive high clouds and instability aloft, but moderate easterly trade winds and a weak low-level ridge have kept shower activity rather minimal and largely confined to windward and mauka areas so far today. A robust 1033 mb surface high is centered far north of the state along 40N latitude, but the local pressure gradient has been slightly weakened by a diffuse surface trough sitting several hundred miles northwest of Kauai, resulting in the moderate easterly trade winds. Afternoon soundings revealed an inversion between 4000 to 5000 ft produced by a weak low-level ridge, giving way to instability above 700 mb generated by the upper-level trough. The axis of the upper trough recently triggered a couple of briefly heavy showers over Kauai, and given the shallow stable layer, we still could see a heavy shower flare up as the upper- level trough advances eastward.
Easterly trade winds will ease slightly tonight as developing low pressure several hundred miles north of Kauai continues to erode the surface pressure gradient. The upper-level trough will flatten as it swings northeast of the islands, and a weak ridge at 700 mb will remain in place. As a result of this tenuous stability, showers will likely remain modest and largely confined to windward slopes. That said, we still cannot rule out a brief and isolated heavy shower anywhere. In addition, expect a short-lived decrease in high clouds.
On Sunday and Monday, trade winds will weaken further and will shift out of the east-southeasterly across the western half of the island chain, while another upper-level trough brings instability, mainly Sunday night into Monday morning. For now, it appears that the deepest moisture and highest chances for widespread rainfall will remain along the surface trough northwest of Kauai, but isolated heavy showers or a thunderstorm remain possible around Kauai. Elsewhere, expect afternoon interior showers and possibly a thunderstorm on Big Island, and another item to monitor will be a chance for localized anchored heavy showers along the Koolau Mountains of Oahu Sunday night. Even though moisture does not appear to be significant on Oahu, the expected veering flow with height has a tendency to produce heavy rain events on the Koolau.
Chances for heavy rainfall look to diminish Tuesday and Wednesday. The upper level trough should move off to the east late Monday, allowing a weak ridge aloft to settle over the state. The GFS and ECMWF are hinting that the surface trough that had been lingering northwest of Kauai will be pushed over the western end of the island chain. This would favor light and variable winds over most areas with higher rainfall chances over island interiors during the late morning and afternoon hours. Trade winds are expected to rebuild Thursday and Friday.

Aviation

A mix of moderate to locally breezy easterly or east- southeasterly trade winds with some sea and land breezes will remain in place through the weekend with high pressure to the far northeast and as a surface trough lingers to the west. Overall, VFR conditions are expected to generally prevail across most airports through tomorrow.
Brief showers will move into windward and mountain areas on the trades, and afternoon clouds and showers may also develop over the interior portions of the Hawaiian Islands during the day as sea breezes develop in the slightly veered flow. However, mid to high clouds with embedded light showers streaming in from the southwest from a nearby jet streak have helped to minimize daytime heating and sea breeze development so far today.
AIRMET Tango is in place between 100 and FL400 due to the potential for moderate mid to upper level turbulence. This AIRMET will likely be needed for the eastern half of the state through the early evening. Light icing will also be possible between 140 and FL250 within the deep layered clouds.

Marine

A high pressure system far north of the state will slowly drift eastward this weekend. Easterly trade winds diminish tonight as a trough to the northwest of the islands develops into a weak low pressure system by Sunday. The resulting weakness in the ridge causes winds to veer southeasterly and diminish over the western Hawaii region from Sunday into early next week. Late next week, east to northeasterly trade winds begin to redevelop with weak ridging re-establishing north of the islands.
A small pulse of medium period swell energy from the northwest should produce a slight bump in north facing surf heights tonight into Sunday morning. A small to moderate, medium period northwest swell will move into the region by Monday night and continue through Wednesday. A moderate, medium period northeast swell arrives Tuesday and peaks on Wednesday, bringing moderate surf to north-facing shores, before slowly declining through the end of the week.
A medium-period south swell will continue to fade tonight, allowing surf along south shores to return to background levels by Sunday. A series of overlapping small south swells will move into the region into next week. Surf heights along east-facing shores remain on the smaller side due to weaker trade winds until the arrival of the aforementioned northeast swell on Tuesday. Small background energy from the west will be possible, especially later in the week, from Severe Tropical Storm Sinlaku.

HFO Watches/Warnings/Advisories

None.

Data Courtesy of NOAA.gov