West Kaua’i

Today: Mostly sunny. Isolated showers in the afternoon. Highs 86 to 92 near the shore to around 79 above 3000 feet. Northeast winds up to 15 mph shifting to the west in the afternoon. Chance of rain 20 percent.

Tonight: Partly cloudy with isolated showers in the evening, then mostly cloudy with scattered showers after midnight. Lows around 74 near the shore to around 63 above 3000 feet. Northeast winds up to 10 mph. Chance of rain 50 percent.

Wednesday: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 86 to 92 near the shore to around 78 above 3000 feet. Light winds becoming east up to 15 mph in the afternoon. Chance of rain 50 percent.

South Kaua’i

Today: Mostly sunny with isolated showers. Highs 83 to 91. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows around 74. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Wednesday: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 83 to 91. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Kaua’i Mountains

Today: Partly sunny with scattered showers. Highs 77 to 86 in the valleys to around 69 above 4000 feet. East winds up to 10 mph. Chance of rain 50 percent.

Tonight: Numerous showers in the evening, then showers after midnight. Lows 66 to 72 in the valleys to around 60 above 4000 feet. East winds up to 10 mph. Chance of rain near 100 percent.

Wednesday: Showers in the morning, then numerous showers in the afternoon. Highs 77 to 85 in the valleys to around 68 above 4000 feet. East winds up to 10 mph. Chance of rain near 100 percent.

North Kaua’i

Today: Mostly sunny with isolated showers. Highs 79 to 89. East winds up to 15 mph. Chance of rain 20 percent.

Tonight: Mostly cloudy. Scattered showers in the evening, then showers after midnight. Lows 66 to 74. East winds up to 10 mph in the evening becoming light. Chance of rain 90 percent.

Wednesday: Showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 78 to 88. East winds up to 15 mph. Chance of rain near 100 percent.

East Kaua’i

Today: Mostly sunny with isolated showers. Highs 76 to 88. Northeast winds up to 10 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 20 percent.

Tonight: Scattered showers in the evening, then showers after midnight. Lows 66 to 78. Northeast winds up to 10 mph in the evening becoming light. Chance of rain near 100 percent.

Wednesday: Showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 76 to 87. Northeast winds up to 10 mph. Chance of rain near 100 percent.

Detailed Forecast

Synopsis

The subtropical ridge north of the Hawaiian Islands will slowly weaken this week with light to moderate trade winds in the forecast lasting through Wednesday. The ridge north of the islands will weaken and winds will veer out of an east to southeast direction from Thursday onward, allowing an expansion of onshore sea breezes over each island during the daylight hours.An unstable band of clouds, the remnants of an old East Pacific cold front, will drift through the islands on the trade winds from Wednesday morning into Thursday enhancing showers statewide. Unsettled bands of showers will continue to ride into the islands from Thursday on into the weekend.

Discussion

The satellite picture this morning shows a weakening upper low with embedded thunderstorms roughly 400 miles northwest of Kauai. Bands of stable stratocumulus clouds are drifting into the islands on the moderate easterly trade winds today. An unsettled cloud band roughly 230 miles northeast of the state continues to ride towards the island chain. Upper air balloon soundings at Lihue and Hilo from 2 AM HST (12Z) this morning show subsidence temperature inversion heights today in the 5,000 to 6,000 foot elevation range. At this range only brief passing showers are forecast, mainly limited to windward and mountain areas. These stable weather conditions will continue through tonight.
Trade wind speeds will trend lower this week as the high pressure ridge north of the islands weakens in response to a passing high latitude cold front. A band of unstable clouds associated with an old East Pacific cold front will ride into the islands from early Wednesday morning into Thursday. Expect wet weather conditions during this time period as these unstable clouds are forced orographically over island mountain ranges. The best shower coverage and rainfall amounts will favor windward island mountain ranges during this time period. Some of the stronger showers with this system will reach the drier leeward areas.
These weakening trade winds will veer from a more east to southeast direction from Thursday onward. Decreasing large scale winds will allow thermally driven local scale sea breezes to strengthen during the daylight hours over all islands. Shower trends in this sea breeze pattern will tend to favor island mountain and interior sections each afternoon. The latest weather model solutions also show good chances for converging winds along the leeward sides of island mountains producing shower bands extending from west to northwest of each island. These developing shower bands may briefly enhance rainfall amounts over Oahu and Kauai in this southeasterly wind regime from Thursday through Saturday.

Aviation

Moderate trade winds will persist through the forecast period with clouds and showers focused mainly over windward and mauka areas. An uptick of moisture is expected tonight through Wednesday from remnants of an old frontal band. Localized MVFR possible within passing showers, but most sites will remain VFR.
AIRMET Sierra remains in effect for mountain obscuration due to low clouds and showers over windward Big Island. Conditions expected to improve later this morning but may return tonight.

Marine

Moderate to locally fresh easterly trade winds will persist through Wednesday before slowly diminishing through the latter have of the week into early next week.
A small medium period north swell filled in yesterday and will gradually fade today. A series of small medium period swells from the north and northwest will trickle in through the rest of the week. A moderate medium period northwest (330 degrees) swell is expected to arrive on Saturday and Sunday, building surf heights along north and west facing shores late this weekend into early next week.
A series of small medium period south swells will keep surf along south facing shores from going flat through the week.
Surf along east facing shores will remain on the small side in moderate trade wind flow, despite some small medium period swell moving into the region over the next couple of days from former tropical cyclone Narda.

Fire weather

Wind speeds will remain below critical fire weather thresholds for the next seven days. Temperature inversion heights near the Big Island and Maui will range from 5,000 to 6,000 feet today.

HFO Watches/Warnings/Advisories

None.