Photo Credit: Jason Weingardt

West Kaua’i

Today: Sunny. Isolated showers in the morning. Highs around 83 near the shore to around 71 above 3000 feet. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Tonight: Mostly clear. Lows around 67 near the shore to around 58 above 3000 feet. East winds 10 to 15 mph.

Sunday: Sunny. Highs around 83 near the shore to around 72 above 3000 feet. Northeast winds around 10 mph.

South Kaua’i

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Kaua’i Mountains

Today: Breezy. Partly sunny with scattered showers. Highs 72 to 78 in the valleys to around 62 above 4000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Breezy. Partly cloudy with scattered showers. Lows 61 to 66 in the valleys to around 56 above 4000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Sunday: Partly sunny in the morning then becoming mostly sunny. Scattered showers. Highs 72 to 78 in the valleys to around 64 above 4000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

North Kaua’i

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East Kaua’i

Today: Mostly sunny with isolated showers. Highs 70 to 83. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Tonight: Partly cloudy with isolated showers. Lows 61 to 73. East winds 10 to 15 mph. Chance of rain 20 percent.

Sunday: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 72 to 82. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Increasing sunshine, decreasing showers, and easing trade winds are expected through the remainder of the holiday weekend. A weak front will move down parts of the island chain Tuesday, bringing a brief increase in northeasterly trade winds and some increased shower activity along windward and northern slopes. Another shallow front may affect the state later in the week.

Discussion

There have been some changes to the forecast with the morning package, but nothing major. The general flavor of the update is to lower PoPs both in the short and medium range, but for different reasons.
High pressure far northeast of the islands will continue to move away from the region with a lingering ridge north of the islands that will drift southward over the weekend. The resultant pressure gradient over the islands will weaken over the weekend, leading to a weakening of the trade wind flow.
High clouds have continued to thin overnight, and that trend is expected to continue today. An upper level trough to the west continues to move away from the region, and even as an upper low develops along the trough, its continued trek away from the islands is drawing the high clouds away. In addition, a mid-level ridge is building over the islands, bringing a more stable airmass over the area. The overnight soundings from Lihue and Hilo have shown the expected lowering of the trade wind inversion, and the global models suggest that will lower a bit more by the afternoon sounding. With the drier, more stable airmass moving over the islands, have trended the forecast PoPs through Sunday night towards the National Blend of Models (NBM) that depicts fewer showers. The showers that do make it in will be focused over the typical windward and mauka areas, as well as during the afternoon hours over the Kona slopes of the Big Island.
On Christmas day, and approaching surface high pressure near the Dateline will begin to cause the weak trade winds to shift to a more northeasterly direction. This pattern can create a few more showers due to subtle low-level convergence, and the effects would likely be confined to around Kauai and Oahu. There is low confidence on this scenario, and any shower increase would probably not be significant.
A very weak front will move down parts of the island chain Tuesday, and it may bring an increase in rainfall mainly to the northern and windward slopes of Kauai and Oahu. The latest runs of the GFS and ECMWF have begun to back off on the PoPs, and as a result, have begun to trend the forecast towards the NBM solution of a drier forecast. Meanwhile, a high behind the front will send a brief shot of breezy trade winds for much of the island chain late Tuesday/Wednesday, before easing once again by Thursday. Another front may reach the islands Thursday or Friday, but the GFS and ECMWF are showing significant differences in how far south this feature will go.

Aviation

Surface high pressure well northeast of state will help maintain moderate to locally breezy trades today. Scattered showers will affect mainly windward locations this morning, then decrease in coverage as drier air filters in by afternoon. VFR conditions expected leeward with partly cloudy skies.
A dip in the jetstream over Hawaii could bring moderate to isolated areas of severe turbulence to the mid and upper levels this morning, then weaken by the afternoon.
AIRMET TANGO for tempo moderate turbulence remains in effect below 8000 feet over and immediately south through west of all island terrain. AIRMET TANGO also remains in effect for tempo moderate turbulence between FL180 and FL320 due to strong winds aloft.
AIRMET SIERRA remains in effect for tempo mountain obscuration above 2500 feet for windward sections of the Big Island.

Marine

Fresh to strong easterly trades will continue through today, then trend down Sunday into Christmas Day as the surface ridge weakens to the north due to a passing front. A Small Craft Advisory (SCA) remains in effect for the typically windier waters around Maui County and the Big Island through tonight. The tail end of this front will slowly advance down the island chain late Monday through midweek. Guidance shows light winds on Christmas Day followed by a shift out of the north in the moderate to fresh range over the western end of the state by sunset. North-northeast trades will briefly strengthen Tuesday to fresh to strong speeds, before shifting out of the east and trending down Tuesday night through midweek as another front approaches.
Latest guidance from offshore NDBC buoys 51001 and 51101 continue to show a slow decline in the the current northwest (310-330) swell since Friday evening. Near shore buoys are still showing the northwest swell holding on locally but should start to trend down slowly later this morning.
Forerunners of a moderate to large, long- period north-northwest (330 deg) swell will arrive later today and build through tonight, keeping surf elevated. This swell will peak Sunday and hold into Christmas Day. ASCAT imagery for Dec 21 and 22 showed a large fetch area of hurricane force winds in the (320-340) window. This could mean this next swell will come in larger than anticipated pushing surf heights well into advisory levels. The forecast has been updated to include a 2 foot increase above guidance for Sunday that will produce advisory level surf for north and west facing shores and possible require the issuance of SCA for building seas if the forecast materializes. As this swell reaches the far northwest offshore buoys later this morning, more confidence can be gained on exactly how much larger this swell may get.
Models show a pair of gale lows rapidly strengthening to hurricane force far northwest of state in the next 24 hours that could bring back- to- back, dangerous, warning level north- northwest (330-340) swells and the continuation of the SCA due to high seas. The first swell will build Monday, peak Tuesday. The second storm could produce an extra large swell, since already acting on existing seas. This swell is expected Tuesday night, holding well above warning level surf into Wednesday. As these swell sources continue their eastward track, can expect a mix of northwest and north swell components through the week which will keep surf elevated along north and west facing shores.
Yet another potentially extra large northwest swell may fill in Thursday, peak Thursday night, producing dangerous warning- level surf. Although confidence is lower being so far out, guidance depicts this trend of progressive swells moving through the islands to continue into the new year.
Surf along east facing shores will steadily lower through early next week as the trades ease locally and upstream of the islands. Surf along south facing shores will trend down over the weekend as a background south-southwest (190 deg) swell moves out. A small, medium period south-southwest swell is possible Tuesday night through Wednesday.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 AM HST Sunday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov