West Kaua’i

Tonight: Partly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows 70 to 75 near the shore to around 62 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 50 percent.

Tuesday: Mostly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs around 88 near the shore to around 75 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 40 percent.

Tuesday Night: Partly cloudy with isolated showers. Lows around 73 near the shore to around 62 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 20 percent.

South Kaua’i

Tonight: Mostly cloudy. Breezy. Scattered showers in the evening, then showers likely after midnight. Lows around 74. Northeast winds 10 to 20 mph. Chance of rain 70 percent.

Tuesday: Partly sunny in the morning then becoming mostly sunny. Breezy. Scattered showers. Highs 81 to 89. Northeast winds 15 to 20 mph. Chance of rain 50 percent.

Tuesday Night: Partly cloudy in the evening then becoming mostly cloudy. Breezy. Scattered showers. Lows around 74. Northeast winds 15 to 20 mph. Chance of rain 40 percent.

Kaua’i Mountains

Tonight: Breezy. Mostly cloudy with showers likely. Lows 64 to 70 in the valleys to around 60 above 4000 feet. Northeast winds up to 20 mph. Chance of rain 70 percent.

Tuesday: Partly sunny. Breezy. Showers likely in the morning, then scattered showers in the afternoon. Highs 73 to 82 in the valleys to around 65 above 4000 feet. Northeast winds up to 20 mph. Chance of rain 70 percent.

Tuesday Night: Breezy. Mostly cloudy with scattered showers. Lows 65 to 70 in the valleys to around 60 above 4000 feet. Northeast winds 10 to 25 mph decreasing to up to 25 mph after midnight. Chance of rain 50 percent.

North Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then showers likely after midnight. Lows 66 to 73. East winds 10 to 15 mph decreasing to up to 15 mph after midnight. Chance of rain 70 percent.

Tuesday: Partly sunny with showers likely in the morning, then mostly sunny with scattered showers in the afternoon. Highs 76 to 87. East winds 10 to 15 mph. Chance of rain 70 percent.

Tuesday Night: Partly cloudy in the evening then becoming mostly cloudy. Scattered showers. Lows 66 to 74. East winds 10 to 15 mph. Chance of rain 50 percent.

East Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then showers likely after midnight. Lows 66 to 77. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Tuesday: Partly sunny. Breezy. Showers likely in the morning, then scattered showers in the afternoon. Highs 74 to 86. Northeast winds 10 to 20 mph. Chance of rain 70 percent.

Tuesday Night: Mostly cloudy with scattered showers. Lows 66 to 78. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Breezy trades are expected to continue through the next few days and potentially become windy by the end of the week. Showers will move through fairly rapidly with this flow, making it over the mountains to leeward areas, but with only light rainfall. A weak upper level low will move away to the east, leaving a trough in its wake and maintaining weak instability over the next few days as the trades strengthen.

Discussion

Showers are moving east to west across most islands this afternoon in breezy trades, most numerous on Oahu, but still not particularly wet. Satellite trends indicate that periods of showers should continue for the next several days. Rainfall amounts are not expected to be high due to the motion of the showers.
An upper level low to our northeast will weaken and become a trough as it drifts east. A trough will be left laying across the state, clearly seen at 500 mb. A piece of energy from the trough will break off to our west midweek, and form a closed low several hundred miles to our west on Friday. The only impact of this on our weather will likely be an increase in higher clouds. We can find no indications of large areas of enhanced moisture that would significantly increase shower coverage or strength through the end of the week. There remains a slight chance of thunderstorms on the Kona side of the Big Island, extending up to the peaks, over the next few days. But odds seem to be getting smaller with each forecast.
Trade winds are forecast to strengthen toward the end of the week, with the strongest winds expected Thursday night through Saturday night. Sustained winds on mountains above 4000 ft could reach 30 mph with gusts to 40 mph.
Models diverge with how they handle our flow, including surface winds, beyond Sunday. The GFS brings a low- to mid-level trough into the region, greatly reducing surface winds. The ECMWF keeps this feature to our north, with only a minor reduction in wind speeds. For now, the forecast splits the difference as we await future model runs which should converge on a solution in a few days. Overall, changes to the forecast were minimal with this new package.

Aviation

Moderate to locally breezy trade winds will continue to focus low clouds and showers over windward and mountain areas through the forecast period. However, with elevated inversion heights as seen on this afternoon's upper air soundings at Hilo and Lihue, showers will occasionally spill over into interior and leeward areas as well. MVFR or even brief IFR will be possible in passing showers. Otherwise, VFR conditions will prevail.
AIRMET Sierra for tempo mountain obscuration is in effect for windward Oahu and the Big Island this afternoon. Looking upstream on satellite, this AIRMET will likely be dropped later this afternoon as some drier air filters in, though it may be needed again for windward areas overnight as another band of moisture moves through.

Marine

High pressure to the north will dominate through the week. As a result, trade winds will remain moderate to locally strong and are expected to strengthen through the week. A small craft advisory (SCA) is in place for the typical windy waters around the Big Island and will likely expand in coverage this week as trade winds strengthen.
A series of small to moderate, long-period northwest to north- northwest swells (320-340 degrees) will fill in throughout the week, keeping surf along north and west facing shores elevated. The current northwest swell (320-330 degrees) is currently declining and will be reinforced by another small long period northwest swell (330 degrees) tomorrow evening. This next swell will peak near average late Tuesday before subsiding Wednesday afternoon. A moderate, long- period swell (330-340 degrees) could produce advisory level surf for north and west facing shores late Wednesday into Thursday. Another slightly larger, moderate, long- period swell (330-340 degrees) will fill in Friday through Saturday, keeping surf elevated along north and west facing shores above advisory levels.
Background, medium- to long-period south to southwest swells will continue to filter into the area through the week, keeping surf along south facing shores from going flat. Small, choppy surf along east facing shores will increase and become rough for the latter half of the week as the trades strengthen.

Fire weather

Winds will remain breezy through the week, but humidity values are expected to be high enough to prevent critical fire weather from occurring. Temperature inversion heights near Maui and the Big Island will range from 6,500 to 8,500 feet today.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST Tuesday for Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.