Hilo

Today: Partly sunny with isolated showers. Highs around 82 near the shore to 65 to 70 at 4000 feet. South winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 20 percent.

Tonight: Mostly cloudy with isolated showers. Lows around 70 near the shore to around 56 at 4000 feet. South winds around 10 mph. Chance of rain 20 percent.

Wednesday: Mostly cloudy with showers likely and slight chance of thunderstorms. Locally heavy rainfall possible. Highs around 81 near the shore to 65 to 70 at 4000 feet. South winds 10 to 15 mph decreasing to up to 15 mph in the afternoon. Chance of rain 70 percent.

Kona

Today: Partly sunny with scattered showers. Highs 80 to 85 near the shore to around 71 near 5000 feet. West winds up to 10 mph. Chance of rain 30 percent.

Tonight: Mostly cloudy with isolated showers. Lows around 72 near the shore to around 57 near 5000 feet. South winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

Wednesday: Mostly cloudy with showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs 80 to 85 near the shore to around 71 near 5000 feet. Light winds. Chance of rain 90 percent.

Waimea

Today: Partly sunny with scattered showers. Highs around 81 near the shore to 71 to 78 near 3000 feet. Southwest winds up to 10 mph in the morning becoming light. Chance of rain 30 percent.

Tonight: Mostly cloudy with isolated showers. Lows around 69 near the shore to 55 to 63 near 3000 feet. Light winds. Chance of rain 20 percent.

Wednesday: Partly sunny with scattered showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs around 80 near the shore to 71 to 78 near 3000 feet. Light winds. Chance of rain 50 percent.

Kohala

Today: Partly sunny with scattered showers. Highs around 81 near the shore to 71 to 78 near 3000 feet. Southwest winds up to 10 mph in the morning becoming light. Chance of rain 30 percent.

Tonight: Mostly cloudy with isolated showers. Lows around 69 near the shore to 55 to 63 near 3000 feet. Light winds. Chance of rain 20 percent.

Wednesday: Partly sunny with scattered showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs around 80 near the shore to 71 to 78 near 3000 feet. Light winds. Chance of rain 50 percent.

South Big Island

Today: Breezy. Partly sunny with scattered showers. Highs around 83 near the shore to around 72 near 5000 feet. East winds 10 to 20 mph. Chance of rain 30 percent.

Tonight: Mostly cloudy with showers likely. Locally heavy rainfall possible. Lows around 72 near the shore to around 56 near 5000 feet. East winds around 10 mph. Chance of rain 70 percent.

Wednesday: Mostly cloudy with showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs around 83 near the shore to around 72 near 5000 feet. East winds around 10 mph. Chance of rain 90 percent.

Puna

Today: Partly sunny with isolated showers. Highs around 82 near the shore to 65 to 70 at 4000 feet. South winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 20 percent.

Tonight: Mostly cloudy with isolated showers. Lows around 70 near the shore to around 56 at 4000 feet. South winds around 10 mph. Chance of rain 20 percent.

Wednesday: Mostly cloudy with showers likely and slight chance of thunderstorms. Locally heavy rainfall possible. Highs around 81 near the shore to 65 to 70 at 4000 feet. South winds 10 to 15 mph decreasing to up to 15 mph in the afternoon. Chance of rain 70 percent.

Waikoloa

Today: Partly sunny with scattered showers. Highs around 82 near the shore to 66 to 73 above 4000 feet. West winds up to 10 mph. Chance of rain 30 percent.

Tonight: Mostly cloudy with isolated showers. Lows around 73 near the shore to 52 to 60 above 4000 feet. Light winds. Chance of rain 20 percent.

Wednesday: Partly sunny with scattered showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs around 82 near the shore to 66 to 73 above 4000 feet. Light winds. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Winds will gradually transition to southerlies as a more unsettled weather pattern develops today through Friday. A kona low will increase potential for heavy rain and thunderstorms across Kauai and Oahu as early as this morning before spreading this potential to the remaining islands over the following days. Shower coverage will gradually diminish Friday through the weekend, though the potential for locally heavy showers may continue as the low aloft lingers.

Discussion

A high remains far north-northeast of the main Hawaiian Islands, while a surface trough continues to develop west of Kauai. The resulting pressure gradient drives moderate east-southeast to southeast winds across the island chain this morning. Radar shows scattered showers embedded within this flow, mainly over water. However, numerous showers and isolated thunderstorms are beginning to edge into local waters west and southwest of Kauai. Models show a low will close off from the surface trough west of Kauai today. On Wednesday, once associated upper troughing closes off and aligns with the surface low, the system will become a kona low a few hundred miles west of Kauai. Moderate southerly winds will develop across the islands by late today and hold in place through Wednesday. Deep southerly flow will take shape over the western islands today and tonight as the kona low matures. Expect increased shower coverage and intensity across Kauai and Oahu, along with the potential for heavy rainfall and a few thunderstorms. Expect more limited shower activity across Maui County and the Big Island through tonight, although deeper moisture moving in from the south may reach the Big Island toward daybreak Wednesday. A Flood watch remains in effect for the islands of Niihau, Kauai and Oahu from this morning through Thursday afternoon and has been expanded to include all of the main Hawaiian Islands from Wednesday morning through Thursday afternoon. Although details are uncertain, heavy rainfall and thunderstorms will gradually encompass the entire state, most likely Wednesday through Thursday.
Expect winds to shift more southeasterly for the second half of the week as the kona low begins to break down. The pressure gradient will relax significantly over the weekend with light and variable winds expected due to broad surface troughing in place over the islands. Shower coverage should gradually decrease Friday through the weekend, though brief heavy rainfall and thunderstorms can't be completely ruled out.

Aviation

Winds will continue to veer southerly as an area of low pressure develops west of the state. Expect shower coverage to increase across Kauai and Oahu this morning then spread to the remaining islands throughout the day. MVFR or lower conditions will impact western islands this morning as showers increase, with reduced flight conditions likely spreading to the other islands later today. Unsettled conditions will continue for the next several days.
AIRMET Sierra for tempo mountain obscuration remains in effect for Kauai and Oahu this morning and may need to be issued for the remaining smaller islands today as showers increase. AIRMET Tango for tempo moderate turbulence between FL200-300 remains in effect for the near future. Will also continue to see tempo light icing across the islands for the near future.

Marine

A surface trough associated with an upper level low located northwest of Kauai and surface high pressure far north- northeast of the state will produce moderate to fresh east to southeast flow across the waters. As the kona low develops within the surface trough in the northwest offshore waters, the southeast winds will weaken. Mainly variable to light south winds are expected to continue through the end of the week.
Thunderstorms have continued through the night for the waters northwest of Kauai near the upper level low. Expect the chance for thunderstorms to continue over the northwest offshore and coastal waters today, and then spread to the east over the offshore and coastal waters by Wednesday, with the chance persisting through the end of the week.
The current medium period north-northwest swell will continue to subside through the day. A new small to moderate, medium period northwest (320 degree) swell generated by the nearby kona low is expected to arrive Wednesday, resulting in a rise of surf along exposed north and west facing shores that will persist through Friday. Background south-southwest swells are expected through the week. Surf along east facing shores is expected to remain below average due to the trade winds weakening through the week.

HFO Watches/Warnings/Advisories

Flood Watch from 6 AM HST this morning through Thursday afternoon for Niihau, Kauai Leeward, Kauai Mountains, Waianae Coast, Oahu North Shore, Olomana, Central Oahu, Waianae Mountains, Kauai North, Kauai East, Kauai South, East Honolulu, Honolulu Metro, Ewa Plain, Koolau Windward, Koolau Leeward.
Flood Watch from Wednesday morning through Thursday afternoon for Lanai Mauka, Kahoolawe, Maui Windward West, Maui Leeward West, Haleakala Summit, Kona, Kohala, Big Island Interior, Big Island Summits, Molokai, Lanai Windward, Lanai Leeward, Lanai South, Maui Central Valley North, Maui Central Valley South, Windward Haleakala, Kipahulu, South Maui/Upcountry, South Haleakala, Big Island South, Big Island Southeast, Big Island East, Big Island North.

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Data Courtesy of NOAA.gov