Photo Credit: Sebastien Gabriel

West Kaua’i

Rest Of Today: Breezy. Mostly sunny with isolated showers. Highs around 85 near the shore to around 75 above 3000 feet. East winds up to 20 mph. Chance of rain 20 percent.

Tonight: Partly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows 65 to 70 near the shore to around 56 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 50 percent.

Friday: Mostly sunny with scattered showers. Highs around 85 near the shore to around 75 above 3000 feet. East winds up to 15 mph. Chance of rain 40 percent.

South Kaua’i

Rest Of Today: Breezy. Mostly sunny with isolated showers. Highs around 83. Northeast winds 15 to 20 mph. Chance of rain 20 percent.

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Breezy. Scattered showers. Lows 63 to 69. Northeast winds 10 to 20 mph. Chance of rain 50 percent.

Friday: Breezy. Partly sunny with scattered showers. Highs around 83. East winds 10 to 20 mph. Chance of rain 50 percent.

Kaua’i Mountains

Rest Of Today: Breezy. Partly sunny with scattered showers. Highs around 78 in the valleys to around 67 above 4000 feet. East winds up to 20 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 40 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 59 in the valleys to around 51 above 4000 feet. East winds 10 to 15 mph decreasing to up to 15 mph after midnight. Chance of rain 50 percent.

Friday: Partly sunny with scattered showers. Highs around 78 in the valleys to around 67 above 4000 feet. East winds up to 15 mph. Chance of rain 50 percent.

North Kaua’i

Rest Of Today: Mostly sunny with scattered showers. Highs 77 to 84. East winds up to 15 mph. Chance of rain 30 percent.

Tonight: Mostly cloudy with scattered showers. Lows 59 to 68. East winds up to 15 mph. Chance of rain 50 percent.

Friday: Partly sunny in the morning then becoming mostly sunny. Scattered showers. Highs 77 to 84. East winds up to 15 mph. Chance of rain 50 percent.

East Kaua’i

Rest Of Today: Partly sunny with scattered showers. Highs 76 to 82. East winds 10 to 15 mph. Chance of rain 30 percent.

Tonight: Mostly cloudy with scattered showers. Lows 57 to 70. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Friday: Partly sunny with scattered showers. Highs 76 to 82. East winds 10 to 15 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

The strong high pressure system far north of the state will continue to slowly weaken with breezy trade winds decreasing into the moderate to locally breezy range through Saturday. Embedded weak upper lows north of the state will enhance some shower activity over eastern islands near Maui and the Big Island today with isolated thunderstorms. The ridge will weaken further on Sunday and Monday producing drier southeasterly winds over most areas. A low pressure system and cold front approaching the islands from the northwest on Tuesday will produce southerly winds with increasing shower activity over the western islands by Wednesday.

Discussion

The satellite water vapor imagery this morning shows the high level cirrus clouds with the subtropical jet stream are slowly drifting eastward. Scattered to numerous showers are showing up on the local radar imagery. However, most of these shower bands are moving towards the west-northwest direction, and passing just north of the smaller islands west of Maui. Upper air balloon soundings from 2 AM HST this morning show the trade wind inversion height around 8,000 feet, which supports this mornings shower coverage.
The subtropical jet stream over the islands will continue to weaken today with decreasing upper level cloud cover over the western half of the state. This jet stream remains just south of a narrow upper level trough with embedded weak upper lows. Weather guidance suggests one of the upper lows will become strong enough to cause a low level trough to form near Maui and the Big Island today increasing shower trends for both islands. Some of these showers will be locally heavy at times with the potential for isolated thunderstorms. The rest of the islands west of Maui will see less rainfall activity as the ridge north of the state continues to weaken into the weekend.
By Sunday and Monday, the weather pattern changes as the ridge north of the state weakens in response to a cold frontal system moving into the region from the northwest. Easterly trade winds will veer from a more southeasterly direction, causing a rain shadow effect for all islands west of the Big Island, and keeping passing showers moving more parallel to Maui, Lanai, Molokai, Oahu, Niihau and Kauai. Drier conditions are expected for all areas except over the Big Island where southeasterly winds will enhance showers over the southeastern slopes near the Ka'u and Puna Districts.
Big changes are coming to the long range Hawaii weather forecast from Tuesday through next Thursday. Recent forecast guidance continues to show a low pressure system and cold front approaching the western half of the state on Tuesday. Weather model solutions are unfortunately still highly variable on the timing and location of showers associated with this next unstable weather system. A trailing cold front extending south from the approaching low moves into western islands from late Tuesday to Wednesday. This system may bring large scale southerly winds to the western islands. Unstable deep tropical moisture, converging surface winds and upper level forcing are all favorable ingredients for widespread moderate to locally heavy showers and possible thunderstorms.
The European (ECMWF) 13/00Z model is now trending towards the American (GFS) 13/06Z model with a large band of showers ahead of the cold front stalling out near Kauai and Oahu. Our confidence remains only moderate that some islands, especially Kauai, Niihau, Oahu and Molokai, will see a round of widespread rainfall by the middle of next week. The confidence in the potential onset timing and duration of heavy rain and thunderstorm impacts for each island remain low at this time.
Please stay tuned to next weeks weather forecast as locally heavy rain and thunderstorms are possible over the western half of the state. Expect changes to the current weather forecast with evolving weather impacts as the time period grows shorter.

Aviation

High pressure well north of the state will maintain moderate to locally breezy trade winds today. This trade wind pattern will focus a majority of showers and lower cloud cover across windward areas through the TAF period. Mainly VFR conditions are expected, with MVFR conditions at times in heavier showers. AIRMET Sierra is in effect for Windward Big Island. Conditions are expected to improve after noon.
AIRMET Tango remains in effect for moderate low level turbulence over and downwind of higher terrain across all islands. Conditions are expected to improve this evening when trade winds are forecast to weaken.

Marine

The 1039 mb high pressure cell centered approximately 1,300 miles northeast of Oahu will begin to move east as a new high expanding down from the Aleutian Islands replaces it far northwest of the state Friday. This will relax the pressure gradient upstream of the islands as the high slowly meanders east southeast the next day or so. This morning's ASCAT satellite pass did show continued Small Craft Advisory (SCA) winds through more eastern island bays and channels, as well as south of Big Island. The local buoys are observing a slow decline in short period north northeast wind waves this morning with many buoys currently reporting 8 to 10 foot combined seas. A Small Craft Advisory (SCA) is in effect through early Friday morning for the typically windy waters surrounding Maui County and Big Island along with the Maui and Big Island windward waters. The SCA may need to be extended into Friday afternoon for the more notoriously winder marine zones surrounding Maui County and Big Island. After a weekend of relatively light trade flow, fresh to locally strong trades will return early next week as the new high establishes itself east northeast of the state and re-tightens the upstream pressure gradient.
An ongoing small, short period north northeast (10-40 degree) swell will maintain near waist high surf along many north to northeast-facing shores through tonight. A small, medium period northwest (320 degree) swell is scheduled to arrive late Friday and fill in Saturday morning. This swell will maintain waist to near head high surf along many north-facing shores…near knee to waist high surf along west-facing shores Saturday. East-facing shores will experience continued choppy conditions through Friday in response to several days of this upstream fresh trade fetch. Surf along east-facing shores will slowly subside going into this weekend as trades fall off a notch. Small, background south swell energy will maintain knee to chest high surf along many southern shores. A very small, medium period south southwest bump in swell arriving tonight into early Friday will result in a small boost in weekend surf.

HFO Watches/Warnings/Advisories

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

Data Courtesy of NOAA.gov