Photo Credit: Sebastien Gabriel

West Kaua’i

Rest Of Today: Cloudy. Scattered showers and slight chance of thunderstorms in the morning, then showers and slight chance of thunderstorms in the afternoon. Highs around 80 near the shore to around 68 above 3000 feet. East winds up to 15 mph. Chance of rain 90 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 67 near the shore to around 56 above 3000 feet. East winds up to 10 mph. Chance of rain 50 percent.

Tuesday: Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs around 79 near the shore to around 68 above 3000 feet. East winds up to 15 mph shifting to the south in the afternoon. Chance of rain near 100 percent.

South Kaua’i

Rest Of Today: Cloudy. Showers likely and slight chance of thunderstorms in the morning, then showers and slight chance of thunderstorms in the afternoon. Highs around 79. Northeast winds 10 to 15 mph. Chance of rain 80 percent.

Tonight: Mostly cloudy with scattered showers. Lows 63 to 68. Northeast winds up to 10 mph. Chance of rain 50 percent.

Tuesday: Mostly cloudy with showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs around 78. West winds up to 10 mph shifting to the southwest in the afternoon. Chance of rain 90 percent.

Kaua’i Mountains

Rest Of Today: Showers and slight chance of thunderstorms. Highs 67 to 75 in the valleys to around 62 above 4000 feet. East winds up to 15 mph. Chance of rain near 100 percent.

Tonight: Mostly cloudy with scattered showers. Lows 56 to 61 in the valleys to around 50 above 4000 feet. Southeast winds up to 10 mph. Chance of rain 40 percent.

Tuesday: Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs 67 to 75 in the valleys to around 62 above 4000 feet. South winds up to 15 mph. Chance of rain near 100 percent.

North Kaua’i

Rest Of Today: Showers and slight chance of thunderstorms. Highs 72 to 79. East winds up to 15 mph. Chance of rain near 100 percent.

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

Tuesday: Mostly cloudy with showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs 72 to 79. Southwest winds up to 15 mph. Chance of rain 90 percent.

East Kaua’i

Rest Of Today: Showers and slight chance of thunderstorms. Highs 72 to 79. East winds 10 to 15 mph. Chance of rain near 100 percent.

Tonight: Mostly cloudy with scattered showers. Lows 57 to 69. East winds up to 10 mph shifting to the southwest after midnight. Chance of rain 40 percent.

Tuesday: Showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs 72 to 79. Southwest winds 10 to 15 mph. Chance of rain near 100 percent.

Detailed Forecast

Synopsis

Trade wind weather today will gradually transition over to a more unsettled pattern on Tuesday across the western islands and across the entire state from Wednesday through Friday as a Kona low matures and gradually breaks down to the west of the island chain. The kona low will bring the potential for heavy rain and thunderstorms to the islands, with the threat for heavy rain beginning as early as Tuesday across Kauai and Oahu. Shower coverage will gradually diminish over the weekend although some locally heavy showers may continue with a low aloft remaining over the islands.

Discussion

Currently at the surface, a 1028 mb high resides around 1150 miles north of Honolulu, while a surface trough is positioned around 1150 miles west of Kauai. The resulting pressure gradient is producing moderate to locally breezy trade winds across the island chain early this morning. Infrared satellite imagery shows extensive jet stream cirrus resulting in mostly cloudy to overcast conditions across the state. Radar imagery shows scattered to numerous showers moving into windward areas, with a a few of these showers spilling into leeward communities at times. The main forecast challenge continues to revolve around the potential impacts and duration of a kona low event expected to affect the islands through much of the week.
Moderate trade winds will persist today, with an easing expected over the western end of the state late today Monday as a new broad surface trough develops a couple hundred miles west of Kauai. The surface trough will amplify tonight and high pressure to the distant north will weaken, allowing for winds to shift southeast. A low will form along the sharpening surface trough on Tuesday, awaiting the closing off and alignment of an upper low Tuesday night to become a true kona low a few hundred miles west of Kauai on Wednesday. Moderate southerly winds will develop across the islands by late Tuesday and hold in place through Wednesday. Winds appear to shift more southeasterly at generally moderate speeds for the second half of the work week as the kona low begins to break down with the surface low moving westward and the upper low shifting to the east. The pressure gradient then appears to really relax over the weekend with broad surface troughing in place over the islands resulting in light and variable winds.
As for the remaining weather details, some rather wet trade wind weather is expected this morning across Kauai and Oahu, with shower coverage and intensity decreasing moving southeast across the remainder of the island chain. The shower coverage should decrease considerably from Oahu eastward by this afternoon, with some showery conditions likely persisting across Kauai. A transition period is expected tonight with the best forcing for ascent remaining to the northeast and northwest of the state. Shower coverage as a result should not be particularly high, with a windward focus in the evening transitioning to a leeward focus overnight as the flow becomes more southerly.
Things become more interesting Tuesday through Thursday. Deep southerly flow will take shape over the western islands Tuesday and Tuesday night as the kona low is in its development stages. This should lead to a marked increase in shower coverage and intensity across Kauai and Oahu, along with the potential for heavy rainfall and perhaps even a few rumbles of thunder. Across Maui County and the Big Island a dry slot, weaker forcing for ascent, and considerable cloud cover will make shower activity much more limited, although some deeper moisture moving into from the south may move into the Big Island toward daybreak Wednesday. A Flood watch has been issued with the morning forecast package for the islands of Niihau, Kauai and Oahu beginning Tuesday morning and continuing through Thursday afternoon.
Although there remains considerable uncertainty in the details, the main potential for heavy rainfall and thunderstorms encompassing the entire state appears to be during the Wednesday through Thursday time frame as the kona low matures and gradually breaks down to the west of the islands. If trends continue the Flood Watch will likely need to be expanded to the rest of the state.
Shower coverage should then gradually decrease Friday through the weekend although some locally heavy rainfall and a few rumbles of thunder can't be ruled out given lingering deep moisture and instability associated with an upper low pivoting overhead.

Aviation

Moderate to locally breezy trade winds will continue today, before weakening and veering from the southeasterly direction by tonight. A band of showers moving in from the east will enhance shower activity across much of the state, favoring the windward and mountain areas through the morning hours. A trough aloft just west of Kauai will increase clouds and showers over Kauai by this afternoon and evening with MVFR conditions likely.
AIRMET Sierra in effect for Mountain Obscuration above 025 due to clouds and showers over north thru east sections of Kauai and Oahu this morning. These conditions will likely diminish a few hours after sunrise.

Marine

Surface high pressure centered approximately 800 nautical miles north of the state is producing a tight enough pressure gradient down across the islands to result in moderate to locally strong trades through tonight. A deepening trough is forecast to evolve into a kona low over the far northwest waters by mid week. This will significantly increase the threat for widespread rain, periods of locally heavy rain, along with isolated thunderstorms. Confidence remains fairly low on the timing of the onset of the main event but a rough target window for the greatest marine impacts from visibility-reducing localized heavy rainfall and storms will be mid to late week.
A recent moderate size, medium period north-northwest (320 degree) swell that prompted this weekend's advisory level surf along many north and west-facing shores will continue to fall through Tuesday. Swell heights have declined enough to cancel this morning's High Surf Advisory (HSA). The next moderate size, medium period northwest (320 degree) swell originating from vicinity gales in association with the kona low will move in Wednesday and lift higher sets to slightly over head high, but stay well under HSA heights, through Friday. The Small Craft Advisory (SCA) is still in effect for the typically windier waters surrounding Maui County and Big Island. Near coastal seas have fallen below 10 feet but will still remain somewhat elevated due to the passage of a series of small north and slightly larger northwest swells. Surf along south-facing shores will remain small with only background south southwest swells expected through the week. East-facing shore chop will hold steady or steadily lower into the week due to disrupted trade flow caused by the area of low pressure to the west.

HFO Watches/Warnings/Advisories

Flood Watch from Tuesday 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.
Small Craft Advisory until 6 PM HST this evening for Maalaea Bay, Pailolo Channel, Alenuihaha Channel.
Small Craft Advisory until 6 AM HST Tuesday for Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov