Photo Credit: James Grenz

Hilo

Today: Partly sunny with showers likely and slight chance of thunderstorms. Highs around 78 near the shore to 63 to 68 at 4000 feet. Northwest winds up to 10 mph shifting to the east in the afternoon. Chance of rain 70 percent.

Tonight: Mostly cloudy. Showers likely and slight chance of thunderstorms early in the evening, then occasional showers and slight chance of thunderstorms in the late evening and overnight. Lows around 64 near the shore to around 50 at 4000 feet. Northwest winds up to 10 mph. Chance of rain 80 percent.

Thursday: Mostly cloudy with showers likely and slight chance of thunderstorms. Highs around 79 near the shore to 64 to 69 at 4000 feet. Northeast winds up to 15 mph. Chance of rain 70 percent.

Kona

Today: Mostly sunny in the morning, then partly sunny with showers likely and slight chance of thunderstorms in the afternoon. Highs 78 to 83 near the shore to around 69 near 5000 feet. Light winds. Chance of rain 60 percent.

Tonight: Mostly cloudy with showers likely and slight chance of thunderstorms early in the evening, then partly cloudy with slight chance of thunderstorms and isolated showers in the late evening and overnight. Lows 64 to 70 near the shore to around 51 near 5000 feet. Light winds. Chance of rain 60 percent.

Thursday: Mostly sunny with slight chance of thunderstorms in the morning, then partly sunny with showers likely and slight chance of thunderstorms in the afternoon. Highs 79 to 84 near the shore to around 70 near 5000 feet. West winds up to 10 mph. Chance of rain 60 percent.

Waimea

Today: Partly sunny. Scattered showers and slight chance of thunderstorms in the morning, then showers likely and slight chance of thunderstorms in the afternoon. Highs around 77 near the shore to 69 to 76 near 3000 feet. East winds up to 15 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy. Showers likely and slight chance of thunderstorms early in the evening, then scattered showers and slight chance of thunderstorms in the late evening and overnight. Lows around 63 near the shore to 49 to 57 near 3000 feet. East winds up to 15 mph. Chance of rain 70 percent.

Thursday: Partly sunny. Breezy. Scattered showers and slight chance of thunderstorms in the morning, then showers likely and slight chance of thunderstorms in the afternoon. Highs 63 to 79. East winds 10 to 20 mph. Chance of rain 70 percent.

Kohala

Today: Partly sunny. Scattered showers and slight chance of thunderstorms in the morning, then showers likely and slight chance of thunderstorms in the afternoon. Highs around 77 near the shore to 69 to 76 near 3000 feet. East winds up to 15 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy. Showers likely and slight chance of thunderstorms early in the evening, then scattered showers and slight chance of thunderstorms in the late evening and overnight. Lows around 63 near the shore to 49 to 57 near 3000 feet. East winds up to 15 mph. Chance of rain 70 percent.

Thursday: Partly sunny. Breezy. Scattered showers and slight chance of thunderstorms in the morning, then showers likely and slight chance of thunderstorms in the afternoon. Highs 63 to 79. East winds 10 to 20 mph. Chance of rain 70 percent.

South Big Island

Today: Sunny in the morning, then partly sunny with scattered showers and slight chance of thunderstorms in the afternoon. Highs around 81 near the shore to around 70 near 5000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Tonight: Partly cloudy with scattered showers and slight chance of thunderstorms. Lows around 66 near the shore to around 50 near 5000 feet. Northeast winds up to 15 mph. Chance of rain 50 percent.

Thursday: Mostly sunny in the morning then becoming partly sunny. Breezy. Scattered showers and slight chance of thunderstorms. Highs around 82 near the shore to around 71 near 5000 feet. East winds up to 25 mph. Chance of rain 50 percent.

Puna

Today: Partly sunny with showers likely and slight chance of thunderstorms. Highs around 78 near the shore to 63 to 68 at 4000 feet. Northwest winds up to 10 mph shifting to the east in the afternoon. Chance of rain 70 percent.

Tonight: Mostly cloudy. Showers likely and slight chance of thunderstorms early in the evening, then occasional showers and slight chance of thunderstorms in the late evening and overnight. Lows around 64 near the shore to around 50 at 4000 feet. Northwest winds up to 10 mph. Chance of rain 80 percent.

Thursday: Mostly cloudy with showers likely and slight chance of thunderstorms. Highs around 79 near the shore to 64 to 69 at 4000 feet. Northeast winds up to 15 mph. Chance of rain 70 percent.

Waikoloa

Today: Mostly sunny with slight chance of thunderstorms and isolated showers in the morning, then partly sunny with showers likely and slight chance of thunderstorms in the afternoon. Highs around 80 near the shore to 64 to 71 above 4000 feet. East winds up to 10 mph shifting to the north in the afternoon. Chance of rain 60 percent.

Tonight: Partly cloudy. Showers likely and slight chance of thunderstorms early in the evening, then slight chance of thunderstorms and isolated showers in the late evening and overnight. Lows around 68 near the shore to 46 to 54 above 4000 feet. Southeast winds up to 15 mph. Chance of rain 60 percent.

Thursday: Mostly sunny with slight chance of thunderstorms and isolated showers in the morning, then partly sunny with showers likely and slight chance of thunderstorms in the afternoon. Highs around 81 near the shore to 65 to 72 above 4000 feet. Northeast winds up to 15 mph. Chance of rain 60 percent.

Detailed Forecast

Synopsis

An extended period of unsettled weather is expected, with a surface trough lingering in the vicinity of the islands and several slow moving disturbances aloft moving through the region. Prevailing moderate northeast to east trade winds will have showers favoring windward and mauka areas during nights and mornings, and interior and leeward areas during the afternoon and early evening hours. A few of the showers could produce some locally heavy downpours each afternoon, and a thunderstorm or two will be possible mainly during the afternoon hours Wednesday and Thursday as a very cold upper level low moves over the island chain.

Discussion

Currently at the surface, a southwest to northeast oriented ridge of high pressure is positioned around 750 miles northwest of Kauai, while broad north to south oriented trough is located very near the Big Island. Light to moderate trade winds prevail across the island chain early this morning, with land breezes in some of the more sheltered locations. Infrared satellite imagery shows partly cloudy conditions in most areas, with a bit more cloud cover associated with showers moving through. Radar imagery shows scattered to numerous showers moving into windward slopes and coasts, particularly over Kauai and Oahu, with a few showers spilling over into leeward communities at times as well. Main short term focus revolves around rain chances and thunder potential during the next several days.
A broad upper level low just north of the state will shift southward over the central and eastern islands today, then meander in this vicinity through Thursday. The upper low is forecast to open up into a trough Thursday night and linger linger over the eastern half of the state through Friday. Upper ridging may briefly build over the islands Friday night, with a shortwave trough then moving in from the west Saturday and Saturday night, with a weakness aloft remaining over the state through early next week.
Meanwhile at the surface, a broad trough will linger over or near the eastern/central islands through the entire forecast period, with some deviations in position expected from day to day as a result of the positioning and movement of the upper level features. Overall, only minor fluctuations in trade wind strength are expected through the forecast period, with moderate northeast trades expected over the western half of the state, while moderate easterly trades prevail over the eastern islands.
With the forcing aloft and the surface troughing in the vicinity of the islands, we expect a persistent unsettled pattern across the islands. Showers will generally favor windward and mauka areas, particularly during nights and mornings. With scattered showers expected over leeward and interior areas each afternoon. Despite relatively low precipitable water values in the 1.0 to 1.3 inch range, the unstable conditions will likely result in some locally heavy downpours, particularly during the afternoon hours. The most unstable weather associated with the upper level low is forecast today through Thursday, and as a result a slight chance of thunderstorms will remain in the forecast during this time, with the highest potential for thunder over the eastern end of the state. Conditions may remain unstable enough for a thunderstorm or two to pop each afternoon over the Big Island Friday through early next week, with thunder not out of the question over the other isles Monday and Tuesday as some deeper moisture moves in from the east.
Lastly, it will remain cold enough for some wintry weather over the Big Island summits in convection through the entire forecast period. Additionally, some frozen precipitation can't be ruled out over the Haleakala summit above 9 kft today through Thursday night before wet-bulb 0 temperatures climb high enough to preclude any wintry weather.

Aviation

The Upper low just north of the islands has destabilized the air mass over the state. This, combined with an ample moisture supply in the lower levels, creates the potential for moderate to heavy showers over the next twenty-four hours and beyond. Possible IFR conditions in isolated thunderstorms statewide through this coming evening. Mountain obscuration due to heavy rain will likely become an issue for Windward Big Island Wednesday afternoon or evening.
As of 2 am HST Wednesday, IR satellite imagery revealed multiple bands of high topped cumulus approaching the islands from the northeast. Radar indicates light to moderate showers associated with these bands. Shower activity is currently greatest along windward slopes and coasts of Kauai and Oahu but will likely spread across the remainder of the smaller islands this morning and the Big Island this afternoon.

Marine

A very large west-northwest swell continues across the coastal waters early this morning. A Small Craft Advisory (SCA) remains in effect for the waters from Kauai to Maui County with seas of 10 feet or greater that are exposed to this swell. A surface trough passing by to the north of the state has caused the trade winds to weaken slightly overnight. Therefore, the SCA has been discontinued for the typically windier waters adjacent to the islands of Maui County and the Big Island. The forecast guidance continues to show a new surface trough developing near or just east of the eastern islands Thursday. Depending on the location and strength of this feature, moderate northeasterly winds are expected to develop over portions of the state, especially over the central and western islands, from late Thursday into this weekend. Note that we do not currently have SCA conditions due to winds or seas in the forecast from tonight through this weekend.
The large west-northwest swell, which was mentioned above, continues to be at least 2 feet above the model guidance. The surf produced by this swell will likely remain near the High Surf Warning thresholds along north and west facing shores of Kauai, Niihau, and Oahu, as well as west facing shores of the Big Island, into this afternoon. A High Surf Advisory (HSA) also remains in effect for north and west facing shores of Molokai, and for north facing shores of Maui. This west-northwest swell will gradually lower later today and tonight, but expect the surf to remain well within the HSA criteria along exposed north and west facing shores through early Thursday morning. This west-northwest swell will continue to subside from Thursday into Friday before a new northwest swell arrives Friday night. Surf produced by this new northwest swell is expected to peak below the HSA criteria along north and west facing shores on Saturday, before it starts to lower Saturday night. In addition, a new north swell with a wave period of around 10 seconds may spread down over the state later this weekend. Otherwise, expect surf heights to gradually lower from Sunday through the George Washington's Birthday Holiday.
An out-of season south swell will keep surf along south facing shores well above our typical wintertime flat conditions. This south swell will eventually lower from Thursday night through Friday, with surf returning to more typical background conditions along south facing shores from this weekend into early next week. A small northeast swell with a wave period of around 9 seconds combined with wind waves due to the moderate trade winds is keeping modest surf along east facing shores. Expect surf to be near the seasonal average along east facing shores into this weekend.

HFO Watches/Warnings/Advisories

High Surf Warning until 6 PM HST this evening for Niihau, Kauai Leeward, Waianae Coast, Oahu North Shore, Kona, Kohala, Kauai North, Big Island South.
High Surf Advisory until 6 PM HST this evening for Maui Windward West, Molokai Windward, Molokai North, Molokai West, Windward Haleakala.
Small Craft Advisory until 4 PM HST this afternoon for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Kaiwi Channel, Maui County Windward Waters.

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Data Courtesy of NOAA.gov