A cold front stalling out near Kauai and Oahu today will produce periods of moderate to locally heavy showers and isolated thunderstorms across the western half of the state in an unstable southerly wind pattern. More stable conditions are forecast over the eastern islands as a weak ridge and southeasterly winds limit shower activity. On Thursday the front weakens into a trough and lifts northwestward back over Kauai and Niihau, providing another round of showers over the Garden Isle with improving weather trends for Molokai and Oahu. The high pressure ridge aloft will slowly build back in over the islands as the trough north of the state fills in and lifts northward, heralding a return statewide to more stable easterly trade wind weather just in time for the weekend. These more typical and stable moderate to breezy trade winds with brief windward and mountain showers will last through Christmas Day.
The short term weather picture looks like a tale of two patterns currently unfolding across the region. A weakening front stalling out near Kauai and Oahu will continue to produce southerly winds drawing up additional unstable and deep tropical moisture across the western half of the state. Expect cloudy skies, periods of moderate to locally heavy showers, and isolated thunderstorms for these western islands through tonight. Meanwhile the eastern islands of Maui and the Big Island are under the more stable influence of a weak high pressure ridge aloft, keeping a drier southeasterly wind regime in place. The central islands of Molokai and Lanai are caught in between these two distinct weather patterns, and a mix of clouds and scattered showers will continue. These western wet weather conditions will continue through tonight with improving weather trends over the western islands from Thursday onward.
By Thursday, the high pressure ridge aloft will slowly build back in over the islands as the trough north of the state fills in and lifts northward, heralding a return statewide to more stable easterly trade wind weather just in time for the weekend. However, clouds and showers will linger over the islands in Kauai County for one more day as the trough slowly drifts across the island. Subsidence temperature inversion heights will range from around 5,000 to 7,000 feet elevation as the ridge builds back in, limiting cloud heights and shower potential. These more typical and stable moderate to breezy trade winds with brief windward and mountain showers will last through Christmas Day.
In the extended range outlook, another cold front may approach the Hawaiian Islands from the northwest a few days after Christmas, possibly triggering yet another round of wet weather conditions. Stay tuned.
A weakening cold front near Kauai and a prefrontal wind convergence boundary near Oahu will continue to produce southerly winds, driving unstable tropical moisture into the western islands of Niihau, Kauai and Oahu through tonight. Expect MVFR ceilings with cloudy skies along with periods of moderate to heavy showers and isolated thunderstorms. Isolated IFR conditions are possible in the heaviest showers or near Thunderstorms. Gusty winds will develop along the northern slopes of each island due to the southerly wind flow over the island mountains. The eastern half of the state will remain fairly stable and dry with prevailing VFR conditions.
The front will weaken into a trough as it slowly lifts northward back over Kauai on Thursday, as a mid level ridge slowly builds in across the state with improving weather trends lasting into the weekend as easterly trade winds gradually return.
AIRMET Sierra remains in effect for tempo mountain obscurations Kauai, Oahu, and Molokai today. IFR conditions over Lanai this morning will likely transition to mountain obscuration around 16Z.
A cold front just NW of Kauai waters will drift over Kauai and stall near Oahu tonight. Moderate to locally strong S winds will hold over the western half of the state ahead of the front before easing to light to gentle speeds tonight and Thursday. A Small Craft Advisory (SCA) remains in effect for strong winds and seas to 10 ft through tonight. Isolated thunderstorms and heavy showers remain possible mainly over Kauai and Oahu waters through Thursday. A strong high pressure system will build N of the state Friday allowing light to moderate trades to return and gradually strengthen to moderate to strong speeds Sunday into early next week.
A moderate, medium period N swell (350-360 degree) will fade today as a moderate, short to medium period WNW swell (280 to 320 degrees) fills in. This swell will elevate surf along north and west facing shores to just below the HSA threshold and will create fairly choppy conditions. Seas will build to 10 feet and higher around Kauai and Oahu waters. A large, medium period NNW (330-350 degree) swell will fill in tonight and peak Thursday right around advisory levels. The SCA may need to be extended through Friday and expanded to include windward Maui waters as this swell will keep seas elevated. This swell will gradually fade out through the remainder of the week with small to moderate N and NW swells mixed in through the weekend.
E shore surf will remain small except where NW-N swell energy wraps through the week. By next week, a large, short period NE swell could elevate surf along E facing shores as strong trade winds return. S shore surf will remain choppy as persistent S-SW winds hold through today, before gradually easing through the rest of week. Small background SW swells will keep surf from going flat through the weekend.
Flood Watch through Thursday afternoon for the islands of Niihau, Kauai and Oahu.
Small Craft Advisory until 6 AM HST Thursday for waters and channels around Kauai and Oahu.
