West Kaua’i

Tonight: Mostly clear. Breezy. Isolated showers after midnight. Lows 66 to 71 near the shore to around 57 above 3000 feet. East winds 10 to 20 mph. Chance of rain 20 percent.

Friday: Mostly sunny with isolated showers. Highs around 83 near the shore to around 73 above 3000 feet. East winds up to 15 mph. Chance of rain 20 percent.

Friday Night: Partly cloudy with isolated showers. Lows around 68 near the shore to around 57 above 3000 feet. Northeast winds up to 15 mph increasing to 10 to 15 mph after midnight. Chance of rain 20 percent.

South Kaua’i

Tonight: Breezy. Partly cloudy with isolated showers. Lows 65 to 70. Northeast winds 10 to 20 mph. Chance of rain 20 percent.

Friday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs around 82. East winds 10 to 15 mph. Chance of rain 40 percent.

Friday Night: Mostly cloudy with scattered showers. Lows 64 to 69. East winds 10 to 15 mph. Chance of rain 40 percent.

Kaua’i Mountains

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Scattered showers. Lows 56 to 64 in the valleys to around 54 above 4000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Friday: Partly sunny with scattered showers. Highs 72 to 77 in the valleys to around 65 above 4000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Friday Night: Mostly cloudy with scattered showers. Lows 56 to 64 in the valleys to around 53 above 4000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

North Kaua’i

Tonight: Partly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows 61 to 69. Southeast winds 10 to 15 mph. Chance of rain 40 percent.

Friday: Mostly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs 74 to 81. East winds up to 15 mph. Chance of rain 40 percent.

Friday Night: Mostly cloudy in the evening then becoming partly cloudy. Scattered showers. Lows 61 to 68. Southeast winds up to 15 mph becoming around 10 mph after midnight. Chance of rain 40 percent.

East Kaua’i

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Scattered showers. Lows 59 to 72. East winds 10 to 15 mph. Chance of rain 50 percent.

Friday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 73 to 83. East winds 10 to 15 mph. Chance of rain 50 percent.

Friday Night: Mostly cloudy with scattered showers. Lows 59 to 71. East winds 10 to 15 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Breezy and gusty trade winds will gradually decline through Friday as a surface high pressure to the northeast slowly weakens. Showers will remain focused over windward areas, with some afternoon showers developing across the leeward Big island slopes. East to southeast trades will be light enough for afternoon sea breezes and spotty showers to develop over all leeward areas Saturday into early next week. A upper level disturbance will bring some instability, resulting in some briefly heavier showers.

Discussion

No changes to the forecast this evening, with the forecast in the near term remaining on trade. The overall forecast philosophy remains the same, but the morning forecast package will likely have some modifications made to it based on the latest trends in the overall model forecast.
A 1028 mb high pressure system remains to the northeast of the islands, with trade winds continuing the downward trend as the high has gotten closer and weakened. The high is expected to remain nearly stationary through Friday as is continues to slowly weaken. By Saturday the high will have weakened further and moved to the east. Expect the trade winds to remain on the locally gusty side into Friday, with lighter winds in place across the islands by Saturday. Trade wind showers will continue to be carried in on the trades Friday, focusing the showers over the windward slopes of most islands, though a small pocket of moisture will produce more shower activity on windward Big Island this evening. A sharp upper-level trough just south of the state will drift over the Big Island tonight and produce some instability. As a result, showers on the Big Island could be heavier, with a couple of freezing rain showers possible on the high summits.
More instability is expected to develop over the islands late Friday and into Saturday, which coupled with the surface high weakening will help to weaken the trades over the region. Additionally, a surface low pressure system is expected to develop about 700 miles northwest of Kauai, which will help to turn the winds to the east and southeast.
An upper level trough already exists about 1000 miles to the northwest of Kauai, and it is from this system that the surface low is expected to form. The upper level trough is expected to remain to the west of the surface feature, but 500 mb heights near the islands will fall as a weak subtropical jet creeps toward the islands. This will result in the developing instability as early as Friday night that will wipe over the inversion and could lead to an increase in shower activity. Rainfall would continue to favor the east and windward slopes, but with the weaker winds, there could be the development of afternoon and evening showers over leeward areas. The global models keep the highest instability just north of the islands, so will need to continue to monitor their handling of this over the next several model runs.
A brief increase in trade winds is possible Sunday as the surface low weakens. The GFS weakens the low faster than the ECMWF, so will have to watch future runs to see which ends up happening. In either event, additional unstable light to moderate east to east- southeast winds are expected early next week. Persistent upper- level troughing over the region will keep the atmosphere slightly unstable, while surface troughing to the northwest will favor weakened and veered winds Sunday night into Tuesday. The GFS and ECMWF are showing the highest chances for rainfall just north of the island chain during this time, but there will be a chance of heavy showers over windward slopes, with spotty afternoon showers leeward.
A return to a more stable trade wind flow is expected by Wednesday.

Aviation

Breezy to locally strong trade winds will continue overnight before declining slightly Friday. Scattered showers, mainly along windward areas, may bring brief periods of MVFR ceilings and visibility. Otherwise, VFR conditions are generally expected elsewhere.
AIRMET Tango remains in effect through Friday for moderate low level turbulence over and downwind of island terrain.

Marine

Surface high pressure about 660 nm northeast of Oahu will continue to drift east and weaken over the next couple of days. Fresh to locally strong trades will ease to moderate to locally fresh speeds by Friday afternoon and hold through through Saturday. The Small Craft Advisory (SCA) for all waters has been trimmed back to windward waters east of the Kauai Channel. Another strong high will pass north of the state Sunday into early next week as a trough develops west of the state. This may strengthen trades to moderate to locally strong speeds briefly Sunday. The trough is expected to shift slightly east towards the state Monday before stalling Tuesday, veering winds out of the east- southeast and winds and easing wind speeds to light and variable over the western half of the state and moderate to locally fresh over the eastern half of the state. Moderate to locally strong trades will gradually return mid- week next week as the trough weakens and shifts far northwest of the state.
The current small to moderate, medium period north northwest swell will continue to fade. Forerunners of a new small to moderate, long period west northwest (300-320 degree) swell have reached offshore buoys 51101 and 51001. This swell should arrive locally during the early morning hours Friday and fill in throughout the day before peaking Friday afternoon into early evening. North and west- facing shore surf will remain under advisory thresholds. This swell will gradually fade over the weekend. A moderate, medium period north northwest swell (340-350) may overlap the fading northwest swell Sunday night into Monday. In the long range, a series of small, long period northwest swell look to keep surf elevated through much of next week.
Surf along east-facing shores will remain rough and choppy but may ease a couple of notches Friday into early next week as winds ease locally and upstream.

Fire weather

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.HFO WATCHES/WARNINGS/ADVISORIES… Small Craft Advisory until 6 PM HST Friday for Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Windward Waters- Big Island Leeward Waters-Big Island Southeast Waters.