Photo Credit: Sebastien Gabriel

West Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows 71 to 78 near the shore to around 65 above 3000 feet. Light winds. Chance of rain 50 percent.

Sunday: Mostly cloudy. Widespread showers and isolated thunderstorms in the morning, then showers likely and isolated thunderstorms in the afternoon. Highs 82 to 89 near the shore to around 74 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 80 percent.

Sunday Night: Mostly cloudy with chance of showers and isolated thunderstorms. Lows 69 to 76 near the shore to around 64 above 3000 feet. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

South Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows around 75. Northeast winds around 10 mph. Chance of rain 50 percent.

Sunday: Cloudy and breezy. Widespread showers and isolated thunderstorms in the morning, then showers likely and isolated thunderstorms in the afternoon. Highs 80 to 89. Northeast winds 10 to 20 mph. Chance of rain 80 percent.

Sunday Night: Mostly cloudy with chance of showers and isolated thunderstorms. Lows around 73. Northeast winds around 15 mph. Chance of rain 50 percent.

Kaua’i Mountains

Tonight: Mostly cloudy. Scattered showers in the evening, then showers likely after midnight. Lows 66 to 73 in the valleys to around 62 above 4000 feet. Light winds becoming northeast up to 10 mph after midnight. Chance of rain 70 percent.

Sunday: Cloudy with showers and isolated thunderstorms. Highs 73 to 80 in the valleys to around 65 above 4000 feet. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 90 percent.

Sunday Night: Mostly cloudy with showers likely and isolated thunderstorms. Lows 65 to 72 in the valleys to around 61 above 4000 feet. Northeast winds 10 to 15 mph. Chance of rain 60 percent.

North Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then showers likely after midnight. Lows 68 to 77. Light winds becoming southeast up to 10 mph after midnight. Chance of rain 60 percent.

Sunday: Mostly cloudy. Widespread showers and isolated thunderstorms in the morning, then showers likely and isolated thunderstorms in the afternoon. Highs 75 to 84. East winds up to 15 mph. Chance of rain 80 percent.

Sunday Night: Mostly cloudy with showers likely and isolated thunderstorms. Lows 67 to 76. East winds 10 to 15 mph. Chance of rain 60 percent.

East Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then showers likely after midnight. Lows 68 to 79. North winds up to 10 mph. Chance of rain 60 percent.

Sunday: Cloudy with showers and isolated thunderstorms. Highs 74 to 85. Northeast winds 10 to 15 mph. Chance of rain 90 percent.

Sunday Night: Mostly cloudy with showers likely and isolated thunderstorms. Lows 67 to 78. Northeast winds 10 to 15 mph. Chance of rain 60 percent.

Detailed Forecast

Synopsis

An early season cold front moving toward Hawaii from the north, will enhance rainfall chances across a portion of the islands this weekend. Widespread rainfall is expected across the western half of the state tonight, before overspreading the smaller central islands Sunday as the front weakens and drifts slowly eastward. Moderate trade winds will develop across Kauai and Oahu Sunday as the front exits eastward. The boundary will further weaken and slow over the central islands through Tuesday, before dissipating. Light to moderate trade winds are expected statewide next week.

Discussion

Current radar shows isolated to scattered showers across the islands this afternoon, with the most persistent showers favoring Kauai and Oahu. Satellite shows limited cloud cover across the state this afternoon, while the band of much denser cloud cover, associated with the approaching boundary, continues to drop southward.
The moisture gradient varies greatly across the state, with deeper moisture and higher inversion heights over the western end of the state near Kauai (PHLI 00z sounding inversion height around 9k ft), trending toward lower moisture depths and inversion heights over the Big Island.
Showers will remain limited across the eastern end of the state tonight, as satellite based estimates show dry layered precipitable water (PW) values (1.15 to 1.30 inches) streaming in from the east. However, the areal coverage of showers will increase across the western islands tonight through Sunday, as the boundary drops southward. The areal coverage of showers will continue to expand slowly eastward across the central smaller islands through Sunday evening and into the beginning of next week, as the weakening boundary nearly stalls as it advances eastward. Deeper layered moisture overspreading the island chain this weekend, will begin to dry across the western end of the state by Monday while deeper layered moisture lingers over the central and eastern islands into Wednesday.
Periods of heavier rainfall are possible across the islands this weekend, as moisture and instability increase in advance of the approaching boundary. Current model trends show a band of increased precipitable water values (1.75 to 1.95in) drifting across Kauai tonight before stalling across the central smaller islands through Monday. The higher modeled PW values are supported well by current satellite data estimating PWs nearing 1.90 inches in the band of enhanced mstr north of the state. Additionally, a few thunderstorms are also possible as current lightning data shows a few strikes occurring in the enhanced band of clouds north of the state, in an area of weak instability (Showalter Index 0 – 2). The latest model trends show this band of instability dropping south across Kauai this evening, before becoming anchored over the central smaller islands Sunday and Monday.
Slightly drier and more stable air returns later next week across much of the state with light background trade wind flow. This pattern will allow for another period of locally driven afternoon sea breeze showers across leeward areas and island interiors, along with showers across favored windward areas, especially overnight and mornings. However, model trends indicate a weakly organized surface boundary potentially lifting up toward the eastern end of the state by early Thursday. This feature could enhance moisture and increase the chance of showers across the Big Island Thursday into next weekend.

Aviation

A frontal boundary will approach the state this evening and will decrease the trade winds and bring an increase of showers tonight. As this moisture stalls over the state on Sunday, frequent showers will bring MVFR conditions across the smaller islands with periods of IFR conditions possible under heavier showers or thunderstorms.
No AIRMETs are currently in effect. But AIRMET SIERRA for mountain obscurations are expected across the smaller islands late tonight throughout the day Sunday.

Marine

Light to moderate trades will hold into tonight, then trend up and shift out of the northeast over the Kauai and Oahu waters Sunday as a cold front drifts southward into the area. Winds will near the Small Craft Advisory level in the windier areas around Kauai Sunday. Recent ASCAT pass confirmed this potential and showed a good sized area of 25 knot northeast winds trailing the front. Similar winds will fill in over the Oahu and Maui County waters late Sunday into Sunday night. The front will stall out and dissipate over the state Monday through Tuesday, with light to moderate northeast to easterly trades generally prevailing through midweek. This trend will likely prevail through the latter half of next week as the ridge remains weak to the north.
A fresh north-northwest swell that arrived Friday will hold and shift out of a more northerly direction tonight, then ease Sunday into Monday. A medium period north-northeast swell arriving late Sunday night into Monday from low pressure centered around 1000 NM northeast of the islands will drive the surf up along northerly exposures trough the day Monday. Surf associated with this swell should peak below the advisory level by sunset Monday, hold through Tuesday, then fade Wednesday.
For the extended, guidance shows low pressure moving eastward over the western Aleutians Sunday into Monday, then dropping south-southeastward after it passes east of the Date Line Tuesday through Wednesday. A large batch of north-northwest winds focused at the islands down the 350 degree directional band relative to the area associated with it should send another northerly swell through the islands, with an arrival centered around Thursday night. If this materializes, surf will peak around the advisory level for north facing shores Friday, then ease next weekend.
A small, long-period south-southwest swell arriving through the day Sunday will lead to increasing surf along southern exposures. This should peak Monday, then fade by midweek. A similar south- southwest swell is possible next weekend. For the extended, guidance shows an active trend continuing with a large area of gales expanding northward within Hawaii's swell window Tuesday through midweek (seas climbing to around 30 ft) on the backside of low pressure passing south of New Zealand. If this unfolds, a solid south-southwest swell will be possible by mid month.
Surf along east facing shores will remain small and well below normal through the upcoming week due to a lack of strong trade winds over and upstream of the islands.

HFO Watches/Warnings/Advisories

None.

Data Courtesy of NOAA.gov