Photo Credit: Jason Weingardt

West Kaua’i

Today: Mostly cloudy. Scattered showers and isolated thunderstorms in the morning, then numerous showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs around 78 near the shore to around 66 above 3000 feet. Southeast winds up to 10 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy. Numerous showers and isolated thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows around 62 near the shore to around 51 above 3000 feet. Light winds. Chance of rain 70 percent.

Saturday: Partly sunny with scattered showers. Highs around 79 near the shore to around 67 above 3000 feet. Light winds. Chance of rain 40 percent.

South Kaua’i

Today: Mostly cloudy. Scattered showers and isolated thunderstorms in the morning, then numerous showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs around 77. Light winds. Chance of rain 60 percent.

Tonight: Mostly cloudy. Numerous showers and isolated thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows 58 to 63. Light winds. Chance of rain 70 percent.

Saturday: Partly sunny with scattered showers. Highs around 78. Light winds. Chance of rain 40 percent.

Kaua’i Mountains

Today: Mostly cloudy. Scattered showers and isolated thunderstorms in the morning, then numerous showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 65 to 73 in the valleys to around 60 above 4000 feet. Light winds. Chance of rain 70 percent.

Tonight: Mostly cloudy. Numerous showers and isolated thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows 51 to 56 in the valleys to around 45 above 4000 feet. Light winds. Chance of rain 70 percent.

Saturday: Partly sunny with scattered showers. Locally heavy rainfall possible. Highs 66 to 74 in the valleys to around 61 above 4000 feet. Light winds. Chance of rain 50 percent.

North Kaua’i

Today: Mostly cloudy. Scattered showers and isolated thunderstorms in the morning, then numerous showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 70 to 77. Light winds. Chance of rain 70 percent.

Tonight: Mostly cloudy. Numerous showers and isolated thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows 54 to 63. Light winds. Chance of rain 70 percent.

Saturday: Partly sunny with scattered showers. Locally heavy rainfall possible. Highs 71 to 78. Light winds. Chance of rain 50 percent.

East Kaua’i

Today: Mostly cloudy. Scattered showers and isolated thunderstorms in the morning, then numerous showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 70 to 77. South winds up to 10 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy. Numerous showers and isolated thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows 52 to 64. Light winds. Chance of rain 70 percent.

Saturday: Partly sunny with scattered showers. Locally heavy rainfall possible. Highs 71 to 78. Light winds. Chance of rain 50 percent.

Detailed Forecast

Synopsis

A front moving down the island chain from the northwest will continue to bring unsettled weather to the state into Saturday. Showers, along with some thunderstorms, will spread over Maui and the Big Island today, tapering off early Saturday morning. Another round of unsettled weather and increasing winds, is expected Sunday through Tuesday, as a stronger front approaches the state.

Discussion

Have made some short term forecast modifications, nudging PoPs upward in some locations to be in better alignment with the latest run of the HREF. Have also added some areas of locally heavy rainfall based on the HREF probabilities. This suggests some locally heavy rainfall could come into the east side of the Big Island this morning, and over Kauai this afternoon.
In the evening hours, we saw a several thunderstorms pop up over the coastal waters, and just outside the coastal waters. As the evening progressed we had some thunderstorms approach Kauai and other develop and move across Oahu. Most of the showers and thunderstorms are moving quickly and not becoming anchored, keeping flooding concerns at a minimum. Will continue to monitor stream response and radar in the event additional advisories are needed.
The front itself remains northwest of Kauai, but is already becoming a diffuse boundary, difficult to discern on satellite. The clouds, showers and thunderstorms we have experienced overnight are associated with the increased moisture being drawn northward ahead of the front. Water vapor imagery shows the islands at the base of a short-wave trough that is swinging through, likely aiding in the development of thunderstorms overnight. Satellite derived precipitable water (PW) shows 1.4 inches extending northward towards Kauai this morning, with some higher PW pockets around the islands. This is on the high end of normal for December, and a little below what earlier model runs had indicated we would have tonight.
While the front is bringing a more unstable airmass to the islands, the front itself is expected to wash out over the central islands later today. Meanwhile, the moisture and colder upper level temperatures are already in place over the islands, so expect showers to persist into Saturday. However as the upper level trough associated with the front swings through the islands, exiting east of the Big Island tonight, thunderstorm activity should die out. Thunderstorms remain in the forecast for all the islands today and many areas tonight, however it looks like the greatest chance will be over Maui County and the Big Island and waters to the north of them.
Our attention then turns to the next frontal system to impact the islands during the Sunday through Monday/Tuesday time frame. The upper level trough associated with this front is expected to be colder/deeper than this current system, but precipitable water values are a little lower. While winds are not a concern with the current front, they are expected to be more of an issue with this next front. Kona winds will strengthen Sunday, becoming strongest Sunday night through Monday with potential impacts downwind of terrain and across windward areas. Expect the winds to then trend downward Tuesday. The strong upper level trough will bring very strong winds to the Big Island summits during this event, and to a lesser degree on Haleakala. Winds are expected to reach High Wind Warning levels for these areas with this next event. Some lower elevations in Maui County and on the Big Island may see advisory level winds. Similar to the current frontal system, thunderstorms will again be possible with this next front.
A drier and more stable airmass is expected over the state Tuesday through Thursday, so look for a decrease in shower activity. However this is expected to be short-lived, as another weaker front will approach the islands for the end next week.
Temperatures will be hovering around freezing through the week on the Big Island summits, and with plenty of opportunities for moisture to reach the summits, snow will remain in the forecast.

Aviation

A front will move down the island chain today, bringing periods of light to moderate rainfall and pockets of heavy rain with isolated thunderstorms. AIRMET Sierra for mountain obscuration is currently in effect for Kauai and Oahu, though conditions seem to be improving on Oahu for now. AIRMET Sierra may be needed for mountain obscuration on Maui later this morning. Some periods of light icing are also possible within the layered clouds ahead of the front. Overall conditions will improve later today as the front washes out over the central islands, though a slight chance of thunderstorms will remain through Friday night. Winds will remain mostly light through the forecast period.

Marine

A cold front will slowly dissipate across the western and central islands today with light to gentle southeast to southwest winds along with some thunderstorms. Lighter winds will continue through Saturday night, with increasing southwesterly wind speed trends on Sunday as another cold front approaches the Hawaiian Islands from the northwest. This second stronger cold front will sweep southeastward through the islands chain on Monday, spreading showers and thunderstorms as it quickly passes through the state. More stable westerly winds will fill in behind the front with decreasing shower trends as the front quickly passes through the region. A Small Craft Advisory (SCA) has been issued through tonight as seas are expected to exceed the 10 foot SCA threshold as the northwest swell moves through.
A large northwest (320-330 degree) swell will build down the island chain today producing extra large and dangerous surf along most north and west facing shores through tonight with decreasing trends on Saturday. Swell heights at buoy 51001 have spiked a few feet above guidance this morning. A High Surf Warning (HSW) remains in effect for north and west facing shores of Niihau, Kauai, Oahu, Molokai, and north facing shores of Maui through tonight. These surf heights will trend down to advisory levels by Saturday. A Small Craft Advisory (SCA) also remains in effect for the same time period covering all northern waters and southern waters around Kauai and Oahu for seas 10 feet or higher. Another extra large northwest (310-320 degree) swell will build into the region from Sunday night to Monday morning. This long period swell energy will produce hazardous surf heights well above warning levels for both north and west facing shores on Monday, and large mixed period swell from the nearby low pressure source will likely hold these surf heights above HSW levels through Tuesday.
A small pulse of south-southwest swell is holding today, otherwise no significant surf is due for east and south shores into the first half of next week.

HFO Watches/Warnings/Advisories

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

Data Courtesy of NOAA.gov