Photo Credit: Brendan Stephen

West Kaua’i

Today: Mostly sunny in the morning, then partly sunny with isolated showers in the afternoon. Highs around 83 near the shore to around 71 above 3000 feet. East winds up to 15 mph. Chance of rain 20 percent.

Tonight: Mostly cloudy with isolated showers. Lows around 66 near the shore to around 56 above 3000 feet. Light winds. Chance of rain 20 percent.

Thursday: Partly sunny with isolated showers. Highs around 82 near the shore to around 71 above 3000 feet. Light winds. Chance of rain 20 percent.

South Kaua’i

Today: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 78 to 84. East winds 10 to 15 mph. Chance of rain 40 percent.

Tonight: Mostly cloudy with isolated showers. Lows around 65. East winds around 10 mph. Chance of rain 20 percent.

Thursday: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 79 to 85. Northeast winds around 10 mph shifting to the southeast in the afternoon. Chance of rain 40 percent.

Kaua’i Mountains

Today: Partly sunny with scattered showers. Highs 70 to 77 in the valleys to around 63 above 4000 feet. Southeast winds up to 10 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 58 to 64 in the valleys to around 54 above 4000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Thursday: Partly sunny with scattered showers. Highs 72 to 78 in the valleys to around 63 above 4000 feet. East winds around 10 mph. Chance of rain 50 percent.

North Kaua’i

Today: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 73 to 83. Southeast winds up to 15 mph. Chance of rain 30 percent.

Tonight: Mostly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows 59 to 68. Southeast winds around 10 mph in the evening becoming light. Chance of rain 40 percent.

Thursday: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 73 to 82. Southeast winds around 10 mph. Chance of rain 40 percent.

East Kaua’i

Today: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 70 to 80. East winds up to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 58 to 70. East winds around 10 mph. Chance of rain 50 percent.

Thursday: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 72 to 82. East winds around 10 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Dry and stable weather will continue through Thursday, with light showers favoring the windward sides of the islands. Moderate to locally breezy trades will prevail over the eastern islands, while lighter east-southeast winds are expected over the western end of the state. An upper level low moving down the chain Friday and Saturday could increase shower coverage and intensity. A cold front moving through the islands Sunday night through early next week could bring the chance for some heavy rainfall.

Discussion

Very few changes have been made to the forecast in the short term (first 36-48 hours). However, have made several changes to the forecast Friday onwards. With the global models in general agreement, have opted to lean the forecast heavily towards the NBM which also factors in the ensembles. The result has been a general increase in showers for the Friday/Saturday time frame in response to an upper level trough moving through the islands from the northwest. And another increase in showers statewide Sunday through Tuesday in response to a cold front. What has not been added to the forecast at this time is any mention of thunderstorms or snow, both of which have a non-zero chance during the upcoming events.
High pressure remains far to the northeast of the state, with a ridge extending to the west-southwest of the high, and a cold front is northwest of the ridge. Moderate to locally breezy trade winds are expected to persist over the eastern islands, while winds over the western islands are weakening as they are closer to the ridge. The lighter winds will allow for local land and sea breezes to form. In the upper levels of the atmosphere, an upper level trough is moving away from the islands to the east, while an upper level ridge is building over the islands. This upper level ridging will help to maintain a dry and stable airmass over the region through Thursday.
The front to the northwest is expected to weaken to a trough and stall northwest of the islands, however another upper level trough is expected to approach the islands from the northwest Friday. There is good model agreement with the upper level feature moving down the island chain Friday and Saturday. This upper level trough will increase instability over the islands, resulting in an uptick in showers both in coverage and intensity as the upper trough moves down the island chain. The global models, and as a result the NBM, all indicate that this airmass will bring a small chance for thunderstorms Friday through Saturday morning. The chance is low and there is some run to run variability, so at this time have opted to not put it in the forecast, however we will be keeping an eye on the possibility. The upper level trough will also bring colder temperatures aloft, and if those cold temperatures coincide with moisture reaching the Big Island summits, snow is a possibility. At this time the greatest chance for that would be Friday night and early Saturday morning when the coldest temperatures reach the Big Island. Again have not added this to the forecast quite yet, but looking at the possibility.
Heading into the new week, continued unsettled weather is expected as a new cold front approaches from the northwest. There is generally good model agreement with the front approaching the islands, but still quite a few differences in the timing and placement. The ECMWF is quite a bit faster than the GFS, having the front approaching Kauai by Sunday night, while the GFS depicts it getting there around Monday evening. Both models bring southerly winds to the islands ahead of the front during the time, with an uptick in showers in convergence areas ahead of the front. The forecast has been nudged heavily towards the NBM resolution as medium ground. The models show a weak mid-level trough moving through the islands along with the front. While this would bring some additional instability to the region, it remains unclear how much.

Aviation

Surface high pressure centered well to the northeast of the state will continue to generate moderate easterly trades through the remainder of this morning and into the afternoon. Thus, expect most isolated/scattered showers to be concentrate along the windward coasts and slopes. There may be brief periods of MVFR ceilings/visibility with any of the more robust showers.
Similar conditions are anticipated tonight.
AIRMET TANGO remains in effect below 8000 feet over and immediately south through west of mountains on Maui and the Big Island for tempo moderate turbulence.

Marine

Surface ridge will remain north of the waters and slowly weaken to keep southeast flow over the islands. Winds will ease into the light to moderate range over the western end of the state but expect the fresh to strong winds to hold over the windier waters around Maui County and the Big Island. A Small Craft Advisory (SCA) remains in effect through tonight and likely will be extended for Big Island and Maui Windward waters where accelerations typically occur with this wind direction. Guidance remains in decent agreement and shows this wind pattern holding into the upcoming weekend as a couple of fronts pass far north of the islands.
Surf along exposed north and west facing shores will steadily decline today as a moderate, medium period north northwest swell moves through. Surf along north and west facing shores will trend back up to advisory levels late tonight into Thursday as a fresh, long-period northwest swell arrives from a hurricane- force system centered around 1700 nm to the northwest. Satellite and altimeter observations are in agreement with recent model guidance as the swell is expected to hold around advisory levels Friday and slowly ease over the weekend. Windward waters of most islands may need to be added into the SCA as this swell comes in tonight due to combined seas greater than 10 feet.
Surf along east facing shores will steadily climb through the week due to a combination of the southeasterly winds locally and the upstream trade wind belt expanding with fresh to strong breezes.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 AM HST Thursday for Maui County Windward Waters, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov